Discussing audio amplifier design -- BJT, discrete

[Jon Kirwan]

On Fri, 29 Jan 2010 01:35:05 -0500, "Paul E. Schoen"
<paul@peschoen.com> wrote:

snip
OK. So I made a similar amplifier output stage using two 2N3055s, and a
2N3904 and a 2N3906. With 8.4 VRMS input the output is 7.3 VRMS into 8 ohms
for 6.66 watts. Input power is 9.97 watts, efficiency is 67%. Some very
slight crossover distortion. 6 mA drive current (about 1.2k input
impedance). Looks good 20 Hz to 20 kHz. I added an output inductor which
affects output at higher frequencies. LTSpice ASCII follows.
snip

Got it and saved it. Sziklai pair on one side, Darlington on
the other. 3 NPN, 1 PNP. Now I'm thinking about tubes (they
don't come in complementary form) and using a signal splitter
and NPNs "all the way down!"

Jon

 

[pimpom]

Jon Kirwan wrote:

On Thu, 28 Jan 2010 19:19:06 +0530, "pimpom"
pimpom@invalid.invalid> wrote:


Q2 needs about +/-50uA peak of base
current at full drive. At signal frequencies, R2 (plus the
much
smaller input impedance of Q1) is effectively in parallel with
the output.

R2 is connected from the output to an input, which
effectively doesn't move much after arriving at it's DC bias
point. As you later point out, the _AC_ input impedance is
lowish (near 600 ohms), so the 10k is pretty close to one of
the rails at AC, anyway. Is that a different way of saying
what you just said? Or would you modify it?

That's another way of putting it, yes.

The output swings by about 4V peak at max power,
which has 400uA of negative feedback current going back
through
R2. The input current requirement goes up by a factor of 9.
IOW,
a negative feedback of 19db. This is substantially better than
nothing and should significantly reduce distortion and improve
frequency response.

Okay. This goes past me a little (as if maybe the earlier
point didn't.) I'd like to try and get a handle on it.

Let's start with the 4V peak swing at max power.

Since you are discussing AC and converting it 400uA current
via the 10k, I would normally take this to mean 4Vrms AC.
Which in Vp-p terms would be 2*SQRT(2) larger, or 11.3V which
I know is impossible without accounting for the BJTs, given
the 9V supply. So this forces me to think in terms of
something else. But what? Did you mean 4Vpeak, which would
be 8Vp-p? If so, that would be about 2.8Vrms.

Yes. It's 4Vp, 8Vp-p and 2.8Vrms. I wanted to give you a mental
picture of how much the output voltage can swing. Each output Q
has about 4.4V of Vce available, and about 4V before hard
saturation is reached (these are all round figure values). That's
4V peak for a sinusoidal wave form.

In that case,
wouldn't a better "understanding" come from then saying that
the negative feedback is closer to 280uA?

Yes, it's 280uA rms. But I was talking in terms of the maximum
amplitude of instantaneous change, which is why I used the terms
"swing" and "peak".

The next point is on your use of "goes up by a factor of 9."
Can you elaborate more on this topic? Where the 9 comes
from? For volts, not power, I think I can gather the point
that 20*log(9) = 19.085), so I'm not talking about that
conventional formula. I'm asking about the 9, itself, and

Without feedback, the input transistor Q1 needs 50uA of AC input
signal to drive the output Qs to full power output (still talking
in terms of peak to avoid confusion). With NFB, we need an
additional 400uA to overcome the current fed back from the
output. That's a total of 450uA peak, which is 9 times the
original 50uA.

Actually, I made an error when I cited the 50uA figure. Q1 is
biased at Ic = 7.6mA, Ib = 50uA. But only 5mA peak is needed from
Q1's collector to drive the output transistors. Divide that by
Q1's hfe of 150 and you get 33uA (peak) of AC signal current
needed into the base of Q1. The corrected total needed from the
signal source is now 433uA. The gain reduction factor due to NFB
is now 13 instead of 9. That's 22db (feedback is usually given in
db).

also your thinking along the lines of concluding that it
significantly reduces distortion.

The basic principle of NFB is that it reduces THD and extends
frequency response by a factor equal to the feedback ratio. So,
in our example, if you have 10% THD without feedback, it will
drop to 0.77% with the feedback factor of 13. But there are
caveats. E.g., phase shifts can cause undesireable effects,
especially with large amounts of feedback. I'm afraid a detailed
treatment of such things is really outside the scope of this
discussion - unless someone else is willing to take it up.

How does one decide how much is enough?

For one thing, how much distortion one is willing to put up with.
Another factor is input sensitivity, or IOW, how much gain is
needed. E.g., to drive the 1W amp to full output, we need 433uA
peak (306uA rms) from the signal source into 1k. That's 306mV
rms, plus some millivolts at the b-e junction. Say about 0.32V
rms total input voltage into about 1k input impedance.

To present the basic concepts, I've made several approximations.
E.g., I neglected the shunting effect of R2. Besides, the input
resistance of Q1 is constant at 600 ohms only for very small
signal amplitudes relative to the quiescent dc levels. This
dynamic input resistance changes significantly with large signal
swings and adds distortion while also complicating precise
calculations.

 

[George Herold]

On Jan 27, 8:43 am, "Phil Allison" <phi...@tpg.com.au> wrote:

"Jon Kirwan is a FUCKWIT TROLL  "

DO  NOT  FEED  THE TROLL  !!!!!!!!!!!!!!!

DO  NOT  FEED  THE TROLL  !!!!!!!!!!!!!!!

TROLLS  are  DESTROYERS of all  NEWSGROUPS.
-------------------------------------------------------------

...   Phil

Dang Phil, take it easy. This seems like the first good thread that
there's been on this news group for a while.

George H.

 

[George Herold]

On Jan 27, 2:19 pm, Jon Kirwan <j...@infinitefactors.org> wrote:

On Wed, 27 Jan 2010 13:23:28 GMT, Bob Masta wrote:
On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan
j...@infinitefactors.org> wrote:

I'd like to take a crack at thinking through a design of an
audio amplifier made up of discrete BJTs and other discrete
parts as an educational process.

snip

When I was first getting interested in power amp
design (back in the '70s) I started collecting
schematics for all the power amps I could get my
hands on, to compare them.  I noticed that the
schematics for simple bipolar op-amp ICs were
remarkably similar to those for big discrete power
amps.  If you have an old National Linear Databook
(or don't mind a lot of rooting around on the Web
for individual datasheets), you might take a look.

You can build a pretty decent amp with only a
handful of transistors.  The same basic circuit
can be used for a wide range of output powers,
just by changing the power supply voltages and the
output device ratings.

Best regards,

Thanks, Bob.  Audio amplifiers, especially ones delivering
_some_ power, seem to offer such an excellent way to learn.
The basic idea, at a behavioral level, is fairly simple.  An
implementation requires some knowledge and thought in the
end.  So the destination is arrived at by taking a great path
to walk, with such wonderful vistas to see, I think.  Much of
interest is along the way of getting there.

I may have an old National databook on linear parts
somewhere.  I keep a lot, but I also have several thousand
books in my library which covers all of the walls in one of
the rooms.  I'm at a point now where to get room for more
books, others must be boxed and stored or simply destroyed
and pulped.  So it's a _maybe_.

One of the nice things (to me) about this kind of a path,
too, is that what I learn can be used for lots of things.  An
audio amplifier is, in effect, not that much different from
an op amp.  There is the usual basic idea of open loop gain
and closed loop gain with negative feedback, phase margins,
problems to solve over a frequency range spanning many
decades, and so on.

A completely separate project I'd like to play with, which
this learning will help prepare me for, is designing a pin
driver.  I'd like to sink or source a programmable current
spanning decades from perhaps 100nA to perhaps 100uA while
reading the voltage at the node, as well as being able to
program a low impedance voltages spanning from -15V to +15V
there and read the current, or read a voltage at the same
node while presenting a fairly high impedence to it.  I
imagine what I learn here will aid me there.  And I'd like to
do this at some speed, as well.  I may then start with a BJT
tester, for example, making up only three of these to start
and tying them into a micro for playing.  Expanding that for
other purposes, later.  It would be fun.

Jon- Hide quoted text -

- Show quoted text -

Hi Jon, I'm enjoying your posts. What's a pin driver? I made a nice
switchable current source (10nA to 1mA) from a voltage reference,
opamp and switchable resistors. (circuit cribbed from AoE.)

George H.

 

[George Herold]

On Jan 27, 9:51 pm, Jon Kirwan <j...@infinitefactors.org> wrote:

On Thu, 28 Jan 2010 11:17:02 +1000, David Eather





eat...@tpg.com.au> wrote:
Jon Kirwan wrote:
On Wed, 27 Jan 2010 17:31:00 +1000, David Eather
eat...@tpg.com.au> wrote:
snip

My particular bias for an amp this size is to go class AB with a split
power supply. The majority of quality audio amps follow this topology
and this is, I think, I great reason to go down this design path (what
you learn is applicable in the most number of situations). I should hunt
down a schematics of what I'm seeing in the distance (which can/will
change as decisions are made) - some of the justifications will have to
wait

I'm fine with taking things as they come.

As far as the class, I guessed that at 10 watts class-A would
be too power-hungry and probably not worth its weight but
that class-AB might be okay.

I have to warn you, though, that I'm not focused upon some
20ppm THD.  I'd like to learn, not design something whose
distortion (or noise, for that matter) is around a bit on a
16-bit DAC or less.  I figure winding up close to class-B
operation in the end.  But I'd like to take the walk along
the way, so to speak.

10 watts / PPM thd? Mmmm... maybe more like .1 - .05 % are realistic and
a few detours to see what would help or harm that.

Hehe.  I'm thinking of some numbers I saw in the area of
.002% THD.  I hate percentages and immediately convert them.
In this case, it is 20e-6 or 20 ppm.  Which is darned close
to a bit on a 16-bit dac.  That's why I wrote that way.  I
just don't like using % figures.  They annoy me just a tiny
bit.

Regarding .1% to .05%, I'm _very_ good with that.  Of course,
I'm going to have to learn about how to estimate it from
theory as well as measure it both via simulation before
construction and from actual testing afterwards.  More stuff
I might _think_ I have a feel for, but I'm sure I will
discover I don't as I get more into it.

But speaking from ignorance, I'm good shooting for the range
you mentioned.  It was about what I had in mind, in fact,
figuring I could always learn as I go.





The first step is to think about the output. The basic equations are

(1).....Vout = sqrt(2*P*R)

With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts -
actually +/- 12.7 volts with a split power supply.

If you don't mind, I'd like to discuss this more closely. Not
just have it tossed out.  So, P=V*I; or P=Vrms^2/R with AC.
Using Vpeak=SQRT(2)*Vrms, I get your Vpeak=SQRT(2*P*R)
equation.  Which suggests the +/-12.7V swing.  Which further
suggests, taking Vce drops and any small amounts emitter
resistor drops into account, something along the lines of +/-
14-15V rails?

Or should the rails be cut a lot closer to the edge here to
improve efficiency.  What bothers me is saturation as Vce on
the final output BJTs goes well below 1V each and beta goes
away, as well, rapidly soaking up remaining drive compliance.

(2).....Imax = sqrt(2*P/R)

This comes out to 1.6 amps. You should probably also consider the case
when R speaker = 4 ohms when initially selecting a transistor for the
output 2.2 amps - remember this is max output current. The power supply
voltage will have to be somewhat higher than Vout to take into account
circuit drive requirements, ripple on the power supply and transformer
regulation etc.

Okay.  I missed reading this when writing the above.  Rather
than correct myself, I'll leave my thinking in place.

So yes, the rails will need to be a bit higher.  Agreed.  On
this subject, I'm curious about the need to _isolate_, just a
little, the rails used by the input stage vs the output stage
rails.  I'm thinking an RC (or LC for another pole?) for
isolation.  But I honestly don't know if that's helpful, or
not.

Mostly not needed, if you use a long tailed pair for the input / error
amplifier, but you might prefer some other arrangement so keep it in
mind if your circuit "motorboats"

Okay.  I've _zero_ experience for audio.  It just crossed my
mind from other cases.  I isolate the analog supply from the
digital -- sometimes with as many as four caps and three
inductor beads.  There, it _does_ help.





Are you OK with connecting mains to a transformer? or would you rather
use an AC plug pack (10 watts is about the biggest amp a plugpack can be
used for)? The "cost" for using an AC plug pack is you will need larger
filter capacitors.

I'd much prefer to __avoid__ using someone else's "pack" for
the supply.  All discrete parts should be on the table, so to
speak, in plain view.  And I don't imagine _any_ conceptual
difficulties for this portion of the design.  I'm reasonably
familiar with transformers, rectifiers, ripple calculations,
and how to consider peak charging currents vs averge load
currents as they relate to the phase angles available for
charging the caps.  So on this part, I may need less help
than elsewhere.  In other words, I'm somewhat comfortable
here.

Ah, then there are questions of what voltage and VA for a transformer.
So there are questions of usage (music, PA, PA with an emergency alert
siren tied in etc) and rectifier arrangement and capacitor size /
voltage to get your required voltage output at full load.

I figure on working out the design of the amplifier and then
going back, once that is determined and hashed out, with the
actual required figures for the power supply and design that
part as the near-end of the process.  Earlier on, I'd expect
to have some rough idea of how "bad" it needs to be -- if the
initial guesses don't raise alarms, then I wouldn't dig into
the power supply design until later on.  The amplifier, it
seems to me, dictates the parameters.  So that comes later,
doesn't it?





I should also ask if you have a multi meter, oscilloscope (not necessary
but useful)and how is your soldering? But it would be wise to keep this
whole thing as a paper exercise before you commit to anything.

I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS
handheld, two oscilloscopes (TEK 2245 with voltmeter option
and an HP 54645D), three triple-output power supplies with
two of them GPIB drivable, the usual not-too-expensive signal
generator, and a fair bunch of other stuff on the shelves.
Lots of probes, clips, and so on.  For soldering, I'm limited
to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and
somewhat wider spade tips in the 1.5mm area.  I have tubs and
jars of various types of fluxes, as well, and wire wrap tools
and wire wrap wire, as well.  I also have a room set aside
for this kind of stuff, when I get time to play.

OK. Next serious project, I'm coming around to your place!

You come to the west coast of the US and I'll have a room for
you!

Your gear is
better than mine. I had to ask, rather than just assume just in case my
assumptions got you building something you didn't want to, and got you
splattered all over the place from the mains, or suggesting you choose
the miller cap by watching the phase shift of the feedback circuit - I
don't read a lot of the posts so I didn't know what you could do.

To be honest, I can do a few things but I'm really not very
practiced.  My oscilloscope knowledge is lacking in some
areas -- which becomes all too painfully obvious to me when I
watch a pro using my equipment.  And I'm still learning to
solder better.  It's one of a few hobbies.

Jon

Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier

Done.

The bits on class A might be interesting as it says 25% efficiency and
50% obtainable with inductive output coupling (i.e. with a transformer)
which is what I said, not what blow hard Phil said.

What I first see there is the amplifier sketch at the top of
the page (I don't really care too much about arguing about
efficiencies right now -- I'm more concerned about learning.)
The input stage shown is a voltage-in, current-out bog
standard diff-pair.  First thing I remember about is that R4
shouldn't be there and better still both R3 and R4 should be
replaced with a current mirror.  R5 should be a replaced with
a BJT, as well.  I assume the input impedance of that example
is basically the parallel resistance of R1 and R2, but if we
use split supplies I'd imagine replacing the two of them with
a single resistor to the center-ground point.  There's no
miller cap on Q3, I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)
The feedback ... well, I need to think about that a little
more.  There's no degen resistors in the emitters of Q4 and
Q5.

Um.. okay, I need to sit down and think.  Mind is spinning,
but I've not set a finger to paper yet and there is lots to
think about in that one.  I could be way, way off base.

Jon- Hide quoted text -

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"I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)"

First Jon I know less about amplifier design than you do... That said,
I would be careful about replacing the diodes in the push-pull stage.
Way back in college I had a Sony stero amp that I had to fix. It came
with a nice circuit diagram. I seem to recall that the bias diodes
in the push pull stage were thermally attached to the same heat sink
that held the output transistors. As the output transistors warm up
their Vbe drop decreases. You want the bias diodes to track this
change. Or else the whole thing could 'run-away' on you. ...
degenerative emmiter resistors (as you suggest) will help some.

George H.

 

[pimpom]

George Herold wrote:

On Jan 27, 9:51 pm, Jon Kirwan <j...@infinitefactors.org
wrote:

"I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)"

First Jon I know less about amplifier design than you do...
That said,
I would be careful about replacing the diodes in the push-pull
stage.
Way back in college I had a Sony stero amp that I had to fix.
It came
with a nice circuit diagram. I seem to recall that the bias
diodes
in the push pull stage were thermally attached to the same heat
sink
that held the output transistors. As the output transistors
warm up
their Vbe drop decreases. You want the bias diodes to track
this
change. Or else the whole thing could 'run-away' on you. ...
degenerative emmiter resistors (as you suggest) will help some.

I like the biasing scheme mentioned by Jon and use it for all my
designs except the early ones using germanium transistors, though
I don't know the name either. The biasing transistor can be
mounted on the output transistors' heatsink for temperature
tracking.

I like it because it's versatile and a single transistor can be
used to bias several transistors with their b-e junctions in
series as long as they are mounted on a common heatsink.
http://img691.imageshack.us/img691/2075/bias.png

My personal preference is to place the bias adjustment pot R3 in
this position rather than with R1. It ensures that any accidental
loss of contact by the pot's wiper arm will reduce the total bias
whereas placing it with R1 will have the opposite effect and
could cause excessive quiescent current in the output
transistors, possibly getting them to overheat.

 

[Jon Kirwan]

On Fri, 29 Jan 2010 09:19:31 -0800 (PST), George Herold
<ggherold@gmail.com> wrote:

Hi Jon, I'm enjoying your posts.

Thanks. I feel like I'm way behind some curves, but it's fun
taking a moment to think about things and it is fantastic
that anyone else is willing to help talk about things with
me. That is priceless. So the real thanks go to those who
are sharing their knowledge and experience here.

What's a pin driver?

Hmm. I think I first heard the idea when talking about
testing ICs, to be honest. But imagine instead a micro with
software to test some discrete part (could be an IC, too,
that that's more complex.) For example, to automatically
derive some modeling parameters for a BJT.

Take a look at this datasheet, for an example of the features
one might support:

http://www.analog.com/static/imported-files/Data_Sheets/AD53040.pdf

I made a nice
switchable current source (10nA to 1mA) from a voltage reference,
opamp and switchable resistors. (circuit cribbed from AoE.)

I'd require at least one that can either sink _or_ source to
the pin. And that would be only one of the pin driver's
required features. I think the datasheet mentioned above
provides some more. But that part is expensive and not
readily available to us hobbyist types and doesn't teach me
anything about various trade-offs I might want to make or how
to design it at all, besides.

Jon

 

[George Herold]

On Jan 29, 2:41 pm, "pimpom" <pim...@invalid.invalid> wrote:

George Herold wrote:
On Jan 27, 9:51 pm, Jon Kirwan <j...@infinitefactors.org
wrote:

"I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)"

First Jon I know less about amplifier design than you do...
That said,
I would be careful about replacing the diodes in the push-pull
stage.
Way back in college I had a Sony stero amp that I had to fix.
It came
with a nice circuit diagram.   I seem to recall that the bias
diodes
in the push pull stage were thermally attached to the same heat
sink
that held the output transistors.  As the output transistors
warm up
their Vbe drop decreases.  You want the bias diodes to track
this
change.  Or else the whole thing could 'run-away' on you.  ...
degenerative emmiter resistors (as you suggest) will help some.

I like the biasing scheme mentioned by Jon and use it for all my
designs except the early ones using germanium transistors, though
I don't know the name either. The biasing transistor can be
mounted on the output transistors' heatsink for temperature
tracking.

I like it because it's versatile and a single transistor can be
used to bias several transistors with their b-e junctions in
series as long as they are mounted on a common heatsink.http://img691.imageshack.us/img691/2075/bias.png

My personal preference is to place the bias adjustment pot R3 in
this position rather than with R1. It ensures that any accidental
loss of contact by the pot's wiper arm will reduce the total bias
whereas placing it with R1 will have the opposite effect and
could cause excessive quiescent current in the output
transistors, possibly getting them to overheat.- Hide quoted text -

- Show quoted text -

Ahh Thanks Pimpom, I've never done any temperature tracking type
calculations.

George H.

 

[David Eather]

On Jan 28, 12:51 pm, Jon Kirwan <j...@infinitefactors.org> wrote:

On Thu, 28 Jan 2010 11:17:02 +1000, David Eather

Sorry Jon,
I'm stuck on google groups for a little while - I can't believe people
actually use it full time or that google could make an interface this
bad. (I suspect it is very fine for simple threads) anyway...

eat...@tpg.com.au> wrote:
Jon Kirwan wrote:
On Wed, 27 Jan 2010 17:31:00 +1000, David Eather
eat...@tpg.com.au> wrote:
snip

My particular bias for an amp this size is to go class AB with a split
power supply. The majority of quality audio amps follow this topology
and this is, I think, I great reason to go down this design path (what
you learn is applicable in the most number of situations). I should hunt
down a schematics of what I'm seeing in the distance (which can/will
change as decisions are made) - some of the justifications will have to
wait

I'm fine with taking things as they come.

As far as the class, I guessed that at 10 watts class-A would
be too power-hungry and probably not worth its weight but
that class-AB might be okay.

I have to warn you, though, that I'm not focused upon some
20ppm THD.  I'd like to learn, not design something whose
distortion (or noise, for that matter) is around a bit on a
16-bit DAC or less.  I figure winding up close to class-B
operation in the end.  But I'd like to take the walk along
the way, so to speak.

10 watts / PPM thd? Mmmm... maybe more like .1 - .05 % are realistic and
a few detours to see what would help or harm that.

Hehe.  I'm thinking of some numbers I saw in the area of
.002% THD.  I hate percentages and immediately convert them.
In this case, it is 20e-6 or 20 ppm.  Which is darned close
to a bit on a 16-bit dac.  That's why I wrote that way.  I
just don't like using % figures.  They annoy me just a tiny
bit.

Sorry.

Regarding .1% to .05%, I'm _very_ good with that.  Of course,
I'm going to have to learn about how to estimate it from
theory as well as measure it both via simulation before
construction and from actual testing afterwards.  More stuff
I might _think_ I have a feel for, but I'm sure I will
discover I don't as I get more into it.

A little experience will get you into the right ballpark when
estimating what you could expect for distortion. It is basically the
same "rules" as you would see with op-amps - the more linear it is to
start with the better. Higher bandwidth stages generally mean you can
use more negative feedback to eliminate distortion - but the lower the
final gain the more instability is likely to become a problem. And bad
circuit layout can increase distortion (and even more so hum and
noise) easily by a factor of 10.

As for how low you need distortion to be one rule of thumb (I forget
the reference) is to be clearly audible the message must be 20db above
the background noise and to be inaudible distortion has to be 20db
below the background noise - which pretty much sets "low" distortion
for PA and similar uses at 1% or 10000 ppm. For HiFi the "message" has
a high dynamic range and you (allegedly) want a distortion figure at
least 20db below that. So a 60 db signal range 0.0001% (or 100PPM).
The you start getting into all kinds of trouble with power output /
dynamic range of the amp etc and you relies that it is all a
compromise anyway. You do the best you can within the restrictions of
the job description.

But speaking from ignorance, I'm good shooting for the range
you mentioned.  It was about what I had in mind, in fact,
figuring I could always learn as I go.





The first step is to think about the output. The basic equations are

(1).....Vout = sqrt(2*P*R)

With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts -
actually +/- 12.7 volts with a split power supply.

If you don't mind, I'd like to discuss this more closely. Not
just have it tossed out.  So, P=V*I; or P=Vrms^2/R with AC.
Using Vpeak=SQRT(2)*Vrms, I get your Vpeak=SQRT(2*P*R)
equation.  Which suggests the +/-12.7V swing.  Which further
suggests, taking Vce drops and any small amounts emitter
resistor drops into account, something along the lines of +/-
14-15V rails?

Or should the rails be cut a lot closer to the edge here to
improve efficiency.  What bothers me is saturation as Vce on
the final output BJTs goes well below 1V each and beta goes
away, as well, rapidly soaking up remaining drive compliance.

(2).....Imax = sqrt(2*P/R)

This comes out to 1.6 amps. You should probably also consider the case
when R speaker = 4 ohms when initially selecting a transistor for the
output 2.2 amps - remember this is max output current. The power supply
voltage will have to be somewhat higher than Vout to take into account
circuit drive requirements, ripple on the power supply and transformer
regulation etc.

Okay.  I missed reading this when writing the above.  Rather
than correct myself, I'll leave my thinking in place.

So yes, the rails will need to be a bit higher.  Agreed.  On
this subject, I'm curious about the need to _isolate_, just a
little, the rails used by the input stage vs the output stage
rails.  I'm thinking an RC (or LC for another pole?) for
isolation.  But I honestly don't know if that's helpful, or
not.

Mostly not needed, if you use a long tailed pair for the input / error
amplifier, but you might prefer some other arrangement so keep it in
mind if your circuit "motorboats"

Okay.  I've _zero_ experience for audio.  It just crossed my
mind from other cases.  I isolate the analog supply from the
digital -- sometimes with as many as four caps and three
inductor beads.  There, it _does_ help.



Are you OK with connecting mains to a transformer? or would you rather
use an AC plug pack (10 watts is about the biggest amp a plugpack can be
used for)? The "cost" for using an AC plug pack is you will need larger
filter capacitors.

I'd much prefer to __avoid__ using someone else's "pack" for
the supply.  All discrete parts should be on the table, so to
speak, in plain view.  And I don't imagine _any_ conceptual
difficulties for this portion of the design.  I'm reasonably
familiar with transformers, rectifiers, ripple calculations,
and how to consider peak charging currents vs averge load
currents as they relate to the phase angles available for
charging the caps.  So on this part, I may need less help
than elsewhere.  In other words, I'm somewhat comfortable
here.

Ah, then there are questions of what voltage and VA for a transformer.
So there are questions of usage (music, PA, PA with an emergency alert
siren tied in etc) and rectifier arrangement and capacitor size /
voltage to get your required voltage output at full load.

I figure on working out the design of the amplifier and then
going back, once that is determined and hashed out, with the
actual required figures for the power supply and design that
part as the near-end of the process.  Earlier on, I'd expect
to have some rough idea of how "bad" it needs to be -- if the
initial guesses don't raise alarms, then I wouldn't dig into
the power supply design until later on.  The amplifier, it
seems to me, dictates the parameters.  So that comes later,
doesn't it?

Yes and No. All the published circuits are made by people who want to
sell transistors, not audio systems, power supplies or transformers.
As a result the power supply is often assumed to be regulated, which
is not true in this case, or the power supply is treated in a very
perfunctory manner that is not at all compatible with good design.

In this case you have the voltage you need for the 10 watts, plus
voltage drop for the driver circuitry and output stage , plus ripple
voltage, plus whatever is required for transformer regulation and
mains regulation. When you add it all up you might find that a chosen
transistor/component is actually not at all suitable for the job. Back
to the drawing board. Change this change that recheck everything again
etc.

If you do the power supply first you have the figures needed for your
worst case already. It saves time and makes a better result (no
tendency to comprimise to save all the calculations already done).

I should also ask if you have a multi meter, oscilloscope (not necessary
but useful)and how is your soldering? But it would be wise to keep this
whole thing as a paper exercise before you commit to anything.

I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS
handheld, two oscilloscopes (TEK 2245 with voltmeter option
and an HP 54645D), three triple-output power supplies with
two of them GPIB drivable, the usual not-too-expensive signal
generator, and a fair bunch of other stuff on the shelves.
Lots of probes, clips, and so on.  For soldering, I'm limited
to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and
somewhat wider spade tips in the 1.5mm area.  I have tubs and
jars of various types of fluxes, as well, and wire wrap tools
and wire wrap wire, as well.  I also have a room set aside
for this kind of stuff, when I get time to play.

OK. Next serious project, I'm coming around to your place!

You come to the west coast of the US and I'll have a room for
you!

Your gear is
better than mine. I had to ask, rather than just assume just in case my
assumptions got you building something you didn't want to, and got you
splattered all over the place from the mains, or suggesting you choose
the miller cap by watching the phase shift of the feedback circuit - I
don't read a lot of the posts so I didn't know what you could do.

To be honest, I can do a few things but I'm really not very
practiced.  My oscilloscope knowledge is lacking in some
areas -- which becomes all too painfully obvious to me when I
watch a pro using my equipment.  And I'm still learning to
solder better.  It's one of a few hobbies.

Jon

Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier

Done.

The bits on class A might be interesting as it says 25% efficiency and
50% obtainable with inductive output coupling (i.e. with a transformer)
which is what I said, not what blow hard Phil said.

What I first see there is the amplifier sketch at the top of
the page

I wasn't going to prompt, but it is close to the sort of thing, I
think, you should be aiming for . As someone has already noted (I
would attribute you if I wasn't on GG, I'm sorry) it has been drawn up
for a single supply, rather than a more common (for this size /
configuration) split supply.

(I don't really care too much about arguing about

efficiencies right now -- I'm more concerned about learning.)
The input stage shown is a voltage-in, current-out bog
standard diff-pair.  First thing I remember about is that R4
shouldn't be there

Correct. Theory says it does nothing. I practice the theory but have
the occasional heretical belief about that.

and better still both R3 and R4 should be

replaced with a current mirror.  

This would provide more differential gain.

R5 should be a replaced with

a BJT, as well.  

In the right configuration it would reduce the common mode signal gain
of things like mains hum and supply ripple (you mentioned power supply
isolation before).
Also, from another (what do you call it branch? thread?) you were
discussing boot-strapping R6. This is not done so much as amplifiers
get bigger but a BJT configured in the same way as the replacement for
R5 is very common. I'm leaving the details to you - perhaps there is a
way to reduce component count without affecting performance. (I am
hoping this is what you wanted "nutting it out for yourself")

I assume the input impedance of that example

is basically the parallel resistance of R1 and R2, but if we
Yes.

use split supplies I'd imagine replacing the two of them with
a single resistor to the center-ground point.  
Yes, but you should probably think of a whole passive network to

filter out low and high frequency - (think what happens if you amp is
operated near a source of RF)
There's no

miller cap on Q3,

Depending on transistors layout etc it might not be needed, but more
often it is the size that is the question.

I'd probably replace the two diodes with

one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)

Vbe multiplier...

The feedback ... well, I need to think about that a little
more.  There's no degen resistors in the emitters of Q4 and
Q5.

Why would/should you use them?

Um.. okay, I need to sit down and think.  Mind is spinning,
but I've not set a finger to paper yet and there is lots to
think about in that one.  I could be way, way off base.

Not at all.

Jon

Is there a way you could post a schematic of where your thinking is
and what you would like to discuss - there is no need for a complete
circuit.

 

[Jon Kirwan]

On Fri, 29 Jan 2010 13:49:16 -0800 (PST), David Eather
<eather@tpg.com.au> wrote:

On Jan 28, 12:51 pm, Jon Kirwan <j...@infinitefactors.org> wrote:
On Thu, 28 Jan 2010 11:17:02 +1000, David Eather

Sorry Jon,

I'm stuck on google groups for a little while - I can't believe people
actually use it full time or that google could make an interface this
bad. (I suspect it is very fine for simple threads) anyway...

Cripes. Google didn't even show the thread when I'd looked,
a day ago or so. And it had been around for at least 24
hours by then. Used to be the case that google groups would
show the posts within an hour or so. Doesn't seem to be
true, anymore. If not, there is no possibility of having a
discussion very quickly via google. It would greatly
lengthen out the interactions. Maybe that's on purpose, now,
to cause people to find some other solution?

eat...@tpg.com.au> wrote:
Jon Kirwan wrote:
On Wed, 27 Jan 2010 17:31:00 +1000, David Eather
eat...@tpg.com.au> wrote:
snip

My particular bias for an amp this size is to go class AB with a split
power supply. The majority of quality audio amps follow this topology
and this is, I think, I great reason to go down this design path (what
you learn is applicable in the most number of situations). I should hunt
down a schematics of what I'm seeing in the distance (which can/will
change as decisions are made) - some of the justifications will have to
wait

I'm fine with taking things as they come.

As far as the class, I guessed that at 10 watts class-A would
be too power-hungry and probably not worth its weight but
that class-AB might be okay.

I have to warn you, though, that I'm not focused upon some
20ppm THD.  I'd like to learn, not design something whose
distortion (or noise, for that matter) is around a bit on a
16-bit DAC or less.  I figure winding up close to class-B
operation in the end.  But I'd like to take the walk along
the way, so to speak.

10 watts / PPM thd? Mmmm... maybe more like .1 - .05 % are realistic and
a few detours to see what would help or harm that.

Hehe.  I'm thinking of some numbers I saw in the area of
.002% THD.  I hate percentages and immediately convert them.
In this case, it is 20e-6 or 20 ppm.  Which is darned close
to a bit on a 16-bit dac.  That's why I wrote that way.  I
just don't like using % figures.  They annoy me just a tiny
bit.

Sorry.

Don't be. I was just explaining myself, not complaining
about your usage.

Regarding .1% to .05%, I'm _very_ good with that.  Of course,
I'm going to have to learn about how to estimate it from
theory as well as measure it both via simulation before
construction and from actual testing afterwards.  More stuff
I might _think_ I have a feel for, but I'm sure I will
discover I don't as I get more into it.

A little experience will get you into the right ballpark when
estimating what you could expect for distortion. It is basically the
same "rules" as you would see with op-amps - the more linear it is to
start with the better. Higher bandwidth stages generally mean you can
use more negative feedback to eliminate distortion - but the lower the
final gain the more instability is likely to become a problem. And bad
circuit layout can increase distortion (and even more so hum and
noise) easily by a factor of 10.

As for how low you need distortion to be one rule of thumb (I forget
the reference) is to be clearly audible the message must be 20db above
the background noise and to be inaudible distortion has to be 20db
below the background noise - which pretty much sets "low" distortion
for PA and similar uses at 1% or 10000 ppm. For HiFi the "message" has
a high dynamic range and you (allegedly) want a distortion figure at
least 20db below that. So a 60 db signal range 0.0001% (or 100PPM).
The you start getting into all kinds of trouble with power output /
dynamic range of the amp etc and you relies that it is all a
compromise anyway. You do the best you can within the restrictions of
the job description.

Understood.

But speaking from ignorance, I'm good shooting for the range
you mentioned.  It was about what I had in mind, in fact,
figuring I could always learn as I go.


The first step is to think about the output. The basic equations are

(1).....Vout = sqrt(2*P*R)

With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts -
actually +/- 12.7 volts with a split power supply.

If you don't mind, I'd like to discuss this more closely. Not
just have it tossed out.  So, P=V*I; or P=Vrms^2/R with AC.
Using Vpeak=SQRT(2)*Vrms, I get your Vpeak=SQRT(2*P*R)
equation.  Which suggests the +/-12.7V swing.  Which further
suggests, taking Vce drops and any small amounts emitter
resistor drops into account, something along the lines of +/-
14-15V rails?

Or should the rails be cut a lot closer to the edge here to
improve efficiency.  What bothers me is saturation as Vce on
the final output BJTs goes well below 1V each and beta goes
away, as well, rapidly soaking up remaining drive compliance.

(2).....Imax = sqrt(2*P/R)

This comes out to 1.6 amps. You should probably also consider the case
when R speaker = 4 ohms when initially selecting a transistor for the
output 2.2 amps - remember this is max output current. The power supply
voltage will have to be somewhat higher than Vout to take into account
circuit drive requirements, ripple on the power supply and transformer
regulation etc.

Okay.  I missed reading this when writing the above.  Rather
than correct myself, I'll leave my thinking in place.

So yes, the rails will need to be a bit higher.  Agreed.  On
this subject, I'm curious about the need to _isolate_, just a
little, the rails used by the input stage vs the output stage
rails.  I'm thinking an RC (or LC for another pole?) for
isolation.  But I honestly don't know if that's helpful, or
not.

Mostly not needed, if you use a long tailed pair for the input / error
amplifier, but you might prefer some other arrangement so keep it in
mind if your circuit "motorboats"

Okay.  I've _zero_ experience for audio.  It just crossed my
mind from other cases.  I isolate the analog supply from the
digital -- sometimes with as many as four caps and three
inductor beads.  There, it _does_ help.



Are you OK with connecting mains to a transformer? or would you rather
use an AC plug pack (10 watts is about the biggest amp a plugpack can be
used for)? The "cost" for using an AC plug pack is you will need larger
filter capacitors.

I'd much prefer to __avoid__ using someone else's "pack" for
the supply.  All discrete parts should be on the table, so to
speak, in plain view.  And I don't imagine _any_ conceptual
difficulties for this portion of the design.  I'm reasonably
familiar with transformers, rectifiers, ripple calculations,
and how to consider peak charging currents vs averge load
currents as they relate to the phase angles available for
charging the caps.  So on this part, I may need less help
than elsewhere.  In other words, I'm somewhat comfortable
here.

Ah, then there are questions of what voltage and VA for a transformer.
So there are questions of usage (music, PA, PA with an emergency alert
siren tied in etc) and rectifier arrangement and capacitor size /
voltage to get your required voltage output at full load.

I figure on working out the design of the amplifier and then
going back, once that is determined and hashed out, with the
actual required figures for the power supply and design that
part as the near-end of the process.  Earlier on, I'd expect
to have some rough idea of how "bad" it needs to be -- if the
initial guesses don't raise alarms, then I wouldn't dig into
the power supply design until later on.  The amplifier, it
seems to me, dictates the parameters.  So that comes later,
doesn't it?

Yes and No. All the published circuits are made by people who want to
sell transistors,

A concern I care not the least about. My _real_ preference,
were I to impose it on the design, would be to use ONLY
PN2222A BJTs for all the active devices. One part. That's
it. Why? Because I've got thousands of them.

Literally. Something like 22,000 of the bastards. I give
them away like popcorn to students at schools. Got them
_very cheaply_. So if I were pushing something, I'd be
pushing a 10W PN2222A design, use signal splitting approach
probably (because it's the only way I think think of, right
now), and distribute the dissipation across lots and lots of
the things.

What to go there?

not audio systems, power supplies or transformers.

Got it.

As a result the power supply is often assumed to be regulated, which
is not true in this case, or the power supply is treated in a very
perfunctory manner that is not at all compatible with good design.

In this case you have the voltage you need for the 10 watts, plus
voltage drop for the driver circuitry and output stage , plus ripple
voltage, plus whatever is required for transformer regulation and
mains regulation. When you add it all up you might find that a chosen
transistor/component is actually not at all suitable for the job. Back
to the drawing board. Change this change that recheck everything again
etc.

In this case, though, there is nothing particularly
remarkable about the rails. Taken across the entire span,
even, doesn't exceed the maximum Vce of a great many BJTs. So
no real worry there. But I see some of where problems may
arise. Luckily, at this level I can side-step worrying about
that part and get back to learning about amplifier design,
yes?

If you do the power supply first you have the figures needed for your
worst case already. It saves time and makes a better result (no
tendency to comprimise to save all the calculations already done).

Well, does this mean we should hack out the power supply
first? I'm perfectly fine with that and can get back to you
with a suggested circuit and parts list if you want to start
there. We could settle that part before going anywhere else
and I'd be happy with that approach, too, because to be
honest I don't imagine it to put a horrible delay into
getting back to amplifier design. So I'm good either way.

I should also ask if you have a multi meter, oscilloscope (not necessary
but useful)and how is your soldering? But it would be wise to keep this
whole thing as a paper exercise before you commit to anything.

I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS
handheld, two oscilloscopes (TEK 2245 with voltmeter option
and an HP 54645D), three triple-output power supplies with
two of them GPIB drivable, the usual not-too-expensive signal
generator, and a fair bunch of other stuff on the shelves.
Lots of probes, clips, and so on.  For soldering, I'm limited
to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and
somewhat wider spade tips in the 1.5mm area.  I have tubs and
jars of various types of fluxes, as well, and wire wrap tools
and wire wrap wire, as well.  I also have a room set aside
for this kind of stuff, when I get time to play.

OK. Next serious project, I'm coming around to your place!

You come to the west coast of the US and I'll have a room for
you!

Your gear is
better than mine. I had to ask, rather than just assume just in case my
assumptions got you building something you didn't want to, and got you
splattered all over the place from the mains, or suggesting you choose
the miller cap by watching the phase shift of the feedback circuit - I
don't read a lot of the posts so I didn't know what you could do.

To be honest, I can do a few things but I'm really not very
practiced.  My oscilloscope knowledge is lacking in some
areas -- which becomes all too painfully obvious to me when I
watch a pro using my equipment.  And I'm still learning to
solder better.  It's one of a few hobbies.

Jon

Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier

Done.

The bits on class A might be interesting as it says 25% efficiency and
50% obtainable with inductive output coupling (i.e. with a transformer)
which is what I said, not what blow hard Phil said.

What I first see there is the amplifier sketch at the top of
the page

I wasn't going to prompt, but it is close to the sort of thing, I
think, you should be aiming for . As someone has already noted (I
would attribute you if I wasn't on GG, I'm sorry) it has been drawn up
for a single supply, rather than a more common (for this size /
configuration) split supply.

I had assumed we'd be using a split supply.

I had assumed a speaker would be hooked up via a cap to the
output, so DC currents into a speaker coil would be removed
from any concern. But I was also holding in the back of my
mind the idea of tweaking out DC bias via the speaker and
removing the coupling cap as an experiment to try. And if
so, I'd pretty much want the ground as a "third rail."
(Playing just a bit upon the Chicago parlance about the once
dangerous rail in their transit system.)

(I don't really care too much about arguing about
efficiencies right now -- I'm more concerned about learning.)
The input stage shown is a voltage-in, current-out bog
standard diff-pair.  First thing I remember about is that R4
shouldn't be there

Correct. Theory says it does nothing. I practice the theory but have
the occasional heretical belief about that.

Actually, I think I've read that theory says it is _better_
to be removed. The reason seemed pretty basic, as it's
easier to get close to a balanced current split; and this, I
gather, lowers 2nd harmonic distortions produced in the pair
-- notable more on the high frequency end I suppose because
gain used for linearizing feedback up there is diminishing
and can't compensate it.

In other words, it's not neutral. It's considered to be
better if I gathered the details. Then even better, the
current mirror enforces the whole deal and you've got about
the best to be had.

Of course, mostly just being a reader means I have no idea
which end is up. So I might have all this wrong.

and better still both R3 and R4 should be
replaced with a current mirror.  

This would provide more differential gain.

_and_ improve distortion because the currents are forced to
be balanced in the pair, yes?

R5 should be a replaced with
a BJT, as well.  

In the right configuration it would reduce the common mode signal gain
of things like mains hum and supply ripple (you mentioned power supply
isolation before).

Yes, that's how I thought about it.

Also, from another (what do you call it branch? thread?) you were
discussing boot-strapping R6. This is not done so much as amplifiers
get bigger but a BJT configured in the same way as the replacement for
R5 is very common. I'm leaving the details to you - perhaps there is a
way to reduce component count without affecting performance. (I am
hoping this is what you wanted "nutting it out for yourself")

Yes! I don't want things handed on a platter. But I also
don't want to have to rediscover all of the ideas by making
all of the mistakes, either. This is the kind of "pointer"
towards something that I like a lot. It gives me a place to
think about something, but leaves me some reason to have to
do so and that helps me own it better.

One general truth about learning is that you don't present
someone with a problem so out of their depth that they have
no chance at it. Doing that means they fail, they feel like
a failure, and it causes a student to just want to go away.
They lose motivation, usually, in cases like that. On the
other hand, providing no difficulty at all merely means
repetition of what they already know and they grow bored from
that, too. Finding the sweet spot where a student is faced
with interesting problems that are not already known, but
perhaps within reach of grasping at with some effort, is the
key. Then it can be fun, educational, and motivate.

That's what you just did for me.

I assume the input impedance of that example
is basically the parallel resistance of R1 and R2, but if we

Yes.

Okay.

use split supplies I'd imagine replacing the two of them with
a single resistor to the center-ground point.  
Yes, but you should probably think of a whole passive network to
filter out low and high frequency - (think what happens if you amp is
operated near a source of RF)

Well, every trace picks up like little antennae. All kinds
of trace voltages appearing here and there. Not good.

So. Can you make an audio amplifier that can withstand a
microwave oven environment and deliver good performance while
irradiated with 1kW banging around in there?

There's no
miller cap on Q3,

Depending on transistors layout etc it might not be needed, but more
often it is the size that is the question.

I was thinking it helped locally linearize the VAS section
and that such would be "good" most anywhere. But I am just
taking things without having worked through them on my own.
So...

I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)

Vbe multiplier...

Okay. Thanks.

The feedback ... well, I need to think about that a little
more.  There's no degen resistors in the emitters of Q4 and
Q5.

Why would/should you use them?

I'm still thinking about that. In general, I was thinking
about them because of the "little re" that is kT/q based in
each BJT, and varies on Ie. Since Ie is varying around, I
was thinking about something fixed there to overwhelm it and
"make it knowable" for the design, I suppose. Maybe that's
all wet, given your query. I'll toss the idea off the side,
for now.

Um.. okay, I need to sit down and think.  Mind is spinning,
but I've not set a finger to paper yet and there is lots to
think about in that one.  I could be way, way off base.

Not at all.

Thanks for that. I'm just glad to be able to talk to someone
about any of this, at all. So please accept my thanks for
the moments you are offering.

Is there a way you could post a schematic of where your thinking is
and what you would like to discuss - there is no need for a complete
circuit.

Yes. I can use ASCII here, for example. But before I go off
into the wild blue with this, do you want to focus on the
power supply first? Or just jump in on the amplifier?

Jon

 

[Jon Kirwan]

On Fri, 29 Jan 2010 15:06:40 -0800, I wrote:

A concern I care not the least about. My _real_ preference,
were I to impose it on the design, would be to use ONLY
PN2222A BJTs for all the active devices. One part. That's
it. Why? Because I've got thousands of them.

Literally. Something like 22,000 of the bastards. I give
them away like popcorn to students at schools. Got them
_very cheaply_. So if I were pushing something, I'd be
pushing a 10W PN2222A design, use signal splitting approach
probably (because it's the only way I think think of, right
now), and distribute the dissipation across lots and lots of
the things.

What to go there?

I meant "Want to go there?"

One part of the brain says "type out word for concept X on
the keyboard," and that gets passed down to a low-level
manager function which maps concept to English word, gets the
wrong hash bucket ID for a "nearby word" and the passes on
motor instructions for mirror neurons driving the hand which
then types the wrong word. Meanwhile, eye reads "want" text,
this gets translated into a concept which is promptly ignored
because the concept already resides in local cache storage
and doesn't need replacement. So the brain checks that what
I wrote is what I intended, glancing quickly over it and gets
the cached version and matches everything up nicely and moves
on.

hehe. This is either a hardware problem or a software
problem, depending upon point of view...

Jon

 

[Jon Kirwan]

On Fri, 29 Jan 2010 15:22:42 -0800, Jon Kirwan
<jonk@infinitefactors.org> wrote:

eye reads "want" text

hehe. Or "What" text....

Nesting all the way down...

Jon

 

[Jon Kirwan]

On Sat, 30 Jan 2010 01:11:28 +0530, "pimpom"
<pimpom@invalid.invalid> wrote:

George Herold wrote:
On Jan 27, 9:51 pm, Jon Kirwan <j...@infinitefactors.org
wrote:

"I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)"

First Jon I know less about amplifier design than you do...
That said,
I would be careful about replacing the diodes in the push-pull
stage.
Way back in college I had a Sony stero amp that I had to fix.
It came
with a nice circuit diagram. I seem to recall that the bias
diodes
in the push pull stage were thermally attached to the same heat
sink
that held the output transistors. As the output transistors
warm up
their Vbe drop decreases. You want the bias diodes to track
this
change. Or else the whole thing could 'run-away' on you. ...
degenerative emmiter resistors (as you suggest) will help some.

I like the biasing scheme mentioned by Jon and use it for all my
designs except the early ones using germanium transistors, though
I don't know the name either. The biasing transistor can be
mounted on the output transistors' heatsink for temperature
tracking.

I like it because it's versatile and a single transistor can be
used to bias several transistors with their b-e junctions in
series as long as they are mounted on a common heatsink.
http://img691.imageshack.us/img691/2075/bias.png

Yeah. That's what I was thinking about. Just some thought
here to add. As you carefully point out, tying it to the
heat sink of the output BJTs to hold them near each others'
temperatures seems important. Vbe varies a great deal over
temperature.

The voltage across this structure is:

Vbe * (1 + R1/(R2+R3))

However, it's also true that the output BJTs are also
experiencing similar (but _not_ the exact same as they aren't
necessarily even from the same manufacturer or family)
changes in Vbe. So it's actually a kind of "good thing" to
have the voltage held between the output BJT bases vary as
the output BJTs temperatures vary.

Question is, is a random selection of a BJT for this purpose
okay? Or does it need to be carefully considered, taken
together with the output BJT characteristics? It seems to me
that some care is needed here, even assuming good temperature
coupling occurs.

Also, I think I've seen some examples where there is a
collector resistor added to this structure, with Q2's base
kept tied directly to Q1's collector lead. What is the
reasoning here? (I believe in the cases I saw, there was a
current source [not a resistor] feeding at the top. I
started to work the equations to show the relationship, but
then realized that there is also base current drive to the
upper side of the output transistors involved and then
decided to just ask, instead of wandering all over the place
right now.)

My personal preference is to place the bias adjustment pot R3 in
this position rather than with R1. It ensures that any accidental
loss of contact by the pot's wiper arm will reduce the total bias
whereas placing it with R1 will have the opposite effect and
could cause excessive quiescent current in the output
transistors, possibly getting them to overheat.

Makes sense.

Thanks,
Jon

 

[Phil Allison]

"George Herold"


Dang Phil, take it easy. This seems like the first good thread that
there's been on this news group for a while.


** As usual - you are 100% fucking WRONG.

It is easily one of the VERY WORST bullshit fests here ever.


Piss off and die ASAP -

you know nothing TROLLING TWAT.




..... Phil

 

[Paul E. Schoen]

"Jon Kirwan" <jonk@infinitefactors.org> wrote in message
news:l6n6m5pd53835g9q745t2t4ifbavkghaua@4ax.com...

A concern I care not the least about. My _real_ preference,
were I to impose it on the design, would be to use ONLY
PN2222A BJTs for all the active devices. One part. That's
it. Why? Because I've got thousands of them.

Literally. Something like 22,000 of the bastards. I give
them away like popcorn to students at schools. Got them
_very cheaply_. So if I were pushing something, I'd be
pushing a 10W PN2222A design, use signal splitting approach
probably (because it's the only way I think think of, right
now), and distribute the dissipation across lots and lots of
the things.

I design a lot of things that way as well. I have (hundreds of) thousands
of parts that I got about 20 years ago, and I really like to use them
wherever possible. You can check my website where I have some of these
parts listed as surplus sales and I'd really like to get rid of them where
they might be used rather than hauling them to the dump. Take a look and if
you can use anything I'll see if it's worthwhile to send them to you for
little more than the cost of shipping (probably USPS flat rate). My website
is www.pstech-inc.com, and just look for the link to surplus parts. If that
doesn't work, try http://www.smart.net/~pstech/surplus.htm and
http://www.smart.net/~pstech/PARTS.txt and
http://www.smart.net/~pstech/PARTS.xls.

I have a lot of MPSA06 NPN transistors, so I use them wherever possible. I
also have a few thousand MJE170 PNP Power transistors (40V, 3A, 12W). And
lots of 2N6312 in TO-66 metal cans (PNP 40V 5A 75W) and about 600
Thermalloy 6060 heat sinks that can be used for them, as well as other case
types. If you like SCRs I have about 600 of 2N6504 which is 35A at 50 V.

If you need a transformer for a power supply I have a couple hundred Signal
241-6-16 which can be used to make a raw +/- 8-10 VDC supply and with a few
more capacitors and diodes makes a nice +/- 20 VDC supply at about 1 amp.
And I have an armload of capacitors such as 500 uF 50V, 1500 uF 50V, and
even some 4500 uF 50V in big blue metal cans. And a few handsful of 1N4003
and 1N4004 rectifiers.

If you could come to my place near Baltimore, MD I could give you a
"shopping spree" where you could fill a few bags and boxes with all sorts
of goodies. Lately I am realizing that almost any new design I do will be
with SMT components and newer parts, and there are only a few one-off
projects that I might make using these older components. Some of them have
been stores so long in a damp, unheated building that the leads are
difficult to solder, and some resistors have actually soaked up enough
moisture to change value. (That is what a friend told me, and he also said
they were restored to normal by baking them for a while).

I've sent "care packages" to others in the past. I don't expect to make any
money selling/giving away these parts but I just want to be compensated for
shipping cost. I don't know if I have some of these parts and if I do I
might not even be able to find them, but I think I can supply enough parts
for you to build a good amplifier and other projects.

Paul

 

[Paul E. Schoen]

"Jon Kirwan" <jonk@infinitefactors.org> wrote in message
news:iou6m5haf0vm46v085k50slhsi6mtgsgs6@4ax.com...

On Sat, 30 Jan 2010 01:11:28 +0530, "pimpom"
pimpom@invalid.invalid> wrote:


My personal preference is to place the bias adjustment pot R3 in
this position rather than with R1. It ensures that any accidental
loss of contact by the pot's wiper arm will reduce the total bias
whereas placing it with R1 will have the opposite effect and
could cause excessive quiescent current in the output
transistors, possibly getting them to overheat.

Makes sense.

As a quick precaution against thermal runaway, a thermistor from base to
emitter, and thermally tied to the case, should shut off base drive if
things get too hot. Or just put a thermal switch on the heat sinks and use
it to shut off the supply to the whole shebang.

Paul

 

[Jon Kirwan]

On Fri, 29 Jan 2010 21:05:02 -0500, "Paul E. Schoen"
<paul@peschoen.com> wrote:

"Jon Kirwan" <jonk@infinitefactors.org> wrote in message
news:l6n6m5pd53835g9q745t2t4ifbavkghaua@4ax.com...

A concern I care not the least about. My _real_ preference,
were I to impose it on the design, would be to use ONLY
PN2222A BJTs for all the active devices. One part. That's
it. Why? Because I've got thousands of them.

Literally. Something like 22,000 of the bastards. I give
them away like popcorn to students at schools. Got them
_very cheaply_. So if I were pushing something, I'd be
pushing a 10W PN2222A design, use signal splitting approach
probably (because it's the only way I think think of, right
now), and distribute the dissipation across lots and lots of
the things.

snip

If you could come to my place near Baltimore, MD I could give you a
"shopping spree" where you could fill a few bags and boxes with all sorts
of goodies. Lately I am realizing that almost any new design I do will be
with SMT components and newer parts, and there are only a few one-off
projects that I might make using these older components. Some of them have
been stores so long in a damp, unheated building that the leads are
difficult to solder, and some resistors have actually soaked up enough
moisture to change value. (That is what a friend told me, and he also said
they were restored to normal by baking them for a while).

I've sent "care packages" to others in the past. I don't expect to make any
money selling/giving away these parts but I just want to be compensated for
shipping cost. I don't know if I have some of these parts and if I do I
might not even be able to find them, but I think I can supply enough parts
for you to build a good amplifier and other projects.

Paul

Paul, I'll write under separate cover, directly. There are
some thoughts I'd like to explore more, if that's okay. I
can also provide a 501(c)3 for tax purposes, as well as cash
compensation. That may also help a little. But we can talk
about that off-line.

Jon

 

[John Larkin]

On Fri, 29 Jan 2010 10:34:49 -0800 (PST), George Herold
<ggherold@gmail.com> wrote:

"I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)"

"Vbe multiplier."

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors. Two alternates are:

1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments. Bothe need negative feeback to kill
crossover distortion.

Or...

3. Use mosfets


John

 

[Phil Allison]

"John Larkin is lying IDIOT

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

** Done correctly, either way produces a stable bias situation in the output
stage.

Larkin has no idea how it is done - cos Larkin is bullshitting asshole.

1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

** Guarantees serious x-over distortion.

Zero bias can be done, but never so crudely as that.

2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

** No need to ever use emitter resistors of more than 1 ohm.

With the usual 0.33 to 0.47 ohm resistors, parallel diodes have barely any
effect.

Very few power amp designs have ever used them - SAE brand amps from the
late 1970s being one exception.

In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments.

** Vbe multipliers always need adjustment to suit the actual devices in use.

Both need negative feeback to kill crossover distortion.

** One thing that NFB is notoriously very poor at doing.



..... Phil

 

[John Larkin]

On Sat, 30 Jan 2010 15:18:55 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"John Larkin is lying IDIOT

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

** Done correctly, either way produces a stable bias situation in the output
stage.

Larkin has no idea how it is done - cos Larkin is bullshitting asshole.


1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

** Guarantees serious x-over distortion.

Zero bias can be done, but never so crudely as that.


2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

** No need to ever use emitter resistors of more than 1 ohm.

With the usual 0.33 to 0.47 ohm resistors, parallel diodes have barely any
effect.

Very few power amp designs have ever used them - SAE brand amps from the
late 1970s being one exception.


In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments.

** Vbe multipliers always need adjustment to suit the actual devices in use.


Both need negative feeback to kill crossover distortion.


** One thing that NFB is notoriously very poor at doing.



.... Phil

Discrete-transistor audio design, being such an ancient practice,
tends to refer to history and authority rather than design from
engineering fundamentals.

If I were designing an audio amp nowadays (which I certainly aren't)
I'd use mosfets with an opamp gate driver per fet. That turns the fets
into almost-perfect, temperature-independent, absolutely identical
gain elements. That's what I do in my MRI gradient amps, whose noise
and distortion are measured in PPMs.

ftp://jjlarkin.lmi.net/Amp.jpg

Why keep repeating a 50-year-old topology when you could have a little
fun?

John