Different Audio Design

[Robert Baer]

"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote:

"Rich Grise" <rich@example.net> wrote in message
newsan.2004.11.17.16.43.28.553634@example.net...
On Wed, 17 Nov 2004 05:42:37 -0800, Watson A.Name - "Watt Sun, the
Dark

"Robert Baer" <robertbaer@earthlink.net> wrote in message
...
Zhoe, vee zee zhat zee soykut iss a mess.
sCRAP.

But they say it works. Well.
^^^^

Is this an adverb, or an interjection? ;-)

Sorry for the confloosion. It's an adverb. "It works well," he said,
with low 'torsion' [distortion]. Hey, give him a break, he has also
learned German, and 'Engrish' seems to be his 3rd language(!) Once you
learn German, there's no more room for Engrish in your head! :-[

Thanks,
Rich

A long time ago i made an RF amplifier using an Esaki ("tunnel")
Diode.
It was rather puzzling, in that the gain was very low and could not be
improved.
Further analysis showed that the dern thing was oscillating like a
banshee at a substantially higher frequency.
Another thought: the "joke" that if you want an oscillator, build or
design an amplifier (and vice-versa).

 

[Phil Allison]

"Robert Baer"

A long time ago i made an RF amplifier using an Esaki ("tunnel")
Diode.
It was rather puzzling, in that the gain was very low and could not be
improved.
Further analysis showed that the dern thing was oscillating like a
banshee at a substantially higher frequency.
Another thought: the "joke" that if you want an oscillator, build or
design an amplifier (and vice-versa).

** Looks to me like young Robert has just come up with another corollary
to the ever growing set of "Laws" popularly attributed to the famous USAF
Major Murphy that likely rates recording somewhere for posterity .

Or has he perhaps been beaten to it by another ??





............ Phil

 

[N. Thornton]

"Watson A.Name - \"Watt Sun, the Dark Remover\"" <NOSPAM@dslextreme.com> wrote in message news:<10po4ltg49m2c3a@corp.supernews.com>...

Swing thru rather quickly I would guess. And doing so with the
possibility of overcorrecting?

inevitably, since opamps are frequency and slew limited, there will
always be a little undershoot then overshoot. But this is not a
problem, it just introduces very small ultrasonic artefacts. It
happens in every class B with feedback, and is pretty much a non
issue. Note that feedback on class B doesnt eliminate distortion, it
just moves it up out of the useful frequency band.

Or hunting about trying to find a place
it can never find?

It might possibly do that during idle, but the consequences are none.
The output still stays at 0, or extremely close to it, with any
artefacts being ultrasonic. again, same as any class B with large nfb.
In practice output trs have very low gain when theyre just beginning
to conduct, so it would probably be stable in that respect.

Or having two opamps, the phase shift might be too
great at high freqs. Or..

Thats a problem, especially with a capacitive load. Combined with the
amps output R you get an RC that youre taking feedback from. However,
I've used simple clas B output tr pair plus corecting opamp before
without any problem. The problem in the design offered is the fighting
joined feedback loops - but its easily fixed.


NT

 

[N. Thornton]

"Arny Krueger" <arnyk@hotpop.com> wrote in message news:<C5mdneYGxNIXxQbcRVn-1A@comcast.com>...

"Watson A.Name - "Watt Sun, the Dark Remover"" <NOSPAM@dslextreme.com
wrote in message news:10pm2aoeda3eqc0@corp.supernews.com

Opinions on this, especially the 2.5W amp schem at the bottom.
http://www.intio.or.jp/jf10zl/EF.htm

When the amp is idle, and there's no(?) current flowing in either
output transistor because there's no forward bias, then the V drop
'resistance' of the E-B junction adds to the 100k feedback resistor,
so the loop gain increases to the open loop gain. So it would seem
that the amp would attempt to hunt in this region, possibly
oscillating?

Depends on the op amp. I stopped taking this circuit page seriously when I
saw "741" on the upper two circuit diagrams.

Lets at least be fair, the earlier circuits on that page make it quite
clear its not an attempt at quality audio. He's playing with ideas,
and quality is not an inherent part of some designs. But the 741, I
would be in no rush to use them - maybe 20 years ago, and even then I
found 748s superior.


NT

 

[N. Thornton]

"Karl Uppiano" <karl.uppiano@verizon.net> wrote in message news:<EzQmd.7146$GV5.6858@trnddc04>...

We're taking a flawed design, and adding band-aids on top of band-aids. You
can make a perfectly good amplifier with one opamp and two push-pull emitter
followers biased with a couple of diodes, similar to the first design
sketched in the OP's link. If that design draws too much idle current,
increase the emitter resistor. There's lots of stuff you can do to improve
that design without resorting to the demented design being proposed.

I cant agree really. Correcting the design with 2 .25w resistors is
not a problem. Ditto moving the feedback point for opamp 1.

NT

 

[John Larkin]

On Thu, 18 Nov 2004 10:09:19 GMT, "Kevin Aylward"
<salesEXTRACT@anasoft.co.uk> wrote:

It's actually not a bad idea to have an opamp per output transistor,
if you do it right, which this guy clearly hasn't. I make a power amp
that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt
rails) and do just that.

Bloody hell. That's some amp, probably about 20KW out. This will
certainly kick the shit out of you at 50 Hz.

How did you get the opamp voltage rating, or was it a discrete one?

It's a transconductance amp, so the fet sources are at the rails
(through current-sense resistors) and the drains are the output. So
the gate-drive fets work on the +-200 power supply rails (with small
floating local supplies) and only need to swing enough to drive the
gates.

Nice thing here is that all 32 fet drains are common, so they are
bolted (actually clamped) to copper heat spreaders on the main
heatsink, without insulators.

So long as the loop is stable its ok.

Well, sure!

John

 

[John Larkin]

On Thu, 18 Nov 2004 17:37:26 GMT, "Kevin Aylward"
<salesEXTRACT@anasoft.co.uk> wrote:

John Larkin wrote:
On Thu, 18 Nov 2004 10:09:19 GMT, "Kevin Aylward"
salesEXTRACT@anasoft.co.uk> wrote:


It's actually not a bad idea to have an opamp per output transistor,
if you do it right, which this guy clearly hasn't. I make a power
amp that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch,
+-200 volt rails) and do just that.

Bloody hell. That's some amp, probably about 20KW out. This will
certainly kick the shit out of you at 50 Hz.

How did you get the opamp voltage rating, or was it a discrete one?


It's a transconductance amp, so the fet sources are at the rails
(through current-sense resistors) and the drains are the output. So
the gate-drive fets work on the +-200 power supply rails (with small
floating local supplies) and only need to swing enough to drive the
gates.

I have recently been playing with both source out and drain out
configurations for that super duper amp I mentioned in one post. One
issue with the drain out is x-over spikes. I was getting 100A thick
x-over spikes in the drain output, verses essentially, none in the
source output. The gates tied together prevents the spike when slewing.
So, worth noting if you want the utmost in speed without frying the
devices in a linear amp.

I'm driving a mostly-inductive load, with overall current feedback, so
the current-out stage makes sense for me; it keeps the loop dynamics
from depending on the load, which is valuable. An audio or such amp is
better of it has an inherently low z-out, sor the same sorts of
reasons.

As far as crossover goes, that is interesting. My output stage drive
signal is grounded, and I have a splitter circuit to convert it into
sinking and sourcing currents to get up to the floating +side and
-side power stages, which are essentially current mirrors. The
splitter must sustain a small idle current on both outputs and also
perfectly split the input signal, so that positive inputs increase the
sink current and negative sigs increase the source current. I've been
tuning this circuit for years, gradually converging on something that
works pretty well. It's an interesting, philosophically disturbing,
problem.

For a voltage-out amp, it would be cool to float a, say, +-12 volt
supply on the output node and hang an opamp to close the loop around
each fet, making an ideal follower. Then you'd be able to run the fets
all the way to the power rails, and you could use very small source
sense resistors, with just millivolts of idle voltage.

John

 

[Arny Krueger]

"Watson A.Name - "Watt Sun, the Dark Remover"" <NOSPAM@dslextreme.com>
wrote in message news:10pm2aoeda3eqc0@corp.supernews.com

Opinions on this, especially the 2.5W amp schem at the bottom.
http://www.intio.or.jp/jf10zl/EF.htm

When the amp is idle, and there's no(?) current flowing in either
output transistor because there's no forward bias, then the V drop
'resistance' of the E-B junction adds to the 100k feedback resistor,
so the loop gain increases to the open loop gain. So it would seem
that the amp would attempt to hunt in this region, possibly
oscillating?

Depends on the op amp. I stopped taking this circuit page seriously when I
saw "741" on the upper two circuit diagrams.

Do you know what a 741 is from the standpoint of quality audio? Anathema!

Maybe a 1k resistor E to B on the output transistors would 'bypass'
this. Or should the amp be biased to work class AB.

It's a variation on what some designers call "current dumping".

However let's say the truth - this is an outdated, amateurish design with no
known merits over established technology.

 

[John Larkin]

On Wed, 17 Nov 2004 22:49:08 GMT, "Karl Uppiano"
<karl.uppiano@verizon.net> wrote:

We're taking a flawed design, and adding band-aids on top of band-aids. You
can make a perfectly good amplifier with one opamp and two push-pull emitter
followers biased with a couple of diodes, similar to the first design
sketched in the OP's link. If that design draws too much idle current,
increase the emitter resistor. There's lots of stuff you can do to improve
that design without resorting to the demented design being proposed.

It's actually not a bad idea to have an opamp per output transistor,
if you do it right, which this guy clearly hasn't. I make a power amp
that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt
rails) and do just that. It forces essentially perfect current
sharing, nukes the device tc and part-part variations, and makes lots
of gate drive available.

John

 

[Kevin Aylward]

John Larkin wrote:

On Thu, 18 Nov 2004 17:37:26 GMT, "Kevin Aylward"
salesEXTRACT@anasoft.co.uk> wrote:

John Larkin wrote:
On Thu, 18 Nov 2004 10:09:19 GMT, "Kevin Aylward"
salesEXTRACT@anasoft.co.uk> wrote:


It's actually not a bad idea to have an opamp per output
transistor, if you do it right, which this guy clearly hasn't. I
make a power amp that uses 32 300-watt fets in the output (16
p-ch, 16 n-ch, +-200 volt rails) and do just that.

Bloody hell. That's some amp, probably about 20KW out. This will
certainly kick the shit out of you at 50 Hz.

How did you get the opamp voltage rating, or was it a discrete one?


It's a transconductance amp, so the fet sources are at the rails
(through current-sense resistors) and the drains are the output. So
the gate-drive fets work on the +-200 power supply rails (with small
floating local supplies) and only need to swing enough to drive the
gates.

I have recently been playing with both source out and drain out
configurations for that super duper amp I mentioned in one post. One
issue with the drain out is x-over spikes. I was getting 100A thick
x-over spikes in the drain output, verses essentially, none in the
source output. The gates tied together prevents the spike when
slewing. So, worth noting if you want the utmost in speed without
frying the devices in a linear amp.



I'm driving a mostly-inductive load, with overall current feedback, so
the current-out stage makes sense for me; it keeps the loop dynamics
from depending on the load, which is valuable. An audio or such amp is
better of it has an inherently low z-out, sor the same sorts of
reasons.

As far as crossover goes, that is interesting.

Of course in a real audio amp, one doesn't put a 500Khz square wave in
to see this particular "problem". For specmanship it is. One cant do the
test with out a fire.

My output stage drive
signal is grounded, and I have a splitter circuit to convert it into
sinking and sourcing currents to get up to the floating +side and
-side power stages, which are essentially current mirrors. The
splitter must sustain a small idle current on both outputs and also
perfectly split the input signal, so that positive inputs increase the
sink current and negative sigs increase the source current. I've been
tuning this circuit for years, gradually converging on something that
works pretty well. It's an interesting, philosophically disturbing,
problem.

For a voltage-out amp, it would be cool to float a, say, +-12 volt
supply on the output node and hang an opamp to close the loop around
each fet, making an ideal follower.

Again, potential issue of stability. I do have an interesting, somwhat
original output circuit that does this with a first order loop enclosing
the outputs. It gets me wonderfully low distortion at 20Khz. In one
sense topologically its essentially the same as other compound pairs,
but this does behave a little different. The feedback diodes are zeners
at 10V. The drive circuit on its own < 0.0001% THD < 20Khz, according to
spice that is. Note the feedback transisters can be low voltage ones,
hence fast ones. DC loop gain is 135db, open loop more!

http://www.anasoft.co.uk/Mospoweramp.jpg


Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[John Larkin]

On Fri, 19 Nov 2004 19:02:29 GMT, "Kevin Aylward"
<salesEXTRACT@anasoft.co.uk> wrote:

Yes, but as I noted, there is the pole of the opamp, and the pole of
rout with the fets, plus all the other op-amp parasitics. Its the old
frequency verses accuracy trade off. Anyway, its cheating to use an
op-amp

But Kevin, inside every opamp are lots of little transistors! That
should make you happy.

John

 

[Phil Allison]

"John Larkin"

One problem here is current sharing. Linear-mode paralleled mosfets
don't do it very well.

** Not all power mosfets are the same. One needs to be careful to
distinguish between the usual fast switching types and the lateral mosfets
( aka audio fets) as made by Hitachi and Semelab.

One of my earlier gradient amps was done with
paralleled fets (with source resistors, at least) and we wound up
making a fixture so we could match sets of fets for production... a
real pain in the sternparts.

** Switching fets, when used in linear mode, need to be carefully matched
and ballasted - while the lateral kind have very different
characteristics and do not need ballasting or careful matching.

The advantage of an opamp per fet is that
the gate drive becomes very simple and sharing is forced to be
perfect.

** Agreed.




................ Phil

 

[Kevin Aylward]

John Larkin wrote:

On Fri, 19 Nov 2004 19:02:29 GMT, "Kevin Aylward"
salesEXTRACT@anasoft.co.uk> wrote:


Yes, but as I noted, there is the pole of the opamp, and the pole of
rout with the fets, plus all the other op-amp parasitics. Its the old
frequency verses accuracy trade off. Anyway, its cheating to use an
op-amp


But Kevin, inside every opamp are lots of little transistors! That
should make you happy.

You don't climb mountains or sky dive then? Its the personal challenge
dude. Its just not the same.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Stewart Pinkerton]

On Sat, 20 Nov 2004 07:16:10 GMT, "Kevin Aylward"
<salesEXTRACT@anasoft.co.uk> wrote:

John Larkin wrote:
On Fri, 19 Nov 2004 19:02:29 GMT, "Kevin Aylward"
salesEXTRACT@anasoft.co.uk> wrote:


Yes, but as I noted, there is the pole of the opamp, and the pole of
rout with the fets, plus all the other op-amp parasitics. Its the old
frequency verses accuracy trade off. Anyway, its cheating to use an
op-amp

No, it's making best use of available technology. Anything else isn't
engineering, it's whimsical indulgence.

But Kevin, inside every opamp are lots of little transistors! That
should make you happy.

You don't climb mountains or sky dive then? Its the personal challenge
dude. Its just not the same.

Ah, so you're going to use only valves to make a DAC? That should keep
you challenged for a while!
--

Stewart Pinkerton | Music is Art - Audio is Engineering

 

[Kevin Aylward]

Stewart Pinkerton wrote:

On Sat, 20 Nov 2004 07:16:10 GMT, "Kevin Aylward"
salesEXTRACT@anasoft.co.uk> wrote:

John Larkin wrote:
On Fri, 19 Nov 2004 19:02:29 GMT, "Kevin Aylward"
salesEXTRACT@anasoft.co.uk> wrote:


Yes, but as I noted, there is the pole of the opamp, and the pole
of rout with the fets, plus all the other op-amp parasitics. Its
the old frequency verses accuracy trade off. Anyway, its cheating
to use an op-amp

No, it's making best use of available technology. Anything else isn't
engineering, it's whimsical indulgence.

Oh dear...you have obviously completely missed the tongue firmly in
cheek bit in this thread. Dont try and teach your granpa to suck eggs
dude.

Secondly, in this particlar example, using two transistors, as in my
referenced circuit (http://www.anasoft.co.uk/Mospoweramp.jpg) is almost
certainly the most effective way to achieve its function. Using an op
amp, as I indicated, has all sorts of issues. For instance, it would be
way more complicated to set it up to run on +/-100V supplies.

That's pretty much the fundamental reason behind my architecture. A very
simply way of putting feedback around the output devices. If you have a
simpler way, that achieves the particular characteristics I require, I
would certainly be interested in it.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Stewart Pinkerton]

On Sat, 20 Nov 2004 11:54:43 GMT, "Kevin Aylward"
<salesEXTRACT@anasoft.co.uk> wrote:

Stewart Pinkerton wrote:
On Sat, 20 Nov 2004 07:16:10 GMT, "Kevin Aylward"
salesEXTRACT@anasoft.co.uk> wrote:

John Larkin wrote:
On Fri, 19 Nov 2004 19:02:29 GMT, "Kevin Aylward"
salesEXTRACT@anasoft.co.uk> wrote:


Yes, but as I noted, there is the pole of the opamp, and the pole
of rout with the fets, plus all the other op-amp parasitics. Its
the old frequency verses accuracy trade off. Anyway, its cheating
to use an op-amp

No, it's making best use of available technology. Anything else isn't
engineering, it's whimsical indulgence.

Oh dear...you have obviously completely missed the tongue firmly in
cheek bit in this thread. Dont try and teach your granpa to suck eggs
dude.

If you were old enough to be my grandpa, I'd be seriously impressed
that you were still posting!

Besides, egg-sucking was grandma's job................

Secondly, in this particlar example, using two transistors, as in my
referenced circuit (http://www.anasoft.co.uk/Mospoweramp.jpg) is almost
certainly the most effective way to achieve its function. Using an op
amp, as I indicated, has all sorts of issues. For instance, it would be
way more complicated to set it up to run on +/-100V supplies.

That would be about the only real issue.

That's pretty much the fundamental reason behind my architecture. A very
simply way of putting feedback around the output devices. If you have a
simpler way, that achieves the particular characteristics I require, I
would certainly be interested in it.

No, with these voltages, it's probably close to optimum. It's not as
if PA design isn't a mature technology these days...........
--

Stewart Pinkerton | Music is Art - Audio is Engineering

 

[Jim Meyer]

"Watson A.Name - \"Watt Sun, the Dark Remover\"" <NOSPAM@dslextreme.com> wrote in message news:<10pm2aoeda3eqc0@corp.supernews.com>...

Opinions on this, especially the 2.5W amp schem at the bottom.
http://www.intio.or.jp/jf10zl/EF.htm

When the amp is idle, and there's no(?) current flowing in either output
transistor because there's no forward bias, then the V drop 'resistance'
of the E-B junction adds to the 100k feedback resistor, so the loop gain
increases to the open loop gain. So it would seem that the amp would
attempt to hunt in this region, possibly oscillating?

I just simulated the circuit in LTSpice using LT1113's instead of
the TL084. It simulates quite nicely. The feedback causes the base
drives to swing through the "crossover" region so that's no problem.

Here's a "cut-n-paste" ASCII file for the input to LTSpice:

Version 4
SHEET 1 880 680
WIRE -16 384 16 384
WIRE 144 144 48 144
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FLAG 176 96 +14
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FLAG 176 160 0
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FLAG -400 0 +14
FLAG -256 0 +7
FLAG -400 128 0
FLAG -256 128 0
FLAG -128 448 +7
FLAG 592 240 out
FLAG 48 144 1st
FLAG -48 416 0
SYMBOL Opamps\\LT1113 176 304 R0
SYMATTR InstName U1
SYMBOL Opamps\\LT1113 176 64 R0
SYMATTR InstName U2
SYMBOL Opamps\\LT1113 -48 320 R0
SYMATTR InstName U3
SYMBOL pnp 288 416 M180
SYMATTR InstName Q1
SYMBOL npn 288 80 R0
SYMATTR InstName Q2
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[N. Thornton]

Robert Baer <robertbaer@earthlink.net> wrote in message news:<419B1FE6.3C4E456B@earthlink.net>...

"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote:

Opinions on this, especially the 2.5W amp schem at the bottom.
http://www.intio.or.jp/jf10zl/EF.htm

When the amp is idle, and there's no(?) current flowing in either output
transistor because there's no forward bias, then the V drop 'resistance'
of the E-B junction adds to the 100k feedback resistor, so the loop gain
increases to the open loop gain. So it would seem that the amp would
attempt to hunt in this region, possibly oscillating?

Maybe a 1k resistor E to B on the output transistors would 'bypass'
this. Or should the amp be biased to work class AB.

Start with the "obvious" DC initial conditions: pins 5 and 6 are at
1/2 of the supply voltage.
Note average DC current thru R4 must be exactly zero (am assuming zero
input bias current in op amps and zero leakage current for C6 and C7).
Then pins 7, 9, 10, 12 and 13 are all within an op-amp Vos of 1/2 of
the supply voltage.
Now we see the dicey part; theoretically (acting seperately), opamp C
will drive Q1 until the inputs "see" its Vos, and opamp D would drive Q2
until the inputs "see" its Vos.
Housesomever, zee soykut iss a mess.
The input conditions that opamp C will try to satisfy is virtually
*guaranteed* to be different than the input conditions that opamp D will
try to satisfy (the Vos of one will be different than the Vos of the
other).
Ignoring that feedback, the conclusion seems to be that the opamp with
the greater open loop power gain (that includes the transistor) will
(mostly) win, meaning the other transistor will either be driven to
saturation or zener E-B breakdown.
But, remember that no DC current can flow thru R4.
It *does* appear that your supposition of oscillation is correct.
Now, replace the transistors with E-to-B resistors and one still has
the problem.
Zhoe, vee zee zhat zee soykut iss a mess.
sCRAP.

Yep, I agree with it all... except 'scrap'. The problem is there are 2
feedback loops on the output end opamps, and theyre _connected
together_. Thus the 2 feedbacks cant happen independantly. They might
cooperate, or they might fight and lead the op trs to an MS-release
event (Magic Smoke).

But rather than scrap it, this is easily fixed. Add a couple of 0.1
ohm emitter Rs on the op trs, then you can complete the loop
separately for each tr. The LS is then being driven thru a 0.1 ohm R,
which is quite ok. Think that through... it all works nicely, no
fighting. V offsets become a non event, 2mV across 0.2 ohm is trivial.

The one issue is opamp speed. Call me cautious but I wouldnt be
surprised if they were using something like an LM324. While that does
have workable audio performance (reduced max amplitude above 6kHz is
fairly ok), it can not shift fast enough to deal with crossover
distortion cleanly. To do that would require performance at way above
20kHz.

I also have some hesitations about opamp A see the output load
capacitance, but again thats easily fixed.


NT