Building a class A audio amplifier - no audio out

I tried to build this Class A amplifier circuit as a learning
experience:
http://www.techlib.com/electronics/audioamps.html#class-a

with the following modifications:
1. I have no MPSW45, so I built a Darlington out of a 2N2222 and
TIP31A.
2. 150k resistors: I only had two 100k resistors in my collection.
But as long as they're identical, should work...???
3. speaker: I had only a 10W, 4 ohm car speaker readily available.

Powered up with a 6V battery, input was the headphone out from a
portable radio. Nothing except for an impressive Thump when I
connected power to the circuit.

What could have gone wrong?

I'm not sure if I would have 200mV p-p on the radio headphone out.
Would that be it?

Thanks,

Michael

 

[Nobody]

On Sun, 10 May 2009 23:47:30 -0700, mrdarrett wrote:

I tried to build this Class A amplifier circuit as a learning
experience:
http://www.techlib.com/electronics/audioamps.html#class-a

with the following modifications:
1. I have no MPSW45, so I built a Darlington out of a 2N2222 and
TIP31A.
2. 150k resistors: I only had two 100k resistors in my collection.
But as long as they're identical, should work...???
3. speaker: I had only a 10W, 4 ohm car speaker readily available.

Powered up with a 6V battery, input was the headphone out from a
portable radio. Nothing except for an impressive Thump when I
connected power to the circuit.

What could have gone wrong?

You chose a crap circuit.

Analogue isn't exactly my strong point, but the circuit appears to be
relying upon the MPSW45's relatively low hfe (25) to draw enough base
current through the 150K resistor to avoid saturation.

The 2N2222/TIP31A pair has a combined gain of 2500, so the
transistors are probably saturated.

A more realistic circuit would use an emitter-follower resistor to
stabilise the DC operating point, bypassed with a capacitor so that you
still get AC voltage gain.

 

[Eeyore]

mrdarrett@gmail.com wrote:

I tried to build this Class A amplifier circuit as a learning
experience:
http://www.techlib.com/electronics/audioamps.html#class-a

Why why why ?

It puts DC through the speaker is is horribly badly biased. It's a JOKE.

Find some decent websites.

Graham

 

[Tim Wescott]

Eeyore wrote:

mrdarrett@gmail.com wrote:

I tried to build this Class A amplifier circuit as a learning
experience:
http://www.techlib.com/electronics/audioamps.html#class-a

Why why why ?

It puts DC through the speaker is is horribly badly biased. It's a JOKE.

And how is a beginner supposed to know?

Do you have any suggestions for a site to look at, or a book?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Tim Wescott]

mrdarrett@gmail.com wrote:

On May 11, 8:16 am, stratu...@yahoo.com wrote:
On May 11, 7:32 am, Tim Wescott <t...@seemywebsite.com> wrote: > Eeyore wrote:

snip
Why why why ?

It puts DC through the speaker is is horribly badly biased. It's
a JOKE.

And how is a beginner supposed to know?

Do you have any suggestions for a site to look at, or a book?

--

Tim Wescott

Lots of good stuff here

http://sound.westhost.com/

G˛


Ah, thank you for the reply.

One thing I noticed is, the larger amps on westhost.com (10W+) need
split power supplies (+/-).

That's because if you want high power you need high voltage swing, which
you get either with a pair of amplifiers in a bridge configuration, with
a split supply, or with a honking big electrolytic capacitor. That
capacitor has to be able to flow all the current from that high-power
audio through an 8-ohm speaker while not interfering with the sound --
it's going to be big and expensive.

I'd like to start with something needing
just 0V-6V or 0V-12V.

So consider an amp in a bridge configuration, but consider also all the
extra work you'll do to build two (fairly complex) amps instead of one
fairly complex amp and one dirt-simple power supply.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

On May 11, 7:32 am, Tim Wescott <t...@seemywebsite.com> wrote:

Eeyore wrote:
snip
Why why why ?

It puts DC through the speaker is is horribly badly biased. It's
a JOKE.

And how is a beginner supposed to know?

Do you have any suggestions for a site to look at, or a book?

--

Tim Wescott

Lots of good stuff here

http://sound.westhost.com/

G˛

 

On May 11, 8:16 am, stratu...@yahoo.com wrote:

 On May 11, 7:32 am, Tim Wescott <t...@seemywebsite.com> wrote: > Eeyore wrote:

 <snip
 > > Why why why ?
 
 > > It puts DC through the speaker is is horribly badly biased. It's
a JOKE.
 
 > And how is a beginner supposed to know?
 
 > Do you have any suggestions for a site to look at, or a book?
 
 > --
 
 > Tim Wescott

Lots of good stuff here

http://sound.westhost.com/

G˛

Ah, thank you for the reply.

One thing I noticed is, the larger amps on westhost.com (10W+) need
split power supplies (+/-). I'd like to start with something needing
just 0V-6V or 0V-12V.

Thanks again,

Michael

 

On May 11, 8:40 am, Tim Wescott <t...@seemywebsite.com> wrote:

mrdarr...@gmail.com wrote:
On May 11, 8:16 am, stratu...@yahoo.com wrote:
 On May 11, 7:32 am, Tim Wescott <t...@seemywebsite.com> wrote: > Eeyore wrote:

 <snip
 > > Why why why ?

 > > It puts DC through the speaker is is horribly badly biased. It's
a JOKE.

 > And how is a beginner supposed to know?

 > Do you have any suggestions for a site to look at, or a book?

 > --

 > Tim Wescott

Lots of good stuff here

http://sound.westhost.com/

G˛

Ah, thank you for the reply.

One thing I noticed is, the larger amps on westhost.com (10W+) need
split power supplies (+/-).

That's because if you want high power you need high voltage swing, which
you get either with a pair of amplifiers in a bridge configuration, with
a split supply, or with a honking big electrolytic capacitor.  That
capacitor has to be able to flow all the current from that high-power
audio through an 8-ohm speaker while not interfering with the sound --
it's going to be big and expensive.

 I'd like to start with something needing
just 0V-6V or 0V-12V.

So consider an amp in a bridge configuration, but consider also all the
extra work you'll do to build two (fairly complex) amps instead of one
fairly complex amp and one dirt-simple power supply.

Ah. I'd neglected to say that a 1W amp would be fine for learning
purposes.

If I just wanted power I'd go for an LM3886 or just buy an Onkyo
system. I'd like to understand how the amps work.

Any comments on this circuit as a beginner project?
http://www.redcircuits.com/Page33.htm

Thanks,

Michael

 

[pimpom]

mrdarrett@gmail.com wrote:

Ah. I'd neglected to say that a 1W amp would be fine for learning
purposes.

If I just wanted power I'd go for an LM3886 or just buy an Onkyo
system. I'd like to understand how the amps work.

Any comments on this circuit as a beginner project?
http://www.redcircuits.com/Page33.htm

Looks ok to me. You have to pay heed to the instructions, especially about
adjusting the quiescent (no-signal) current.

I'd recommend mounting the two output transistors Q3 and Q4 on a small
heatsink, making sure that they are electrically isolated from each other
and from ground and other components. The two transistors can theoretically
dissipate nearly 1W each with a 4-ohm load. That may seem small, but it's
enough to get a bare TO220 transistor quite hot.

One 0.47-ohm 1/2-watt resistor in series with the emitter of each output
transistor will aid stability.

Q2 dissipates about 0.25W which will also cause the small transistor to run
quite hot to the touch. I suggest using a BD135-16 or a BD137-16 instead of
the BC337. Choosing a sub-type with the -16 suffix ensures that it will have
about the same gain level as the BC337.

 

[Eeyore]

Tim Wescott wrote:

Eeyore wrote:
mrdarrett@gmail.com wrote:

I tried to build this Class A amplifier circuit as a learning
experience:
http://www.techlib.com/electronics/audioamps.html#class-a

Why why why ?

It puts DC through the speaker is is horribly badly biased. It's a JOKE.

And how is a beginner supposed to know?

He's not that much of a beginner. I'd have known that was crap by my mid
teens. It's about as simple as it gets. I was building stuff more complex at
age 12.

Do you have any suggestions for a site to look at, or a book?

Application notes are good, as long as you can find one that covers the
subject. Discrete design is rare these days and apart from the experts, the
standard of it seems to be going downhill fast as people aren't taught
'circuits' and it just becomes 'building blocks'.

A decent audio site is here.
http://www.dself.dsl.pipex.com/ampins/ampins.htm

Graham

 

[Eeyore]

mrdarrett@gmail.com wrote:

On May 11, 8:16 am, stratu...@yahoo.com wrote:
On May 11, 7:32 am, Tim Wescott <t...@seemywebsite.com> wrote: > Eeyore wrote:

Why why why ?

It puts DC through the speaker is is horribly badly biased. It's
a JOKE.

And how is a beginner supposed to know?

Do you have any suggestions for a site to look at, or a book?

Lots of good stuff here

http://sound.westhost.com/

Another decent one.

Ah, thank you for the reply.

One thing I noticed is, the larger amps on westhost.com (10W+) need
split power supplies (+/-). I'd like to start with something needing
just 0V-6V or 0V-12V.

Then you'll *need* capacitor coupling which is generally considered a 'bad thing' (
for 101 reasons ). All top-notch audio uses split supplies.

Graham

 

[Eeyore]

mrdarrett@gmail.com wrote:

Ah. I'd neglected to say that a 1W amp would be fine for learning
purposes.

First you need to learn how discrete circuits work and how to measure / troubleshoot
them. You'll have fun finding a book that'll teach that. My first came from 1969 (
Mullard ) followed by an RCA handbook and a full blown theory book my Dad bought me and
people were already well into ICs by the mid 70s.

Graham

 

[Eeyore]

mrdarrett@gmail.com wrote:

Any comments on this circuit as a beginner project?
http://www.redcircuits.com/Page33.htm

Grossly obsolescent design.

I'm actually having some trouble figuring some aspects of it's operation. It's a bit
oddball.

Learn how low level preamps work first, then tack on emitter followers to see how power
amps work.

Graham

 

[Eeyore]

mrdarrett@gmail.com wrote:

"pimpom" <pim...@invalid.invalid> wrote:
mrdarr...@gmail.com wrote:

Ah. I'd neglected to say that a 1W amp would be fine for learning
purposes.

If I just wanted power I'd go for an LM3886 or just buy an Onkyo
system. I'd like to understand how the amps work.

Any comments on this circuit as a beginner project?
http://www.redcircuits.com/Page33.htm

Looks ok to me. You have to pay heed to the instructions, especially about
adjusting the quiescent (no-signal) current.

I'd recommend mounting the two output transistors Q3 and Q4 on a small
heatsink, making sure that they are electrically isolated from each other
and from ground and other components. The two transistors can theoretically
dissipate nearly 1W each with a 4-ohm load. That may seem small, but it's
enough to get a bare TO220 transistor quite hot.

One 0.47-ohm 1/2-watt resistor in series with the emitter of each output
transistor will aid stability.

Q2 dissipates about 0.25W which will also cause the small transistor to run
quite hot to the touch. I suggest using a BD135-16 or a BD137-16

You'll have trouble getting any BD135 or 137 IME these days at least at a sensible
price. They're 30 year old devices in an little used package these days.

Half the trouble is the most half-assed DIY websites use exclusively obsolete
parts. Thinks 2N2222 for example. Metal can, expensive and outperformed now by a
2c TO-92 device.

instead of
the BC337. Choosing a sub-type with the -16 suffix ensures that it will have
about the same gain level as the BC337.

Hahahahaha ! Just try getting suffixed types. LMAO.

Ah, thank you very much. I'll heatsink the transistors, and thanks
for the tip about emitter resistors.

Sinclair ( of Sir Clive Sinclair fame ) made audio modules for DIYers in the 70's.
Their Z30 and Z50 power amps neglected to use emitter resistors and subsequently
failed regularly.

I even wrote to the company pointing out the design error. FINALLY after God knows
how many returns they found PCB space to fit some. Problem fixed.

What you need to understand is WHY they were important. That comment is to you Mr
Darrett or you'll never be a designer.

Graham

 

[Eeyore]

mrdarrett@gmail.com wrote:

Eeyore <rabbitsfriendsandrelati...@hotmail.com> wrote:
Tim Wescott wrote:
Eeyore wrote:
mrdarr...@gmail.com wrote:

I tried to build this Class A amplifier circuit as a learning
experience:
http://www.techlib.com/electronics/audioamps.html#class-a

Why why why ?

It puts DC through the speaker is is horribly badly biased. It's a JOKE.

And how is a beginner supposed to know?

He's not that much of a beginner.

He's not? You might be surprised. My background is in chemistry,
not electronics.

Well, you post often enough here and usually get good info. Just on that basis I'd
hope you'd picked something up.

I'd have known that was crap by my mid
teens. It's about as simple as it gets. I was building stuff more complex at
age 12.

Ah, good for you. I wasted my teen years teaching myself C
programming, assembly language programming, and modifying Michael
Abrash's VGA Mode X graphics routines. Then Windows 95 came out and
made my experience worthless. Eh. That's life.

I was offered a job programming in C back around 1984. I took one look at the
bizarre syntax and ran away. My career would have been surely very different and
probably wealthier had I taken the offer but I might have missed a lot of
interesting challenges.

For some odd reason, electronics seem much harder to me than
chemistry... I wonder why... but I enjoy exercising the old brain
cells.

Do you have any suggestions for a site to look at, or a book?

Application notes are good, as long as you can find one that covers the
subject. Discrete design is rare these days and apart from the experts, the
standard of it seems to be going downhill fast as people aren't taught
'circuits' and it just becomes 'building blocks'.

A decent audio site is here.http://www.dself.dsl.pipex.com/ampins/ampins.htm

Been there, borrowed Self's book from the library, still a bit over my
head.

I found this interesting, though.
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part1/page3.html

First look seems promising.

BUT ! Before anything else. Do you know how to bias a single transistor small
signal amplifier. Don't touch anything else until you've mastered that together
with all its variants and effects.

Even better ... start with an emitter follower and ask yourself why they're used
and then do the same with the complementary version ( may not be instinctively
obvious but every audio power amp of any note uses one ).

Actually, I nearly forgot. UK magazine Wireless World had a 3 or so part monthly
article on discrete design by a practical college lecturer as opposed to an Ivory
Towers University type. I learnt more from that, faster than anything else. I must
see if I still have it. It dropped all the ultra-theoretical bunk and got on with
how to make circuits that work and why.

Graham

 

[Eeyore]

mrdarrett@gmail.com wrote:

I found this interesting, though.
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part1/page3.html

Ah, it's maintained by Jim Lesurf. That explains why it's good.

Graaham

 

On May 11, 11:32 am, "pimpom" <pim...@invalid.invalid> wrote:

mrdarr...@gmail.com wrote:

Ah.  I'd neglected to say that a 1W amp would be fine for learning
purposes.

If I just wanted power I'd go for an LM3886 or just buy an Onkyo
system.  I'd like to understand how the amps work.

Any comments on this circuit as a beginner project?
http://www.redcircuits.com/Page33.htm

Looks ok to me. You have to pay heed to the instructions, especially about
adjusting the quiescent (no-signal) current.

I'd recommend mounting the two output transistors Q3 and Q4 on a small
heatsink, making sure that they are electrically isolated from each other
and from ground and other components. The two transistors can theoretically
dissipate nearly 1W each with a 4-ohm load. That may seem small, but it's
enough to get a bare TO220 transistor quite hot.

One 0.47-ohm 1/2-watt resistor in series with the emitter of each output
transistor will aid stability.

Q2 dissipates about 0.25W which will also cause the small transistor to run
quite hot to the touch. I suggest using a BD135-16 or a BD137-16 instead of
the BC337. Choosing a sub-type with the -16 suffix ensures that it will have
about the same gain level as the BC337.

Ah, thank you very much. I'll heatsink the transistors, and thanks
for the tip about emitter resistors.

Michael

 

[Eeyore]

mrdarrett@gmail.com wrote:

Been there, borrowed Self's book from the library, still a bit over my
head.

Buy this. It's ANCIENT and refers to a lot of obsolete op-amps which with
intelligence you can substitute with better modern parts but it covers some good
ground like noise calculations for example. It helps you get the feel.

I have an original print copy of course. Finally it''s been reprinted.

http://www.amazon.com/National-Semiconductor-Audio-Radio-Handbook/dp/1882580354

TI have some super and huge IC mainly handbooks too that you should have. I'll
need to recheck their filenames.

Graham

 

On May 11, 11:50 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:

Tim Wescott wrote:
Eeyore wrote:
mrdarr...@gmail.com wrote:

I tried to build this Class A amplifier circuit as a learning
experience:
http://www.techlib.com/electronics/audioamps.html#class-a

Why why why ?

It puts DC through the speaker is is horribly badly biased. It's a JOKE.

And how is a beginner supposed to know?

He's not that much of a beginner.

He's not? You might be surprised. My background is in chemistry,
not electronics.

I'd have known that was crap by my mid
teens. It's about as simple as it gets. I was building stuff more complex at
age 12.

Ah, good for you. I wasted my teen years teaching myself C
programming, assembly language programming, and modifying Michael
Abrash's VGA Mode X graphics routines. Then Windows 95 came out and
made my experience worthless. Eh. That's life.

For some odd reason, electronics seem much harder to me than
chemistry... I wonder why... but I enjoy exercising the old brain
cells.

Do you have any suggestions for a site to look at, or a book?

Application notes are good, as long as you can find one that covers the
subject. Discrete design is rare these days and apart from the experts, the
standard of it seems to be going downhill fast as people aren't taught
'circuits' and it just becomes 'building blocks'.

A decent audio site is here.http://www.dself.dsl.pipex.com/ampins/ampins.htm

Graham

Been there, borrowed Self's book from the library, still a bit over my
head.

I found this interesting, though.
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part1/page3.html

Thanks,

Michael

 

[Jon Kirwan]

On Mon, 11 May 2009 19:57:07 +0100, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:

mrdarrett@gmail.com wrote:

Ah. I'd neglected to say that a 1W amp would be fine for learning
purposes.

First you need to learn how discrete circuits work and how to measure / troubleshoot
them.

I agree. As you point out elsewhere, things have moved from learning
these details to using functional blocks created by others who know.
That's too bad in some ways. In others, it means more people can have
at it without having to spend so much time informing themselves well
and fully, I suppose.

I'm just a hobbyist and in no way do I imagine I understand all of the
important details. But I've cobbled together my own designs for a few
usably working audio amplifier circuits, when I was much younger. And
at least for me, learning things in discrete steps helped a lot. At
some point, it's time to figure out how to combine functions a bit.
But to start out, separating them seems to help. For me, it did.

Part of this will be learning about conditioning the input source
appropriately and well. Degenerative voltage amplifiers aren't that
hard to understand and design and may be a good place to actually
start. In that regard, the student manual for the Art of Electronics
is important -- the book doesn't cover the details well enough on its
own -- if that book is to be used. The student manual includes a
"compute this first, then that second, then..." approach. Very easy
to follow. (Then include bootstrapping of the input from the BJT
emitter as a 'next thing' to gather well.) As you also point out in
another post, emitter followers for output.

Although some of these functions can be combined in a given design, I
don't think I could have handled it when I was going through this. It
was the fact that they could be taken in steps that allowed me to
succeed, as much as I did. So I recommend taking this in parts.

I started out trying my hand at understanding the basic degenerative
BJT amplifier, then gaining a foothold on the bootstrap to stiffen the
input a bit. I used a well designed power supply, a well designed
signal generator, and a well designed oscilloscope as tools --
designed by professionals while learning about simply voltage
amplification. (It's remarkable to me how complicated it all seemed
at first and how so much simpler it seems in after-thought, looking in
hindsight.) It was after that when I began worrying about how to work
with various input sources worked; sometimes their own supplies and in
all cases some kind of matching to avoid distortion or avoidable loss
of signal. I actually built my own carbon granule microphone, in
fact. Most of my experience was with dynamic microphones, but later
this included electret. Then I worked on outputs (mostly just the
typical 4-8 ohm speaker, but also various kinds of headphones.) One
such I also built by hand, winding wire, using fixed magnets, and
using tiny metal plates as diaphrams.)

It was fun but took a lot of time. I made stuff that didn't work
well, too.

You'll have fun finding a book that'll teach that. My first came from 1969 (
Mullard ) followed by an RCA handbook and a full blown theory book my Dad
bought me and people were already well into ICs by the mid 70s.

I learned this stuff in the late '60s and early 70's, when I had the
time as a kid and young adult, too.

Jon