Building a class A audio amplifier - no audio out

[[email protected]]

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]

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]

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

 

[[email protected]]

Eeyore wrote:
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/

G²

 

[[email protected]]

 <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/

G²

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

 

[[email protected]]

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.


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]

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.

 

[[email protected]]

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]

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]

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]

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

 

[[email protected]]

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 complexat
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.

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

 

[Eeyore]

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

 

[[email protected]]

On May 11, 8:16 am, [email protected] wrote:

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

Well then perhaps this 'golden oldie' might be of interest.

http://www.swtpc.com/mholley/PopularElectronics/Dec1967/PE_Dec1967.htm

G²

 

[Eeyore]

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.


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]

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.

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.


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

 

[[email protected]]

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

30 cents is unreasonable to you? Available at mouser.com

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.

I bought a mixed bag of NPNs at the local Radio Shack, and they were
all plastic TO-92s, not metal cans.

BC337s are 6 cents at mouser... what TO-92 NPNs go for 2 cents?

Hahahahaha ! Just try getting suffixed types. LMAO.


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.

Thanks for the warning.

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.

Exactly. The "why" is what makes for slow going... else I'd just buy
an Onkyo or an LM3886, as I mentioned earlier.

Graham

Michael

 

[[email protected]]

On May 11, 8:30 am, [email protected] wrote:> On May 11, 8:16 am, [email protected] wrote:

<snip>
 > > Lots of good stuff here
 >
 > >http://sound.westhost.com/
 >
 > > G²
 >
 > 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

Well then perhaps this 'golden oldie' might be of interest.

http://www.swtpc.com/mholley/PopularElectronics/Dec1967/PE_Dec1967.htm

G²

Thanks a bunch!

Michael

 

[Eeyore]

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

 

[[email protected]]

On May 11, 1:44 pm, Eeyore <[email protected]>
wrote:

.....

I was offered a job programming in C back around 1984. I took one look atthe
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.

How could you be offered a job in C programming if you didn't know C
programming? (thoroughly confused) That would never fly over here.

On that note... I took a biochemistry lab class where I had to
calculate the A, C, G and T fractions from DNA analysis of a
bacterium. It involved *a lot* of punching numbers into a
calculator. I thought, "screw this" and wrote a Pascal program to do
it. I turned in the source code with my lab report. The grader wrote
"Mabey (sic) you're in the wrong major" on the top of my report.
Mabey he was right. ;D

.....

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.

Nope. Will start there. Thanks.

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 ).

You mean a Sziklai?
http://en.wikipedia.org/wiki/Sziklai_pair

I noticed that a lot of power amps use the Sziklai pair vs. the
Darlington. Is the Sziklai demonstrably superior?