Building a class A audio amplifier - no audio out

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.

Why why why ?

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

Find some decent websites.

Graham

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

On May 11, 7:32=A0am, Tim Wescott wrote: > Eeyore wrote: > > Why why why ? >

Lots of good stuff here

G=B2

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

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.

--

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

s

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?

Thanks,

Michael

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.

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

Michael

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.

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.

Graham

Another decent one.

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

Graham

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

KE.

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

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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.

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

I found this interesting, though.

Thanks,

Michael

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

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

G=B2

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.

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

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

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.

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

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

Graaham

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

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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?

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the 70's.

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Thanks for the warning.

God knows

o you Mr

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

Michael