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Simple audio amp: your comments wanted

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carbonzit

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So I built this simple little 2-watt* audio amp shown here. It's for a project of mine, but I also did it as in instruction exercise and for fun. (A web page describing it can be found here. Look there if you want to see component values.)

**broken link removed**

It seems to work OK, but I have some questions about it I'd appreciate answers to.

  1. The first time I ran it, the output transistors got very warm (I had the circuit breadboarded with no heat sinks; since then I've put on heat sinks). Since this is a class B output stage (right?), there shouldn't be very much power dissipation there, should there? How hot should these transistors get when operating, say at about half volume (not cranked all the way up)?
  2. Speaking of which, I tried to adjust the quiescent current using the trimmer pot as suggested by the author, but found it problematic. With no signal, the amp seems to draw much less than the 10-15 mA recommended, with the trimmer all the way over (0 Ω). However, as soon as I turn it to a higher resistance, the collector current goes waaaaay up, almost to a full amp. ????? I'm wondering if there's something wrong with this diode biasing circuit.
  3. Lacking the proper test equipment, I adjusted the trimmer for least current as described above. Since I understand that this affects the operating point of the output stage, and therefore crossover distortion, any suggestions for adjusting this without a 'scope? How about using a test tone?
  4. Should the diode be mounted on or near the output transistors in order to track their temperature? or doesn't that matter?
  5. What size heat sink would you think would be appropriate for the output xistors?
  6. Speaking of which, I was thinking that I might could mount the transistors in pairs on two heat sinks. Since the TIP3x metal tabs connect to their collectors, how about if I mounted both upper transistors (Q3) on a common heat sink, connected to V+, and the other two on another, connected to ground; do you see any problem with this? After all, both amps (in a stereo amp) would be connected this way anyhow, right?
I'm interested in general comments on this circuit. From what I can tell, the people who put this web site together (RED Circuits) seem to be competent and know what they're talking about. However, if you have any suggestions about this project, I'd like to hear them.

*According to the author, anyhow.
 
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2. Is the value of R9 equal to 100 ohms as the parts list states?

4. Yes the diode should be mounted on Q3 to track its temperature.

6. Connecting the transistors Q3 to a common heat sink and Q4 to a different common heat sink should be ok.
 
So I built this simple little 2-watt* audio amp shown here. It's for a project of mine, but I also did it as in instruction exercise and for fun. (A web page describing it can be found here. Look there if you want to see component values.)

**broken link removed**

It seems to work OK, but I have some questions about it I'd appreciate answers to.

  1. The first time I ran it, the output transistors got very warm (I had the circuit breadboarded with no heat sinks; since then I've put on heat sinks). Since this is a class B output stage (right?), there shouldn't be very much power dissipation there, should there? How hot should these transistors get when operating, say at about half volume (not cranked all the way up)?
  2. Speaking of which, I tried to adjust the quiescent current using the trimmer pot as suggested by the author, but found it problematic. With no signal, the amp seems to draw much less than the 10-15 mA recommended, with the trimmer all the way over (0 Ω). However, as soon as I turn it to a higher resistance, the collector current goes waaaaay up, almost to a full amp. ????? I'm wondering if there's something wrong with this diode biasing circuit.
  3. Lacking the proper test equipment, I adjusted the trimmer for least current as described above. Since I understand that this affects the operating point of the output stage, and therefore crossover distortion, any suggestions for adjusting this without a 'scope? How about using a test tone?
  4. Should the diode be mounted on or near the output transistors in order to track their temperature? or doesn't that matter?
  5. What size heat sink would you think would be appropriate for the output xistors?
  6. Speaking of which, I was thinking that I might could mount the transistors in pairs on two heat sinks. Since the TIP3x metal tabs connect to their collectors, how about if I mounted both upper transistors (Q3) on a common heat sink, connected to V+, and the other two on another, connected to ground; do you see any problem with this? After all, both amps (in a stereo amp) would be connected this way anyhow, right?
I'm interested in general comments on this circuit. From what I can tell, the people who put this web site together (RED Circuits) seem to be competent and know what they're talking about. However, if you have any suggestions about this project, I'd like to hear them.

*According to the author, anyhow.

1) They will get hot when running at any kind of decent level, it's why they have heatsinks.

2) Only that diode biasing is pretty poor, and with the bias incorrectly adjusted it WILL take masses of current, and destroy itself. The amp is a poor design, it's missing critical emitter resistors, and is fairly low quality anyway.

3) Adjust for the recommended current.

4) Yes, absolutely critical, as is using a special diode intended for biasing, but it's a poor method of biasing anyway.

5) One that keeps them cool enough.

6) Either separate heatsinks (and insulate them), or use transistor mounting insulators on a single heatsink.

But I would seriously recommend adding emitter resistors, 0.5 or 0.22 ohm, it's essential to prevent thermal runaway.
 
The high stray capacitances of a breadboard will probably cause the amplifier to oscillate at a high frequency which will cause the output transistors to get hot.
Build the circuit compactly on a pcb or on stripboard instead.

The polarity of C2 is shown backwards.
 
The amp is a poor design, it's missing critical emitter resistors, and is fairly low quality anyway.

Point about emitter resistors taken.

Apart from that, could you elaborate on why you think it's a poor design? And what exactly do you mean by "low quality"? I know it's not an audiophile (or audiophool) quality design, but is it lacking in the distortion department? power bandwidth? what?

Audioguru said:
The polarity of C2 is shown backwards.

Good catch.
 
The "2W" amplifier produces only 1W into 8 ohms with 0.2% distortion that some people can hear.
Its frequency response is not bad: 30Hz to 20kHz at -1dB.
 
Apart from that, could you elaborate on why you think it's a poor design? And what exactly do you mean by "low quality"? I know it's not an audiophile (or audiophool) quality design, but is it lacking in the distortion department? power bandwidth? what?

It's a late 60's, early 70's design, things have moved on a LOT since then.

Distortion and power bandwidth are probably the biggest 'problems'.

But adding emitter resitors, changing the bias to a Vbe multiplier, and changing C2 round will make it more relaible.
 
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