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.
*According to the author, anyhow.
**broken link removed**
It seems to work OK, but I have some questions about it I'd appreciate answers to.
- 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)?
- 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.
- 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?
- Should the diode be mounted on or near the output transistors in order to track their temperature? or doesn't that matter?
- What size heat sink would you think would be appropriate for the output xistors?
- 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?
*According to the author, anyhow.
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