P3A on +/-12V rails


2003-09-03 12:17 pm
Hello :)

I've got a pair of old 8ohms speakers I've been asked to turn into self-powered speakers. It's for a small room and on a very tight budget. I've got at hand a 10-0-10VAC/2A transformer. This should be good for around 2X8W, which is more than enough considering the size of the room and the efficiency of the speakers.

So my questions are:

- is the attached schematic correct ? It's obviously inspired heavily by the P3A from Rod Elliot.
- can I use BC550C/560C for everything but the output transistors (which won't be heavily biased, sound quality is of secondary concern) ?
- are the bd241c-242c good enough for my purpose ?

Thanks in advance.


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add emitter resistors to Q1, Q2 & Q4.
Increase R5 to 3k9 and be prepared to trim it slightly lower to balance the currents in the two halves of the LTP.

Add a resistor above the Vbe pot and take the base of Q8 to the bottom of this extra resistor.

Allow for a Zobel on the output.

C2 could be lowered down to 33uF without loosing any more bass or alternatively increase C1 to 3u3F to extend the bass response.

I am a bit doubtful about using bc550s for the drivers.


2003-09-03 12:17 pm
First of all, thanks to everyone:

While it would have been more interesting to build a P3a-like, I think Andy is right. It's going to be a bit faster and probably more reliable to build an amp based on a pair of tda2030.

The discrete amp would have cost 1€ more per channel and be like two cm wider and longer.... not something I would worry about.