P3A on +/-12V rails - diyAudio
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Old 29th February 2008, 09:19 PM   #1
00940 is offline 00940  Belgium
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Default P3A on +/-12V rails

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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Old 29th February 2008, 11:04 PM   #2
jaycee is offline jaycee  United Kingdom
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Should work, but even simpler would be an opamp-follower type amp. Take a look at P76 or even P113 for example.
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Old 1st March 2008, 04:50 AM   #3
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R7 and R8 should be 10k or less.
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Old 1st March 2008, 06:58 AM   #4
00940 is offline 00940  Belgium
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Thanks

sawreyrw: r7 and r8 are 1500 ohms, which is less than 10K. Do you mean 1500R is too low ?
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Old 1st March 2008, 08:11 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
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.
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Old 1st March 2008, 11:25 AM   #6
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20V-CT transformer + tight budget > let's build a chip amp, TDA2030 or similar!
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Old 5th March 2008, 12:44 PM   #7
00940 is offline 00940  Belgium
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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.
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