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bostjancek 29th March 2005 05:00 AM

Need help on P27
Hello !

I've build the P27 (P27) on the PCB made by me. First I must say that the preamp sounds greate. I tried it with TL 072 and then I put inside the OPA 2134 - resoult dead quiet. For testing I connected the preamp on my P3A - PCB made by me - cool combinatin with nice sound. Then I connected the preamp with proper amp and there was a problem. If I put all the commands to max (gain, volume, ....) there is a strange noise from the speaker - like strange oscilation but not in the HF (this doesn't happend with the P3A). Then I've tried to disconect the feedback from -spk throu the R23 and it was much better, but not OK. Now I'm a bit puzzled. Strange thing is that the drivers Q7 and Q6 are getting too hot (cca. 65C). I've measured the voltage drop trough the R14 and R15 and the resoult was 11V drop. 11V means that there is a current flow of 19,6 mA. Is this OK ? The PCB is not the first I made - all double checked (layout and components placing and components values also) from me and another two friends of mine.

I must ask - is the +/- 23V OK for this amp ? The mentioned +/- 23V are from the power supply from amp I actualy use (toroidal transformer 2x17V 60VA, 20A rectifier, 2x4700mF/50V capacitors, 2x100nF blocking capacitors) and I want to use it for P27 (because I only need 30W amp). If you have some idea what could be wrong, please help me !

Thanks in advance !

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