Help with power amp

[peter5]

Hi, I'm still trying to complete my two channel Leach Amp but I have been having some problems. When turned on with load and source, it amplifies the signal but the highs aren't very good and neither are the lows. Could this be the result of a bad connection somewhere? Any help would be appreciated because I'm very new to this sort of thing. Thanks a lot.


Peter

 

[Herman]

Hi Peter,
its really strange that you miss both highs AND lows...
I've been lucky obviously since I did not have that trouble with my Leach Amp.

To help you, I'd suggest you re-check the components, especially the caps, since they can be mixed up quite easily. First, please check C1, C10 and C11. If they are too big, your amp will roll off at a few kHz, stealing you the highs.
If everything is correct, we have to think about deeper reasons :-(

For the missing lows, I do not have any real idea since the whole thing is dc-coupled. Maybe one of the transistors is gone ?

Hope I could help a little. Feel free to ask again.

Good luck
Herman

 

[peter5]

Thanks for the help Herman. I checked all of the capacitor part numbers and unfortunately they are all correct. I also checked to make sure that the transistors aren't shorted and none on them are. The resistors are also fine. Several more questions...

For C6 I used a single 220 microFarad nonpolar 50 volt electrolytic. The directions call for the same except for a 16 volt capacitor instead of a 50 volt. I don't think this should make a difference should it? I also don't have access to an amp meter so I set the bias current using a voltmeter across the collectors of Q12 and Q13. Is this accurate enough?

Thank you for your time
Peter

 

[Herman]

Peter,
too bad that the simple suggestions didn't work

The usage of that 220uF/50V instead of 16V is absolutely okay...hope you installed the jumper for the other cap (C6b)!

Bias setting via the voltage drop over Q7 is not good, but should give a resonable behaviour anyway...maybe you have some audible crossover distortion at higher levels-or the output stage ist running hot when idling-whatever, this has absolutely nothing to do with your losses at high and low frequencies. I checked the schematics again, and, following your fault description, IMHO the problem must be somewhere around R17..R20 and C6..C9.