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In Reply to: RE: MP Engineering OTL posted by Kasanay on June 06, 2016 at 08:11:38
You use the plural, "gems". Do you have a stereo pair? Nice.
I don't see an output coupling capacitor in the schematic.
Yes I have a pair of these gems. Actually, I have 3 - a stereo pair and a spare (not really sure how that happened, but I'm glad it did).
So I may be looking at the amp incorrectly - I was referring to C4 and C5 as "output coupling capacitors" as they are between the inverter and the output tubes. Is this incorrect? Are these caps considered "interstage coupling caps"?
Without referencing the schematic a second time, a capacitor between the input stage and the driver stage can correctly be called a "coupling capacitor", but when I said there was no output coupling capacitor, I was referring to the interface between the output tubes and the speaker. Thus you need to carefully adjust DC offset before hooking these to a speaker, which I think is why there is a meter placed into the circuit by activating that switch shown above the 3.3R resistor. Counterpoint SA4 and KSS both I think were Futterman designs with no coupling capacitor too. I think Counterpoint used some sort of servo mechanism to keep DC offset low or absent. But both of those amplifiers could occasionally cause problems.
You are correct about the meter. I set to bias to -20V and then press the offset test button and adjust it to 0.
I was fortunate to receive a message from Matthew Polk of Polk Audio giving me clear instructions on how to do this. Matthew in familiar with the process as he also owns a pair of these amps.
I can get the left channel spot-on but the right channel - not so much.
I hoping that replacing the coupling and other caps will correct this.
There does not appear to be a separate bias adjustment for each power tube. So you do have to get the tubes to work together. For example if two power tubes are weak, you may not want them to be in the same bank of power tubes. That may be why the one channel is a little off.
This could very well be the issue. I think I will replace the coupling caps first. That will narrow down the number of possible causes.
I did come across this post:
Although it references a different amp, I believe the same applies.
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