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In Reply to: RE: Pas 2 balance bypass posted by Cleet Torres on March 22, 2014 at 12:14:34
My understanding thus far is that the balance control can be left out. That is, just run the respective wires from the blend switch, or from the selector switch (bypassing the blend switch, as well), directly to the volume control.However, when I completed the first rebuild of my PAS in this way, I felt the sound was somewhat distorted, or fuzzy, or having some breakup as the volume was turned up. This could have different causes that I am investigating.
But, I've have wondered if it might be better to replace the balance control with a 750K resistor to ground to emulate its presence. Could there be a need to emulate the presence of the loading the balance control provides? That is, the load either looking into the PAS from the outside or looking back from the first section of the 12AX7? If so, then could the problem I noticed be exacerbated by using a 100K volume pot vs. the original value of 250K? I used 100K because I recall Joe mentioning in another post a while back the high frequency performance of the PAS is improved..?
Edits: 03/24/14 03/24/14Follow Ups:
Bypassing the balance pot is relatively simple - as you outlined. In effect, connecting the center lug wire to the hot outer lug of each section of the balance pot (and reming them both from the balance pot) is all that is needed. There is no need (nor is it desired) to add a resistor to simulate the load of the balance pot (the less loading the better with the PAS outputs)
I cannot comment to the distortion you observed afterwards - there should be little or no audible change (any would be to the better).
Thanks for your response, Joe. I am fairly stumped as to the breakup I heard. I am working on a second rebuild with a few changes but nothing I am terribly confident would make a difference.
There was one thing, though. It uses a cap board that emulates the original power supply topology but with low ESR electrolytics. I jacked-up the capacitance quite a bit. I think, but am not sure, that the problem might have gotten a bit better when I added a bleeder resistor. Besides bleeding the electrolytics when the preamp is shut off, I understand that a bleeder resistor could also offer a form of regulation during operation, is this correct?
Could it be possible that the breakup had something to do with the power supply? The "breakup" I heard when the volume increased reminded me of the sound of power supply capacitors draining when power is turned off. Again, same circuit as the original, but capacitance was increased to over 500uF for the B+ and something like 24,000uF for the filament supply...
I understand that some feel large increases in capacitance will not yield much benefit, but I am wondering if it can become so much that it could actually cause a problem like what I was hearing?
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