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In Reply to: RE: PAT5 Gain posted by petercapo on April 18, 2015 at 21:35:54
The preamp is stock. I tried a Carver amp and had the same gain issue. The amp I am using is actually is a Van Alstine creation built on a Stereo 120 chassis. Sounds pretty good. I did check every voltage test point and everything is spot-on. The resistors marked for replacement for different phono gain are stock. In spite of this, the gain is waaaay to high. I am using a Shure ML140He that outputs under 5.0 Mv. Did Dynaco screw up the resistor value/gain specs?
I will investigate your links.
Thank you.
Follow Ups:
It looks to me like the overall gain is set by resistors R320 and R321 (both 470k) in the feedback loop around the main opamp. I would start there. (Note, the schematic also shows a phono gain switch - if the gain issue is only on phono inputs, and not line level, look there, too.)
Only the BI-FET version has a phono gain switch. I will certainly look into the resistors on the op-amp [tonight] and go from there.
What do you expect for $99.00 Canadian plus shipping to Brooklyn?
Here's the parts list.
But, you said you are also having the same problem with the high level inputs such as with a CD player, correct? I would not think Dynaco made such a mistake. Something is amiss. Perhaps a previous owner of your PAT-5 changed something somewhere, or maybe some part(s) is/are failing.
UPDATE
I reduced the phono gain with 200 ohms in series with the factory resistor.
Doing this only reduced volume slightly BUT-by reducing phono gain, I reduced all that other noise one finds with the volume up and set to 'phono'. This permitted me to hear some noises I was not hearing before. The phono section does have a bad cap on one side. So it needs a simple rebuild. Reducing overall gain is the next thing to do.
Sounds good.
Yes. Too much gain on everything. But all voltages are correct, so not power supply. When an A.C. compressor is failing, pressures can actually go up. But if a capacitor or transistor is going south, wouldn't volume or voltage decrease? I am having difficulty finding possible alterations in the circuitry. The alteration would not be on the phono board, but in the board with the final gain stage and tone control. Also, the volume pot and balance pot are original. I was hoping for a poor replacement. Would have been an easy fix.
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