Do you mean on 330A's , my amp, or old amps of this ilk, generally?
OK So 1st, is the loudness tap just an extra tap at a point other than center? Could that be simulated with something along the lines of a resistor off the center tap? 2nd, are you saying 1st choice might be the "law faking" arrangement, to provide best tracking, vs even a good quality audio taper pot?
Thanks a bunch...
Any variation in input impedance between channels would be a real one off-fault on any amp.
The loudness tap is a fixed point some predetermined way "along" the resistive track. You can't simulate it in any way.
Unless you can guarantee any log pot is accurately matched then the linear pot + resistor might be the best option.
Also if your source components and the amp sensitivity mean that the pot is never turned more than a few 10's of degrees to get all the volume you need then adding some attenuation somewhere can help. Pot mismatching always shows up at the extremes of the resistive tracks.
The loudness tap is a fixed point some predetermined way "along" the resistive track. You can't simulate it in any way.
Unless you can guarantee any log pot is accurately matched then the linear pot + resistor might be the best option.
Also if your source components and the amp sensitivity mean that the pot is never turned more than a few 10's of degrees to get all the volume you need then adding some attenuation somewhere can help. Pot mismatching always shows up at the extremes of the resistive tracks.
Actually hard to find a stereo pot, linear or log, with a split splined shaft. Definitely not that combination with a loudness tap. Was thinking, even if it costs a bit more, something like a Dale 23-step stereo attenuator might work (there appears to be room) and would allow me to take a loudness tap wherever I wanted, no? I see them on ebay for like $27.
Split shafts aren't common, add the requirement of a tap and you are looking at original parts more often than not. Split shafts alone are available,
PC2C16BU Concentric Potentiometer Enquiry Form
A switched attenuator is a possibility if you want to go down that route.
PC2C16BU Concentric Potentiometer Enquiry Form
A switched attenuator is a possibility if you want to go down that route.
OK So the new plastic Alps pot is in. Didn't fix the problem
Argh. Will do some more tests today.
To recap (the thread; amp's fully recapped I've isolated the problem to be at least upstream of the power amps: cross-wiring the preamp/amp in/outs switches which speaker is louder. I thought I'd found the signal balanced before the volume pot and different after, but I'm going to have to recheck that.
The imbalance sounds strongly in the lower frequencies but I believe in overall amplitude as well. One of my earlier scope tests reflected this as well.
The other thing is I get some distortion when I hit the halfway mark on volume. Going to run the preamp into another power amp this a.m. to see if that's also an upstream thing. Running KLH 5's with it right now.
Argh. Will do some more tests today. To recap (the thread; amp's fully recapped
I've isolated the problem to be at least upstream of the power amps: cross-wiring the preamp/amp in/outs switches which speaker is louder. I thought I'd found the signal balanced before the volume pot and different after, but I'm going to have to recheck that. The imbalance sounds strongly in the lower frequencies but I believe in overall amplitude as well. One of my earlier scope tests reflected this as well.
The other thing is I get some distortion when I hit the halfway mark on volume. Going to run the preamp into another power amp this a.m. to see if that's also an upstream thing. Running KLH 5's with it right now.
Schematic here...http://www.markmalmberg.com/330A/schematic.pdf
I have traced the problem to the tone board. Approx. in the center of the drawing, look for the word CONTOUR, VR903 is the balance pot. The signals at either end of the balance pot are off-balance. The signals at the TAPE (source/monitor) switch are same.
Any best guesses within that area?
I have traced the problem to the tone board. Approx. in the center of the drawing, look for the word CONTOUR, VR903 is the balance pot. The signals at either end of the balance pot are off-balance. The signals at the TAPE (source/monitor) switch are same.
Any best guesses within that area?
So you need to prove it. Try bypassing the tone stage on both channels. If the problem really is in that area then its just a case of checking the pots and passives.
Attachments
Good idea and done. Jumpered from TAPE switch to ends of balance pot, signal was equal on both sides and balance pot worked properly. Tempting to leave the balance controls bypassed
Any pointers for checking the components of the tone board? Should I just compare resistances R vs L? I don't think any of the caps are electrolytics (all electrolytics have been replaced).
Although it now appears the problem is indeed in the tone control section one other component is a suspect. That is the coupling cap that drives the stage. C502 or C602. Have you replaced those ? The lower impedance of driving the tone control would show up a problem in that cap.
If it really is a component in the tone stage then its an odd one tbh You would have to check all the resistors and substitute the caps but its all only a few minutes work.
If it really is a component in the tone stage then its an odd one tbh
You would have to check all the resistors and substitute the caps but its all only a few minutes work.Although it now appears the problem is indeed in the tone control section one other component is a suspect. That is the coupling cap that drives the stage. C502 or C602. Have you replaced those ? The lower impedance of driving the tone control would show up a problem in that cap.
If it really is a component in the tone stage then its an odd one tbh
C502/602 look in the schem like electrolytics and I replaced all the electrolytics in the amp but I'll double check for those. Thx...
Wow, that turned out much simpler than I expected. After so much work chasing this down, I should feel pretty dumb but I actually feel educated
Measured the resistance on each of the four tone pots, treble/bass L/R. 3 of the 4 were 50K, one of the bass pots only measured 30K. Thought about it a minute or two, re-cleaned it (not so easy to get at) and it now measures 50K like the others. I guess some sort of conductive film or dirt must have been in there? I'd have expected higher rather than lower resistance. Anyway, both channels now equal.
Measured the resistance on each of the four tone pots, treble/bass L/R. 3 of the 4 were 50K, one of the bass pots only measured 30K. Thought about it a minute or two, re-cleaned it (not so easy to get at) and it now measures 50K like the others. I guess some sort of conductive film or dirt must have been in there? I'd have expected higher rather than lower resistance. Anyway, both channels now equal.
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