B1 Buffer Preamp

I tried a 2k74 resistor between wiper and ground on my 25k linear pot; it made a small difference in the range of movement but not a lot. Could I use a 100k log pot or is this too high;seem to think this has been mentioned before but can't find it. This might be a stupid question but would placing a 100k resistor across the the outer pins turn it into a 50k pot?
using a higher resistance pot than the minimum acceptable load for the source is liable to make your problems worse not better.

Adding a log faking resistor changes the curve of resistance/attenuation with respect to angle of actuation.
It cannot change a 100k pot into a 50k pot.

At 0dB attenuation the source sees 100k//50k//Rin, the receiver sees Rsource
At -6dB attenuation the source sees 50k+[50k//50k//Rin], the receiver sees 50k//50k//[50k+Rsource]
At -100dB attenuation the source sees 100k, the receiver sees 0r0.

I would not want that between my source and my receiver.
 
Right / Left channel balance

Folks,

I found the volume of right and left channels are different coming out from the preamp. I used a Alps Blue Stereo pot. The left channel is louder so the image is not centered. I compared with using the HK 635 as preamp, used the same amp, and the image is centered. Did I screw somewhere with the Pass B1? How can I diagnose what went wrong?

Thanks,
Ken
 
Folks,

I found the volume of right and left channels are different coming out from the preamp. I used a Alps Blue Stereo pot. The left channel is louder so the image is not centered. I compared with using the HK 635 as preamp, used the same amp, and the image is centered. Did I screw somewhere with the Pass B1? How can I diagnose what went wrong?

Thanks,
Ken

Just to be certain that the "problem" is with the B1, reverse the input connections to verify that the image remains shifted to the left. Do the same test for the output leads.

Measure the resistance from the wipers to ground with no input connections and, of course, no power to the B1. If the pot is at fault you would expect higher resistance to ground on the left channel. If this is the case then there may be a way to torque the pot to align the wipers. Of course, you want to verify that the "grounded" terminals of the pot are indeed at ground as indicated by 0 ohms resistance to ground or nearly so. Verify that there is the same resistance to ground for both input terminals and that this resistance equals the stated value of the pot. Measure the resistance of the output jacks to ground to verify equal resistance to ground at approximately 221K.

If all of the above pans out, then verify that the gate voltages applied by R103/203 are equal and near 1/2 the supply voltage. Verify the same for the source voltages of Q100/200.

If all of the above check good then look at C100/C200 and C101/201 to verify that they have nearly the same capacitance and no leakage.

You might try temporarily replacing the pot with a voltage divider network. I would try a 20k (input side) and a 5K (ground side) voltage divider with matched resistors for each channel. If this balances the sound stage then the pot is apt to be out of alignment. Meanwhile your source, amp, ears, and room would be shown to be balanced.

If the above is inconclusive then an oscilloscope and a signal generator would be helpful.

I assume that the shift to the left is not subtle. Correcting a subtle asymmetry is why the B1 calls for separate pots.
 
Addenum. One more thing. If you did not use matched fets then it might matter where you put the higher Idss device. For the current limiters (sinks) to limit current to a constant the signal fets must "try" to draw more current than the Idss of the current limiter. Otherwise there may be a signal voltage drop across the current limiter due to a varying depletion zone. Thus, the higher Idss device should be the signal fet. At least, that is the way I see it. Someone else may have a different interpretation. If Idss is a source of asymmetry in your case then I predict that the signal fet is the lower Idss device for the right channel.

This is one aspect of the B1 that puzzles me. NP makes the point that the best performance is when Idss for the limiter and signal fets are nearly the same. I would have predicted that intentionally using a higher Idss for the signal device would assure that the current limiter was at pinch off and that maximum linearity would result.

Regarding my earlier suggestion to reverse the output connections. In this case, the sound stage should shift to the right if the B1 is asymmetrical. If, instead, the sound stage remained shifted to the left then the power amp would be suspect.
 
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lae2,

Thanks for the explanation! I think that I screwed the Alps pot.

When I switch channel the image shifted to the right. I remembered that the thing I did to cause this was to mount the pot.

There is a small "bump" on the Alps pot. If I put the washer on the external panel, it will not mount properly. So I bought a nut from HD. I used the original nut on the inside and use the HD nut on the outside. The HD nut was too tight for Alps and I kind of force it into the position. This may have changed the balance inside the pot. As you suggested, there is a way to torque the pot. The pot was "torqued" to lose its balance. Where can I get more information on torquing the Alps pot? What's the right way to mount the Alps pot on the panel?

That's really a lot to learn!

Just to be certain that the "problem" is with the B1, reverse the input connections to verify that the image remains shifted to the left. Do the same test for the output leads.

Measure the resistance from the wipers to ground with no input connections and, of course, no power to the B1. If the pot is at fault you would expect higher resistance to ground on the left channel. If this is the case then there may be a way to torque the pot to align the wipers. Of course, you want to verify that the "grounded" terminals of the pot are indeed at ground as indicated by 0 ohms resistance to ground or nearly so. Verify that there is the same resistance to ground for both input terminals and that this resistance equals the stated value of the pot. Measure the resistance of the output jacks to ground to verify equal resistance to ground at approximately 221K.

If all of the above pans out, then verify that the gate voltages applied by R103/203 are equal and near 1/2 the supply voltage. Verify the same for the source voltages of Q100/200.

If all of the above check good then look at C100/C200 and C101/201 to verify that they have nearly the same capacitance and no leakage.

You might try temporarily replacing the pot with a voltage divider network. I would try a 20k (input side) and a 5K (ground side) voltage divider with matched resistors for each channel. If this balances the sound stage then the pot is apt to be out of alignment. Meanwhile your source, amp, ears, and room would be shown to be balanced.

If the above is inconclusive then an oscilloscope and a signal generator would be helpful.

I assume that the shift to the left is not subtle. Correcting a subtle asymmetry is why the B1 calls for separate pots.
 
Any help to align the Alps pot? or I should get two pots and one for each channel. Any recommendation for "reasonably priced" mono pots?

lae2,

Thanks for the explanation! I think that I screwed the Alps pot.

When I switch channel the image shifted to the right. I remembered that the thing I did to cause this was to mount the pot.

There is a small "bump" on the Alps pot. If I put the washer on the external panel, it will not mount properly. So I bought a nut from HD. I used the original nut on the inside and use the HD nut on the outside. The HD nut was too tight for Alps and I kind of force it into the position. This may have changed the balance inside the pot. As you suggested, there is a way to torque the pot. The pot was "torqued" to lose its balance. Where can I get more information on torquing the Alps pot? What's the right way to mount the Alps pot on the panel?

That's really a lot to learn!
 
Any help to align the Alps pot? or I should get two pots and one for each channel. Any recommendation for "reasonably priced" mono pots?

Ken, I am using the GOLDPOINT MONO MINI-V STEPPED ATTENUATORS. They are not inexpensive though.

Are you able to measure the resistance from the wiper to ground for each section of the Alps pot? I am not familiar with this pot.

In an earlier post you wrote "When I switch channel the image shifted to the right." Were you switching the source inputs or the B1 outputs when the image shifted to the right?
 
I switched the B1 outputs to different speakers and the image shifted. I haven't measured yet since I don't have the right equipment to do that. I was a bit scared to see a volume pot could lost alignment quickly and perhaps two pots is the way to go.

Ken, I am using the GOLDPOINT MONO MINI-V STEPPED ATTENUATORS. They are not inexpensive though.

Are you able to measure the resistance from the wiper to ground for each section of the Alps pot? I am not familiar with this pot.

In an earlier post you wrote "When I switch channel the image shifted to the right." Were you switching the source inputs or the B1 outputs when the image shifted to the right?
 
I switched the B1 outputs to different speakers and the image shifted. I haven't measured yet since I don't have the right equipment to do that. I was a bit scared to see a volume pot could lost alignment quickly and perhaps two pots is the way to go.

So perhaps the quickest way is to try using two pots. I have my eyes on PEC Log 25K mono pot:

Digi-Key - KA2531S28-ND (Manufacturer - KA2531S28)

Any other suggestions of pots in this price range?
 
So perhaps the quickest way is to try using two pots. I have my eyes on PEC Log 25K mono pot:

Digi-Key - KA2531S28-ND (Manufacturer - KA2531S28)

Any other suggestions of pots in this price range?

That pot certainly looks good to me. Still, there may be something amiss in the B1 circuitry. For other than the image shift, does each channel have the same sonic clarity? Do I understand correctly that the image was centered before you mounted the Alps stereo pot?
 
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That pot certainly looks good to me. Still, there may be something amiss in the B1 circuitry. For other than the image shift, does each channel have the same sonic clarity? Do I understand correctly that the image was centered before you mounted the Alps stereo pot?

The image was centered before I "force" one nut into the Alps stereo pot. When forcing it, I might have damaged the pot. Also, it could be the pot has issue balancing/matching channels. I read that from another threat saying the possibility of a PEC stereo pot missing 6db between channels. So two pots for a reason I guess. It's hard to believe the pot would cause such an issue given that there are so many pots in other equipment and most of them are stereo pots. ;)
 
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The image was centered before I "force" one nut into the Alps stereo pot. When forcing it, I might have damaged the pot. Also, it could be the pot has issue balancing/matching channels. I read that from another threat saying the possibility of a PEC stereo pot missing 6db between channels. So two pots for a reason I guess. It's hard to believe the pot would cause such an issue given that there are so many pots in other equipment and most of them are stereo pots. ;)


Hi ken, sorry for the delay in replying to your email. It clearly sounds like you damaged the pot somehow since it was fine before you got all Rambo on it :p . The stereo PECs channel matching is not representative of stereo pots in general (clearly, since most people nowadays use stereo volume pots). Bottom line is, you should probably just replace that alps...
 
Hi ken, sorry for the delay in replying to your email. It clearly sounds like you damaged the pot somehow since it was fine before you got all Rambo on it :p . The stereo PECs channel matching is not representative of stereo pots in general (clearly, since most people nowadays use stereo volume pots). Bottom line is, you should probably just replace that alps...

In the case of the B1, there is not a balance control other than by independently adjusting the channel gain by non-ganged pots. Even if you have a perfectly synchronized stereo pot there is still merit to using separate pots for each channel. Your ears, room, or equipment, may not entirely agree with the recording engineers.