Bypassing Volume Control

markw51

Member
2006-12-02 1:18 pm
Picked up an Altec Lansing 944A power amp and am making it more friendly for home use.

I bypassed the input level controls by shorting the wiper to the input of the pot. Now the volume control does nothing but why is that actually the case?

If the pot is turned all the way down does that not short the signal to ground or to common? In the attached schematics, the input pot is R7. One is a closer up look at the input stage.

Also, the next stage is another op amp. Is that yet another stage of amplification or is it acting as a buffer? This boat anchor sure has a lot of drive and has a power transformer the size of a cantaloupe.

Any other suggestions to improve performance would be appreciated.
 

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rayma

Member
2011-04-29 8:37 pm
i wouldn't do that, the first op amp's output would be shorted to ground if the pot
is turned down. Just move the wire going to the wiper, instead to the top terminal
of the pot (and remove the jumper wire that you added).

The U101 op amp is inside the amplifier's feedback loop, and cannot be changed.
 
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markw51

Member
2006-12-02 1:18 pm
i wouldn't do that, the first op amp's output would be shorted to ground if the pot
is turned down. Just move the wire going to the wiper, instead to the top terminal
of the pot (and remove the jumper wire that you added).

The U101 op amp is inside the amplifier's feedback loop, and cannot be changed.

I believe what you are suggesting is what I did - I connected the wiper to the top terminal of the pot. I didn't explain it very well. I see what you are saying about shorting to the output of the op amp to ground. That would result in no output. But if I turn the pot all the way down the volume does not change. That's why I can't figure out what's going on.
 

rayma

Member
2011-04-29 8:37 pm
If the sound level stays the same when you turn the pot, the wiper could still be connected,
and could short the op amp's output (when the volume knob setting is at full CCW).

If the center (wiper) terminal of the pot is not connected to anything at all, then it should be fine.
 
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markw51

Member
2006-12-02 1:18 pm
altec-lansing-9444a-629741.jpg
You move that wire to the top terminal of the volume control. See post #2.

OK understood. Problem is that the pots are hard to get to but the circuit board is easily accessible. I have attached a photo of the innards of the amp. The input board is the small board toward the top of the photo. If you look to the far right you can see the three circuit board connections of one of the two pots. Its easy just to short two of those circuit board connections with a piece of wire.
 
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Some connectors of that type have a tiny retaining tab that, with a small-enough screwdriver (try your jeweler's screwdriver with the 1 mm blade), can be depressed a little, while pulling gently on the corresponding wire. If you ascertain which is the wiper and which is the full-CW end, remove those and tape them together. Then you haven't done anything that can't be undone.

Cheers
 

markw51

Member
2006-12-02 1:18 pm
Some connectors of that type have a tiny retaining tab that, with a small-enough screwdriver (try your jeweler's screwdriver with the 1 mm blade), can be depressed a little, while pulling gently on the corresponding wire. If you ascertain which is the wiper and which is the full-CW end, remove those and tape them together. Then you haven't done anything that can't be undone.

Cheers

Good idea. I can try that without removing the board.