Audio Research LS-3 Balance Pot

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I just acquired an AR LS-3 (solid state) preamp. I have several concerns about it; but first in line is to understand the functioning of the channel balance mechanism. (I/m getting some confusing results balancing results.)

The parts list states that the component is a 100K Dual Balance Pot, part # 45100529. I can't find that number, but there is a Bournes pot that could be the one.

I'd like to be able to understand how the pot changes the balance, and by how much. Questions like: are the changes linear (or seem linear to the ear)? Does one channel increase at the same level as the other decreases? How could I could I calculate the decibel increase/decrease at a given output?

Balance circuits come in many flavours...

The two most common are the Swing Ground which essentially takes one channel to ground while allowing the other to pass more freely, and the Contra-Volume which uses two volume control pots one one shaft wired so that turning one side up is also turning the other side down.


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I think the most common type of balance circuit (very common in Japanese gear) may be one using an "MN" law pot as Technics called it, usually with center detent. This uses two sections of track with very different conductivity, so that as you turn it to one side, one channel is attenuated while the other is passed basically uninhibited. So basically log from left to midpoint and a super wide track from there to the right, while the other channel gets the same mirrored (first wide track, then antilog).


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The Audio Research LS-3 bal pot is as they say: half resistance and half "zero" resistance. Two different wafers.

The Bournes pots in the first post are all two same wafers.

A fast-fix is to use a dual linear. This will give 6dB loss at center. If you never turn Vol full up, then you can do this and just run Vol a couple notches higher; it is the same thing.

If you are very brave, take apart a 200k dual linear (100k will work here). Get Car Window Defroster Repair Paint. Cover one half of one wafer, the other half of the other wafer. You probably want overlap at center so both will be zero loss for several degrees around center. Re-assemble. In theory this gives two 50k half-pots on opposite sides of center. In practice the parts may never go back together or the conductive paint may wear-out too quick.


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