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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Audio Research LS-3 Balance Pot
Audio Research LS-3 Balance Pot
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Old 22nd November 2019, 05:19 PM   #1
wanders is offline wanders  United States
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Default Audio Research LS-3 Balance Pot

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.

https://www.mouser.com/datasheet/2/54/PDB18-1013655.pdf

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?

Thanks
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Old 22nd November 2019, 08:33 PM   #2
Douglas Blake is online now Douglas Blake  Canada
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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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Old 22nd November 2019, 11:16 PM   #3
sgrossklass is offline sgrossklass  Germany
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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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Old 22nd November 2019, 11:35 PM   #4
Mark Tillotson is offline Mark Tillotson
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Or even metallized over the conductive stretch?


Anyway the schematic ought to give enough information to figure out what kind of pot is intended.
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Old 23rd November 2019, 03:25 AM   #5
PRR is online now PRR  United States
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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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Old 23rd November 2019, 04:02 AM   #6
rayma is online now rayma  United States
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Quote:
Originally Posted by wanders View Post
first in line is to understand the functioning of the channel balance mechanism.
Pots like this have been used since the 60s. Look at the Dyna PAS-3 manual for example.
That one is 750k though, so you couldn't use it.
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