A Wheatstone Bridge Matches Your Pots

Sometimes the simplest hacks can be the most useful or ingenious, and such is the case with [Keri Szafir]’s method of ensuring that potentiometers used in audio devices are matched. If you consider a typical stereo amplifier for a moment, you’ll see two amplifiers in one box with a single volume control. Two channels, one knob? Volume knobs are ganged stereo potentiometers.

All potentiometers are not created equal, and particularly in the cheaper devices they may not have a consistently matched resistance across both pots and across their travel. This messes up the stereo balance, so naturally it’s worth selecting a part with good matching. [Keri] selects them not with his golden ears, but by wiring both pots together as a Wheatstone bridge. A meter between the two wipers would detect any current due to a mismatch.

A Wheatstone bridge is one of those handy circuits that has plenty of uses in both AC and DC measurements. We probably see them most often in a strain gauge.

15 thoughts on “A Wheatstone Bridge Matches Your Pots

  1. Probably a stupid question but: “A meter between the two wipers would detect any current due to a mismatch”. But with a stereo source, Left/Right signals are obviously most of the time different, so won’t that induce a permanent mismatch?

    1. As the picture of the test circuit shows, you wire both pots in parallel to a voltage source, and measure the current or voltage between the two wipers.
      A potentiometer in this configuration acts as a voltage divider. The voltage on the wiper is determined by the position of the wiper and the total voltage across the pot (the actual resistance of the potentiometer is less important).
      For a stereo potentiometer, where two separate potentiometers are controlled by the same knob, you want the voltage division to be the same on both pots when you turn the knob — even if the signal is different into both pots. If one potentiometer is dividing the left channel by 3, you want the other pot to also divide the right channel by 3.
      By feeding the same signal into both pots (in this case, 5-12 volts DC, but it doesn’t matter), and comparing the output of both pots, you can see how well they are matched. If they are well-matched, then the output of both should always be the same, so there is a null reading on the meter. If they aren’t, then the outputs will differ, and you won’t get a null reading on the meter. If the meter changes as you turn the pot, then that is also a problem.
      You can feed any signal in for this test, not just DC. Perfectly matched pots will give a rock-steady null result regardless of if you feed it Mozart or Metallica, a sine wave or a square wave, a gnat’s fart or an SLS launch.
      Yes, if you feed one pot the left channel and the other pot the right channel then the meter will likely jump like crazy, as the two signals differ. That would tell you very little about how well matched the pots are.

  2. This is a method of testing the dual potentiometers out of circuit, to ensure the center position is equal on both pots. The meter won’t be used while the amp is running. It’s not that important, since you can turn the pots in question to compensate for any variance, but it is useful to have them set up correctly.

    1. The method applies to building new amps and servicing / upgrading existing ones, and it’s not just about the center position, but the resistance ratios along the pot’s travel. The pot is dual-ganged, no real possibility to manipulate channels separately…

  3. If you are dealing with audio, consider the ear is a pretty forgiving device. Back in the day, broadcast stations used high reliability Shallco stereo pots which used switched wiper contacts on fixed attenuator resistors instead of a wiper on a resistor plate. The attenuator steps were based on the minimum amount of change detected by the ear which was 2 dB or a voltage variation of about 20% of total signal between the two (power is 40%).
    The ear is actually more critical of phasing for stereo location than amplitude.
    Oh, by the way, if an audio pot gets “scratchy” the problem is more likely the decoupling capacitor either into or out of the potentiometer leaking DC into the pot, instead of the pot becoming defective.

    1. Stepped attenuators are generally better, more reliable and easier to get consistent resistance than pots. You can mostly find them in good grade hi-fi amps and preamps. They can be made in different topologies too – balanced, unbalanced, whatnot…
      Even the blue Alps audio pot is in fact a stepped attenuator made out of SMD resistors.

      1. It was a different era. The Shallco pots were really expensive but it didn’t matter because once you had one it almost never needed to be replaced even when it was cranked thousands of times per day as DJs spun records. They had a separate audio cue monitor output by twisting hard to far left (detent) and constant 600 ohm input and output impedance back in the day when people cared about such things. When you pulled an annual audio proof of performance on a station you always had a nearly *exact* match between the channels no matter what setting the pot was at. When they were turned off, they were off, not 40 or 50 dB down, but off.
        Most broadcast engineers regarded use of ganged resistive wiper pots as something they wouldn’t ever do (ever) on a console. You could actually build a mixing console by wiring the outputs of a bunch of pots together without any electronics, connect directly to cross-town balanced audio lines (football or church feeds etc.). Like I say, different time. Lost art.

        1. Oh yes, definitely lost… They don’t do it this way anymore.

          Also, I made my rackmount 8ch line + 2ch mic mixer with potentiometers, but they’re not in the signal path – instead, they are tied between +/-15V and provide control voltage for coupled LM13700N operational transconductance amplifiers. No mismatch between channels, no cracking.

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