Wiring Balanced Volume Pots?

[studiodog]

hi folks,

I'm new to the forum, and hoping someone can help me out with a project. I'd like to wire up a passive stereo volume control with balanced line-level XLR or TRS 1/4" inputs and outputs.

I plan to use a dual-gang 10K Ohm log taper pot, but can't find any resources for wiring the balanced cable. How should I handle the second signal wire in the XLR connnector? I suppose I could connect it to ground, but that would negate the benefits of the balanced signal and add to the noise potential.

Any links to wiring diagrams or other help would be very much appreciated!

[David Davenport]

Check out figure 17 in this article: Chapter 4

I use "configuration B" as a passive volume control in my system when I need one and it works fine. I use 5K series resistors and a 10K pot.

You can ignore the surrounding circuitry for your application - just connect pin 2 (or tip) to one input phase and pin 3 (or ring) to other phase. Do not ground either phase. Same for the outputs which go to the grids in the figure.



Dave

[studiodog]

-- Sorry if this is a repeat post. My first attempt didn't seem to work! --

Thanks for the reply, Dave! I have a few questions about the configuration you linked to, if you don't mind answering:

If I understand the schematic, and that may be a long shot, you're suggesting that both + and - signals from the XLR input connect to the pot via their respective series resistors, the + phase to the wiper and + output, the - phase to the pot's input lug(?) and - output. Input and output grounds connect via common ground (or simply to each other? perhaps via the pot housing?). The ground lug on the pot remains empty, yes? My guess here is that the audible signal is attenuated by adding more of the inverse phase signal via the pot?

Thanks for the help and the series resistor values, too!

[Davey]

The following stage will be a balanced (differential amplifier) so if you reduce the variable resistor value to zero ohms the two outputs would be at the same potential (no difference) and you'd have zero output.

The circuit Dave mentioned is fine. The only recommendation I would make is to tie the unused potentiometer lug to the wiper so if the pot develops noise or intermittent contact it wouldn't create an open circuit.

Since you have a log-taper pot it will work differently depending upon which "direction" you wire it in the circuit.

Hope that makes sense.

Cheers,

Dave.

[studiodog]

Hey Davey. Thanks for the clarification, and the tip for the open ground lug. I actually prototyped the circuit this morning with parts I had lying around, and it seems to work just fine! I've ordered some better-quality parts for the actual build, so I'm now waiting for the postman!

Cheers,

[David Davenport]

Hi Studiodog,

Yes, your supposition is correct as you have found by modeling it. Davey's point is well taken for the unused pin on the potentiometer.

One other thing - if the frame of the pot is metal, it should be grounded to the chassis. There may be a ground lug connected to the frame of the pot but this pin is not connected to the resistive element of the pot. The resistive element should not be connected to ground.

Dave

[studiodog]

Cheers, gents!

[coolnose]

Hi all,

just stumbled over this thread looking for a quick & dirty volume control for test running an Aleph P.

Could I stay with the given values of stereo 10k log and 5K resistor in series each rail ?

Thanks in advance for your help!

Max

[AndrewT]

Fig17 confB will give an impedance balanced output.
ConfA will be virtually impossible to set to balanced output if the tracking of the pots varies even very slightly.

You could substitute a precision matched stepped attenuator, but that is either going to be very expensive or take you a very long time to match all the steps to <0.01%