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I built the following circuit (physically):

enter image description here

V1 is my audio signal and the circuit is a simple tone and volume control. When both the volume and the tone pot are turned all the way down (so each wiper is closest to ground), some small amount of the signal can be heard on the output. It decreases as I turn the volume pot up again.

With higher values for C1, this gets worse. With lower values, say <1 µF, this behaviour disappears, however, the cutoff frequency of the low-pass filter will be too high for my purposes.

I'm at a loss as to why this happens. One reason that surely contributes to it is that I can measure a residual resistance between the Rvol wiper and ground even when it is fully closed (around 5 Ohms). I believe this is common though. I've tried to increase the value for Rvol, thinking that the output voltage between those 5 Ohms will then be lower, but this didn't help.

Any help would be greatly appreciated.

one_three_three_seven
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  • use a potentiometer with an on/off switch that activates when the pot is turned all the way down... wire the switch so that it shorts the pot wiper – jsotola Mar 23 '20 at 21:46
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    You shouldn't need a shorting switch. I think we need some context. It looks like an electric guitar setup. Can we have a photo? //// Note that when you use the CircuitLab button on the editor toolbar and "Save and Insert" on the editor an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. – Transistor Mar 23 '20 at 21:57
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    You need a series resistor ahead of the tone control. Otherwise with the wiper at GND, C1 is acting like a stort circuit across the signal source, overloading it and causing large signal currents in your ground. This is probably what you are hearing. –  Mar 23 '20 at 21:59
  • Guitar pickups have high enough output impedance that this isn't a problem. That's why I've asked for clarification. – Transistor Mar 23 '20 at 22:01
  • I think the wiper should go to the volume input, not just short the wiper to the top terminal... This maintains constant impedance on the Input signal for the most part... (Ignoring the downstream loading) – MadHatter Mar 23 '20 at 22:13
  • Thanks to all of you. A series resistor of 1k and connecting the Rtone wiper to the Rvol input seems to have solved the problem. It's working very well now. – one_three_three_seven Mar 23 '20 at 22:50
  • By the way, it's not a guitar circuit, but a line level signal. – one_three_three_seven Mar 23 '20 at 22:50

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