MCP41100 DigiPot Volume Control Noisy Buzz, Fuzz, and Distortion

Hi, I just started incorporating digipots into my guitar pedal circuits. I've successfully built one type of circuit whose digipot controls volume and gain, but on my second guitar pedal circuit, I'm running into the following issue:

When the digipot (MCP41100) is set to a lower volume (physical pot is on a Arduino Nano's analog input and dictates the digipot), I get all kinds of nasty buzzy, almost gated or "spitting" fuzz type of distortion. Only when I crank the volume mostly up does it go away, but then I'm left with a huge, overpowering volume.

I've attached the schematic of the circuit and circled where the digipot is in the circuit. Nothing too complicated, just shunting the audio signal to ground. There's already a coupling cap to block DC before the pot input.

Here's what I've already tired:

-another 1uf or 10uf coupling cap after the digipot's wiper
-2.5v bias offset?: 2x 47k resistors forming a voltage divider on the digipot's input, as such (hope I did this right):
--5v ---47k---DigiPotIN---47k---Gnd

I'm not sure why the MCP41100 works on one guitar pedal circuit and not the other, as the digipot/arduino circuits are identical to each other.

Any and all comments are appreciated, thank you!

I'm not sure why the MCP41100 works on one guitar pedal circuit and not the other, as the digipot/arduino circuits are identical to each other.

Maybe you've got a bad part? Are they socketed so you can swap them?

Is it noise or distortion? Is the noise present with no signal, or with the 22uF capacitor removed? Noise is always a potential problem with analog electronics and even more-so when mixed with digital.

-another 1uf or 10uf coupling cap after the digipot's wiper
-2.5v bias offset?: 2x 47k resistors forming a voltage divider on the digipot's input, as such (hope I did this right):
--5v ---47k---DigiPotIN---47k---Gnd

Actually no.... A 100K resistor (to 5V) in series with the 100K pot will give you the 2.5V bias. (But your bias should still be about 2V and this shouldn't cause the symptoms you describe.)

Thanks for the reply!

I did socket and swapped out, but no go :frowning:

I've built this circuit many times with mechanical pots and this is not how it distorts haha

It's really like a fuzz circuit that's starved of voltage. Kind of like a static sound lingering in the audio signal. Not pleasant haha

The noise is only present when there's an audio signal.

This noise is also, not, the ticking that some digipots do when adjusting, etc.

I haven't tried removing the 22uf and I don't think I should since it's a coupling cap haha

I haven't tried removing the 22uf and I don't think I should since it's a coupling cap

The point about a digital pot is that it can only work for voltages between the power rails. In this case you only have between 0V and 5V. Other types of digital pot have a negitave voltage input so it can cope with negative voltages.

Once you put a coupling capacitor in you generate a signal that goes above and below the ground. To stop this horribly distorting you have to follow any AC coupling with a circuit that restores the DC bias to 2.5V.

I've built this circuit many times with mechanical pots and this is not how it distorts

Because mechanical pots don't care about negitave voltages.

Thanks for the reply, Mike!

That last point I just wanted to clarify that the noise I'm getting isn't part of the original circuit haha The circuit is a distortion/overdrive pedal whose job IS to distort, so I wanted to rule out the possibility that it's distorting "correctly" puahah

I just read up on that yesterday! The original circuit is going off of 9v. Arduino/digipots are running at 5v, so I made a power supply I made that shifts the negative/positive side down/up 2v or so to "center" it around 9v.

The issue with that is is I've just introduced so much hum/noise into the circuit, I'm just going to pull it out...

For the 2.5v Bias, I thought the voltage divider I tried earlier should have solved that issue? Unless I did that wrong...

Do you have a schematic of such a circuit? I think I might have seen you post one on a similar post about digipot noise. If so, I may have already tried that :frowning:

Here's the power supply I found:

Original post: Write-Up: Adding MIDI Control to a Guitar Effects Pedal - Audio - Arduino Forum

Well the circuit above doesn’t contain any capacitors and that will add to the noise. In fact if you want to generate noise one very good way to do it is to use a Zener diode.
I would add a ceramic capacitor across the Zener and capacitors from the generated ground and 5V and the battery’s terminals.

I have always had good results using two equal resistors with capacitors across each to generate a signal ground. The capacitors are vital in stopping the noise on supplies.

Also audio circuits in a digital environments are always tricky because of the pickup of digitally induced noise. I have found that a good way to reduce pickup is to make input impedance as low as possible, as well as the standard techniques of layout and separating ground runs with star wiring.

Ooo, good to know about Zeners!

I actually tried to add a 100uf filter cap on the 9v in and also a 100uf cap parallel with the zener. It quieted things a bit, but the noise was still unacceptable.

I'll add another filter cap to the 5v as well!

I'm going to assume it's really noisy because the guitar circuit is a high gain circuit, so some noise is normal at higher gain settings, and it seems the PS just adds to it.

Since I'm unsure about the 2.5v offset, would you mind proofreading my schematic/layout hybrid? haha I soldered the two resistors RIGHT on the digipots' legs.

https://drive.google.com/open?id=1I03N0zZ3SNkyve_ceF4WcUgt7PbcsTVd

The schematic looks good.
As to capacitors many times when it comes to suppressing g noise. 0.1uF ceramic capacitor is a lot better than a 100uF electrolytic one. This is because the ceramic one has a much lower self inductance and so will work to suppress much higher frequencies of noise. This is why a 0.1 if capacitor is often seen in parallel with a 100uF one.

Thank you, Mike!

Of course, I do have another coupling cap after the wiper hehe! I had to re-read your comment about removing the 22uf until it clicked about AC going positive and negative, and that putting my MCP41100 out of spec! Thanks for the push!

And good to know about the 0.1uf caps vs the 100uf. In my simpleton thinking, I just thought electrolytic "reservoir" + the bigger capacitance = BETTER than a tiny 0.1uf haha :stuck_out_tongue:

Hmm... sadly, that didn't seem to do much haha

I wired per the schematic I sent and with a 1uf electrolytic cap after the wiper. Positive leg facing the wiper, negative to the output jack.

This was wired without the power supply I posted above, just to clarify that I'm not hopeless haha

I'm going to try the power supply route again with some 100n caps and see where that takes me, but the static/buzzing noise was still there from what I remember, when I tried the PS initially... sigh...

Man, at this point, I might just replace the Volume pot with a dual supply... Any part # anyone can recommend?