Ye olde hum when turning down the guitar volume [Grease Gun]

[midwayfair]

This happens once in a while, and I have no satisfactory explanation for it: You turn down the guitar pot and you get a bunch of hum. Put it at full and everything's fine and dandy.

In this case, it's the grease gun I just built, though I think I've had it happen in at least one pedal before. I know other people have posted about it, and I've never seen a good explanation.

My Grease Gun might not be too useful for troubleshooting (basically I've made a bunch of subs -- like all my coupling caps are 100nF now, so it's going to be hard to just say what the main problem is), but what might be useful is knowing what I've done to try to get rid of it:

1) Input series resistance? -- I see this floated as a solution sometimes. Nope.
2) Change bias resistors? -- No meaningful effect.
3) Cut the bass? -- To some extent this works (it's low frequency hum after all), but then I lose a bunch of distortion and signal!
4) Change the input device? -- No difference in my case with a BJT, MOSFET, or FET (with the positive bias removed) in Q1. Hum persists.
5) Buffer the input? -- I was certain this would do it -- after all, the problem only manifests when the guitar pot is turned down, so if the guitar pot is decoupled from the circuit, then it must ... Nope!
6) Move some wires around -- no change (so it's not RF, but I knew that)
7) Remove the pulldown? -- Nope.
Extra power filtering/100R on the voltage input? -- No change.

I am just mystified. I mean, I'll live with it, but I wish I knew what caused it. There's nothing about Q1 that I haven't built before without issue, and it's basically half a big muff, which despite overall thermal noise being an issue doesn't exhibit the volume pot issue I've got here.

So this is more of a general troubleshooting thread I guess.

[rullywowr]

How about that ye olde ungrounded AC plug in your jam space?  Does it hum when plugged in a grounded outlet?

[midwayfair]

How about that ye olde ungrounded AC plug in your jam space?  Does it hum when plugged in a grounded outlet?

Hm, good thought. I'll check tonight (my practice space has proper 3-prong outlets -- just the soldering iron and bench amp are on a 2-prong adapter), though I haven't had this problem with other gain circuits recently.

[rullywowr]

Just thought I'd mention it.     Hope you nail the hum thing, it's certainly annoying when you turn down your volume and hear "bzzzzz"

One thing that is always in my gig bag is this (well worth the couple bucks):


It could save your life someday.  Ever played at a venue and your lips keep getting shocked when near the microphone?  Yep, scary!   

[G.G.]

I have this problem too but it seems to be more an issue with my guitar than any particular pedal. I rewired my Tele a while back & followed the standard Tele wiring layout from the Seymour Duncan web site. It seems to work fine but there's a noticeable hum that shows up when I roll the volume down about 3/4 then goes away the more I reduce the volume, also not present at full volume. I've been meaning to open it back up and re-check the wiring, but I would definitely like to figure out a solution.

edit: So I found this on TDPRI. The last reply mentions putting a resistor & cap in parallel at the output but doesn't detail any values. Anyone familiar with this?

http://www.tdpri.com/forum/tele-technical/281392-hum-goes-up-volume-pot-goes-down.html

[alanp]

Is the guitar wiring alright?

[midwayfair]

Is the guitar wiring alright?

Yes, it's just this pedal.

[Govmnt_Lacky]

Jon,

Since this is a Muff-style circuit, is there any reason why the Gain is being held so far from GND?

What I mean is.... why is R8 such a high value? If you were to replace that with a 1K.... would you get oscillations?

Dont know if that has anything to do with your issues but, it just looks odd to me 

Another possible, but very unlikely, issue could be that your LED on the Grease Gun could be leaking. When you are minimizing your Volume pot, it is allowing a small amount of voltage to drain to ground somehow...... or, I might just be talking out my arse! 

[GrindCustoms]

I have this problem too but it seems to be more an issue with my guitar than any particular pedal. I rewired my Tele a while back & followed the standard Tele wiring layout from the Seymour Duncan web site. It seems to work fine but there's a noticeable hum that shows up when I roll the volume down about 3/4 then goes away the more I reduce the volume, also not present at full volume. I've been meaning to open it back up and re-check the wiring, but I would definitely like to figure out a solution.

edit: So I found this on TDPRI. The last reply mentions putting a resistor & cap in parallel at the output but doesn't detail any values. Anyone familiar with this?

http://www.tdpri.com/forum/tele-technical/281392-hum-goes-up-volume-pot-goes-down.html

I know what you are talking about, i have to dig up that thread where i was having oscilation noise in bypass with my first Heliotrope built.... it did'nt solve the problem for that issue, but the info is in there... give me a sec.

[GrindCustoms]

Here it is!

Quote from Kaycee
« Also a 100R and 100n resistor and cap in parallel to your chassis ground.»

[Morgan]

5) Buffer the input? -- I was certain this would do it -- after all, the problem only manifests when the guitar pot is turned down, so if the guitar pot is decoupled from the circuit, then it must ... Nope!

THAT is shocking!

I get the occasional scratchy guitar volume knob thing, but have not run into this hum issue. Fascinating.

[twin1965]

I got this problem with my Easy Face. Never fixed it. Using a Standard Tele with single coils.

Sent from my GT-I9100 using Tapatalk 2

[mmlee]

I had the exact thing happen today in the studio with a fuzz factory a tele.

[deafbutpicky]

I had this "phenomenon" happening with a cd4049 and lm386 involved circuits
and a Fuzzface and I think it might be an oscillation and bias problem. Solved
it somehow by making the 9v to ground capacitor bigger and tweak the bias a little.