What components can introduce a hum to a circuit?

[Outlaws]

Is there anything in general that I should look for?  I would like general stuff so I can learn.
I am sure everyone wants details though so I just got a Rullywow Glasshammer going.  Basically a Tubescreamer/Landgraff.  Same power supply as I use on my other pedals without issue, and it's the only pedal plugged in the PS or even the signal chain. Sounds excellent except for a hum that is very much in the vein of a ground loop.  Not present when the effect is bypassed.
I have moved around the wiring without hearing any increase or decrease.
The 25k drive pot when all the way off appears to eliminate it possibly, and after turning it clockwise a hair it starts to come in quickly.  So possibly something in the circuit path related to that area.
And on a side note, I haven't played a Tubescreamer in a very long time and I forgot how good they are.
PCB Layout

[R.G.]

We live in a literal ocean of AC power line hum. It's conducted in on power supply leads, and it's all around us in the very air.  There's no one component - save perhaps a ground wire - that keeps it out or lets it in.

First - have you tried your circuit on a battery? An external power supply may or may not be the problem, but trying a battery tells you for sure, as batteries do not hum.

Second- tiny hums are made audible by amplification, just like tiny guitar signals. As you increase the drive pot, it increases the circuit gain, and the hum comes up.That indicates that there is some small hum being amplified.

>> does the hum continue to increase in loudness as you continue to turn the drive pot up?

It is possible that your drive pot is bad, going open as you turn it up. Check that the wiper stays in contact with the resistive element as you turn it up.

Then there is always wiring errors. Check the wiring yet again.

[Outlaws]

We live in a literal ocean of AC power line hum. It's conducted in on power supply leads, and it's all around us in the very air.  There's no one component - save perhaps a ground wire - that keeps it out or lets it in.

First - have you tried your circuit on a battery? An external power supply may or may not be the problem, but trying a battery tells you for sure, as batteries do not hum.

Second- tiny hums are made audible by amplification, just like tiny guitar signals. As you increase the drive pot, it increases the circuit gain, and the hum comes up.That indicates that there is some small hum being amplified.

>> does the hum continue to increase in loudness as you continue to turn the drive pot up?

It is possible that your drive pot is bad, going open as you turn it up. Check that the wiper stays in contact with the resistive element as you turn it up.

Then there is always wiring errors. Check the wiring yet again.

The drive pot is interesting, it appears to work in that it adds dirt has you turn it up, but the way it kicks in is odd.  The first hair of a turn it's very distinctively adding something into the circuit it sounds like, and the hum that gets introduced only gets louder for that part, then the hum is the same perceived volume for the rest of the rotation.

[R.G.]

Is your drive pot really 25K?? Most of the tube screamer knockoffs use much larger values - 250K to 1M is common.

Is one terminal of the drive pot left unconnected?

[Outlaws]

Is your drive pot really 25K?? Most of the tube screamer knockoffs use much larger values - 250K to 1M is common.

Is one terminal of the drive pot left unconnected?

Sorry, it's 1M. And yes there is a leg unconnected.

[R.G.]

It sometimes helps to solder the unconnected contact on the pot to the wiper. Give it a try.

[Outlaws]

It sometimes helps to solder the unconnected contact on the pot to the wiper. Give it a try.

Will do. It's on a PCB but it doesn't appear it jumped.

Also, it feedbacks really easily with the drive up by 3 o'clock and the tone up about the same.  I just don't remember tube screamers doing that either.

[R.G.]

Feeds back acoustically through a loud amp speaker, or electronically on the PCB?  If it gets a squealing note at low acoustic volumes, it's a circuit or layout problem.

Tube screamers by themselves don't feed back. Well, not proper ones.

Stuffing a distortion pedal into a very small box is almost asking for wiring problems to cause oscillation.

[Outlaws]

Feeds back acoustically through a loud amp speaker, or electronically on the PCB?  If it gets a squealing note at low acoustic volumes, it's a circuit or layout problem.

Tube screamers by themselves don't feed back. Well, not proper ones.

Stuffing a distortion pedal into a very small box is almost asking for wiring problems to cause oscillation.

Even with the volume fairly low.  I tried moving wires around with no luck.  Its a fantastic sounding pedal otherwise.

[thermionix]

When you have the hum going, does turning your guitar volume off get rid of it?  Could be hum your pickups are...picking up, then amplified by the gain of the pedal.

[pinkjimiphoton]

check the grounds everywhere, and check your electros for leakage. diodes can also make stuff hum in the power supply especially,
but often hum is a bad cap somewhere. check for solder bridges, and make sure ya have no voltage on the - side of any caps going to ground.

[Outlaws]

When you have the hum going, does turning your guitar volume off get rid of it?  Could be hum your pickups are...picking up, then amplified by the gain of the pedal.

The hum goes away, so that's good.

[Outlaws]

So new info....might be important.

The switch in the downward position (LED) and it only squeezes with the volume at a decent level.  Still not loud loud, but when it's up in the Diode position, it it squeals no matter what the pedals output volume is at all when the tone is maxed clockwise and drive gets about noon.  The LED section is less susceptible to squeal, though still really bad.

Swapped in a different 4558 for fun, same story.

[antonis]

Does anyone hear the NFB loop cap protesting..??

[R.G.]

Someone does, surely. And one speculates that both hum and oscillation may come from the other two huge problems with beginners' work -soldering and component value/placement.

@OP: Gently re-melt all of the solder joints and ensure that they are correctly soldered. Check the components to be sure they're inserted in the correct places. If you had bad solder joints to the feedback components of the opamp, it might cause either of these.

The problem with us speculating like this is that there are a large number of possible causes. There is no clear connection of hum to a specific component, nor is there a connection of oscillation to one or two parts.

[Outlaws]

Does anyone hear the NFB loop cap protesting..??

Which cap is that on my board?
http://www.rullywow.com/wp-content/uploads/2015/05/GlassHammer-rev1C.pdf

[antonis]

Does anyone hear the NFB loop cap protesting..??

Which cap is that on my board?
http://www.rullywow.com/wp-content/uploads/2015/05/GlassHammer-rev1C.pdf

Although there isn't any diagram, I would say C3 because it's the only cap in xx pF neighborhood...

[stallik]

Can I clarify something? When you roll off the guitar volume, the hum goes away?
If that's the case then surely the source of the hum is your pickups which is amplified by the pedal. Are you using single coils or humbuckers?

[thermionix]

^^^

[Outlaws]

Can I clarify something? When you roll off the guitar volume, the hum goes away?
If that's the case then surely the source of the hum is your pickups which is amplified by the pedal. Are you using single coils or humbuckers?

Humbuckers. So it stands to reason the effect is amplifying the inherent hum in a manner that is being exaggerated by self oscillation.