ROG Tonemender ground hum

[waltk]

I've built 3 ROG Tonemenders now.  The first was a couple years ago - my FIRST build.  The second was a year ago - after a little more experience.  I just built the third - and after scores of builds, I know what to expect from a circuit.  The problem is I get a HUGE amount of hum (built on a PCB etched from the ROG site).

I noticed that if I ground it to a large metal object (like my table saw), the hum almost completely disappears.  It's battery-powered, so the hum is not from a misbehaving wall-wart.  Is there something about the PCB latout that's particularly susceptible to hum?

I searched, and read, and studied the schematic and layout - but still can't see anything obvious that would cause this.  It's basically just a buffer (using 1/2 of the opamp), then a tone stack, then a gain stage (using the other half of the opamp).  I've built 3 of them now, so I don't think it's anything I've mis-connected - they all have the same behavior.

Anyone else had this experience with a Tonemender?  Any ideas about what to try?

[keto]

I've built 2 recently (in fact, wired one into a box tonight, combined with another circuit), and one of the (several) things I like about it is that it's *dead* quiet.

I use star grounding (everything to the input jack - circuit, stomp switch, dc adapter, don't really use battery snaps but if so that too, and the output jack as I normally ground pots there), and tho I am a relative neophyte still sucking up knowledge and understanding, have read that it's pretty much *the* way to go for quiet builds.  Try that?

What op-amp did you use?  Ive been successful with a TL072 in my first build, and believe (though it is possibly too soon to say for sure) TL272 in the current build may be quieter. 

[waltk]

Thanks for the reply.

I've built 2 recently (in fact, wired one into a box tonight, combined with another circuit), and one of the (several) things I like about it is that it's *dead* quiet.

Hmmm... Your experience with the Tonemender is definitely different from mine.

I use star grounding (everything to the input jack - circuit, stomp switch, dc adapter, don't really use battery snaps but if so that too, and the output jack as I normally ground pots there), and tho I am a relative neophyte still sucking up knowledge and understanding, have read that it's pretty much *the* way to go for quiet builds.  Try that?

I've just tried it in a test rig (rather than boxed up).  The disturbing thing is that other (even high-gain) circuits are quiet on my test platform.  Everything is pretty well grounded - there's only one ground point on the board, but it gets pretty directly connected to the input and output jack sleeves, and battery.  The opamp is socketed in all three builds, and I've tried other opamps (TL072 in this one).

[GibsonGM]

Sounds like some kind of bad grounding problem or power supply noise, Walt.   Are you using a wallwart? Then try it with a battery.   Are your jacks properly grounded?  Can you isolate what 'part' of the pedal is affected by grounding to the table saw?  Does wiggling wires change anything?

Check out the filter cap in the power section, if that is backwards, soldered badly or blown it could cause the hum.
Mine was very quiet, pretty nice unit, and I built it on perfboard. 
If nothing above helps, post voltages on the opamp and battery.

[waltk]

Sounds like some kind of bad grounding problem or power supply noise, Walt.   Are you using a wallwart? Then try it with a battery.   Are your jacks properly grounded?  Can you isolate what 'part' of the pedal is affected by grounding to the table saw?  Does wiggling wires change anything?

Check out the filter cap in the power section, if that is backwards, soldered badly or blown it could cause the hum.
Mine was very quiet, pretty nice unit, and I built it on perfboard. 

Thanks for the ideas.  I'm still stumped.  Yup, it definitely has something to do with grounding - but what?

It isn't power supply noise - it's running off of a battery.
The caps in the voltage divider section have their negative leads to ground (as in the schematic).
The sleeves on the jacks (on my test rig) are connected directly together, and they are solidy connected to the negative battery terminal and ground point on the PCB.
Wiggling the wires doesn't change anything, but the hum gets louder when my fingers get anywhere near the PCB.
The three Tonemenders I've built (with a year between each build) all have the same hum problem, so I think it unlikely to be a bad solder joint or bridge.

The strange thing is that when I attach the common ground to a large metal object (e.g. table saw), the hum pretty much disappears.  Everything else works as expected.

[PRR]

> if I ground it to a large metal object (like my table saw)

When you ground it to your amplifier (just plug it in), that should do the same thing.

Why isn't your amplifier (which you have not mentioned) as groundy as your table-saw?