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Noise increases when guitar volume is rolled down

PostPosted: 30 Nov 2016, 16:46
by m4j0rbumm3r
Hi all,

Last weekend I've rewired my Gretch Electromatic Bo Diddley guitar as I personally don't really like the standard Gretsch controls of 1x volume per pup, 1x master volume and 1x master tone.

I wanted to try something like this:
* signal from each pickup into its own Fender TBX tone control
* the two pickup signals into a dual gang pot wired for 'stereo' volume panning (intended here as a single control that can cut either pickup level and does nothing at center detent)
* the two outputs from the volume panning pot to the 3 way switch
* the single output from the switch to the master volume and then to jack

I know this description is a bad substitute for a schematic, but it roughly shows the idea for the signal path. All grounds were connected and I didn't forget about grounding the strings etc through the bridge.

The only wiring I actually had to change were the TBX and volume pan pots. The volume pan pot output lugs I then connected to the leads that go to the toggle. The wiring of the toggle and the master volume I left as it came from the factory.

Now, upon testing it, the two TBX tone controls and the one pickup pan control worked as expected. The master volume did not. Well, it does reduce the volume when turned down, but doing so also gradually brings up a hum / noise, which becomes quite loud when the master volume is turned all the way down. As it stands, the only way to make the guitar quiet is to set the master volume at 10 and mute the strings. When playing it with the volume fully up, it doesn't sound bad in any way. Only turning the master volume down causes trouble.

I don't know a lot about electronics (obviously), but I suspect a ground issue. So I have checked with my multimeter that the back (ground) of all controls has zero resistance to the jack ground. Did the same for the bridge, strings, etc. This all looks fine, no bad solder joints or broken leads as far as I could tell. But still the noise problem remains. Also, the problem is unchanged when I flip the toggle, turn the TBX's or the pickup volume pan pots into different positions, etc. The noise seems related only to my master volume control.

I've searched both the forum and the interwebs but my issue seems to be a rare one. At this point, I am stumped.
Does anyone have suggestions as to what may be going on or how to debug this?


Re: Noise increases when guitar volume is rolled down

PostPosted: 01 Dec 2016, 14:41
by phatt
Hi and welcome :thumbsup
Yes ground issue is the likely culprit.
Is the center lug of the *Master pot* actually at *Ground* when you turn down? (Max hum)
With pot turned *OFF* Measure the resistance between center and ground lug with your DMM ,, it should by close to zero Ohms. If not the lug has lost contact with the wiper pad inside.
The lugs are only press fitted by a small eyelet to the internal resistive pad. Not uncommon to have them go intermittiant when twisting and turning wires on those small lugs.

Re: Noise increases when guitar volume is rolled down

PostPosted: 03 Dec 2016, 10:47
by m4j0rbumm3r
Thanks for the input. :thumbsup
Sounds like a very plausible cause. I'm going to test it and report back.

p.s. I'm not really new to the forum though I have not posted here in three years or so. Seems like my account history has been cleared somehow.

Re: Noise increases when guitar volume is rolled down

PostPosted: 03 Dec 2016, 11:56
by m4j0rbumm3r
Measured 2 Ohm resistance from master volume pot center lug to ground when it is fully turned down (max hum). The increase in hum is very smooth/gradual as I turn master volume down, it doesn't produce any pops and crackles as one would expect with bad connections inside the pot.

Some additional observations:
- With master volume all the way *down*, hum is loud.
-- If I then short the center lug to ground with a piece of breadboard wire, there is no change. Hum remains loud. This suggests to me that the 2 Ohm I measured is not the problem.
-- If I then short the center lug to the *other* lug, i.e. the one which goes to jack, the hum disappears entirely. I guess this makes sense as the jack signal lug is now at ground. This was sort of a sanity check.

Could I have a ground *loop* issue with the guitar wiring? Should I look for and cut any redundant ground interconnections to remove such loops?

Re: Noise increases when guitar volume is rolled down

PostPosted: 03 Dec 2016, 12:24
by m4j0rbumm3r
Did some further digging and found sensible info here: viewtopic.php?f=3&t=11384&hilit=hum#p123795

Let me quote the relevant bit from that thread:
I'm a physicist/RF engineer (I build things for MRI scanners) ... You definitely want to star ground your guitar. Any loop will act as a small antennae that will pick up the 50/60Hz electromagnetic field from the electric wiring in your house/studio/stage etc. 50Hz has a wavelength of around 6000km, and so you're definitely within the near field regime unless you are in the middle of the Pacific Ocean. Any loop of wire will couple strongly to this field (it's mostly magnetic), this includes a loop in your wiring in your guitar, this causes a common mode signal down what should be a differential (ie single ended) mode and that's why you hear a ground loop. I always star ground my guitars, it makes a hell of a difference.

Re: Noise increases when guitar volume is rolled down

PostPosted: 03 Dec 2016, 13:18
by phatt
I'm not going to argue with the teck geeks but I've never has issues in 40 Years and *IF* it was that bad then every stock redne7 guitar would be unplayable. [smilie=a_holyshit.gif]
Ground loops are not really an issue with passive guitar circuits as there are no big current differences but yes good practice to have only one ground path.
50/60 hZ hum and RF interference is not your problem!!!!

I'd say you are just missing a ground somewhere.
This is where it's good practice to draw up a schematic of what you wish to build. Working from wiring layouts alone can lead to stuffups.
Yes on the rare occasion you do need a layout but mostly Schematics are less prone to miss wiring.

Simulations and Cad programs use *Nodes* to keep track of complex wiring paths and circuits and they are rarely wrong ,, so try numbering each wire node on your schematic and count how many pots and resistors connect to each node. I've found that works well for me.

Re: Noise increases when guitar volume is rolled down

PostPosted: 03 Dec 2016, 14:05
by m4j0rbumm3r
Yeah, I did find a ground loop and cut it to end up with a star / daisy chain ground path. It just didn't remove the hum.

I'm going to follow your advice and draw up a schematic. It's not something I am good at but I need to practice it anyway.

Re: Noise increases when guitar volume is rolled down

PostPosted: 04 Dec 2016, 07:24
by m4j0rbumm3r
Here is my attempt at a schematic. I am no EE mind you.
The Fender TBX pot pairs have a very particular taper: one has 250K for the first half of its travel and is *open* for the second half. The other has very little resistance for the first half of its travel and 1M for its second half. This is not apparent from the schematic.
Not shown are grounds for pickups and bridge.
I couldn't find the right switch type diagram, it should be a regular three way guitar toggle.

Why would this hum when master volume is down, but not hum when master volume is up?


Re: Noise increases when guitar volume is rolled down

PostPosted: 04 Dec 2016, 10:43
by m4j0rbumm3r
The good news: I finally solved the problem.
The bad bews: I probably set the record for longest time ever spent figuring out than an *instrument cable* is bad.

Since I went over the Gretsch wiring a hundred times and couldn't find problems with it, I plugged in another humbucker guitar that I knew had good wiring. Playing around with that, I found out that it could also produce hum, but *only* with both (Les Paul style) volume controls set at 5. I then left the volume knobs in that position and switched the guitar cable: no more hum for any position of the volume. Tried it with the Gretsch: again, no more hum with master volume at any setting.

So it was all just a weird interaction between guitar and bad instrument cable.


Re: Noise increases when guitar volume is rolled down

PostPosted: 05 Dec 2016, 13:07
by phatt
Yay,, good on you for drawing the circuit. :thumbsup
Just be aware that the master volume is drawn *inverted or reverse*.

If your actual circuit is wired up that way it can cause some issues with hum when turned down.
It's still perfectly valid and was used in some guitars, Hofner comes to mind as the old *Verythin* had no PU selector switch and just relied on the 2 volume pots wired backwards. But they were prone to hum and buzz.

When you wire up a volume pot normally the pot grounds the guitar lead when you turn right down thus shorting/grounding the amplifier input. = No hum
When wired in reverse the amp input is left Live as well as the guitar lead,, it's only the guitar internals that are shorted out.
Potential hum especially if your lead is not so good. :blackeye

Re: Noise increases when guitar volume is rolled down

PostPosted: 07 Dec 2016, 17:14
by m4j0rbumm3r
Thanks for checking the circuit and good call on the master volume wiring. I had not thought of the fact that the amplifier input is not grounded with volume down this way. I left master volume wiring just as it came from the factory, which is indeed inverted.

Since I'm now planning to swap the 500KA master pot for an 1MA pot, I will then also change that master volume wiring around.

After playing around with the guitar a little, I found that it was a bit muddy for my taste. Probably as a result of the extra load on the pickups that my custom wiring introduced. So I have already changed the wiring of the two TBXs into what is suggested here: ... ne-control
That wiring removes R3 and R9 from my circuit and is an improvement both in nominal tone (TBX centered) and in usefullness of the TBX. On the neck humbucker in particular, having a bass cut is really useful. Actually, it offers much more usable tones than the standard treble cut.

I'm thinking that a 1M master volume pot with a parallel cap and resistor for some treble bleed will make the wiring just right for my needs.

Re: Noise increases when guitar volume is rolled down

PostPosted: 28 Dec 2016, 14:08
by m4j0rbumm3r
Today I replaced the 500K master volume pot by one that is 1M and changed the wiring to have jack tip connected to the pot centre lug - as suggested by Phatt. This is quite an improvement. Complete silence with master volume at 0.

Because of the change to 1M, the guitar now also has more output with master volume at 10. When using the control, the decrease in volume is just one would expect. My theory is that the 1M pot value counteracts the effect of extra signal loads introduced by the fancy pots. I threw in a 560pF treble bleed cap with no resistor and this works well for these humbuckers. The amount of treble now feels balanced over the whole range of volumes, whereas before it was dull sounding at low master volume.

So for future reference, this is the version that I feel works well: