Why does my guitar hum when I turn the volume down?

[Morocotopo]

(I had the most strong urge to name the post "Why does it hurt when I pee?" but decided to leave my Zappa hallucinations for later)

Now on to the subject matter.

Why does it do such a thing? Let me explain: when I use it with a dist/od pedal, the position of the guitar´s volume determines the amount of hum I get, and it behaves different according to the type of pedal. For example:

- With a Guvnor clone, vol at max is zero hum, as I turn it down I start getting hum, and when at zero the hum goes away.
- With a Fuzz Face that has a pre "smooth" pot, the amount of hum depends of the smooth pot setting.
- With a Harmonic Percolator, at max vol hum, turning it down NO hum, with vol at zero hum again ().

So, each circuit behaves differently... same guitar, same everything... what are the mechanisms at work here?

[aron]

I hope someone can answer this one. I never understood this myself.

Aron

[R.G.]

You're changing the effective resistance between the amplifier or pedal input and ground.

As a simplified view, imagine your guitar as one pickup feeding a volume control, and the wiper of the volume control wiper going out to the output jack. When the wiper is at max, the resistance the amp/pedal input sees to ground is the total resistance of the volume pot in parallel with the pickup inductance and resistance. For a 2H pickup inductance the impedance is maybe 4K to 8K resistance and 753 ohms or so inductive. This is the opposite of the "lose treble by loading" paradigm; at low frequencies, the inductive impedance vanishes. So the line to the amp/pedal is held to ground by a few K ohms.

At minimum pot setting, it's tied to ground.

In the middle, the impedance to ground is the parallel of the upper and lower halves of the pot resistance, which is 62.5K for a 250K pot and 250K for a 1M pot. So the cable is much higher impedance at the middle resistance position. Easier for hum to get in however it does get in.

[Morocotopo]

R.G, thanks.

Let´s take it step by step.

First, I´m changing the resistance to ground. OK. So I´m actually changing the following circuit´s input impedance, and the higher impedance, the more sensibility to noise. so far so good.

From there, err...

Give me some time to fully grasp this.

[ashcat_lt]

The noise wants to complete it's circuit as "easily" as possible.  You'd prefer that it find it easier to do so through the source than the load.

More technically correct: 
Imagine the noise source connected to the signal source via a largish resistor.  It's actually more like a capacitor, but...  This largish resistance is the "top resistor" of a voltage divider with the signal source impedance (you're guitar) as the "bottom resistor".    The noise heard at the load comes off the "middle" of that voltage divider.  The bigger the "bottom resistor" is compared to the "top", the louder the hum.

[R.G.]

Ashcat's correct. The noise/hum usually comes in through a capacitive divider, with the random capacitance to the rest of the universe from the wire that goes from guitar to amp being the top side of that divider. The bottom side of the divider is the parallel combination of the guitar, including pickup and volume pot on one end and the amp/pedal on the other end.

The amp/pedal end is nearly always 1M resistive plus some other junk that doesn't matter a whole lot. The guitar end goes from nearly zero (when volume pot is at minimum) to 125K to 250K in the resistive middle, back down to a few k at the maximum of the pot rotation. So hum can get in best when it's in the middle.

This is another situation where a buffer in the guitar helps. It converts the few k to 250K of the guitar to a reliable sub-1K impedance and gives the hum-coupling capacitor no chance.

[wavley]

R.G.

[Gurner]

Coming into this late.....so let me see - when the guitar is turned half way down, it starts to hum, if that wasn't bad enough it hums  Rod Stewart songs?

No cure for that one... such a humming guitar needs to be sold.

[duck_arse]

just teach it the words.

[wavley]

just teach it the words.

And join a Faces cover band.

[thehallofshields]

This is another situation where a buffer in the guitar helps. It converts the few k to 250K of the guitar to a reliable sub-1K impedance and gives the hum-coupling capacitor no chance.

Any advantages of going with a Tillman style Common-Source over a basic Common-Drain buffer?



I just got a Jazzmaster with 1M Pots, and I get pretty loud hum from 95%-5% on the Volume Knob.

[amptramp]

This is another situation where a buffer in the guitar helps. It converts the few k to 250K of the guitar to a reliable sub-1K impedance and gives the hum-coupling capacitor no chance.

Any advantages of going with a Tillman style Common-Source over a basic Common-Drain buffer?

I just got a Jazzmaster with 1M Pots, and I get pretty loud hum from 95%-5% on the Volume Knob.

The advantage of common source is you can get voltage gain.  Common drain will always have a gain of less than 1.  Common source also inverts the phase, but this seems to have little effect on the audio.

[thehallofshields]

The advantage of common source is you can get voltage gain.  Common drain will always have a gain of less than 1.  Common source also inverts the phase, but this seems to have little effect on the audio.

I guess what I wanted to ask was; if I don't need any Voltage Gain, is there any reason to not just use a buffer? I know the Fetzer has some nice non-linear behavior that adds some tubeness, but I wouldn't want that much gain onboard.

Do JFET Buffers draw less current?
Do JFET Buffers create less noise?
Can JFET Buffers have non-linear behavior?

[LightSoundGeometry]

it has to be a common drain to be a buffer, it was one of our lab projects when doing classes of amps or it will be an amplifying ratio.

the characteristics of each type make it what it is ..the common collector/drain are the buffer stage/circuits. otherwise you have a gain stage and amplification without the proper Z impedance characteristics; and I also believe I/O are in phase which makes it a voltage buffer circuit

[G. Hoffman]

In real life, this usually means there is a poor solder connection to ground somewhere. 


Gabriel

[robthequiet]

Um, ground loop? I have had bad cables and faulty wiring in guitars do this kind of thing. Not disagreeing with previous explanations but I would look at the wiring in the guitar for sure.

[LightSoundGeometry]

Um, ground loop? I have had bad cables and faulty wiring in guitars do this kind of thing. Not disagreeing with previous explanations but I would look at the wiring in the guitar for sure.

ground loop comes from two or more AC sources with more than one common ground reference.

I think you can run your stuff through a DI box and eliminate some noise

[LightSoundGeometry]

Um, ground loop? I have had bad cables and faulty wiring in guitars do this kind of thing. Not disagreeing with previous explanations but I would look at the wiring in the guitar for sure.

ground loop comes from two or more AC sources with more than one common ground reference.

I think you can run your stuff through a DI box and eliminate some noise
https://en.wikipedia.org/wiki/DI_unit

[Mattnezz]

if I don't need any Voltage Gain, is there any reason to not just use a buffer?

Yes there are things to consider.
For your buffer to eliminate hum coming from high impedance (ie rotating a 1M volume pot) your buffer has to be close to, or inside the guitar. So you'll need to power it aswell. I've seen solutions though, where you solder the whole buffer inside a male jack and power it by the cable.

A guitarsignal straight from a pickup is a complicated signal where all kinds of impedances, reactances, inductances,  are involved. This is one of the reasons this signal combines well with low-input-impedance fuzzes. If you love classic fuzz, inserting a buffer kills al the fun, since you now have a fairly straight low-impedance source.

By lowering impedance you also minimise the influence a (long) cable has, cable capacitance usually rolls of some highs (forming a RC low-pass-filter, where R is guitar Zout and C is the cable). So the signal might sound brighter.

[thehallofshields]

if I don't need any Voltage Gain, is there any reason to not just use a buffer?

Yes there are things to consider.
For your buffer to eliminate hum coming from high impedance (ie rotating a 1M volume pot) your buffer has to be close to, or inside the guitar. So you'll need to power it aswell. I've seen solutions though, where you solder the whole buffer inside a male jack and power it by the cable.

A guitarsignal straight from a pickup is a complicated signal where all kinds of impedances, reactances, inductances,  are involved. This is one of the reasons this signal combines well with low-input-impedance fuzzes. If you love classic fuzz, inserting a buffer kills al the fun, since you now have a fairly straight low-impedance source.

By lowering impedance you also minimise the influence a (long) cable has, cable capacitance usually rolls of some highs (forming a RC low-pass-filter, where R is guitar Zout and C is the cable). So the signal might sound brighter.

I mean is there any reason I wouldn't just want to use a JFET Buffer (Common Drain) instead of a JFET Amplifier (Common Source).
I know with the Buffer I'll get the same ultra-high Input Impedance, and even lower Output Impedance.

So if one is okay with the 99% Voltage Gain of a Buffer, instead of 140% like in the Tillman, is there any incentive to use the Tillman?
Is noise performance better? Is current-drain lower? Will it clip differently?