humbuckers middle position volume swell

[Billy Bones]

Hi guys,

Sorry, this will seem elementary, but I'm looking for a good explanation.

Matched humbuckers in phase with each other. Middle position. When I take one of the volumes down to 7 or 8, there is an output increase.

What's up?

[Possum]

...

What do you mean "matched?" Identical output?
Whatever, you are getting cancellation in the middle position, when you unbalance the two pickups the cancellation goes away and you regain frequencies so the volume goes back up....

[voodoochild]

Hi guys,

Sorry, this will seem elementary, but I'm looking for a good explanation.

Matched humbuckers in phase with each other. Middle position. When I take one of the volumes down to 7 or 8, there is an output increase.

What's up?

This is normal. I can't explain why it does that but I have always noticed it. Nature of the beast. Sorry I can't explain why.

[Sweetfinger]

If you have them wired "vintage style", when you roll off one of the pickups, you are not only removing its output from the signal, but also removing its series path to ground. Each pickup "sees" the other as a 8K or thereabouts inductor connected to ground. That's why two coils or pickups wired in parallel will sound cleaner with slightly lower output. Each pickup is bleeding off signal from the other.

If get a volume increase with the humbuckers wired modern style, with the output taken from the center lug of the volume pot, then you have out of phase pickups.

[Mike Sulzer]

I Each pickup "sees" the other as a 8K or thereabouts inductor connected to ground. That's why two coils or pickups wired in parallel will sound cleaner with slightly lower output. Each pickup is bleeding off signal from the other.

Also, two identical pickups in parallel have half the inductance of one. The cable capacitance stays the same. Since it is larger than the pickup capacitance, the resonant frequency rises by somewhat less than the square root of two, resulting in a cleaner sound.

[Billy Bones]

I mean that they have the same size coils, amount of widings, and the same output. Basically two of the same pickups.

When they are balancing each other out in the middle position, that's completely different than a phasing problem?

[Sweetfinger]

The cable capacitance stays the same. Since it is larger than the pickup capacitance, the resonant frequency rises by somewhat less than the square root of two, resulting in a cleaner sound.

How can cable capacitance be larger than pickup capacitance? Sort of common sense tells me that a coil is going to have a much larger capacitance than a 10 to 25 foot shielded cable, ostensibly designed for low capacitance. To test: If I measure cabes I get somewhere in the low picofarad range, while I've got some dead pickup coils that measure up to 3 or 4 thousand picofarads. Am I missing something?

[David Schwab]

When they are balancing each other out in the middle position, that's completely different than a phasing problem?

If they are out-of-phase you will lose all the low end when they are evenly mixed. It will sound really thin as well as having less output.

[Mike Sulzer]

You might want to check your measurements again. It is relatively easy to measure the capacitance of a guitar cable. 500 pf is typical; it does vary, of course. This is not surprising, given the capacitance per foot of typical cable. 30 pf/foot is not unusual; it is hard to find cable cable less than 12 pf per foot; it gets very big and does not make a good, flexible cable. Guitar cables often have a thick insulating layer; the actual shielded cable is smaller than you think and thus typically pretty high in capacitance.

Measuring pickup capacitance is harder than you think. Many meters cannot handle ~10K of resistance in parallel.

A humbucker consists of two coils in series; this doubles the inductance and halves the capacitance. 80 pf for a PAF type is typical. Measuring it is easy if you have an instrument for measuring the complex impedance as a funcion of frequency. You can derive the capacitance from the measurements above the resonant frequency

Intuition needs some careful training to be useful for determining coil capacitance. You might want to think of its as a lot of elementary capacitors connected in a complicated series/parallel network. Not so easy to figure out.

The effect of the cable on the sound is easy to determine. If you put a buffer in the guitar, you will find that you need to add capacitance across the pickup, equal to the cable capacitance, to keep the sound the same. A buffer makes the sound independent of the cable capacirance, and it allows you to set up the pickup/C combination to get the sound that you want. You will find that moving the resonance above 4 or 5 KHz makes the guitar less bright as you put the resonant boost above the range of a guitar speaker (and maybe above your hearing range if you are getting older and have played loud for a long time).

How can cable capacitance be larger than pickup capacitance? Sort of common sense tells me that a coil is going to have a much larger capacitance than a 10 to 25 foot shielded cable, ostensibly designed for low capacitance. To test: If I measure cabes I get somewhere in the low picofarad range, while I've got some dead pickup coils that measure up to 3 or 4 thousand picofarads. Am I missing something?

[ShannonH]

At last...

What do you mean "matched?" Identical output?
Whatever, you are getting cancellation in the middle position, when you unbalance the two pickups the cancellation goes away and you regain frequencies so the volume goes back up....

Thanks for that. This is how I have always explained it as well. It's sort of like two speakers' cones of sound intersecting and causing volume drop because of frequency cancellation.

I have found that a bridge pu wound with 42 (8.4k), and a bridge with 43 (14.5K) tend to do this less. The only reason I can figure is the way each wire carries freq's differently. Or it could just be a vast difference in tonal sensing because of overwinding the bridge. Most of my customers who want to run vintage wiring want to eliminate the drop.