What are input and ouput impedances?

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zmix

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The common practice was to variac guitar amps down to lower mains voltage to force premature clipping at lower power output.

JR
Yes, Eddie Van Halen's guitar tech told me they set the variac to 90 Vac - the comments Eddie made in that Guitar Player Magazine interview were though to be a practical joke, they imagined people blowing up their amps all over the world. Hardy-har-har..
 

FIX

Paul Wolff
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That would be me, Paul..!

Way, way back in the 1890s (yes the decade of grunge) I did some impedance plots of some passive guitar pickups I was studying. I discovered that a typical dual coil "humbucker" had an impedance of over 160kΩ at resonance (which was around 3.2kHz). In terms of impedance matching this explains why low value pots can reduce the "brightness" of a guitar pickup - they act as a zobel of sorts and damp that resonant peak.
Of course that is when the pots are full up... I think I was referring to DC measurements on the pickups. Maybe not, It was even before the 80s...

Here's a good one:

Everyone knows that Eric Johnson can hear the difference between type of batteries, which is easily explainable. He could also hear that the 5 position switch was darker than the 3 position switch in a Strat, when stuck in the middle between T pickup and M pickup. It was actually quite clear, until I figured out why.

When Fender made the wiring harnesses, one wire looked slightly short, so to get it right it had to be tugged on a little. This wire was connected to the Middle tone control, and cause the contact to rotate ever so slightly. Of course, when switching between the 3 pickups, the T pickup had no tone control, and the M and F did. When switching it into the middle position between T and M to get both, the contact didn't quite touch, disconnecting the M pickup's tone control. This made it every so slightly brighter. When the 5 position switch was made, they made it "correctly" and the M tone control was in the circuit in that same position. Snip that contact and you have the original...

But wait, there's more... Aside from Jimmy Hendrix stringing his guitar upside down, effectively slanting the T pickup the other way, he also swapped the M pic-up and F pick-up on the switch, so it went T-T/F-F-F/M-M, so his sound was a combination of that angled pickup and the F and T pick-up in parallel, with no tone knob.

BOOM
 

abbey road d enfer

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But wait, there's more... Aside from Jimmy Hendrix stringing his guitar upside down, effectively slanting the T pickup the other way, he also swapped the M pic-up and F pick-up on the switch, so it went T-T/F-F-F/M-M, so his sound was a combination of that angled pickup and the F and T pick-up in parallel, with no tone knob.
F & T being the initials of Neck and Bridge?
 

FIX

Paul Wolff
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Treble Mid Front = Bridge Middle Neck

I worked for a guy that called them that, so it's always what I called them, , or sometimes Bass Middle Treble.
 

mightysteve

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That would be me, Paul..!

Way, way back in the 1890s (yes the decade of grunge) I did some impedance plots of some passive guitar pickups I was studying. I discovered that a typical dual coil "humbucker" had an impedance of over 160kΩ at resonance (which was around 3.2kHz). In terms of impedance matching this explains why low value pots can reduce the "brightness" of a guitar pickup - they act as a zobel of sorts and damp that resonant peak.

3.2 kHz is a pretty high resonant peak for most humbuckers (maybe it was a Filtertron?) but that's basically right. The inductance of the pickup coil combined with the parallel capacitance of the rest of the circuit (the coil capacitance, and esp. the cable capicitance) forms a harmonic oscillator with a characteristic resonant frequency, which is much of what differentiates one pickup from another.

But to your point: that's absolutely right. The lower the resistance of the pots, the higher the load on that oscillator, meaning higher damping. The Q (quality factor) of the oscillator is reduced, meaning the height of the resonant peak is reduced. And since 3.2 kHz (e.g.) is on the "bright" part of the spectrum as far as guitar goes, you lose highs when you use lower pots with lower resistance.

On the other hand using a longer cable is like hooking up an additional capacitor between hot and ground (30 pf/ft is not atypical). This lowers the resonant frequency of the pickup (really the resonant frequency of the entire circuit in which the pickup is embedded), and explains why you get a "warmer" or "duller" sound when using longer cables. (The resonant frequency f = sqrt(1/LC).) *That* loss of highs is a movement of the peak to a lower frequency, rather than a reduction in the height of the peak.
 

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