Author Topic: Kingslayer cap sub.  (Read 228 times)

toolguy

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Kingslayer cap sub.
« on: May 27, 2018, 08:24:21 PM »
Hello all.
I have everything but a 3.9n cap for c15 on my Kingslayer build.
Looks like it goes to the middle leg on the tone pot.
I have a 2.2 or a 4.7.
Any idea what the tonal differences will be from the 3.9?
Thanks

reddesert

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Re: Kingslayer cap sub.
« Reply #1 on: June 01, 2018, 04:57:17 PM »
The 3.9 nf cap in the Kingslayer is in the feedback loop of an inverting opamp, and is fed by the middle lug of the tone pot. This is in parallel with R21, a 100K feedback resistor.

The basic ideas here are that:
1. the resistance in the feedback loop of the inverting op-amp controls the gain. If the cap and tone pot were disconnected the gain would be given by R_feedback / R_input = R21 / R19, which is 100K / 100K, so a gain of 1.  (See any page about inverting op-amp amplifiers for a more detailed explanation.)

2. The 3.9 nf cap allows high frequency signals through. This means a lower resistance feedback path for high frequencies, so effectively, R_feedback is reduced, lowering the gain at high frequencies. As you turn the tone pot down, it has lower resistance in series with the cap, so it lowers the feedback resistance and thus the gain at high frequencies.

You could calculate what the effects are by computing the reactance, which is the equivalent frequency dependent resistance (impedance) of the capacitor. Reactance = 1 (2 * pi * frequency * C). So the gain cut depends on the value of frequency * C.

The 4.7 nf cap has a 20% higher value than the 3.9 nf cap, so if you sub the 4.7 nf cap in, you'll have an equivalent gain-cut at a 20% lower frequency.  This could be compensated to some extent by turning the tone pot up a little - increased pot resistance compensates for the tone cap lower reactance.

20% in frequency, by the way, is about three semi-tones. Does a Klon sound substantially different played in the key of A rather than C? I dunno, but my guess is not. This is why I tend to pause and tell myself not to get stressed over minor component value or material differences.