BD-2 Galaxie Mod Question

[Grunfeld]

I'm currently putting together the AionFX Sapphire (BD-2 clone) but using the Galaxie mod. For C11 it lists 3 options, 68n, 82n, or 100n. Before I decide on which to use I'd just like to have a better idea of the effect that capacitor will have on the sound.

Circuit can be found here
https://aionfx.com/app/files/docs/sapphire_documentation.pdf

Thanks!

[nocentelli]

The galaxie mod "replaces half of the passive components, completely transforming the pedal into something new"..... The original value for c11 is 1000n / 1u: The galaxie mod literally decimates this down to 100n, a significant decrease. The suggestion is that a less common value of 82/68n might be even better i would be tempted to just split the difference and go for 82n as a compromise, or if you only have 100n or 68n, use either of them, since I am 99% sure you would not be able to difference in a blind test

[Grunfeld]

The galaxie mod "replaces half of the passive components, completely transforming the pedal into something new"..... The original value for c11 is 1000n / 1u: The galaxie mod literally decimates this down to 100n, a significant decrease. The suggestion is that a less common value of 82/68n might be even better i would be tempted to just split the difference and go for 82n as a compromise, or if you only have 100n or 68n, use either of them, since I am 99% sure you would not be able to difference in a blind test

Yeah I think I might go with 82n.

[bartimaeus]

It looks to me that R18 and C11 are part of a discrete non-inverting amplifier.

Originally R18 (2.2k) and C11 (1uF) are forming a low-shelf filter at 72 Hz, a gentle cut in the lows.

In the Galaxie mod, R18 is raised to 10k. You can find the new filter cutoff frequencies by plugging 10k and the cap values into this calculator: http://sim.okawa-denshi.jp/en/CRtool.php

High cap value means more bass. So 68nF would have the least bass, with a cutoff at 234 Hz.

To be honest, I'd probably recommend 68nF. It cuts bass in a clipping stage, which can help to avoid a "bloated" sound.

The Sapphire pedal has an added bass knob at the end of the circuit, so you can restore any bass you cut.

[antonis]

R18+C11 form a voltage divider with R19+R20//GAINA//C10 setting the amount of local feedback (as C3+R7 with R8+R9//GAINB//C4 do)..
The above combination is a LPF, by itself, turning into a HPF due to inside NFB loop placement..
The higher the C11 value (for a given R18 value) the lower the R18/C11 impedance (the more the bass "leak" through it), hence the lower the NFB effective amount the higher the gain (and vice versa)..
The opposite effect stands for R18 value, of course..

P.S.
Not quite so simple, 'cause Q6 Gate can hardly be considered as "virtual ground", hence gain calculation in accordance to R18/C11 cut-off frequency isn't straightforward but you get the general idea..