Just noticed this one on the Projects page yesterday (yes, I'm slow). I LOVE the concept and definitely want to build one.
But I'm curious as to the progressive "order of magnitude" value escalation on the coupling/decoupling caps from input to output. What's the thinking behind the move from 1n at the front to 1u at the back?
Thanks
The 1n decoupler is due to the high input impedance (R2 @ 10M) into the op-amp. 1n is sufficient to cover all the range of guitar frequency (you end up with a -6db per decade starting at about 15Hz). This is a really common element to all the John Hollis circuits, actually. Anyway, you can see a similar set-up in the AMZ Mosfet boost.
The 1uF is pretty standard as an output decoupler (as you know) although you probably could lower it to 100n. One thing I would like to learn more about is the importance of capacitive reactance as it relates to guitar signals, actually. Like, does the reactance of a 100n decoupler vs. a 1uF decoupler vs. a 1n cap really matter? I should read up on that truth be told.
Ah, right. I should have seen that.
Thanks for the explanation.
I think 1u output cap is sufficient to move the cutoff -3dB point to 15Hz running into a 10k impedance. Targeting a 10k input resistance makes sense, though, it's in the ballpark for a rigidly-biased BJT common-emitter amp input.
I've seen some (MXR Micro Amp) with a 15uF output coupling cap, presumably to feed a low-Z mic input without low-end loss--or maybe it's what they had on hand at the time.
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