Hey Brian: Question about the Wigl circuit

[culturejam]

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

[madbean]

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.

[culturejam]

Ah, right. I should have seen that.

Thanks for the explanation.

[EBRAddict]

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.

I use this site like some people use an online Bible/Torah/Koran http://sim.okawa-denshi.jp/en/Fkeisan.htm