Series Capacitor QQ - Sanity Chect

[karbomusic]

Building a yet another Fuzz Face and I want the input cap switchable between the stock 2.2 uF and 0.68 uF. If I calculate 2.2 uF and 1.0 uF in series I get 0.677. Would this actually give me the same rolloff/frequency as it would literally switching between 2.2 and 0.68? Reason being I'd like to go ahead and mount the 2.2 on the board and it would be easier since I can just throw the 1.0 on the switch.

[Mike Burgundy]

Yup.
Two situations that you switch between:
-one 2u2 cap.

-2u2 and 1u in series. these are the equivalent of a single 0u68 cap, and can be thought of as one such cap.

Go for it.

[karbomusic]

Perfect and thanks. Common sense said so too but just triple checking.

[karbomusic]

So I thought I wired this up right but it doesn't appear to work or my values are off based on what I expect to hear. Basically on the BB I was swapping the 2.2uF out with a .68 uF and it did what I wanted. Wired in the box no audible change (no I did not do the math yet). Is my harebrained method to switch in/out in series using an SPDT flawed?

Code Select Expand

Input ----------------->
           Output |   |   | Cap
                   <-------------

^ Meaning the input signal hits the center pin, the output to the pc board is pin 1 (left), cap is strapped across pins 1/3. My assumption was when engaged, signal travels through the switched cap then to the output pin and on to the board. I assumed the disconnected pin 3 from the cap (when not engaged) would be irrelevant since when switched out that leg isn't connected. What am I missing? Here is a picture if you can make it out:



Apologies for any idiocy here. I can drop it to a really low value to audibly confirm but I expected something based on my breadboard test.

[ashcat_lt]

It's possibly better to short that cap when you don't want it.  It only really needs an SPST for that, but you can wire the input and one end of the cap to the either of the outside lugs, and the other end of the cap with the output on the middle lug.

[karbomusic]

It only really needs an SPST for that

How do I do that when the input signal runs directly through it? It's in series with the input signal and the real decoupling cap on the perf board.

[PRR]

SB like this:

[karbomusic]

SB like this:

Thanks I'll try that, still not clear why the way I'm doing it wouldn't work.

[duck_arse]

looking at your switch, I can't see why it doesn't work. looking at your ascii diagram, I can't figure it (my local style-sheet needs adjustment).

perhaps if you drew your circuit as wired in the style of prr's dia, it would all become clear.

[karbomusic]

Thought you'd never ask! Mine doesn't have the flair PRRs does though..

[duck_arse]

well, I dunno why it doesn't work. you only need to move the 1uF short leg to the switch common to have wot prr drawed.

I'm going to stop looking at it now ........

[karbomusic]

well, I dunno why it doesn't work. you only need to move the 1uF short leg to the switch common to have wot prr drawed.

I'm going to stop looking at it now ........

Yep, agreed. When on the breadboard I was simply removing the 2.2 and replacing it with a .68 and it removed enough woofiness/mud for the feature I wanted but didn't test the series piece (hence this entire thread). I was going to just switch between the two entirely then when building I realized it would be much simpler to exploit dropping the value via series. However, the switch appears to have absolutely zero effect (other than that very brief ms of no connection when switching). Not making much sense to me unless the resulting capacitance is still too large for me to hear what I need but it was pretty darn obvious when testing and swapping them out on the breadboard.

This is bugging my "got's to know" sensibilities + I need to justify that hole in the box and switch LOL. I'm going to put a .001 uF and see what happens, that will surely answer part of it for me.

[karbomusic]

I fixed it by dropping the cap value on the switch. I got down to .047-.068 before it sounded like what I was shooting for. I think I was off by a factor of 10 in my original calculation so it was working the entire time but my target was .047-.068 as the resulting series capacitance not 0.47 - 0.68. I knew that seemed too big but misread what I grabbed out of the bucket of parts at the time.

So Yay! but a slightly painful yay.

[duck_arse]

yearh, I dunno where we got that 1uF from (prr) but I was thinking in the interim that going from 2u2 to 2u2-1uF prolly wouldn't make with the audibles.

well done.

[karbomusic]

yearh, I dunno where we got that 1uF from (prr) but I was thinking in the interim that going from 2u2 to 2u2-1uF prolly wouldn't make with the audibles.

well done.

1uF was my original series calculation. I'm assuming that when I was swapping on the BB I was using .068. Then later when I calculated I for some reason I had .68 in my head which is what you get with 2.2 + 1.0. All my fault, I knew darn well (in the recesses of my mind) that .68 should be too big. Then again the FF uses 2.2 so all of that combined + my own hurry is the root cause.

It works perfectly now so, this simply burns the lesson in my mind a little better anyway. The idea was a de-wool switch and works perfectly. Always a silver lining.

And thank you guys for the input, much appreciated.