Circuit Block Explanation

[SprinkleSpraycan]

Hi All, the schem is of the DOD Carcosa Fuzz. Im very curious to understand the function of C7,C8,R18,R19 & R20.

If im guessing, i think the 2 220k are slowing signal, more so vs the single 220k mode on the spdt. c7 and r20 are a high pass filter. c8 is a mystery to me.

Am i even close? Can someone break it down for me?





And...is it similar to what i circled in the Bumble Buzz(Propolis) Schematic?

[PRR]

http://www.johnhearfield.com/RC/RC4.htm

[idy]

The second is a bridged T notch filter.
The first looks like a twin T... but that has an R to ground between two Cs and C to ground between the two Rs.

[Rob Strand]

Look at the panel at the bottom of:
http://www.geofex.com/article_folders/eqs/paramet.htm
Schematic: (doesn't display)
http://www.geofex.com/article_folders/eqs/twintee4.gif


Look at the bottom two circuits in that panel.  There's two forms of the bridge-T circuit
where the caps and resistors are interchanged.

If you look at your first circuit it's sort of like the bottom right circuit on RG page, expect it has been messed with with the addition of the 220k's across the caps.  Your second circuit is like the bottom left circuit on RG's page.

The impedance of the circuit before the bridge-T and the load impedance following the bridge-T circuit will change how the circuit behaves.  In your first circuit the 220nF + 5k tone pot is going to load down the bridge-T part a lot because 5k is so much lower than 220k.  It's so loosely a bridge-T it needs closer investigation. It's almost like a low-pass filter. 


Not much bridge-T'ing notch left:

[Rob Strand]

After knowing the answer    the DOD Carcosa Fuzz is really just a shelving treble booster.   The 10nF caps across the 220k's does the treble boost; 1/(2*pi*10n*220k_) = 72Hz is the starting frequency of the boost.  The addition of the 100k to ground allows for a high slope on the boost.  The tone control is just a low-pass filter idea.  (The 220k switch leaks more bass through.   You can see that in the plots.)

[SprinkleSpraycan]

Thanks for the help and links. I've done my best to understand it. I'll pop it on a bread board and do my bench time with it.