Help me understand this cap in some Fuzzes. Not covered by RGs article

[Chris S]

Hi all this is a schematic for the red witch fuzz that I stumbled across
the gain can be increased by changing the size of the capacitor between the collector and base on the first transistor. A similar thing is happening on this on knob fuzz
This capacitor isn't in the traditional fuzz face schematic. So my question is what exactly is it doing and what would happen if i put a small capacitor between the base and collector of the first transistor on the run off groove multi-face?

Thanks!

[antonis]

The capacitor between Collector and Base acts as a frequency variable negative feedback resistor..(sic..!!) 

It's capacitive resistance varies conversely with signal's frequency so you can select how much gain (negative feedback amount) you want in relation with respective frequency...

[GibsonGM]

Antonis beat me, lol 

[alexanderbrevig]

As I am a beginner I say this with expectations to be corrected but;

The capacitor between Collector and Base acts as a frequency variable negative feedback resistor..(sic..!!) 

It's capacitive resistance varies conversely with signal's frequency so you can select how much gain (negative feedback amount) you want in relation with respective frequency...

I think capacitive resistance (sic) is called capacitive reactance. Furthermore it is more correct (at least easier) to think of negative feedback as attenuation, not as gain.
This configuration looks to me more like what I'd think of as feedback, or bootstrapping.
Though, as I've said. This is probably all wrong 

[antonis]

I think capacitive resistance (sic) is called capacitive reactance.

You can also call it Myriam or Angella if you like ..(it's OK with me..)

Furthermore it is more correct (at least easier) to think of negative feedback as attenuation, not as gain.

I think that negative feedback amount reflect exactlly what is happening here, which is the partial subtraction of the incoming signal due to 180 degrees phase shift between Base and Collector...

You even are able to calculate the acual gain by the ratio of capacitor's reactance and Base resistor (as long as this gain is much lower than the "open loop" one and the reactance is much bigger than Collectors resistor..)

[antonis]

 

Antonis beat me, lol 

I wouldn't dare to, Sir ..

P.S.
Your answer was more comprehensible and explanatory than mine...

[alexanderbrevig]

But is it really negative if it's between the collector and base? As I said, I'm new so I'm asking because I really wonder. I'd think this is feedback meant to entice, not to reduce/control.

[anotherjim]

Common emitter circuit the collector signal is anti-phase to the base (it is inverting after all). So collector voltage falls as base current rises and vice-versa. High frequency feedback (it's a small capacitor)  is negative. It's a high cut filter or low pass if you prefer.
2 reasons I can think of.
1, Fuzz sounds better if high frequencies are reduced before, within and/or after clipping stages.
2, Many of these circuits first saw use with Germanium transistors. They have higher junction capacitance than modern silicon types so already had some built in. Si versions sound harsher than the Ge originals, so the b-c cap helps restore some of the Ge sound.

[antonis]

To add a (possible) third reason to Jim's wordings:

Reducing gain by negative feedback allows more "controlable" setting of circuit's overall gain..
(it also adds more harmonic distortion - bad for HiFi but sometimes good for Fuzz...)

[alexanderbrevig]

Oh my god.... I get it! For some reason I forgot that NPN works by making it go ON by going LOW so yeah. Thanks! It all makes sense when you pointed it out

[Chris S]

Thanks everyone!

[GibsonGM]

Oh my god.... I get it! For some reason I forgot that NPN works by making it go ON by going LOW so yeah. Thanks! It all makes sense when you pointed it out

Yes!    Think of that cap as a 'volume control'.  When the high freq's allowed to come back ('feedback'!) thru it hit the base, they are 180 degrees out of phase with the signal that is already there (this happens nearly at the speed of light...).   

This turns DOWN the volume on those freq's, so to speak.     If there were no inversion (think buffer, and then it would be 'bootstrapping'), yes, it would reinforce.  But there IS an inversion, and therefore we have this useful phenomena.

Small cap - treble cut.  Bigger cap - overall gain cut.   

[tubegeek]

Resistors have resistance. Capacitors have reactance.

[PRR]

Does it actually "reduce gain"?

I suspect that for an *ideal* voltage source, the values shown have nearly no effect on voltage gain.

What it DOES do is increase input *capacitance*. (Ask Mr Miller about his Effect.) If fed direct from guitar this can have dramatic effect on guitar top-end response.
_______________________________________

> Resistors have resistance. Capacitors have reactance.

Everything has impedance.

[tubegeek]

Everything has impedance.

Except superconductors!

[LightSoundGeometry]

Resistors have resistance. Capacitors have reactance.

both are measured in Ohms ..I know that one haha ! 

[R.G.]

It's a negative feedback component that decreases gain as frequency goes up. Transistors have collector-base capacitances in the one to few pF range normally. This limits their RF gain, as the capacitance acts like a capacitance to ground from the base equal to the actual capacitance times the voltage gain of the stage. So it sucks out more input signal the higher the input frequency components get.

It's a treble response limiting capacitor. It wipes off harsh high frequencies, and is common on circuits trying to get smooth sounding fuzz out of NPN transistors that have high frequency response that extends from audio into the tera-Hz range.

[Chris S]

Thanks RG!