Feedback loop caps

[Stevo]

This may have been said but i missed it...What does the 4.7k and 1.uf cap to ground really do on the shaka...I have seen many different caps to ground and resistors to ground  in the feedback loop, does it add gain or does it just like a rc filter adding bass? If it adds gain the higher the more I would guess? The rat uses two? :?

[Joe Hart]

I don't think it "adds bass" as much as it takes out some highs. But I could be wrong.
-Joe Hart

[Stevo]

The bigger cap the better and a 4.7 is the best resistor maybe a 2.2electro would be more lows..... and a .22 would be more highs?

[Stevo]

Must be I built one with a bigger cap and the bass is more pronounced...It is like an RC filter I guess anybody know what else a bigger cap in the feedback loop will do? :?  :?

[R.G.]

Your answers are all below:
==========================
The gain of an opamp is G = -Zf/Zi for inverting, = 1+ Zf/Zi for noninverting.

Zf is the grand total mishmash (GTM) of all the things from the output to the - input, and Zi is the GTM of everything from inverting input to ground, including all frequency effects.

Resistors have a Z that is constant, equal to the resistance.
Capacitors have a Z that is Zc = 1/(2*pi*F * C); that is, it goes down constantly with frequency.

A resistor and cap in series has an impedance of the resistance in series with the capacitance; that is, the total Z is infinite at DC, and decreases until the frequency where R = Zc, after which however much the cap's Zc decreases, it doesn't change things much. This frequency is F = 1/(2* pi* R * C) and is the characteristic rolloff frequency of the series network.

A resistor in parallel with a cap has an impedance of the two in parallel, so at DC it's just the resistance, as the capacitor's Zc is huge and does not affect things. This holds true until the frequency where Zc=R, and from that point on, the resistor fades into insignificance. This frequency is also F = 1/(2*pi*R *C).

[Stevo]

Thanks RG all the techno stuff is cool..I know a little but in laymans terms what does it do and how does a bigger one or smaller one or two effect the tone.. SORRY... :?

[R.G.]

Let's see. A capacitor has an impedance that decreases with increasing frequency. So the higher the frequency, the lower its impedance.  An opamp has a gain that Zf (feedback impedance) divided by the Zi (input impedance). So if you're using a capacitor in the feedback, and its impedance decreases with increasing frequency, then the gain of the opamp will also decrease with increasing frequency, all the other parts being held constant.

Decreasing gain as frequency goes up - cuts treble. Or increases bass, depending on how you interpret that. Same thing.

A bigger cap cuts treble at a lower frequency, so it has the appearance of making bigger treble cuts or more prominent bass.

[WGTP]

Here is the way I remember it.

.1uf and 10K = 160Hz.

If you double the cap value you have to half the resistor value to keep the same frequency.  If you double one you have to half the other.  

From that the frequency starts doubling or halving if you change the cap or resistor.  Or it moves in 10's

.01uf and 10K = 1600hz

.1uf and 1K = 1600hz

.01uf and 1K = 16000hz

Hope that helps.  

[RLBJR65]

Thanks R.G. and WGTP  Great info!