opamp feedback loop frequency

[Vitrolin]

Hi

I need some help to understand the feedback loop frequency of an op amp.
in a tube screamer (in my example the ggg ts-808 clone) the feedback loop of the clipping amp contains:
2 diodes, provides clipping
1 51K resistor, fixes min. gain
1 500K pot, gain control
1 51pf cap, somekind of filtering i guess
and there is at ground connection which goes through:
1 0,47uF, again i guess somekind of filtering
1 4,7K resistor, something to do with the max and/or min gain setting

What i want to know is:
In what way does the caps filter the loop, and how do they interact with the gain resistors?
How do i calculate the loops center frequency, or better yet high- and lowcut frequencies?
...Or am i way of and the frequency is determend by R-C filters before and after the op amp? In which case i would like to know what are the purpose of those caps.

I really hope someone can help me out at little here

[MohiZ]

Here's some reading for you: http://gaussmarkov.net/wordpress/parts/op-amps/op-amps-4-divided-negative-feedback/

Basically, the caps are used to set the passband of the op-amp, so that the low end and the high end are rolled off at some point. The low frequency roll-off is (approximately) 1/(2*pi*0,47uF*4,7K) ~ 72 Hz, and the high frequency roll-off is at least 1/(2*pi*51pf*551K) ~ 5670 Hz, though it increases with the gain. It might not be easy to intuitively understand why the approximations are the way they are, but they sure are easy.

[aziltz]

i did a spice simulation to learn just that.

in a nutshell,
the cap to ground helps to set the lowest frequency that receives gain in the loop.  .47uF

the cap in parallel with the gain pot/diodes softens the gain of the highest frequencies, at high gain settings.  (51pF)

here's the thread.  there's more details here.
http://www.diystompboxes.com/smfforum/index.php?topic=75775.0

[aziltz]

i did a spice simulation to learn just that.

in a nutshell,
the cap to ground helps to set the lowest frequency that receives gain greater than 1, from the op-amp.  .47uF

the cap in parallel with the gain pot/diodes softens the gain of the highest frequencies, at high gain settings.  (51pF)

here's the thread.  there's more details here.
http://www.diystompboxes.com/smfforum/index.php?topic=75775.0

[brett]

It's a simple concept:
R, L and C all offer impedance in the feedback network.
R is independent of frequency, but the other two are frequency-dependent (C offers maximum impedance at 0Hz and L provides maximum impedance at infinite Hz).
In RC and RL circuits, the -3dB point is where C or L offer equal impedance to R.
The formula for the cutoff frequencies for RC circuits is given above.  RL is similar - the impedance of an L is the inverse of a C.

have a great day

[Vitrolin]

hi thanks for your replies they really cleared up somethings for me. exept that last one by brett:
i suppose that R is resistance, C is capacitance, but what is L?
if you could please explain that again
thanx

[Cliff Schecht]

L is inductance. When you start getting into higher frequencies, long traces and part leads start looking more and more like little inductors sprinkled throughout your circuit. Lucky for us audio guys, we don't ever go higher than about 30 kHz (excluding the digital domain, of course), so we don't have to worry very much about stray inductance and capacitance. Unfortunately, when we do have to deal with inductors in the audio frequency range, they tend to be quite large in value (500 uH isn't a small inductor by any stretch of the imagination..)