What's the purpose of the RC after an opamp?

[the_floyd]

... like this:

I've seen it practically everywhere, yet I don't really understand its function. What does it do, and how would changing the values affect things?

Help would be much appreciated.

EDIT: (sorry for the blurry graphic, that should read 1k and 4.7u)

[R.G.]

I drew it, I guess I should handle the questions. 

Those parts have a couple of possible uses. Exactly why the specific values were chosen depends on what (if anything  ) was in the designer's mind at the moment. I didn't do the original design, so I have to guess too.

The cap is there to block DC from the opamp output from affecting the diode clipping and output signal. You want to NOT have a DC level on the output of any effect, and especially for one which uses a pair of diode clippers like this one. Various bad things happen if there's any significant DC offset on the output of an effect. The value of the cap is chosen for what it has to drive. In this case, it's big because it drives a low impedance; the 1K and a diode to ground in both directions. Notice that here is a series capacitor and resistor to ground after the JFET at the output as well. Same reasoning; block DC.

The resistor is there for one and maybe two purposes. In the Rat, it's there to limit the amount of current the opamp has to provide to the diodes. The value of the resistor determines to some extent how hard the diodes are driven, and so to some degree how hard they clip the signal. Other pedals with a similar resistor before diodes have resistors in that position up to 10K. There is a second possible use for that resistor. Some opamps are fussy about non-resistor loads on their outputs. A small series resistor on their output helps with stability against oscillation. It could be doing that here.  It very much depends on the opamp and the rest of the circuit whether that's needed or not. It's not a bad idea in general, as long as what the opamp+resistor drives is high enough in impedance for it not to cause too much signal loss.

[Earthscum]

As well, the cap and resistor should form a high-pass at about 40Hz, cutting some "flub" out of the bottom.

The cap on the input of circuits serve much the same purposes that R.G. explained... that is, they block DC, and the resistor on the input of circuits limits how much current can get into the gate/grid/base of the active component. The input RC's generally have more attention during design to the frequency roll-offs.

[the_floyd]

Ah, got it. Thanks!

[nexekho]

Another question about op-amps: (not worth starting another thread over) do they have a minimum supply voltage?  I can't find anything specific in the 5532 datasheet, a limit of 22/-22V, but no lower limit.  If I dribble 0.9V and -0.9V into the supply and negative supply, will it still work, just with a weaker output?  (and clipping a lot if I don't adjust the supporting circuitry)

[Earthscum]

TI datasheet: http://www.ti.com/lit/gpn/ne5532

2nd page. I suspect you only looked at the max ratings.

VCC+ Supply Voltage Min:5V  Max:15V
VCC- Supply Voltage Min:-5V  Max:-15V

So you're looking at +/-5V min, or 10V rails. The NE5532 is, as I understand it, basically being abused in this way with stompbox use, although I don't know how. I've used them with 5V regulators with (seemingly) no problems, which is half that.

[nexekho]

Ah, thanks, the store I'd be getting it from provides a Fairchild datasheet which as far as I can tell doesn't list that information:
http://pdf1.alldatasheet.com/datasheet-pdf/view/54235/FAIRCHILD/NE5532N.html
Not too great at reading these things, so it might just do.

[beans_amps]

Also note that the minimums are those that the manufacturer lists for guaranteed operation.  The internal transistors may not bias up properly or whatever.  As a fuzz/distortion builder we probably do not care about that sort or thing.

Sean

[Seljer]

Another question about op-amps: (not worth starting another thread over) do they have a minimum supply voltage?  I can't find anything specific in the 5532 datasheet, a limit of 22/-22V, but no lower limit.  If I dribble 0.9V and -0.9V into the supply and negative supply, will it still work, just with a weaker output?  (and clipping a lot if I don't adjust the supporting circuitry)

An issue is how close you it can get the the voltages of the power rails, if you have a regular run of the mill cheapo opamp in a pedal running from 0 and 9 volts, the output can only swing from slighlty under 1 volts to just a bit over 8 volts