Good slew rates for OD/dist/fuzz?

[Chill]

I've read some things on the bbs and in some of RG's articles suggesting that lower slew rates sound better when the semiconductor itself is genertaing clipping.  I recently built a 3Q fuzz, three cascaded amp stages; it's pretty clippy but very harsh, which I think is partly due to using NPN switching (i.e. fast slew rate) Q's.  

What slew rates are good for clipping for opamps?  -for BJT's?  -for JFET's?

[Mark Hammer]

Slew rate refers to how quickly a device can respond to an instantaneous demand.  This is both a function of the frequency demanded and the amount of gain applied.  So, assuming a gain of 1 (unity) there is essentially no difference whatsoever between devices for anything in the audio range, no matter what their slew rate happens to be.  Just about any device can replicate the input signal with a fairly wide bandwidth.  As the amount of gain applied is increased, the device starts to be a little more hampered in swinging that wide/far at the highest frequencies.  

Note that gain and level are different from each other.  Reproducing a very hot signal at unity gain is a trivial matter and slew rate matters little in such instances.

Of course gain is also one of those multiplicative things, and can be split up several ways.  So, if you have two cascaded gain stages, each set for a gain of 12, then your overall gain at the output is 144.  Though the overall gain is pretty hot, the amount of amplification requested from each stage is quite modest, so the slewing requirements imposed on stage are very modest.  Again, for musical purposes, assuming a bandwidth of maybe 10khz, there is really no functional difference across most commonly used devices, despite substantial differences in their slew rate.

Now, if it were the case that you were asking a single op-amp to provide a gain of 200 or more across the full audio spectrum (e.g., for use as a condensor mic or turntable preamp), with maybe another 10khz for extra measure (so, 20hz-30khz), then slew rate would start to become an issue, and devices that slew at least a couple of volts per uSec would probably be recommended.

If you look at the sorts of chips normally used in everyone's favourite stompboxes, though, the overwhelming majority do not exhibit slew rates of more than 0.5-1V/usec.  Those that do, are generally selected for their input impedance characteristics or something else, and are not selected for their slew rate which is only coincidentally linked with these other attributes  A bit like suggesting that people select girlfriends for their shoe size - in some cases, I suppose yes, but in most cases it is merely a coincidence.

In still other cases, avoiding a high slew rate can be a good strategy to follow.  The MXR Distortion+ and the Proco Rat use a single op-amp to provide large amounts of gain.  Since the upper harmonics lend an unpleasant fizziness to the tone, having a reduced capacity to provide big swings at higher frequencies works in one's favour.  In the case of the Dist+ the capacity is reduced by use of a meagre 741 op-amp.  In the case of the Rat, the reduced capacity to yield high gain of high frequencies is produced by means of the compensation cap between pins 1 and 8.

Bottom line: for most of what you'll build, it's a non-issue.

[Chill]

Interesting, and thanks!