From: till@acid-rain.lucid.com (Don Tillman)
Subject: Re: Onboard preamps????
Reply-To: till@lucid.com
Organization: Lucid, Inc.
Date: 5 Oct 92 10:42:26


   Date: Thu, 1 Oct 1992 19:13:36 GMT
   From: mikem@col.hp.com (Mike McTigue)
   Organization: HP Colorado Springs Division

[...]
   The slew rate for a LT1012 opamp is spec'd at .1V/uS. So calculating
   backward to get what A would have to be to hit the max slew rate

		     A = (.1V/uS) / (2 * pi * 10khz) = 1.59 Volts !!!!!

   I don't believe a pickup puts our anywhere near this kind of voltage
   level. I did some measurements once on my strat and the output was
   more on the order of 10's of mV, maybe 100mV on a hard transient.
   Still plenty of headroom, even if hotter humbuckers are used. 

Actually, during peaks and transients pickups *do* put out signals like
that.  The stock bridge pickup on my Stratocaster puts out a little over
2.0 volts p-p on transients.  Moving the pickup closer to the strings,
employing a hotter pickup or a more aggressive playing style will increase
that.

The slew rate might be less with running off a 9v battery (sub-optimal
power supply), but I haven't looked into it enough to say.

[...]
   Let me know if anyone sees a flaw in my thinking on the opamp circuit,
   but I'm willing to bet that it will not slew rate limit when buffering
   a pickup signal unless it is designed with lots of gain.

This is a classic oversimplified-model-you-learn-in-school vs. real-life
engineering issue: you don't have to actually be at the slew limit for the
effects of slewing to be noticable.  This phenomenon is popularly called
TIM distortion.

The problem with op-amps is that they don't sound good for serious audio.
(If you don't like the sound of most transistor amps, than you really won't
like the sound of op-amps!)  Why?  Here are some potential reasons:
  
  The signal has to go through at least a dozen nonlinear transistors
    within the chip before it gets out.
  Tons of feedback and a lame feedback topology.
  Low gain and high distortion at higher frequencies due to the
    compensation techniques used. 
  Clipping is sudden and recovery ungraceful.
  Slewing/TIM distortion.
  Crossover distortion from Class B output stage.

Audiophile quality preamps, power amps, and CD players go through great
pains to avoid op-amps for these very reasons.  The recording studio I was
at a month ago was chosen for the sound of its mixer, built before op-amps
were available.  On and on.  Remember the claim on rec.audio about tubes
having second order distortion, transistors having third order, and op-amps
having seventh order?  Well it's completely wrong, but there is an
important point being made -- the design topology shows up in the sound,
the dynamics, and the feel.  It's measurable, but not conveniently.

Unlike the tube vs. transistor discussion, the use of op-amps strictly
dictates the circuit topology, so op-amp generalizations are mostly valid.
The magnitude of the effects do vary with circuit details and choice of
device.  

But of course this is all a matter of personal preference, and if you
really like what you got, there's nothing better.  For my ears the
high-feedback op-amp sound drives me up a tree, and I don't mind doing a
little work to get rid of it and get a more "natural" sound.

  -- Don

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