[sdiy] Request brain dump on balanced lines

[Don Tillman]

   > Date: Wed, 16 Oct 2002 15:19:42 -0500
   > From: Grant Richter <grichter at asapnet.net>
   > 
   > >> And the servo works nicely if it is fast enough, and as long
   > >> as it does not hit its limits of operation.
   > > 
   > > But at the same time the output is the gain times the error.  :-)
   > 
   > You can see, or even listen to the error, just by monitoring the
   > differential between the inverting and non-inverting
   > terminals. (difference = servo failure).

Yes.

But still, for there to be an output there will be an error voltage.  

One can claim that the error is negligible or that the error doesn't
sound bad, fine; we're talking subtleties, certainly.

   > For a modern TL074 style op-amp, the fast slew rate and wide
   > bandwidth allow the servo to keep up just fine.

At 10KHz, the open loop gain of a TL074 is only 300, so that's a 0.03
volt error for a 10.0 V output.  

That error is mostly gain error, but it also reflects the opamp
distortion.  And it has some recognizable characteristics... for one
thing it's frequency dependent, increasing level at 6dB/octave.  But
it also changes character at 6dB/octave, the order of the distortion
goes up as the amount of feedback increases at lower frequencies.  (So
as the level of distortion goes down the annoyance factor increases
because of the higher order distortion products.)

It's a wildly complex mechanism.  And it's a perfect example of why I
think a static measurement of 2nd and 3rd harmonic distortion is
misleading.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com