[sdiy] Simple discrete Unity-Gain Follower ?

[Don Tillman]

   > Date: Sat, 03 May 2003 01:06:48 +0200 (CEST)
   > From: Magnus Danielson <cfmd at swipnet.se>
   > 
   > From: Don Tillman <don at till.com>
   > Date: Fri, 2 May 2003 01:52:12 -0700
   > 
   > > There are a number of serious problems with feedback loops,
   > > even when they're working properly.  The most blatent is that
   > > the output is the amplifier gain times the error.  That's just
   > > philosophically wrong at a deep fundamental level.
   > 
   > Don, you didn't really mean to say that, did you?

Yes, absolutely.

   > Again, it is about applying the technique adequately.

I disagree.  It's about the process that's going on.  The
low-feedback-amplifier process is completely different than the
high-feedback-opamp process.  I wouldn't even call the opamp circuit
an amplifier -- it's really a servo.

Servos work very differently than amplifiers.  

The major source of distortion in a low-feedback amplifier is device
nonlinearity.  The major source of distortion in high-feedback-opamp
units are, well, a whole collection of weird side effects of the
feedback loop dynamically responding to input, load, device
nonlinearites, frequency compensation, and so forth.  It's not a clean
process.

For instance...  Slew limiting and TIM distortion don't exist in low
feedback amps.  And simple nonlinearity doesn't exist (as such) in
servos.

   > > And, of all the truly cherished pieces of audio equipment (hifi amps,
   > > guitar amps, signal processors, etc.) it's really hard to name any
   > > that use large amounts of feedback in the audio chain.  And of all the
   > > truly despised pieces of audio equipment, most use a lot of feedback.
   > > The correlation is pretty significant.
   > 
   > The problem here is that "truly cherished pieces of audio
   > equipment" may be apparent to you, but is surely not apparent to
   > me and many others. The objectivity in the truth of the statement
   > remains to be prooved.

Nahhh.  Some amplifiers rule and some amplifiers suck.  I don't know
of too many cases where one person thinks a given amp rules while
another claims it sucks, but you can throw out those cases and do just
fine.

I mean, no one's claiming that a McIntosh MC75 sucks, right?

   > But don't fall for the normal audiophile ways of prooving it to
   > me, I won't care.  I stopped listen to that in the 80thies.

Don't need proof; the goodness should just be obvious.  Follow the
zen. 

I know it sounds like I'm being silly, but I'm completely serious.
Most proofs involve one or more weird abstractions, a measurement or a
metric that can be used to rank an amplifier.  But that abstraction
requires a set of assumptions, and those assumptions won't be relevant
to life in the real world.  Total harmonic distortion measurements,
for instance.

   > As for ways to design things, it is *really* hard to judge a
   > design. You must quantify aspects, and the human hearing and
   > perception is as such not very easy to grasp. There is many
   > concepts you got to get really right, and I've found that some of
   > the concepts is not even well understood in the audio world at
   > large. What I've learned was that there where really few
   > authorities that really grasped all the peculiarities and could
   > rationally explain them, and I mean *REALLY FEW* (approx to 0).

I agree completely.  I have a lot to say about this, but it will have
to wait until I write up properly.

  -- Don

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