OTA question

Don Tillman don at till.com
Thu Oct 12 19:14:50 CET 1995

   Date: Thu, 12 Oct 1995 13:35:00 -0700 (PDT)
   From: JH-3-Man

   > Nasty feedback? Interesting. Can you elaborate a little?

   Don has posted about this before, and he has some good points
   about it, IMO. Pumping signals thru dozens of nonlinear transistors,
   all in one heavy feedback loop, has its disadvantages. (Especially
   when the forward path is rather slow, due to the artificially reduced
   bandwidth, which is necessary for stable unity gain applications.)


Every circuit topology one can come up with has a different set of
characteristic distortions, and it's a matter of personal aesthetics
which annoy you or please you.  Some distortions can be truly
unbearable in the smallest quantities, and some others can be a lot of
fun in comparatively larger quantities.

It turns out that many of the pieces of electronic equipment that
musicians drool over have only a small number of simple stages with
little or no feedback.  For example, many guitar amps (like the
classic Vox and Fender models) have the signal going through roughly
four simple tube stages.  Same with tube hifi amps.  The Moog Modular
has very few opamps in the signal chain.  The ARP Odyssey has none
(except possibly the filter in the sealed box).  Hammond organ,
Mellotron, Clavinet, Wurlitzer piano... I'm talking the really warm
juicey stuff here.

On the other end of the scale, take something like those old Peavey
mixers filled with 741s; they sound like crap.  Or those Japanese
hifi receivers from the 70's during the THD specmanship pissing

Or the Fender Utra Chorus guitar amp; this thing sounds horrible
(personal opinion, of course) and look at the schematic and you'll see
the signal going through 7 opamps, each with about 7 transistors in
the signal path, plus a few discretes, so the signal goes through
something like 54 transistors between the input and the output.
Compare that to the Tweed Fender Bassman where the signal goes through
four tube stages in series and you'll see what I mean.

  -- Don

More information about the Synth-diy mailing list