input attenuation

[Don Tillman]

   From: Martin Czech <martin.czech at intermetall.de>
   Date: Thu, 1 Jul 1999 16:01:42 +0200 (MET DST)

   Quite a while ago Juergen Haible proposed that balanced audio wiring of
   large modular systems is not such expensive, because most "real" inputs
   have allready + and - pins.  

   But some means of input attenuation is needed, this would mean a dual,
   good matching pot, thus this proposal was more or less negated.

   Is it really necessary to have a dual pot ?  

Nope, you can use a single pot.  Here's how: input jack positive to
pot maximum, input jack negative to pot minimum and the negative diff
amp input, pot wiper to the positive diff amp input.

It doesn't look symmetrical, but the math works out.

   The only real expense would be to have a balanced output driver, that
   is one op amp more, and stereo jacks...

JH later reported that the problems he was having were due to grounding
topology and that he didn't think balanced patchcords on a modular
were worth the trouble.

My thoughts on the topic:

Balanced signals are necessary when you are connecting a number of
pieces of audio equipment together and some of them run with high
currents and you care about hum and noise.

For a modular, if you are thinking about something that looks like a
large Moog, balanced signals will not offer any improvement.  There
are no high currents (the highest currents are probably in the LEDs
and the reverb tank drive; that's not going to be a problem).

If you are making an effects box that will connect to devices that
have current-hogging digital circuitry or computers or motors
(Mellotrons, Hammonds) balanced signals can be very important.

If you are going to be interfacing your modular to the outside world,
a set of balanced interface modules would be a good thing.

  -- Don