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(Eric Bartlett) wrote in message . com...
I have been thinking about about analog mixers. I understand
(somewhat)the electronics behind good pre's and eq's, but wonder what
makes a good mix buss. A design I saw on a cheaper board is a
resistor on the output of each channel connecting to the mixbuss
amplifier. What in the design (besides quality components) sets a
cheaper board apart from a good console? Is there a diference in the
way the channels are summed? Or is it just a better line amp?

Most of the cheaper stuff uses a plain opamp in an inverting config.
Problem is as you add more inputs you take more loop gain off the
opamp so it gets pretty dirty with over 24 inputs. Your bandwidth
shrinks a lot too.

The higher end consoles use some sort of transistor in the loop of
an opamp design mostly based on Paul Buff's "trans-Amp" (tm)(Hi
Paul!). This reduces loop gain on the opamp to unity, or like 1 input
summed as the transistors do most of the gain, just like your favorite
solid state transformerless mic pre. With this design many more inputs
can be summed cleanly. Some of these consoles use this topology in a
balanced mode, this decreases the crosstalk as capacitive leakage
currents in the buss resistors are common moded out. I've measured
over 90 db separation (stereo crosstalk)in some of these consoles
while most of the cheap stuff measures -60 db.

Jim Williams
Audio Upgrades