Mixer circuits

Tony Allgood oakley at enterprise.net
Sun Jan 18 18:44:45 CET 1998

Mark wrote:  <Almost all commercial mixing boards use
op-amps.  I've held different Neve channels in my hands and looked at
-- all IC's, every one.  You don't even need discrete components to
quality mic pre's -- most megabuck consoles use chips from Analog
Devices (OP27??).>

The TL072 reigns supreme in the mixer world. Behringer have a dual low
noise op-amp which is rather nice. The 5532 is very good, and unity
stable, unlike the 5534.

Most mic amps use a long tailed pair at the very front end. LM394 in
the Neve desks, and the outputs of the pair feed into the input offset
pins of an 5534, rather then use the op-amps own input pair. The
current is quite large down the pair, 5mA, to keep the noise levels
down. Soundcraft/Soundtracs use a similar thing, although they have an
op-amp in the feedback loop to give a larger gain control.

Mind you, if your at line levels, who needs mic amps. A 5532 is as good
as you want, even for a summing stage. Note: you may need to go for a
balanced summing junction if you want to keep as much noise out of the
summing amp.

Footnote: never use an OP27 for voltage following, it honks at MHz if
the input pd gets too near the -ve supply rail.

Tony Allgood

Cumbria, UK

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