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Subject: Mixers....again.

From: "Paul Schreiber" <synth1@...
Date: 1999-10-10

After reading over the previous inputs, and researching the potentional
op amps that are both low drift and decent audio quality, here is what I've
decided to build:

The mixer has 6 inputs, 2 outputs. Each input has an attenuator.

One output (called the SUB) uses a special switching jack (it is a
Switchcraft 113). This jack
has an isolated contact that is actuated when the plug is inserted. Unlike
the current 112A jacks,
it is not connected to the signal path. It's just a switch.

This switch controls 7(!) analog switches. The mixer is set up "normally"
with 6 inputs and 1 output.
3 inputs have log pots, and 3 inputs have linear pots. If you are mixing 2
or 3 audio signals, you
use the log input channels. Certainly, you can use 1 or more linear
channels, too (but the controls are
more sensitive to small rotational increments). The linear channels are for
DC control voltages, but again
you can use the log ones, too. I am guessing 90% of the time 2 or 3 signals
is all you are mixing.

There are 2 GAIN pots. In the default 6:1 configuration, only 1 GAIN knob is
active: it controls the master
gain. The other knob is shorted out by an analog switch.

NOW.......if you plug a patchcord into the SUB OUT jack, lots of stuff
happens. The mixer is split into 2 seperate
3:1 sections. The aforementioned dead GAIN pot now is active for the linear
section (it's a linear pot, too).
The former master GAIN pot controls the log section (it's a log pot).

When you go to dual 3:1, the feedback resistor in the summer is switched.
So, there will be a gain change of
nearly 2:1 when you do this. You will have to watch out! This is so the
smaller 3:1 sections can get maximum
dynamic range, as well as when we are in 6:1 mode.

The opamps used are (currently, until I find a better one) Analog Devices
AD822s. These have low drift (about
9 times LOWER than a TL072) and about 4 times LOWER offset
voltage.Generally, the ∗drift∗ is what is important,
and also we need decent audio performance. I'm also going to look at the
OP-285. Also, the same low-drift
precision resistors used in the MOTM-300 VCO front end are used.

I think this design is a good tradeoff with the proposed solutions. It offer
6:1 or dual 3:1, does not require expensive dual pots,
and handles audio and CVs. I will bootstrap the output sections to drive
1500pf loads (about 20 feet of cable).

I'll start breadboarding in about 10 days (about how low AD takes for a
sample request) and hopefully can offer
it at the same time the MOTM-440 filter ships. Since there are 8 jacks and 8
pots, the kit will be around $139.

Paul S.