MOTM

This sounds great! I like the idea of changing the gain level when switching 
from dual to mono mode. I'm sure that I would be using dual 3 to 1 mixers the 
majority of the time. It might be a good idea to label the two sides of the 
mixer LINEAR and LOG as a visual reminder of which pots are which.

John B.

In a message dated 10/9/99 9:57:34 PM, synth1@... writes:

>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.