[sdiy] mixer idea?

[thescum at surfree.com]

>i'm envisioning a rack module with 8 or 16 inputs, 8 or 16 level pots, and 8 
>or 16 switches to send the output of each level pot to either the stereo 
>outputs or a dedicated output wired to the interface.
>
>any reason i shouldn't build this entirely passive? i don't need any gain, 
>and i'm not picky about the mix going to the stereo outputs.

If you can deal with some of the compromises involved, there's no problem with fully passive mixers.  Those compromises include:

-Insertion loss.  The more channels driving the buss, the greater the insertion loss.  Depending on the levels involved, this may or may not be significant.

-If you're switching things on and off the mix buss, the insertion loss changes...the circuit decomposes into a voltage divider, and switching sources off the buss changes the denominator of that divider.  One common trick is to not just switch off the buss, but replace the feed with a resistor to ground when switched out.

-You'll need to balance impedances of the stages...if the buss-feed resistors are small, they'll load the faders and mess up their response.  A 10K level control into 100K feed resistors is a reasonable compromise.  Faders are commonly buffered from the loads they're driving with some sort of follower or amplifier.  If you need it to be stereo with panpots, things get even trickier...

-We'll just asume that you're using resistors of a high enough value that everything attached will be able to drive the load.

-We'll also assume that the nominal operating levels of all the attached devices are roughly equivalent, or adjustable at the source, and don't need trimming on the way in.

You can always build up the passive version to give a whirl.  If it stinks, you can easily retrofit a couple of opamps as a virtual earth summing amplifier, to solve the insertion loss problems.

Byron Jacquot