[sdiy] Mega Madness One

[Byron G. Jacquot]

>That's my new name for my Tascam MM-1 mixer. Because listening to my 440hz 
>test tone coming out of it for hours on end has driven me mad. The mega part 
>comes from the fact that I cannot trace a single signal ANYWHERE. This is 
>most madening. Each of the 16 channel cards are bussed through a common 
>backplane. The backplane has 16 traces and a big fat ground. Besides power 
>rails and ground I cannot find the merest shimmer of my sine tone on any of 
>these traces even though I hear it coming from the mixers outputs.

My first thought is that they're using inverting mode opamp summers in the master section.  The - terminal on them is a low-impedance virtual earth, sensing current rather than voltage.  Not much to see there.

If you want to trace signals, you'll have to step above or below that buss.  On the channels, check the wiper of the fader, if you can get at it...it's probably followed by an opamp buffer, then a 10K or 22K resistor feeding the buss.  On the master, see if you can trace the backplane connection to the summing amp, then check it's output pin...or, alternatively, the wiper of the master fader.

Byron Jacquot