abbey road d enfer said:
Bill Kahler said:
It's the 8 channel passive summing mixer listed here as the 8 channel summing buss:
http://www.forsselltech.com/schematics.shtml
Thanks again.
This one is not passive; the first one, listed as " 8 Channel Passive Summing Busses " is passive.
The "8 channel summing buss" has its own virtual-ground summing amps, so a shunt resistor is not needed (in fact not desirable).
We may be looking at different schematics but both Forssell buses (bus correctly spelled with one 's') use a passive summing topology with the latter example followed by a non-inverting active buffer/make up gain stage. This is arguably not passive, and six of one half dozen of another compared to the virtual earth topology which as you surely know uses an opamp in the inverting mode (thus the name).
It is a common myth that the make up gain of passive + gain stage is somehow less offensive than a well executed virtual earth sum bus with it's very similar noise gain. On paper the noise gain of a virtual earth is N+1 vs. N for passive plus make up. For more than a small handful of inputs that +1 term is insignificant.
A completely passive sum bus should have no sound character of it's own, so will take on the sound character of the following gain stage (if that is not neutral). If you want a sum bus that sounds like xyz mic pre or gain stage, this is how to get that.
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Regarding the OP's question, if feeding into a "Seventh Circle pres", which I ASSume is a mic preamp, it will be expecting a "microphone like" source impedance (150-200 ohms). There are two issues with using a mic preamp for make up gain. They will be optimized for low source impedance and operating at high gain. Using a mic preamp with higher than microphone source impedance and lower gain will result in higher than expected noise floor from the mic preamp.
One solution is to terminate the passive sum which drops the source impedance and level to make a better match to the high gain stage. It should be apparent that you are throwing away signal needlessly only to boost it back up again (much more than N+1). A simple virtual earth will require less make up gain so in principle deliver better performance.
Of course if you desire the specific sound character of the Seventh Circle pre, this is how. I would suggest perhaps experimenting with best termination for S/N.
I can't answer your question about ground wiring, without looking at NYDs schematic, it should offer some specific advice. In general chassis would connect to pin 1, and balanced outputs to 2 and 3, if unbalanced outputs signal ground goes to pin 3 directly or through a build out impedance similar to hot leg source impedance.
JR
