Seeking help with my mix bus. API vs. PM2000 schematics.

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Well-known member
Jun 8, 2005
Portland USA
I'm curious if it would be possible to replace the summing circuit on my Yamaha PM2000 console with API. (Another story is I've actually already done this with CAPI's kit for the full mix bus replacement, but this is a little different) Let me explain.

Below is the schematic for the  Yamaha PM2000 master bus. After the input channels' 33K summing resistors, they go into the program bus.  On the schematic, there is an inductor at the input and the dc blocking input cap and we're into IC1 and IC2 which are Hitachi HA1457 chips. After that the signal is sent to a master send, and returns to the fader.
Then comes the discrete opamp for the make up gain. In my case I have 990C's in this position.

I've also attached the schematic of the first stage of the (API) capi ACA mix bus. It's later boosted by a 325 style line amp.

What I'm wondering is, can I replace the the Yamaha IC1 and IC2 network right after the inductor, with the first stage of the API summing network, and come back into the Yamaha's fader? So instead of the 325 makeup amp, it would be my Yamaha's DOA (990C's in my case)

From my previous research it looks like the feedback cap and resistor would need to be changed to match my 33K summing resistors, but otherwise?

I would need to flip polarity it looks like at the output transformer at the very end.

The API transformer flips the polarity on the way to the 325 makeup amp (not attached) Do I need this transformer if I can just flip polarity at the output transformer?

Also, I realize the power rails are +/-24V which means I need to lean toward those opamps which can handle, which  is fine. That's what I want anyway. Just wondering about the circuit replacement.

Right after the inductor (PM2K) I'd go into the 220uF cap of the ACA (?) and then send C16's output to the PM2K's 10K fader?


  • API summing amp.png
    API summing amp.png
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