The idea behind this project is to use a passive summing mixer and two 500-system preamps as makeup gain.

Electronic wise its a very easy task, the one problem is, that the 500 system isnt really designed for beeing used that way.

Most racks got one-in/one-out per slot maximum, so to get a stereo sum out of the module, you would need to build it two slots wide.

But there is a company named Fredenstein, that sells API500 cases, that got two-in/two-out per slot.
They use the aux in/out in the API500 norm for their own models.

Plus, most of their cases got a switchable compressor bus.

So the plan is to use one SUB-D connector, built in the front panel.
The passive summing circuit is basically a resistor network, so i will build it on a small PCB and solder that directly to the SUB-D.


It would be nice to have 12 L-C-R switches on the frontpanel.
Master isn’t needed because of the already huge gaindrop of -18dB.

I ordered two Lindell 6×500, which got no pad nor line in, so i will design my own pad, depending of the final gain loss in the summing part and the possible sweetspots of the preamps.


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