[sdiy] Differences between MG-1 and Rogue filters

[Metrophage]

There was a long while when I had no schematics for the MG-1, so I was
sketching some mods based upon those I had for the Rogue. I noticed
that my MG-1 filter doesn't growl the way a Mini or 55 can. I was
thinking of tweaking the summing amp for the VCOs, etc prior to the
filter... surprise! The MG-1 doesn't use a summing amp there! So when
people ask "what's the difference", there's one for your list.

It occured to me that it would be simple to make a summing amp out of a
TL071, but now looking at the expected signal levels makes me wonder if
I should. On the Rogue schematic, there is indication that after the
summers feedback resistor the signal should be +/-12v. On the MG-1
schematic the "summed" signals are supposed to be between +4mV and
+30mV? So I am nervous about putting a lot more signal through there.

The VCF appears to otherwise be very similar to that in the Rogue.
Other difference I see is that in the MG-1 the resistors in the ladder
are 200 ohms, while the Rogue uses 1k. Any advice? If I put a summing
amp, should I change the ladder resistors to 1k also? Will doing this
make the filter overdrivable? Would there be any benefit to making the
feedback resistor a pot, or would this just let the secret smoke out? I
can't wait for the PCB to dry so I can check this out further!

CJ

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