[sdiy] Moog 914 Fixed Filter Bank

David Ingebretsen dingebre at 3dphysics.net
Thu Jun 20 22:19:04 CEST 2013

I have a fascination with this module and already built Yves Usson's version
and have the PCBs for Jurgen Haible's version, but want to do my own layout
with some other features and make it more amenable for installation into a
euro format.

I like the GIC or Gyrator idea for an inductor simulator, along the lines of
what Jurgen did rather than the Deliyannis topology Yves choose because the
small extra cost for the ICs and PCB real estate is not a big deal to me for
this module. I also like the fact that should I ever become so motivated, I
may try my hand and wind some coils and use them instead of the GICs (I
know, the 5 Henry ones will be huge and a pain to wind). Rather than just
blindly use Jurgen's values, I'd like to better understand the Moog circuit
and make my own choices for capacitors.

That given, I was wondering if anyone knew typical values for the capacitors
Moog used in the 914? The schematic says "selected" and I assume this was
done to determine the center frequency as well as the "Q", right? I know
that Yves mentions using a "Q" of 3.7 and a 12 dB slope. Where does the "Q"
of 3.7 come from? (I can't get Yves' contact page to work, or I'd ask him
directly :).

I take it the two stages in the Moog design give the 12 dB slope of the
filter. I further assume each stage would have the same center frequency,
but this isn't clear to me yet as I haven't "reverse engineered" what Yves
or Jurgen did and the theoretical performance based on their choices for
capacitor and/or resistor values.

I already have a good SPICE simulation running of the Moog design including
the original input and output circuits and have the GICs designed. I've been
experimenting with different values, starting with those specified by
Jurgen, but have yet to put pencil to paper for any calculations until I
better understand the design goal for the original in terms of "Q" and any
other parameters.


David M. Ingebresten, M.S., M.E.
Collision Forensics & Engineering, Inc.
2469 E Ft Union Blvd, STE 114
Salt Lake City, UT 84121

Office: 801 733-5458

dingebre at 3dphysics.net
dingebre at CFandE.com

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