Synth patents, and the SSM 2040

[Sean Costello]

Hi,

I was wondering if anyone out there could point me towards some useful
patent numbers for synthesizer circuits.  I am specifically looking for
numbers pertaining to the Moog ladder filter, but any numbers would be
useful.  I have been unable to find any useful Moog patents (excepting one
patent that I think was referring to the "silver wedge: model Maestro phase
shifter), as the patent databases I have access to only date back to 1975.

BTW, I found the patent that the SSM 2040 filter supposedly falls under:
U.S Patent number 3,969,682.  The inventor was David Rossum, and the
assignee was Oberheim Electronics, Inc.  This patent, "Circuit for Dynamic
Control of Phase Shift," covers the technique of using cascaded OTA/buffer
stages to create an all-pass filter, as well as modifications to the circuit
to obtain highpass and lowpass filters.  The circuit shown is pretty basic,
similar to that shown in Klein's book.  The "exemplary" OTA and buffer
stages shown are very simple:  The OTA is a matched NPN differential pair,
with two PNP transistors forming a current mirror, while the buffer uses a
FET and a PNP.  I have no idea if these are the actual circuits that are
used within the 2040, or just example circuits for the sake of explanation;
maybe someone who knows more about electronics could take a look at the
patent and figure something out.  If anyone wants more info, I can send them
the illustrations from the document via e-mail (I can scan the text too, but
that will take forever to OCR, due to the poor quality of the microfilm used
to make my copy).

Thanks,

Sean Costello

P.S.  The patent was granted on July 13, 1976 - 20 years ago next Saturday.
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