moog sonicV diode ladder

[Gene Zumchak]

Terry,

    What an excellent analysis.  For history buffs, I have a few words about
the Sonic V filter.
The Sonic V was designed by a company in Buffalo, NY that called itself
MuSonics.  I don't know if any were ever sold with MuSonics on them.  At the
time, the Mini-Moog was just becoming a product.  The Moog facility in
Trumansburg, NY (finger lakes region near Ithaca and Cornell where Bob Moog got
his Ph.D. in engineering physics, that absolute toughest engineering discipline
at Cornell), was capable of any production.  Modular units were one at a time.

    The  pricipal investor in MuSonics, decided that Moog needed cash to get
the MiniMoog started.  He need a company name with Moog in it.  An agreement
was made.  Although some Sonic V's were sold under Moog, he had nothing much to
do with them.

    The diode ladder was to circumvent Moog's filter patent.  I doubt that it
could have successfully done so in court.  Since the companies merged, the
question was moot.  There was a competitor at the time, who shall remain
nameless, but who was in the habit of potting his stuff.  His original low-pass
filter sounded much like Moog's ladder.  The principal investor in Moog had one
of these potted filter modules X-Rayed, and guess what was inside?

    Unlike the original MiniMoogs, the Sonic V made use of IC op amps, mainly
the 741 and 748.  For the exponential transistors for the oscillators it used
the uA726 which was a heated pair of transistors.  It was available I believe
for a short time as the LM726 but faded into history.  Too bad.  The CA3046's
now had to do that job and there future is questionable.

Gene (I was there) Z




Terry Michaels wrote:

> Hi list members:
>
> The recent discussions on Moog transistor ladder filters, and diode ladder
> filters, brings up a question:  Is there a consensus that these filter
> types sound significantly different?  If so, one reason might be the
> coupling between filter sections.
>
> The 4 stages of a transistor based ladder filter have "reverse isolation" a
> term used in RF circuitry.  Each section is isolated from the others, each
> section adds a single pole, all 4 poles should fall onto the same point
> (same cutoff frequency).  The diode ladder filter has no "reverse
> isolation", each section is coupled to the one next to it. The poles will
> be in different places, because each filter section has a different amount
> of loading.  The loading depends on where it is in the circuit, and if it
> is connected to the input buffer, or output buffer, or another filter
> section.
>
> Another way to look at is: the transistor ladder is equivilant to 4 RC
> lowpass sections in tandem with a buffer between each one, and the diode
> ladder is equivilant to 4 RC lowpass sections connected together in tandem,
>  with no buffers between them.
>
> If the filter topology is extended to the rf range, a multiple section
> filter with high isolation between sections will have a higher Q, and a
> sharper cutoff, than a filter with low isolation between sections.
>
> I do not have a modeling circuit to graphically analyze this.  Has anyone
> on the list run these filters on Spice or anything else?  I would be
> curious to see what the lowpass functions looks like.
>
> Any comments?
>
> Terry Michaels