Recording - October 2014 - 60

I'm one of those people who's hooked on history. As
a lifelong audio rat, I've developed a keen interest in our
audio heritage, which I've expanded into an ongoing
900-page book project on the roots of audio technology. I also serve as the director of the TECnology Hall of
Fame, which I founded 10 years ago as the historic offshoot of the TEC Awards.
I'm also a flea market junkie. You never know what
you might find in such places, but a year ago, I stumbled
across an item that intrigued me and set me on a path
of audio discovery. Piled in the corner of a stall at a flea
market in Oakland, CA, was a large, odd-looking rackmount chassis. On closer examination, it was a singlechannel 7-band passive equalizer. It had no manufacturer marks, although the engraved front-panel markings
on the aluminum front panel were obviously professionally done. The seller didn't offer any help, but informed
me (in Spanish) that it came out of a storage auction sale
and he wanted $30 for it. I offered $20, which he
accepted. See Picture 1 for my find.
At first I wondered whether this was a prototype from
Pultec, Langevin or possibly Universal Audio. Figuring out
exactly what it was proved to be somewhat of a mystery-
or, as Sherlock Holmes would say, "the game's afoot."

design with no active gain makeup stages; such devices typically
demonstrate insertion losses that can range from about -2 dB to -16
dB. Therefore, most outboard passive filter designs include some kind
of amplification circuit to compensate for any signal attenuation.
The frequency centers of this particular equalizer are identical to the
famed Motown Studio EQ (50/130/320/800/2k/5k/12.5k Hz),
with ± 8 dB boost/cut in 1 dB steps, in/out hardwire bypass switch and
channel I/O on a barrier strip. The Motown Studio EQs are 2-rackspace boxes with an onboard gain makeup stage, and some 40 of
these were fabricated to Motown specs for the label's studios in 1968
by Protofab of nearby Troy, MI. However, the EQ I had seemed different than those-it was a 4-rackspace unit with a global switch that
drops the center points of all the frequency bands by 50%.

Unlocking the mystery
Getting it home, I took a few notes and popped the
covers for an inside look, as shown in Picture 2. It's an
LC equalizer design-meaning its frequency response
changes are made via routing the signal through a network of capacitors and inductors (the latter are sometimes also referred to as chokes or coils). LC circuits are
found in all kinds of audio devices, from speaker
crossovers to vintage studio tools, such as the Pultec
EQP-1 equalizer. This mystery EQ is also a fully passive

1. A $20 find at a local flea market yielded an unexpected score:
a vintage mastering equalizer built by Motown Studios.

RECORDING October 2014

Inside was a lot of intricate and beautifully done point-to-point hand
wiring, from the mil-spec stepped switches and resistor ladders to two
custom Fiberglas circuit boards with toroidal inductors mounted on the
underside, along with Electrocube capacitors and carbon composition
resistors. It clearly wasn't homemade, although it could have been a
custom one-off built for a studio or mastering facility.
Still intrigued, I posted images of the unit on Facebook and thanks to
some help from producer Ron Saint Germain and studio guru/former

Recording - October 2014

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