Hammond B2 // “split” preamp with optical Expression control

Got my magnificent Hammond B2 organ 20yrs ago …
it came with an Ampeg V4 head and a modded 122 Leslie cab

… problem for my ears has always been the hum

for a long time I was suspecting the V4 … but then one day I realized it was internally induced in the organ’s wiring from the organ’s very own preamp … the preamp power tranny to be specific

took me a while to figure that out,

anyhow, … prior to all this, maybe 10yrs ago, I came up with a relatively practical analogue optical-potentiometer circuit … where, I could “tune in” the pot value and input CV control range, etc. … but I never found any good reason to use it – until now

indeed, a bone-fide voltage-controlled analogue potentiometer circuit …

a circuit capable of handling modest voltage swings, ie., without anything like the 5v restriction of digital pots, say …

and if I could make one, I could make two … by using 4 well-matched opto-isolators I could replicate a well-matched dual-pot … etc

the benefit of developing my own simple but accurate opto-matching methodology

with this technology at hand I was able to develope a dual-channel unity-gain preamp/buffer system that emulates George Stevenson’s 1931 “ear-compensated volume control” patent using optical means … (see Fig.1 and Fig. 3 in US1788035)

patent US1788035

which is what Hammond/Hanert based their “expression pedal” volume circuit later on

patent US2646468

the idea in part is to go back to the original source patent and emulate one of the working concepts presented there with my opto-pot circuitry … and doing it times-2 in a matched manner, so both “pots” track each other … with one path used for controlling the volume output of the lower register/keys and another for the upper ones … thus, perceptually acting the same as the original “mono” expression circuit found in the B2’s AO-10 Hammond tube preamp // … an important “signature” thing in Hammond organs



the low-Z passive output circuits can be used for anything really …
feeding effects, direct recording … or driving preamps/amplifiers of course

at the moment prelim testing has me running the outputs to a pair of modded Crate V50
guitar amps … with upper register going thru the dirt channel of that one amp and lower reg going thru clean channel of other amp … it’s pretty good sounding already

the goal is to plug an upper keys amp to Leslie via 50w hot tube amp
and lower register through a bass rig (w 18’s)

photos show modified replacement “doghouse” I got via eBay
took the guts out and mounted the CV pot instead of variable capacitors

so now, the original Hammond foot-pedal can be used to control the new preamp
while preserving the “treasured” expression pedal response !

01 B2-pot

02 B2-wiring

03 B2-doghouse

04 B2-expression

05 B2-back

the output of the Trek-II Percussion unit is paired with the upper keys and works as before … I’ve also included a blend-able FX loop into the upper channel as well, then terminated by one side of the optical expression circuit … the lower channel is simply a buffer paired with the other side of the optical expression circuit … all direct-coupled, so no capacitor distortion

the preamp takes the signal directly off both sides of the organ’s Matching transformer
and preserves the large dynamics potentially present there (several volts pk-pk in the low end)
… in the process allowing for very good Signal/Noise performance

yielding pure and virtually hum-less vintage tone-wheel signals,
with good drive (to run long signal cords),
good isolation specs …

and with little to no added noise from the active buffers …
extremely clean

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