Rob Schwimmer's Hobbs Theremin

Posted: 1/1/2024 2:58:04 PM

From: Northern NJ, USA

Joined: 2/17/2012

Rob's Hobbs

A few months ago Rob got in touch with me, wondering if I might take a look at his Hobbs Theremins.  Rob actually owns two Hobbs, one in a natural wood finish and one in a piano black finish.  I said of course, and Rob sent me additional information on them to help diagnose the issue, which was intermittent operation triggerable by tapping on the enclosure of the black unit.  I took some pix of both, though much more of the natural unit, and thought it would be nice to share them with you, which Rob and Charlie have graciously agreed to.  The album is located here:

Above: the musician's view of the control panel.  Left to right: VOLUME field knob, mute switch, PITCH field knob, LED indicator, DEPTH knob, LOW knob (bass), HI knob (treble), COUPLING knob (oscillator coupling).  Gobs of Rob's Hobbs' knobs!

Above: The audience view.  The coils visible through windows are actually hand wound transformers for the variable pitch, fixed pitch, and volume oscillators.

Above: The bottom of the unit, with female microphone flange and I/O panel.  On the panel is barrel connector power in, 1/4" line out, 1/8" pitch preview out with volume knob and mode switch.  I replaced the microphone flange with a new Atlas unit because this one was stripped out (aluminum doesn't make for the most durable of threads).

Above: Unit with the top removed looking at the back of the control panel.  You can see that the COUPLING control is a variable capacitor.

Above: To the left you can see the 3x RF chokes for the pitch antenna series inductance.  The PCBs are hand wired on phenolic vector board.  I believe the three trim pots to the right are to set the RGB LED internal illumination.

Above: To the right you can see the 4x RF chokes for the volume antenna series inductance.

Above: An LED strip gives the interior a nice warm glow.

With this presentation, Charlie was concerned that both of the units he made for Rob are rather unusual.  Indeed, the wooden one is the first one he made, and the black one was made as a backup with an enclosure built for a previously prototype.  He said that neither is a really great representation of what a typical instrument from him looks like.  To me, the first unit is quite historic and very much worth a particularly close look.  The black unit has a cleaner and more conventional interior, with hand etched PCBs.

The fix for the intermittent operation of the black unit was mechanical in origin (>90% of electrical problems are mechanical).  Apparently, one of the hand wound transformers had flexed the PCB board mount, and over time this worked one of the connections loose - bit of solder fixed it.

A big thank you to Rob & Charlie for giving us the opportunity to see this beautiful Theremin in the buff!

Posted: 1/1/2024 3:46:39 PM

From: Russia

Joined: 9/8/2016

Thanks for the photo review..

And thanks to everyone who makes such photo reviews of different models of theremins.

Posted: 1/1/2024 5:33:33 PM

From: Northern NJ, USA

Joined: 2/17/2012

Rob's Hobbs - The Sampling & Presetting

With Rob's and Charlie's approval I took some audio samples of the natural unit while I had it in my possession.  I like to enlist the help of the owners here as to what timbre(s) they prefer to play with, as it considerably narrows the possible choices of what data to gather.  Rob likes the tone controls at nominal, and the DEPTH knob at both nominal (12:00 noon) and max, so I recorded these two settings:

You can clearly hear the effect of the DEPTH knob here.  I analyzed these samples and came up with D-Lev presets for them.  Here are recordings of single notes, the Hobbs first, D-Lev synth second, and so on:

Above: The hobbs_max.dlp D-Lev preset.  It consists of all harmonics set to max, with a lot of treble roll-off.  The oscillator filter is low-pass, with little resonance and less than full pitch hand tracking.

Above: The hobbs_mid.dlp D-Lev preset.  It consists of all harmonics set quite a bit lower in intensity, with more treble roll-off.  The oscillator filter is again low-pass, but with full pitch hand tracking.  Note that the noise filter is set to low-pass with a fixed cutoff of 761Hz - the noise source isn't actually employed here, a software change (motivated by this preset actually!) routes the oscillator through the noise filter when the noise level is set to zero.

Charlie was similarly concerned that about the presentation of these samples as the "hobbs theremin sound".  He said that he's made some changes in recent versions that give an even wider palette of tone adjustments.

Thanks again to Charlie & Rob!

Posted: 1/5/2024 11:01:00 PM

From: Portland, Oregon

Joined: 2/22/2018

This is an annotated diagram of the internals of a more recent Hobbs instrument, mine looks like this:

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