The Aetherwellen web site [LINK] by the amazing Alexander Klaus is a veritable treasure trove of Theremin mods. I've been intrigued by the "Etherwave Standard Theremin simplest ever ‘Lower-Register’ Modification" (see Chapter 2) and a brave soul recently shipped their EW+ (Moog Music Etherwave Plus) to me to try it out. The results and some discussion are presented here.
The mod is simple, indeed one can hardly imagine anything simpler: just a resistor and capacitor. And the capacitor can be formed from two twisted wires if you are so inclined, making the mod pretty trivial component-wise. Here is the circuit from the Aetherwellen web site:
Above: The mod is in the dotted box.
Basically all it does is couple the two pitch oscillators via the series combination of 150k resistor and small capacitor. At first glance one might think that this could only increase the coupling and further aggravate things on the bass end, but that isn't the case. A simulation on the Aetherwellen site demonstrates that it instead behaves rather like a bridged twin-T notch filter, which introduces sufficient phase shift and damping to extend the easily playable bass range. The end result is similar to what the active buffering ESPE01 / YAEWSBM modules do, but much it is much less invasive and almost free.
When disassembling the EW+ I noticed that the pitch rod plumbing knuckle was rather loose. Closer inspection revealed a cracked side cheek, most likely from a previous fall to the floor (don't look at me). Some Titebond III and overnight clamping fixed that, and coating the knuckle threads with 5 minute epoxy before reinstalling it arrested any free rotation.
Above: Cracked cheek.
I think it's best to use a variable trimmer capacitor instead of twisted wires because it's easier to get the adjustment spot-on. I bought a bag of 100 trimmers from Electronics Goldmine a while back for cheap, and as luck would have it they just happen to be a good fit for this mod. My brand spankin' new LCR meter tells me their range is 0.65pF to 5.8pF (I'm loving this new addition to my Theremin workbench).
Above: Big bag o' trimmers.
The far leg of C2 is actually electrically connected to the near end of L12, which gives us a convenient physical attachment point for the trimmer. And, depending on how solderable the near leg of C6 is, you may not even have to remove the main PCB to do the mod. On this unit C6 was quite flush, so I had to remove the PCB and push the C2 leads up from the back with my soldering iron in order to gain a bit of clearance here. A short section of insulation from some random hookup wire protects the resistor leads from accidentally shorting out to anything.
Above: Two views of the mod.
The brown/red/orange/yellow loose wiring bundle from the 8 pin expansion port on the main PCB to the front panel PCB travels rather close to the mod, and the mod itself is rather high impedance, so I zip tied the bundle together near the mod to help immobilize them (not shown).
If you have a scope you can adjust the trimmer for maximum vertical symmetry of the waveform. For this adjustment set the "WAVEFORM" knob to 12 o'clock and the "BRIGHTNESS" knob fully clockwise. The symmetry varies somewhat with frequency, and we mainly care about bass end smoothness, so I ended up "manning" the "PITCH" knob for zero beat while adjusting the trimmer a tiny bit at a time, looking for the point at which oscillation kicked in at the farthest point. It's easier to do than to explain, and (unlike other EW tuning) I'd feel pretty comfortable doing this adjustment without the aid of a scope.
Above: Scope view of audio playing ~440Hz. Note symmetry of rising and falling edge angles.
At the beginning of the mod article Alexander states: "For those who only like to expand the lower register, preserving the EWS sound and circuit, a very simple solution has been found." Not to disparage Alexander in any way, but this statement led me to believe that the mod would preserve the low end timbre, but that's clearly not the case, even in his video (which to be fair I should have watched more closely - my bad). The thing I don't like about active buffering modules is they kill the low end "raspiness" that can add so much to the low end timbre. Buffering does indeed smooth out the low end playing response, making the far field more linear, but at a price. This simple mod also kills the raspiness, and I suppose that's necessarily a side effect of reduced coupling by whatever means. The mid and high end timbre does indeed seem untouched with this simple mod, and I presume Alexander was referring to this in his statements.
I made an MP3 of pre mod (first three samples) and post mod (last 3 samples): https://d-lev.com/research/aetherwellen_ew_mod_pre_post.mp3
The first two samples I'm holding my closed fist at zero beat, then opening and closing my hand. Before the mod my open hand produced a max of ~120Hz, and after the mod this was reduced to ~95Hz, so the far field is obviously less compressed after the mod, and the transition to/from zero beat doesn't feel so "jumpy". The third sample is going from zero beat to touching the pitch antenna and back to zero beat. Touching pitch antenna before the mod produced 3280Hz, and after the mod this was 3120Hz, so very little change here, and the timbre seems much the same when not near zero beat.
I think this is probably a better solution than buffering modules. For one thing it's so trivial that just about anyone can install and tune it, and if one ends up not liking it it's just as easily defeated or removed without the unit being much the worse for wear. So the risk of damaging the unit doing mod / unmod is significantly reduced. Also, the coupling can be adjusted to some degree, and (who knows?) some players may prefer a level of coupling somewhere between the zero of the buffering modules and the full-on of the stock response.
If anyone is interested in a kit of trimmer cap and resistor, email me. Special low, low one-time introductory price of only $100 USD (I keed!).
