YAEWSBM - Yet Another Etherwave Standard Bass Mod

bisem, I'm glad the modded EWS is working out well for you!  It seems to have found the right home.

Coupling inhibits operation on the other side of null, and can provide an interesting nasal or bowed string sound in the bass.  But, to me anyway, these things don't outweigh the smooth buttery bass response a more decoupled design can provide.  I wonder how often developers come down on the wrong (or maybe just  different) side of the cost / benefit analysis compared to most players?

I vaguely recall someone here saying the EWPro has a coupling control?  Perhaps yours could use some adjustment.

That's interesting....but I think my EW pro sounds fine....as good as the EWS you sold me ;^)

Hi guys

I'm new here but have had an interest in Theramins going back to 1975 When I built a design I found in a Babani electronic projects book - the volume circuit never did work. Anyway I've got an early Etherwave , the one with 4 x coils and have built Dewster's nice little bass mod using his pub design. The bass extension is brilliant but I can't get rid of the "'motorboating' near zero hz. I've even tried balancing using a 100k pot and 2x 47k resistors but this has no effect. Capacitor  C6 is further away than the more recent models so I've had to use a 3" wire to Dewster's pcb.Could this be the reason? Any help would be most great fully received

Marek (uk)

"I can't get rid of the "'motorboating' near zero hz."  - Marekbuk

It could be a mismatch in the transistors, or the input coupling capacitors, or as you say the extra distance to the mod board.

You might try adding a largish capacitor (maybe 100pF or 200pF) in series with one of the inputs to see if it gets better or worse.  Putting a 1M or 470k across one of the base resistors might also make it better or worse.  If that doesn't point you in the right direction, swapping the transistors out one at a time with identically numbered parts might fix things.

My circuit started out life on a breadboard, and after I was happy with it I transferred it to vector board.  You might try the same if simple padding doesn't give you clues.

There is the outside possibility that ESD has damaged C2 (the cap on the EW board that connects directly to the pitch antenna) and changed its value.  These caps might also get damaged through the necessary man handling of the leads when doing the mod.

Im a bit puzzled by this "motorboating" - In fact, I was puzzled by the inclusion of the trimmer in the YAEWSBM.. I have just stuck simple FET buffers in most of my designs (and more complex 4 transistor push-pull output buffers in others) and never had this problem..

Can someone explain the mechanism? As I see it, if the oscillators are isolated, there shouldnt be a path for feedback to cause motorboating.... Is this a PSU issue? Perhaps add a 470n ceramic decoupling capacitor to the buffer supply, and perhaps a small series (100R?) resistor between the supply and the buffer? ... I can see that any supply-side feedback could cause motorboating.. (I always have local decoupling on every circuit I build - I buy great reels of good low ESR/ESL 470n capacitors and sprincle them everywhere I can fit them.. They are the best value cost/capacitance ratio C's you can buy while still retaining good HF properties ;-)

Fred.

"Im a bit puzzled by this "motorboating" - In fact, I was puzzled by the inclusion of the trimmer in the YAEWSBM."  - FredM

When I breadboarded the YAEWSBM I noiticed that very near the null point, when the output was subsonic, it sounded like "ba-BUP ba-BUP" (a non-symmetric waveform like a pulse).  Putting the pot in there and tweaking it made the output "bup-bup-bup-bup" (a symmetric waveform like a square).  A small thing but I was happy that I was able to fix it (on my EWS anyway).

Hi Dewster,

Its real interesting - and im glad you were able to fix it! ;-)

All sorts of horrors can occur close to the null point if oscillators are right at the edge of synchronisation - it was the "ba-BUP ba-BUP" modulation occurring down at the LF audio end that first caused me to propose oscillator buffering..

Perhaps whats happening is that, with buffers fitted, sync still occurs - but only really close to null (say 2Hz as opposed to 'round say 20Hz)...

Im just not sure why the pot (or matched transistors) fixes the problem (or reduces it).. I am thinking on the lines of current 'spikes' from the buffer coupling back via the power lines / radiation from the wires to the oscillator/s - but this is a guess.

Fred.

"I am thinking on the lines of current 'spikes' from the buffer coupling back via the power lines / radiation from the wires to the oscillator/s - but this is a guess."  - FredM

Totally possible.  I'm kind of slack when it comes to adding components, but it seems there is no such thing as too much decoupling, particularly with Theremin stuff.  Also, I was designing for a sample of one which can be problematic when generalized with a model that has changed over time.

I was real lucky - I was tutored as a young engineer by Jeremy Lord (Skywave Synthesisers) who was a brilliant analogue hands-on engineer.. I was mostly doing digital / mixed signal stuff working with video signals, all morphed together by analogue circuits -

When I got stuck, he would come over with a handfull of "point ones" (100nF ceramics) pick up the prototype board and schematic, look at the monitor, and patiently explain what was on the monitor in terms of the circuit operation, show me on the schematic where fitting a 'point one' might help, tack the capacitor on, and the predicted improvement just happened ! In 20 minutes he solved the issues that had caused me great worry for the whole day!

I probably fit about 2x as many decoupling capacitors as are really needed (and 4 times as many as most engineers do ;-) - they are there on my schematics so I dont forget to fit them in the layouts - And I have been interrogated by cost-cutting managers over my "wasteful" use of capacitors and board real-estate ever since I left Jeremy Lord Synthesisers / Lord Medical (about 1980)..

LOL ;-)

Fred.

Good news! I followed Fred's advice and decoupled the  supply rails with 100 ohm and 470nf caps and whilst the pcb was on the 'bench' I replaced the long wire from C6 with a screened cable. Voila! no motorboating! I'm not sure which cured the noise but remembering back to my early travails with noisy guitar effects - de-coupling is always a good idea. Thanks Guys - now I'm going back to tune L5 and L6