Trouble with EM Theremin volume oscillator

Posted: 1/27/2015 1:15:05 AM

Joined: 1/27/2015

Hello everyone!

Before diving into my question, perhaps I should provide some back story.

I am just finishing a college degree in avionics at a specialized school nearby and love tinkering with electronics. I have a small home lab complete with oscilloscope, power supply, frequency generator and anything else I could need (although I seem to keep on buying stuff). I even built my own CNC to route double-sided PCBs and a 3d printer to, well, do "stuff" with.

So last christmas my mom asked for a theremin. "WTF is a theremin?" I asked myself. How I would describe it: A box used to make star wars sound effects.

Now for the questions!

I designed, routed, soldered and got (mostly) working a PCB based on the EM Theremin PDF. Not the "hot rodding" one. Some mistakes were made, some traces missing... Nothing a quick jumper cant fix. All in all, it seems to be quite solid. I got the variable pitch oscillator AND the fixed pitch oscillator working and I can get some (faint) sound on a low power speaker from the detector. HOORAY!! My (first) problem is that the volume oscillator just. won't. work. period. I'll provide a schematic with my measurements down lower. I've observed all the correct measurements around the associated transistors and verified that the resistors are all correct. I've even changed out a few caps just to make sure I hadn't killed them when soldering.

I should also mention that I was unable to find the coil specified in the BOM, but I did order one from coilcraft. The SLOT TEN-5-11 (link) style 5. Also, The shield can is NOT touching a ground plane. I tried grounding it but it made no difference... I didn't expect it to either...

So what I see from the output of the volume oscillator is basically coupling from the variable pitch oscillator. But the noise is in the order of 10mV, nowhere near the 10V i'm SUPPOSED to be getting. I confirmed this because the 2 are in phase, perfectly.

If you guys need more info, scope snaps, measurements, pictures of the board... ANYTHING. please let me know. I also have available to me a huge lab with millions of dollars of equipment, nothing is impossible for me (until I finish school hehe) so please don't hold out for simplicity's sake.




Posted: 1/27/2015 7:47:39 PM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Matthew you are already lifting me out of my slump and thinking again. Did you know you can buy new finished and tuned EtherWave Standard board for $96. This got me to thinking, why copy when I can buy the volume control pcb, which I actually like and do a Hybrid blend of my work with Moog's design.

I am not knowledgeable of Moog's volume approach yet, so can't offer much to help you.

Got to think about this more, Wavefront are you paying attention. (-'


Posted: 1/27/2015 10:25:17 PM

Joined: 1/12/2014

You should use the SLOT TEN-5-10 coils with the right resistor values(these values can be found in other posts on this forum). The position of the components can also slightly influence the behaviour of the oscillators. You should more or less try to place them like they are on the factory made PCBS. I made a working etherwave pcb not so long ago. If you want I can send you my eagle files.

Posted: 1/30/2015 2:48:03 PM

Joined: 1/27/2015

I've found the problem. Suffice it to say it was 10 inches between the theremin and the chair...

Posted: 1/30/2015 4:52:54 PM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Hello Matthew,

Let us see a photo or better yet hear something. Sound is the one thing that tells all and fortunately can be readily improved. I spent the week laying out a 1.8" x 3" board that I call my EWS Break-Out as it allows me to buffer and transfer the raw signal from C2 & C6 for my angelic sound processing board using a short TRS cable. The lack of good brightness of the EWS is something that has always bothered me. Reverb is usually used to fake it.

Hopefully your C2 & C6 have  .1" pin spacing and have a little clearance for a (.3 x .3) terminal.

I should have a brightened EWS audio demo sample to share in two weeks.


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