Earth/Human Ground Problem?

"- in fact, I would also remove power from the reference oscillator, and just look directly at the VFO frequency while debugging the problem (taking care not to load the VFO when doing so)."  - FredM

I think I'd put metal shield around the Theremax, something about the size of a trash can.  Unplug it and let it acclimate to the climate for a few days, say outside on the curb.  I have a feeling that would solve ALL of your problems. ;-)

" something about the size of a trash can " - Dewster

Awww .. Thats a bit harsh! - Its a really lovely looking theremin!

;-)

Fred.

Ooh sorry gnsmith116, no trying to harsh on your most excellent woodwork.  It's just that these experiments leave me with a slightly queasy feeling towards the Theremax and the soundness of its design.

" leave me with a slightly queasy feeling towards the Theremax and the soundness of its design " - Dewster

Yeah -

I share that "queasy feeling" - there are some truly horrible aspects to the design on the audio side, and audio is supposed to be the companies strong point.. Never really bothered to look at the RF side - what I saw on the audio was enough for me! The things they do... "queasy" aint a strong enough word .. nausea is closer -

But the woodwork and layout is beautiful IMO.. And some people love the Theremax, and I have never played one - so who am I to pass judgment based on schematics ?!

Fred.

To all in the Theremin community!!!! Am I the only one that has had any success at building a Theremax? I will have to admit I haven't used any of the "CV" stuff, I don't know if that part works or not. The Theremin itself works just fine. The linearity isn't great, but it is playable. ---- Greg, I wish I could be of some help to you, but I just don't have any knowledge that would be of any help. I did notice from the start that If I would touch the "faceplate" or the metal on the amp that the "pitch" would change. But not meaning to sound like a smart a$$, I just avoid any metal while I play it. By the way, my Wavefront and EWS do the same thing. I just thought it was part of Theremins in general. I wouldn't have a clue to how to fix it. Now my B-3 Deluxe does not seem to have that problem. The "Special" Theremin I am building right now has been "Grounded" from the get-go and doesn't have "any" of those problems. And yes it IS Linear. There again it is the "design" of the Theremin that's the difference, IMHO, and I have Christopher to thank for that. It is His design that is superior, all I did is "put it together". Even my design of the “enclosures” was per-approved by him, before I built them. Greg, if "swapping" boards with you would be of any help, send me a private E-mail to work out the details. You should have my address already. Don’t get discouraged, as I want to see you succeed at your build.

IHS ---- Dana

"I did notice from the start that If I would touch the "faceplate" or the metal on the amp that the "pitch" would change. But not meaning to sound like a smart a$$, I just avoid any metal while I play it. By the way, my Wavefront and EWS do the same thing. I just thought it was part of Theremins in general." - Dana

It is "part of Theremins in general" - Touching a grounded object when in the sensing field of the pitch antenna will change the pitch - this is natural, unavoidable, and due to the way theremins operate.. this will occur and the degree to which it occurs will depend on the players coupling to ground before they touch something grounded.. If the player is standing barefoot on an earthed metal plate, touching a grounded object probably wont alter the pitch!

(unless the "earthed" object they touch isnt actually earthed, but live - in which case anything could happen, and theremin pitch might be the least of the players worries - I say this just as a cautionary note - its not a good idea to connect ground to your feet - even quite a small leakage current from hand to foot can kill you)

What isnt "normal" is for VFO frequency to increase (as in, "difference" ie pitch to decrease) when the player touches something grounded - this is quite bizarre behaviour, as increasing ground coupling to the player should increase the capacitance "seen" by the "antenna" and therebye decrease the VFO frequency.

Also, the distance at which this effect is active is absurd, and leaves me thinking that something REALLY strange is happening with thisv particular theremin.

Fred.

"I think I'd put metal shield around the Theremax, something about the size of a trash can.  Unplug it and let it acclimate to the climate for a few days, say outside on the curb.  I have a feeling that would solve ALL of your problems. ;-)" - dewster

No offense taken, I've considered it from time to time but alas we have plastic curb trash containers...

"But the woodwork and layout is beautiful IMO.. And some people love the Theremax, and I have never played one - so who am I to pass judgment based on schematics ?!" - FredM

Thanks for the kind words on the woodworking.  Dana sent the details on his jig to bend the volume antenna and I made a near duplicate, so thanks to him for that part.

As far as the audio design is concerned, I have made and and am simulating some additional mods to the comparitor component of the volume and pitch CV outputs.  That, along with a better voltage regulator, have largely cleared up the noise issues.  Of course, of course those changes require other mods to get the intended volume response back.  One advantage of the mod I made is the selection of staccato or legato for the volume response.

I've been on travel for work the past few days so have not been able to take another set of linearity measurements.  On the theremax, as you probably know, the pitch adjust is on the oscillator connected to the antenna.  What I was planning to do for a comparison between a tube and a spring was to leave the pitch reference oscillator alone at 899.2 KHz (set to 899.0) but seems to move 200Hz after a few minutes) and set the pitch tune so that both antennas, even though they have different capacitances,  can be nulled by just adjusting the pitch tune.  That may not be a true apples to apples comparison, but I don't have any aluminum tape (is that the shiny metal duct tape?) and I think the results will be very different from what I have seen so far. 

Dana, thanks for the offer to "swap" circuit boards but mine is pretty seriously modified and at this point it is valuable to me as a learning platform and I am no longer concerned to play with it.  I would like to get the "Theremax" to the extent that it is still a Theremax, working as well as possible.  I do think that the dual oscillator volume section might ultimately prove to be very beneficial.  I also like the IF transformer coupling for the pitch outputs because by adjusting the DC offset of the AC ground side of the secondary you can change the waveshape.

Thanks to all for their suggestions and support.  I'll keep you all posted.

Greg