dewster - You're an Engineer with an Open Mind!

As someone else said it was Luck that you showed up at this board, I know through inspiration you will not be here long. There is no way to communicate with you off the board as your email at TW is private.

If I could "hire" you to look at this pcb layout and tell me what you don't like about the RF oscillator section on the left side, I have a clever way to pay you.

The detector is diode D2. I do have a higher voltage than in general across the emitter resistors. Everything works but I need an engineering evaluation, I have great even harmonics which I find to be more a function of the coils and their interaction. I may not listen to a thing you say but would respect it completely. (-'

I have private webpages this came from. I think I am ready to home etch this board again. Once produced commercially it will be given away, not because I have lots of $$ because I live on SS. I would order 50 for the price break but need only 10. I am on a journey.

Circuit Board   The graphic is a bit wide to display here.

To the Newbie the pitch section is made of two oscillators which both must be identical in design to minimize drift.

This is a stand alone volume control but can also be a simple pitch theremin. If your left handed with a right handed theremin that can't be turned around this mounts on your own stand you can place anywhere and plug it into your theremin audio out. Also the direction of the volume hand motion can be switched with a switch.

Isn't this fun stuff?

Christopher

Hi Christopher,

Is there a schematic available?  I only took a peek at it today, but will give it a good rogering tomorrow.  Not promising anything, I'm not a Theremin expert, but two heads I guess...

Though I have looked at a bunch of RF oscillator designs lately (before I gave up on the whole notion of analog oscillators in Theremins).  They dead bug everything on single sided thick copper clad mounted in cast metal enclosures with big machine screws.  My Theremin prototyping analog front end breadboard is in an aluminum box with a UHF connector out to the antenna (UHF connectors are a natural for 3/8 tubing, much easier to set up and tear down than the mickey mouse plumbing fixtures on the EWS).  I wonder why Theremins are generally built on single or double sided PWBs with little or no shielding?  Is it an attempt to not capacitively load the sensitive circuits?  Seems like it's just asking for RF interference and thermal drift.

I can’t speak engineer-eeze but RS ghetto talk should suffice. No schematics here, I just sketch, slide rule, go to breadboard and then direct to real life pcb layout. Your world is modern digital which gives you a perspective I shall never have due to the many issues that are part of my living.

dewster said: Theremins are generally built on single or double sided PWBs with little or no shielding?  Is it an attempt to not capacitively load the sensitive circuits?

IMHO The short answer is yes. You will learn interesting things as you go along. If you have looked at a lot of RF oscillators then maybe my threads purpose is another for you to study, as I know it so well. It is just a standard Hartley oscillator. I recently found better results taking my RF mixing signal off the collectors using 1 Meg resistors. Trial and error lead me here and I get rich even harmonics in the audio signal. I wish the two osc sections could break apart so I could reposition thier conversation with one another.

Once you discover perfect pitch field linearity naturally it is easy to find what damages the theremin pitch field.  The pitch oscillator and the antenna are one resonant circuit. Any conductive mass or grounded surfaces suck energy from this field. Set a full soda can on your theremin about 6” from the antenna and experience what I am saying. The 796 kHz IF transformers I use for coils “are not” grounded though they could be by a terminal connection. Grounding the IF Can or bringing the antenna connecting wire through a metal chassis hole is like placing one hand near the bottom of the antenna and playing with the other hand. Grounding the fixed oscillator IF Can gives a waveform improvement but slightly adds to thermal drift. As you discovered with the scope trick it is desirable to have a maximum energy field developed around the antenna. This has nothing to do with the Null point but more to do with how many linear octaves will be available. In a linear pitch field all octaves are linear with a slight tightness at the lowest octave, opposite of the  spread out like many theremins.

My circuit board is multipurpose, visit this webpage which was generated by the same board layout above but wiring the oscillator slightly different which it is setup for. I believe I was phase modulating a sub-carrier to get a perfect triangular wave with no Null point, the signal goes down to 1hz undistorted then rolls over into the other direction.  Other freaky things were also occurring worth future research like perfect pitch field linearity without the spring but off of an 18" alligator clip lead antenna with extraordinary sensitivity (above normal).

I live in a valley where high powered AM transmitters are hard to pick up on an AM radio and no TV reception, so I can’t address interference.

"I recently found better results taking my RF mixing signal off the collectors using 1 Meg resistors."

Breadboarding, I've found high value resistive voltage dividers give droopy response at the pickoff point for fast signals so I've switched to capacitive dividers.

"Grounding the IF Can or bringing the antenna connecting wire through a metal chassis hole is like placing one hand near the bottom of the antenna and playing with the other hand."

FredM has a very practical idea here which I intend to give a shot:

http://www.element14.com/community/docs/DOC-20436/l/longwirepdf

I'm going to try placing the secondary coil of my analog front end inside the pitch "antenna" (which is currently composed of 3/8" PEX toilet supply tubing, wrapped in wire turns or aluminum foil tape, and covered in heatshrink tubing - it looks like a "rubber duckey" antenna).  I could actually put both coils and capacitor in the antenna and have it all outside if I used the normally grounded part of the UHF connector as the quadrature signal pickoff point, but I hope to avoid that scenario as it exposes sensitive points to ESD and such.

I've been simulating the tank coil & cap / linearization coil combo and quite freaky things can happen when the zeros and poles line up just right - with ideal components (apparently no such thing when it comes to coils) you can get thousands of volts at the antenna with only a couple of volts in (hence your Tesla coil reference?).  These are really narrow operating points, changing component values even 1% destroys the mode so you either have to stumble across them accidentally (how I encounted them) or have a cunning plan up-front (I'm evidently not smart enough to outwit a couple of coils and a cap).

Which brings me to another point: as with subatomic physics, almost everything new is discovered accidentally in the lab.  A tube is glowing purple on the wrong end and bam! you've got x-rays.  Some experiment is scattering weirdly and bam! you have a rough idea of the size of the nucleus.  The theory all flows from someone seeing something weird or accidentally doing something wrong.  Many of my more interesting circuits (of which there are not many) are at least partially due to plugging components into the breadboard wrong, or setting the function generator to the wrong waveform or frequency.  Not saying theory is bad, we need it to completely understand the physical world and advance lab findings, but the physical world is in the lab, and there's no arguing with the physical world.  At the large telecom where I used to work there were people who were always drawing schematics or in the lab, and people who were never in the lab.  I fancy myself as something of a straddler between these two types.  There were also entrenched digital and analog camps, and I'm a straddler there too (& master of none!).

Take this all light hearted I am being illustrative.

Aw yes… this antenna area is where I had to separate from the masses. I will mention it again, until you find the theremins natural Perfectly Linear Pitch Field in which I have mentioned two methods, you will be working in the dark. Your experience will be more like using a prism in moonlight to study the color spectrum rather than in the sunlight.  You may well discover something as I am unaware of a lot, but bring a flashlight.

Constantly I mention to my own builders if a webpage, call it EiEiO or what ever, does not provide at the minimum a final sound byte to reinforce the methods being presented Run. In the world of theremin “everyone is an expert”. This is exactly what gets the Glasgow theremin builders in trouble. That may be why I no longer draw schematics cause I hand out the board or the PDF, building is like paint by number. Kids don't want schematics they want toys!

After linearity is an understanding of the voice, some people like that whistle sound full of the reverb & echo, a Band-Aid because that is all they know and paid for it.

Next is good volume shading, some people like it snappy because that is all they have and had to paid for it.

Next is having a technique in the circuitry that enhances more dynamic sound and playing. Most people never get this far because they have purchased an instrument from the limited number of theremin designs available. Oh and they paid for it!

Now to be sacrilegious…When the EW-pro first came out (circa 2005) I received many emails about the poor sound and the volume snappiness from this instrument of worship. Fortunately a good engineer came along a few years later to work out these issues. In the TW forum these issues were rarely mentioned out of respect I guess or is it all some Thereminist had to play and they paid for it.

There is a need to enhance the theremin instrument from within, but you got to ask the question, do I have True Grit?  The theremin takes no prisoners, it is part of the enigma.

"In the world of theremin “everyone is an expert”."

I suppose that's because the field is so incredibly small.  If you know anything at all about them you know more than the vast majority of musicians and EEs.  That, and people (present company excepted) tend to parade around a piece of wood with a nail in it because few can really do anything.

"...that is all they know and paid for it."

Over on Piano World my sound technology reivews (based on a diagnostic MIDI file and spectral analysis of the resulting MP3) were initially sometimes met with harsh "kill the messenger" responses, despite my hedging them with all the disclaimers I could think of.  People that dropped $7k USD on a DP that didn't test as well as (or sound as good as) a $50 PC sample set howled like I was trying to kill their kid.  Strange how entrenched voluntary self identification as a consumer has become, that possesions merely purchased become extensions of one's ego.

The retro movement - "everything designed / built before I was born reached the apex of perfection" (tubes, LPs, electric guitars, etc.) - is a huge impediment to modern development.  It's gotten so Korg puts glowing tubes on a lot of their solid state stuff just to appease this crowd.  Though I will say the human race has completely lost the ability to build a decent calculator.

OK, I traced out the oscillator section.  Looks like a common base Hartley.  I don't have the capacitor values, but C13|C23|R8 seem to form a high pass capacitive divider.  1/10 amplitude resistive mixing to D2.  The oscillators may be coupling here.

20 ohm Pot-5 does what?  The common bias point might be coupling your oscillators - or is that the intended purpose, some kind of counter coupling to cancel out the mixer coupling? 

I'd have to simulate to go further, short of that I'll probably never really know much more as I'm going down a completely different path.

Where do you hook up the antenna?

dewster:

Your headed in another direction...

Pot-5 could be a 15 ohm resistor. I don't know why it does it but it takes the sharp edge off of audio waveform for a nicer sound. A slight bit of isolation to the RF board? There is a sweet spot between 0 and 20 ohm.

I do wish my coils were further apart for more pitch control but the real purpose of the board is stand alone volume control and that works excellent. This project has a special purpose.

Antenna connects in upper left corner where it says Ant. Thank you for looking.

I brought my EtherWave Standard back into the lab today. Did the scope trick and she popped at 280 kHz. (resonance) Then leave L5 alone. Then adjusted the L6 to bring the tone back with Null in its proper spot. Then I made a recording as I always do cause lurkers like to see what I am up to, I hope? I start the EW at about 40 Hz which shakes my desk, a little rough but unless I had a tuba fetish I would not have much interest in a bottom end. Haven't been married in quite a long time.

The adjustment made it more linear, not perfect. I enjoy it more now. With loose coupling in the pitch field the scope amplitude increased at least 30%.

EWS Bass Sample with no Mod. My focus always is on that Clara Rockmore's sound, not her skill which creates her personal voice.

Edit: The oscillator had a tendency to stop at 280 kHz so I raised the freq to 287 kHz and that did the trick. Also I added a .01 uf capacitor inline on the magic wire to the volume loop to stop the issue of touching it. The peak amplitude is not the sweet spot for EW circuit behavior. Looking good now!

Grasshopper know that a theremin of any design has attitude, be strong!

I wish you the best on your journey, it could be challenging.

Christopher

"Antenna connects in upper left corner where it says Ant."

Doh!  I was concentrating on the lower oscillator and missed that.  For some reason I expect the fine tuning to happen at the fixed oscillator, not the variable.

"Edit: The oscillator had a tendency to stop at 280 kHz so I raised the freq to 287 kHz and that did the trick. Also I added a .01 uf capacitor inline on the magic wire to the volume loop to stop the issue of touching it. The peak amplitude is not the sweet spot for EW circuit behavior. Looking good now!"

Interesting, I'll have to play around with my EW a bit more.  Nice sound sample!

"I wish you the best on your journey, it could be challenging."

My hope is that this is just a phase I'm going through.  Theremins are interesting, and I do like to see them played well, but I hate to hear them played poorly and - truth be told - I don't particularly enjoy playing them myself.  I prefer polyphonic percussive accompaniment / lead instruments like guitar or piano.  My ultimate goal is to make something that is easier and funner to play than guitar.  Now that it is child's play to physically separate the controller from the sound generator, ergonomic factors are much easier to accomodate.  Though the trick is to keep the sound and response rich, controllable, and organic.  You'd think there would be lots of development going on but it seems companies are too profit motivated and risk averse (not to mention poorly managed) to do anything new.