Parts clarification for Etherwave Theremin build

Hello everybody! This is my first post in this forum.

First, let me introduce myself, my name is Konstantin, I am originary from Russia, but residing in Mexico for the last 25 years. I've been an electronics geek since the first memories of my life! Never got a former education in electronics, but have always had a huge interest in the topic, specially related to music and audio as of the influence of my parents who are classical musicians. I have designed and constructed many devices like MIDI controllers and digital audio processors, aswell as other miscelaneous aparatus.

Not long ago I decided to build a theremin and my first option was the old original Moog implementation, EM Theremin. Later on I found out about the newer modification presented in the Understanding, customizing, and hot-rodding your etherwave theremin document, with main modifications being the use of a fixed inductor in series with each of the variable ones, as well as minor capacitor changes in the oscillators.

After browsing and reading some of the very interesting and informative posts about parts selections and other theory present on this forum, some questions arised in my mind. Given the fact that to present day there are not many options to source variable inductors, I landed on the Coilcraft webpage as suggested by some post in the forum, with the inductors of interest being the SLOT TEN-5-10  47uH variable coils.

As noted in some post, 5 resistors in the design require changes, those being R2, R6 and R16, a change to 1.5K from the originally 2.2K, as well as, R15 and R19, from 470R original to 330R.

Also I noted that the antenna linearizing inductors are reduced in quantity, 3 x 10mH for the pitch antenna and 2 x 2.5mH and 1 x 5mH for volume antenna. However, reading the forums later on, I found a post where it was mentioned to use 10mH + 25mH + 5mH for the pitch and 2.5mH + 10mH + 2.5mH for the volume antenna. The linearizing inductors I am talking about here are the ones sourceable from Mouser, Hammond Manufacturing 1535 series. What is the best configuration of linearising coils to use along with the SLOT TEN-5-10 variable ones?

Concerning about the fixed inductors required, 2x 47uH and 1x 22uH, I found suggested in the forums to use EPCOS B82144A2473J and EPCOS B82144A2223K inductors, also sourceable from Mouser.

I drew the schematic in Eagle, and include here a high resolution jpg of it. 

Its worth noticing that I managed to get a working prototype on separate breadboards (called here protoboards, the classic white plastic protoboarding panels), with some miscellaneous variable coils I had gathered from all sorts of devices along the years. After some major tweaking and trying I had both antennas working in a rather unstable and very sensitive way as of all the cabling involved in breadboarding this little monster.

Among the equipment which I own, there is a digital tektronix 60mhz scope, some multimeters and a precision inductance meter, so the build of a prototype was aided with this.

If anyone can give me any advice about the linearising coils configuration, as well as any other aspect of the circuit that would need more attention from my part, I would be greatly thankful!

Coils and sources:

Hi Konstantin,

the optimal configuration for the linearization coils is 3 x 10mH on the pitch side, and 1 x 5mH + 2 x 2.5mH on the volume side. Other variants are outdated or were experimental. The Hammond 1535 series inductors are perfect for that, they give even slightly better results than the Miller 6300 series in the EM/Etherwave circuit.

It is true that using the Coilcraft Slot Ten variable inductors requires slightly different resistor values as I've written multiple times here in the forums.

The small 47uH and 22uH fixed inductors are not critical, but for temperature stability. Some experimenting with different types will lead to big surprises.

Hello! Thank you for your prompt reply! Appreciate the pointers!

Some days ago I got a package of inductors from ebay and was wondering if one of these will work,

Ebay fixed inductors pack

Measured inductance of a 47uH one is around 45uH, and DC resistance is something like 2.5ohms, as opposed to 0.5 ohm on the EPCOS ones. Do you think one of these will work along with the slot ten?

I decided to no longer experiment on the breadboards and make a proper PCB, as the crazy cabling interconnections had too many intermittent aspects, so will try the inductors once the board is done.

Thanks again!

"Some days ago I got a package of inductors from ebay and was wondering if one of these will work..."  -- kostix

You could always try them out and see.  Though the "generic" nature of the parts might make getting identical parts again in the future rather dicey.  

At first I used these types of parts in the linearizing coil position, where they can actually work if you watch the specs like self-capacitance (and maybe Q) and particularly if you string several in series.  But I abandoned them due to rather high temperature dependence which made my oscillator vary too much over time as room changed temp during the day / night.

Stick one in your freezer for a bit with a thermocouple on it, pull it out and record the inductance vs. temperature as it warms up.  Something this small you might have to tape/glue it to a metal bolt or something to get enough thermal mass to slow the temperature change down to give you enough time to get decent readings.  You will be shocked at how temperature dependent things like UHF connectors are (capacitance).  These types of experiments are simple and easy and essential for Theremin work IMO.

Hello!

I appreciate the advice. Makes sense the small dimensions to be prone to temperature changes with consequent frequency shifts. I was wondering if anyone ever tried to make some sort of controlled temperature chamber around the temperature sensitive parts, to first stabilize the temperature to some point above ambient and thus avoiding further de-tunings cause of warm up? I realize this would introduce a certain warm-up time prior to use. I figure some power resistors or maybe a peltier element could be used with a constant current source to heat it up "silently" and thus rising the temperature above certain point. Also a more complex system comes to my mind with the use of digitally controlled potentiometers by an mcu, and a temperature sensor or sensors to automatically compensate for frequency changes. Maybe I am over complicating things in my mind. So many variants are possible, makes me eager hehe.

What would be the best known and tested fixed inductors to use along with the variable ones in the EW? Do the EPCOS ones have acceptable temperature stability figures?

Best regards,

Konstantin

Konstantin,

At one point in time I was also very interested in the EM design with the hot-rodded modifications.  I went so far as to put together a unified schematic, PCB layout and part list.  I even acquired the hard to get TOKO coils.  Alas, I never actually built one of these but others have actually used my article to build one and were very successful (see Roland's comments).

I still have the TOKO coils (actually bought 4 just in case).  I would love to donate them to you or anyone else for that matter that wants to build one of these based on the information I provided. I'm pretty sure I'll never get around to it now and hate to see them end up in the trash at some point in the future.

If you are interested in this build and would like the coils let me know and I'll give you first dibs.  Anyone else interested in the same offer let me know as well.  (Offer not valid for coil collectors :)

"Makes sense the small dimensions to be prone to temperature changes with consequent frequency shifts."  -- kostix

Yes, the general need of the electronics industry is for parts to be as small as possible, so chokes like these tend to utilize ferrite with high field concentration, which brings with it high TCO.  The needs of Theremin builders are the opposite, where large-ish coils are OK and actually help reduce interaction and such.  Outside of the odd stone age RF choke, and IF transformers (which are themselves quickly becoming stone age) the inductor industry has very little to offer Theremin builders.  It takes a fair amount of research and asking around before this is painfully obvious.

"I was wondering if anyone ever tried to make some sort of controlled temperature chamber around the temperature sensitive parts, to first stabilize the temperature to some point above ambient and thus avoiding further de-tunings cause of warm up? I realize this would introduce a certain warm-up time prior to use. I figure some power resistors or maybe a peltier element could be used with a constant current source to heat it up "silently" and thus rising the temperature above certain point. Also a more complex system comes to my mind with the use of digitally controlled potentiometers by an mcu, and a temperature sensor or sensors to automatically compensate for frequency changes. Maybe I am over complicating things in my mind. So many variants are possible, makes me eager hehe."

FredM was thinking along these lines at one point.  Like oveninzed crystals you probably want to insulate and raise the temp to some point above possible ambient.

But honestly, why not start with low TCO components in the first place?  Most Theremin builders probably don't address this sufficiently, but this is where you can nip most temp issues in the bud.  NP0 caps, air core coils, and watch all temperature dependent delays through the active devices as well.  A well designed and built Theremin should snap to attention a power-up and stay there.  A poor one will drift around and require constant readjustment.

"What would be the best known and tested fixed inductors to use along with the variable ones in the EW? Do the EPCOS ones have acceptable temperature stability figures?"

Hand wound air core coils are an option.  If constructed correctly they are remarkably stable over temp and have really high Q.  It might seem like a pain in the ass (actually not all that bad in small quantities with a home brew winder) but looking for coils that probably don't exist is an even more monumental, never ending pain in the ass, snipe hunt.  The inductor industry just isn't geared at present to supply Theremin builders in any meaningful way.  If you can avoid ferrite I'd say go for it.

dewster and FredM are brilliant engineers and they think like engineers. But let's not forget that a theremin is a music instrument which is to be played by a musician.

A musician will not expose his instrument to extreme ambient temperature variations. He will bring it into his practice room or concert venue, let it acclimate for half an hour and then tune it, as a violinist would do, too. After a few minutes of playing he would check and perhaps slightly adjust the tuning, and that's all and ok.

You can't naturally prevent drift in theremin oscillators. But that is not forcibly a problem. Since the audio signal is generated through heterodyning, everything is about the frequency difference. Thus, the circuit designer has to make sure that both oscillators drift the same amount which is not difficult when using identical components for both oscillators.

Hello Johnthom!

I have definitely seen your design sometime when browsing the forums and it would most definitely be interesting to build it and try it out! I would of course, humbly accept your kind offer and do my best with building it to life! It would be interesting to hear differences in sound from your design and from my own board when I make it as well hehe.

I appreciate it man! Thank you!

Best regards,

Konstantin

"A musician will not expose his instrument to extreme ambient temperature variations. He will bring it into his practice room or concert venue, let it acclimate for half an hour and then tune it, as a violinist would do, too. After a few minutes of playing he would check and perhaps slightly adjust the tuning, and that's all and ok." -- Thierry

And if Theremins were made of wood and taut strings and such I'd be the first to agree with you.  

Engineers who are also musicians is not the empty set - indeed you are one of us.  Why rail against yourself?