Moog Melody DIY coils: construction, paratemers, etc.

Hello everyone!

I'm thinking about building the theremin according to Moog's "A Tranzistored Theremin" 1961 article, but I can't find the parameters (L, SRF, "Q", etc.) of coils, producing of which is described in text - L2, L3, T1, T2. Also any hints on construction (slug-tuned coil forms mentioned in article are very hard-to-get, especially in my country, so i would like to produce them by myself) and any related info will be highly appreciated!

Here's a link to article, just in case:

http://www.clarisonus.com/Archives/Music%20Generation/Moog%201961%20A%20Transistorized%20Theremin.pdf

Hello Heorhii,

I have never built this design but  do like it for learning the fundamentals.

Here is info I collected over the years.

Christopher

Heorhii, that circuit uses fairly complex inductors.  What is the reason you've chosen it?

Jason, if you're reading this could you add this schematic to the TW archive?  And there are two other Moog Theremin articles / schematics at the link, one I've never seen before which talks a lot about audio waveshaping and is quite interesting:

  http://www.clarisonus.com/Archives/Music%20Generation/

"Heorhii, that circuit uses fairly complex inductors.  What is the reason you've chosen it?"

I'm looking for a schematic based on tranzistors with both pitch and volume antennas implemented. At the moment, i stopped on mentioned schematic (Moog, 1961) and EMTheremin ones, but in both I've met some problems with inductors. Bourns 6300 series, Toko inductors are unavaliable in my country (and too expensive for me in case of ordering from abroad), so, if i'll decide to build according to one of those articles, I have to produce them by my self.  

Current schematic attracted me because of it's simplicity and included description of how to produce some coils (L2, L3, T1, T2), but design details of their forms (which I also use to produce by my self in case of choosing this schematic) as well as their performance parameters are remain uknown to me. In addition, this schematic includes Bourns 6300 series rf chokes, which parameters are avaliable in datasheets.

But in  EMTheremin schematic all inductors have their own avaliable datasheets, information from which may be used for their production.

How do you think, which variant is more real to produce?

Also any info on producing, design and parameters of mentioned inductors or schematics with less hard-to-get components will be highly appreciated!

Thanks for your reply!

Heorhii, what country do you live in?

It's certainly possible to wind your own ferrite chokes and coils, but if you are having trouble finding components, where would you get the ferrite?

Coils are the hardest part of Theremin construction IMO.  Few qualify, and those that do are often hard to obtain.  Even back when we had lots of coils (and variable capacitors) to choose from, Bob Moog was winding his own on custom or stock coil forms.

I'd recommend you pursue a design that uses air core coils, and that you try to wind them yourself.  Can you easily purchase varnished magnet wire?  Do you think you are up to winding your own large coils (like Theremin, or Tesla coil)?

Heorhii, my theremin journey was always about finding her voice. I tried many variations of LC coils and never found critical magic. In your first theremin build your focus should be on RF oscillators that oscillate, then mixing the signal but keep the two RF signals in isolation from the opposite oscillator, detection and audio amplification. Then later learn the tricks. Also build in a more modular form so sections can be scrapped and improved.

Matching LC coils of the oscillators will give balance to the circuits. My best coils were by connecting two 100 uh chokes in series operating at 922 khz, this gives an easy center tap and can use voltage controlled tuning. The heavy gauge of wire seen on the chokes using the ferrite core gives a higher Q which developed two more linear octaves on the high side of the pitch field near the antenna, this is remarkable.

A theremin that has no Null Point I think is desirable for a better low end sound. If there is theremin magic it begins with the waveform characteristics found in RF mixing. All my sounds even my vacuum tube designs use a single IN914 diode for RF mixing, nothing more. Clara liked my sound, me too. Good Luck

Christopher

Edit: This pitch board is my first theremin design 10 years ago, the variable oscillator is on a small second pcb board. (modular) In theremin design OLD does not necessarily make for a better theremin, always get sound samples to know where you will end up, theory & practice can be like night & day.

On the coils. I made some but it is a bit tricky. First I found this info here on this site:

"BTW - I calculate the inductance of the Melodia tank at about 290uH"

And someone posted this about the original coil forms:

I got an answer from an engineer at J. W. Miller about the coil forms Bob Moog recommended in his article.

Quote from: "Andy Chow"

On all 3

Form is .375" ID x .500 OD x 1.875" long.

Core is .368 Dia x .75" long Carbonyl E powder Iron with a 6-32 x 1.375"

brass stud.

General Material Properties for RF Materials

Material Mix No.

Basic Iron Powder

Material Permeability (µ°)

Temperature1 Stability (ppm /°C)

Relative Cost

Toroidal Color Code

-1

Carbonyl C

20

280

2.7

Blue/Clear

-2

Carbonyl E

10

95

1.7

Red/Clear

I found some coil forms for sale here that look promising:

http://www.surplussales.com/Inductors/Ind-SlugTu/Ind-SlugTu-1.html"

The original used 36 gauge wire but modern wires are a different diameter so I ended up using a different

size. 

The permeability of the original ferrite was Red so I bought some (expensive!) original style forms from surplus sales which had red ferrite. I then wound and measured the inductance and then rewound with a different diameter until I was close to 290uh. My Melodia is all done I just haven't fired it up yet...

"and is quite interesting:" - dewster

thanx dewster for interesting articles!  Added to my collection.

"On the coils. I made some but it is a bit tricky..."

Fat Bob, thank you very much about info on coils! It's extremely useful!!!

Could you please tell some more about:

- inductances between various terminals ('cause there are 4 leads in each coil and pairs of them (1-2, 3-4) are connected to different windins of single coil, as described in article) for each coil

- specs of wires you used for  final version of chokes design (wire diameter, diameter with isolation material, type code, etc.)

- winding specs for picked wire (number of winds, widths, etc.)

Also there is one intreresting moment about cores, here's the quote from article: "Because of their low "Q" and high disturbing capacitance, air-core or powdered-iron-core chokes cannot be used in place of ferrite-core chokes to make up theantenna coils". It gives some doubts about core info posted by Andy Chow.

"Because of their low "Q" and high disturbing capacitance, air-core or powdered-iron-core chokes cannot be used in place of ferrite-core chokes to make up theantenna coils".

Uh, Theremin himself used air-core coils for the EQ coils, and he pretty much knew what he was doing at the time.  For certain frequencies, air-core coils are probably as high Q as they come (there's a tradeoff between ferrite core magnetization losses and the greater wire resistance losses of air-core coils).