Super Simple Design Idea

Kind of crazy that this topic has popped back up after a year.  Circumstances have changed, and I'm too busy lately with other projects to play with theremin design, but I'd like to compile a list of design features I've been wanting to see, in case anyone has a shortage of ideas and wants to play with implementation of something different.  Probably wouldn't be a good idea to try to cram all these bells and whistles into one theremin, but they're just things that come into my head when I'm recording that would occasionally be nice.

Toggled full-wave rectification after the mixer stage of a heterodyne design, with maybe a low-pass resonant filter after that

In a design like the Theremax where the oscillators can be coupled by placement of a resistor between the oscillator caps, why not put a pot there instead and have an extra tone control?

Band noise voice: A noise source is controlled by the amplifier, and the pitch side controls the center of a bandpass filter after the noise source, with a knob controlling the bandwidth of the filter. This would make nice synthesizer woodwind tones, or rough metallic percussive sounds when playing staccato.

Bowed string voice: In a setup similar to an Etherwave Standard, add in a fairly high-frequency noise source to the mixer, then modify the mixer output with a high-pass filter, the cutoff frequency of which is controlled by the volume side.

Just throwing those out if anybody does a lot of prototyping and is looking for a new feature to play with. I'd like to hear the results if anybody uses any of these.

Hi Dewster,

My plan was to do comprehensive testing and documentation - but I got side-tracked (as usual).. This is still on my "to do" list.

I was heavily into the Lev oscillator some months back - trying to find an off-the-shelf transformer to replace the air wound tank transformer.. I had used CMC's in the past for antenna inductors and these had seemed to work well, so I first went this route for the Lev tank transformer - and I had apalling results.. So I set about winding my own transformers / coils, and using a peltier fridge / warmer to see the effects.

I started doing detailed logging of results - but it just takes too much time - I wanted to get the Lev clone running, so hurried through the process.. when it was obvious that the Lev oscillator was not going to do what I wanted it to do, I moved to other things.

As I say, Comprehensive evaluation is still on my to-do list.. But its importance to me had dropped a bit because I am actively correcting the resulting errors presenting at the output CV - I do however still want a comprehensive set of test results - this is indformation that will be gold dust.

But will need to wait until I have moved - My biggest problem right now is having a tiny work area and having to pack everything away in order to make space to do anything else...

All I can say at this time is that there was a minor improvement when using air coils rather than the 6300 series inductors, a minor but more  significant improvement when using an air wound equivalent of the 42IF106 IF transformer vs the Ferrite slug IFT, and an absolutely massive improvement using an air wound transformer for the Lev oscillator rather than a off-the-shelf CMC, and air wound also beat other broad-band Coilcraft ferrite transformers I was trying for the Lev oscillator, and beat my hand wound (on a 42IF106 former) transformer, although this was the best of the ferrites.

If things go to plan, I will be moving from my home within a few months and into a vacant 3 bedroom house - I will be able to set up my ATE and fridge, and run full automated test - and I will publish what I find! - I think that what is really needed is a table of inductance vs temperature - or perhaps SRF vs temperature ? I was just looking at the oscillator frequency vs temperature - but what do you think the best measure would be?

"I imagine thermal variation of an air core is most likely due to the expansion / contraction of the copper wire, which the coil former might help minimize.  Hence the use of ceramic formers with the wire tightly wound in ham VFOs.." - Dewster

I think you are right, but there was no sophistication with my air coils - the best were wound on Tufnol (epoxy paper) tube, some were wound on ABS pipe. I use enameled silver plated copper wire of 0.1mm or 0.2mm diameter depending on expected current, and "tight" or "tidy" is not how I would describe them (they are not works of art like the coils Charlie winds!) .. but I do fix the windings rigidly with hard adhesive / varnish.

All I can say is that air coils beat ferrites WRT thermal stability - or at least this is what my limited tests lead me to believe.. I do not know how various air coils or the construction of these will compare against each other - but I do suspect that a coil wound by Lev or by CHobbs will outperform any coil I could ever wind!

Winding coils is not something I enjoy at all - I have built several winding jigs, but think I should have just gone out and bought a winder when I could afford to - I have probably spent as much on my home-brew winders by now. My aim is always to just wind one working coil that I can take to a local contractor (or send to China) who can manufacture them.

Oh - buying copper wire from electronics suppliers can be a pain - big spools costing a fortune which are difficult to handle.. I found a UK supplier who sells small reels of really good ECW in lots of really pretty colours (great when winding transformers etc)

http://wires.co.uk/acatalog/colour_copper.html

Fred.

Hi Jo,

All the things you suggest (or at least most) fall neatly into the "theremin controlled synthesiser" catigory..

>>Toggled full-wave rectification after the mixer stage of a heterodyne design, with maybe a low-pass resonant filter after that

When one has a pitch tracking LPF, one can do what one likes with its resonance / tuning etc

>>In a design like the Theremax where the oscillators can be coupled by placement of a resistor n the oscillator caps, why not put a pot there instead and have an extra tone control?

I believe some theremins have this

>> Band noise voice: A noise source is controlled by the amplifier, and the pitch side controls the center of a bandpass filter after the noise source, with a knob controlling the bandwidth of the filter. This would make nice synthesizer woodwind tones, or rough metallic percussive sounds when playing staccato.

Bowed string voice: In a setup similar to an Etherwave Standard, add in a fairly high-frequency noise source to the mixer, then modify the mixer output with a high-pass filter, the cutoff frequency of which is controlled by the volume side.

All of these are easily obtained using subtractive synthesis - connect a EW+ to a VC synth and its available.

Noise through a tracking VCF is a lovely addition for the theremin IMO - it allows all sorts of wonderful effects to be created - wind, rain, storms - all controlled by hand / arm movement.

Fred.

"I think that what is really needed is a table of inductance vs temperature - or perhaps SRF vs temperature ? I was just looking at the oscillator frequency vs temperature - but what do you think the best measure would be?"  - FredM

If the response to temperature is roughly linear, then delta ppm/C should characterize it pretty well, at least for comparison purposes.

"...the best were wound on Tufnol..."

Is that one of those Spinal Tap composites?

Is that one of those Spinal Tap composites? - Dewster

Huh ? .. Dont see a connection between spinal tap and tufnol .. bit obscure.. ;-)

Tufnol is available in different grades, determined by the filler and the binder - most common fillers are paper and cotton, most common binders are phenolic and epoxy - the laminates are formed under high pressure.. Its a bit like PCB, but can be obtained as sheet, rod or tubes, or formed to specific shapes if you have the money to pay for the tooling and require large quantities - but this is expensive! - it is similar, I think, to some Bakelite products..

I use Tufnol quite a lot - It has been my material of choice for instrument panels, particularly when capacitive sensors must be incorporated.. The epoxy / paper material composite is tough, and has good dielectric properties - it is also reasonably inexpensive to have the production panels produced with all the printing etc in the laminate, and a circuit board containing the sensors is easily fixed to the back with epoxy.

I have mainly used the kite brand 1P/13 (phenolic paper) - and have started to use this tube for antennas and antenna extention arms - it is really tough, easy to machine (I can tap the end of the tube to allow multi-pin connectors to be inserted - something I need for my multi-dimensional expression loop) light and fairly priced (but a lot more expensive than ABS or other common plastics)

For coils, I have used 1P/13 tube with 32mm diameter, but am looking at having some 80mm (3.25") OD, 3mm wall tube specially made - the price for this depends on the quantity I order, but would clock in at about £40 / metre if I order 10 metres..

Is anyone interested in buying some of this tube if I had say 9 metres spare ?  LOL. ;-)

.. probably enough tube to fit every RCA clone, Keppinger or other valve theremin built for the next five years...

Fred.

Sorry, Nigel Tufnel...

This looks like nice material to do all sorts of Theremin-y stuff with.  Aren't there standard tube sizes close enough to 80mm OD that you can use?  If my spreadsheet is to be believed, there is an optimal former diameter for a given wire length (which seems to be where the turns height is ~0.4 times the diameter).  But this peak is so broad that anything in the ballpark should work almost as well.

One trick I read some Tesla coil winders do is to warm up the copper wire just before winding the coil.  This supposedly gives a tighter wind once the copper cools down and shrinks.

I set up a 100w lamp just above my spool a few minutes before winding and leave it on until im done.     I dont know how well this works- the spool gets pretty warm, but i assume that a thin, individual strand of wire has plenty of time to cool between when it leaves the spool and meets the form.   I usually leave a few feet between the two to so I dont need to readjust the angle of the winder as frequently.

One trick Ive learned that does make a things waaay easier is to  rub a thin film of bees wax onto the form before winding.  Makes the wire  'stick'  and helps keep everything in place.

" Aren't there standard tube sizes close enough to 80mm OD that you can use? " - Dewster

This is a standard tube size - but they dont have it in stock, and havent had it in stock for quite a while.. So they will make a batch if I order some, but have a MOQ..  Oh, if they do get some other order and have it in stock, then the MOQ is 1 metre.... But they arent too keen to make just one metre for me (they may yet do this - I have told them I want to use it in a prototype which could mean an order for several hundred metres if it goes into production.. its in the balances right now.. ;-)

One problem I have with my consultancy work is that I seldom accumulate the buying 'cred' that I deserve - I get samples, design, and pass the design to my client - the client / their buyer places the (volume) order(s) for the parts I specified - but I seldom have any dealings with the suppliers other than scrounging samples or small quantities.. I have managed to get some clients to name me when placing the orders, but this is rare.. Tufnol is one of the products that I have often specified (but this goes back some time - last job was probably 9 years ago) but over the last 9 years I have probably generated / spent no more than £100 on their products - so I am probably not a "high priority" customer ;-)

"I set up a 100w lamp just above my spool a few minutes before winding and leave it on until im done" - Chobbs

I have heard that heating the wire is a good idea..

But I am puzzled a bit about the theory behind "tight" winding - Sure, I accept that it does give a lovely looking coil - but in terms of the effect of expansion, I am a little bit lost...

As I see it, there are a few major aspects governing the behaviour of a coil - one of these is the diameter, another is the number of turns, another is the interwinding capacitance, another is the through-former capacitance..

Expanding copper will increase the wire length, which will (?) increase the coils inductance but I  dont see how expansion will change the effective former diameter (but here is where I am most likely to be wrong I think) as the expansion will occur on all axis, so as the wire lengthens it will also be getting thicker.. If the wire expands to a greater degree than the former, it will (I imagine) pack more tightly against the former as it heats up (?).. In which case it would be better to cool the wire (and former) when winding it (?)

Then there is the capacitance - but (as Dewster highlighted in prior posts) interwinding capacitance on an air coil is a non-issue.. capacitance effects are much more likely to be due to through-former coupling.. Here, I can see that a tightly would coil could have less capacitive variation from temperature change than a loosley wound one.. One reason why I wondered whether measuring SRF as a function of temperature might be a good idea.

Then I also wonder about the effect of bonding the winding to the former - If tight winding is beneficial, then why wouldnt rigid bonding of the wire with a hard adhesive like cyanoacrylate or a hard epoxy be just as effective?

I dont really have a clear mechanical visualization of how expansion of copper wire 'works' in a coil - how the 'deformation' of the winding occurs must surely play a part in the change to inductance caused by heating / cooling ? ... For example, if one has a coil wound such that, at the starting and ending turns, the winding doubles back for say 1/4 turn - change in the length of these 1/4 turns should (?) have no effect on the coils overall inductance, an any increase in the length will be countered by an equivalent change in the 'contra' length (?) .. if these 1/4 turns are free to take up any expansion..

Fred.

It's dumb, but I posted a video of HIVE performing a simple "bouncing ball" activity:

Trivial, I know, but as its father I'm kind of proud (which I'm told is a sin) of it taking a baby step.

Chobbs: wouldn't the beeswax prevent the varnish from adhering to the tube?  (Or do you not use varnish?)

FredM: you raise some interesting expansion/contraction questions, which I need to think about before I reply.

"Trivial, I know, but as its father I'm kind of proud (which I'm told is a sin) of it taking a baby step." - Dewster

LOL ;-) ... Its great! .... Far more impressive than many demo's I have seen for processors which took years to develop, and then, at long last, an evaluation board for this new super arm processor with configutable on-chip peripherals and analogue modules is released.. and the first example they give.. ?  ... Flashing a single LED!  ;-)

As for pride being a sin... well, in that case, all human progress must stop if we are to be sinless - because all the prime motivators (pride, love of money etc) are "sins".. It is only our delusion that we have something (or are striving for something) to be proud about which keeps us going a lot of the time, me thinks.

When it calls you "Dad" then....

ps - thats a neat little board.. are those 0.1" headers on the underside? can the board simply plug into PCB mounted recepticle (sockets) on a board below it? -