Completely newbie planning to build EM theremin

Charlie - That theremin picture you pasted earlier.. Beautiful!

Which theremin is that? an EM or some other? - is it perhaps an RCA clone?

Hazel:

I use the 7812 / 7912 parts rather than the L versions - Unless one really needs the space saving, the L parts are a bit feeble and tend to get warm if the input voltage goes high, or fail / become unstable if one isnt careful.

EDIT>

That inductor is probably a bad choice - I have not found axial inductors of that type to be particularly good.. will  check it out more, but I think you could find better.

From the Mouser list, I would probably choose this: http://es.mouser.com/ProductDetail/Bourns/RLB0608-220KL/?qs=sGAEpiMZZMsg%252by3WlYCkU63OFoSwiulmUau3IAZIv0k%3d

Or actually, this one is better : (it las lower DC resistance (0.086) and higher current - the significance of this is that it wont warm up as much, and temperature affects the inductance and contributes to drift - the axial inductor has 0.74R resistance, and lower current handeling - these parts are not designed for inductive stability)

IMO, This inductor is the better choice ->

http://es.mouser.com/ProductDetail/Bourns/RLB0912-220KL/?qs=sGAEpiMZZMsg%252by3WlYCkUz6k%252bUjgS74ea1Pn3CSADMs%3d

LM13700N is what you want. N just specifies the package..

When looking at parts, download their data sheets! - Even if you dont understand everything on them, you can find out the differences and things like what the 'N' designator means!

You REALLY do need to do this! - You need to gain some understanding of the parts you will be connecting together! - If you dont, you wont have a clue about what to check on your board when things dont work!

I know - I must come across like some grumpy school teacher.. But its fer yer own good, Laddie! ;-)

ps - while at Mouser, buy 3x 42IF106 IF transformers .. these little beasts are wonderful replacements for all manner of tunable inductors - one gets adjustable inductance from about 44uH to 114uH using pins 1 and 2 - great for oscillators.. And one gets from about 700uH to 1.6mH if one connects all the windings in series - great to connect in series with your antenna inductances to fine tune them.

Thanks Fred, Now i know about the importance of the max DC resistance and max current. I will take it into account for the rest of the parts.

But i still have a couple of doubts: what about the Q? and the tolerance? Your inductor has Q 50 and a tolerance of 10%. I guess they are important but not so important as the max resistance and current. Please, correct me if i'm wrong.

PS: what about an IC with 1A of output? Is that ok?

~Hazel

Fred- thanks.  The picture is from my newly finished JFET RCA clone -posted in response to your and Rob's comments about  being busy little elves.  meant to be only a teaser. I would have posted more info but I didnt wanna take over the thread.      

But i still have a couple of doubts: what about the Q? and the tolerance? Your inductor has Q 50 and a tolerance of 10%. I guess they are important but not so important as the max resistance and current. Please, correct me if i'm wrong.

Ok, Lets first discuss tolerance.. What this means is the possible variation in the value of the part - as in, you may get a part having 22uH, or you may get it having 22uH +/- its specified tolerance.. Yeah - its vitaly important IF you are depending on it.. But you are not - There is an adjustable inductance in series which makes the tolerance error correctable.. As long as the combined tolerance errors (this is capacitor and inductor tolerances, which may be cumulative and shift the resonant frequency up or down, or they may be counter-acting, giving the required resonance - always assume worst case) can be corrected with the adjustable component, you are ok.

Q is much more complex with regard to its effect on the oscillator - But the EM oscillator is highly tollerant, and 50 is more than enough. The EM oscillator will work with almost any inductor - But not all inductors will be equally stable.. Those horrible little axial inductors gave me a lot of problems in the early days because they have dynamic effects produced by changes in the current through them - The current through them changes as a function of hand position, so these effects can be a problem - I am not sure of the cause of these stability effects, all I know is that I bought a large quantity in the early days, but they will sit with my stock of other redundant components and not be used on theremins - they make great firework ignitors though! ;-)

PS: what about an IC with 1A of output? Is that ok?

Yeah - What can I say.... LOL.. That IC is a 7812! - The 78L12 is an IC as well....... IC's come in packages of all kinds - some look like transistors.. Some transistors look like IC's .. You can get 78L12's in 8 pin "IC" packages.

You would know this if you had downloaded the data sheets for the 78L12..

You REALLY need to make a BIG psychological shift my friend - You are taking on a huge project for a newbee - You need to get into the "bits" and spend an hour doing research on any question you want to ask before you ask it! - I dont mind answering, but its not doing you any favours - because I wont be there when you power up your construction for the first time - And when you have problems, you wont be able to communicate the problems in a way that anyone will understand, unless you bite the bullet and - well - take things slower and study a bit.

Fred.

" I would have posted more info but I didnt wanna take over the thread." - Charlie

Dont be so darn humble! - That is a work of art.. And even more so because its the new RCA clone! This is sensational! How does it perform ? How does it sound?

There are a couple of threads on the TW Theremin and RCA / Clone stuff which are probably better places than hijacking this thread ;-) .. But Please! Post the details! This IS EXCITING!!!

You are right, i should take things slower. One of my difficulties is that this is almost the only place to me to find some answers. Why? Because this is almost the only place where the people are specialized in theremins. For example, i asked here about this fixed inductor. I read the data sheet and also i'm reading basic electronics, but i'm months away from answering it myself. And I won't trust the answer if it doesn't come from some theremin-expert.

The good thing is that i'm learning a lot and now (in the example) i know the importance of max DC current and resistance. This is like a theremin-specialized school for me.

I also read how important is the temperature coefficient so now i'm picking resistors with 50 ppm, no more. ;)

But you are right, the way is to first learn more and more. Thank you for your patience Fred!

~Hazel

"Those horrible little axial inductors gave me a lot of problems in the early days because they have dynamic effects produced by changes in the current through them - The current through them changes as a function of hand position, so these effects can be a problem..."  - FredM

I believe I'm seeing this in my AFE, particularly since I upped the inductance 100x.  Also lots of ambient temperature perturbation.

Any recommendations for fixed inductors in the neighborhood of 0.1H that will function around 70 kHz?

"Any recommendations for fixed inductors in the neighborhood of 0.1H that will function around 70 kHz?" - Dewster

To be honest, I have not played much with such large inductances - When I have needed them, I tend to use Common Mode Chokes like this:

http://uk.farnell.com/murata/pla10an2230r4d2b/choke-common-mode-2x22mh-0-4a/dp/9528466

And wire the coils in series - 22mH per winding will give about 90mH when series connected - one can get CMC's in all sorts of values, they are quite low cost, have low resistance and reasonable Q.. Alas, the capacitance is quite high, so the SRF is often low - and the data available is often poor as one is not using the part in its intended mode.

However, I have found them suitable for frequencies below 500kHz - and have even used lower inductance CMC's for antenna equalization (I used them in early experiments for saturable reactors).. But again, I never used anything as large as you are looking for ( - the largest I really played with were 7.4mH/winding, and I used one winding in series with antenna and one for DC control - This never really worked, so I ended up using two 3.6mH in series with the DC windings crossed over to give phase cancellation - the only big inductance was then one CMC with 43mH per winding, which was connected to the DC windings to block HF coupling back to the control circuit.. This scheme worked well - but doesant prove anything about your application! )

I found the Murata PLA10 series were the best low cost parts (they are widely available - I can buy them for under 50p)

The Bourns inductor below may also be a good choice - I have used the smaller 5900 series parts, but nothing above 10mH.

Mouser Part No:  542-5900-104-RC 
Manufacturer Part No:  5900-104-RC

http://es.mouser.com/ProductDetail/Bourns/5900-104-RC/?qs=sGAEpiMZZMsg%252by3WlYCkU94zzn%252bPs%2fEGc7Lu1DieqfU%3d

Edit -> Have just looked more closely at the above, and its current handeling (saturation) and heating are quite bad! I think you will get a lot better from a CMC.. Particularly as you are operating below 100kHz.

Fred.

"... one can get CMC's in all sorts of values, they are quite low cost, have low resistance and reasonable Q.."  - FredM

Fred, you are a wannabe Theremin builder's gold mine!  I hadn't thought of using a CMC, and if I had the specs (particularly the lack thereof for this application) wouldn't necessarily have encouraged me to.  Those muRata PLA10 parts come in "regular" and "sectional" winding, with the latter having less capacitance. 

Not sure what they're using for the ferrite - they say it is "high permeability" which would tend to imply high temperature dependence, no?

The inductance value spec is a minimum, I wonder how much this can vary?  Not that it really matters much for my AFE, the operating frequency is exceedingly non-critical, as is the linearizing inductor value.

Looking at the EPE 2008 Theremin, they seem to be saying "screw it" and winding their own linearizing coils on pot core & bobbin (300 turns, I believe the relative permeability of the core is 1200, and they put a 2.5mm plastic "air" gap in it to lower temperature dependence - I have no idea what this ends up as in Henrys).  I gotta say, coil shopping is exhausting work and winding your own might put an end to the endless fresh hell pretty quick.  But pot cores ain't exactly cheap.

My prototype started out in life using some coils I boosted out of an old Radio Shack graphic equalizer - most of the values wouldn't work, too low self resonance I suppose, but two did and pretty well at that.

"Looking at the EPE 2008 Theremin, they seem to be saying "screw it" " - Dewster

Using the EPE / SC designs as any sort of "reference" is a bad idea - The EPE equalizing doesnt work! - If one looks at the component values one quickly sees that someone who never had a clue about theremins simply took a SC and stuck an inductor on it of aprox the right value for the antenna C..  But the SC tank is designed for direct connection to the antenna - the tank values just dont bloo*y work with the new configuration.

I built a load of EPE front-ends (more than 60 - but 40 of these were for an industrial application), they needed a lot of modification once I realised the error - And when modified, they CAN (sometimes) work well with good inductors - but they cannot work well with the coil specified by them.

I have a set of PLA10's which are "scheduled" for testing next time I power up my ATE (its an ancient pile using an old 286 PC/AT - Takes about 1/2 hour just to set it up as a voltmeter ;-) And will run a full gambit of tests on them - Stick them in the controlled oven and test everything.. The ATE is probably going to give up the ghost one of these days, so I use it frugally.

The PLA10's have a high AL - But I think a lot of this is due more to the closed construction - I have split the C core deliberately to reduce the inductance, and the effect is dramatic and I can reduce 1.5mH to a few hundred uH (my first Lev oscillator experiments used a modified PLA10 - but I ended up having to remove some turns).

"The inductance value spec is a minimum, I wonder how much this can vary?  Not that it really matters much for my AFE, the operating frequency is exceedingly non-critical, as is the linearizing inductor value." - Dewster

When using CMC's, you cannot trust the specs for anything other than use as CMC - Even the inductances of windings can be confusing .. The PLA10's give the actual per-winding inductance, which is unusual..

Variation is quite high (10%?) between parts - When I experimented with them in antenna circuits I used them close to the oscillator, with a 42IF106 wired as a variable inductor (0.6 to 1.6mH) in series, and a couple of good 5mH inductors close to the antenna.. But my PLA10s were being used as saturable reactors with controlled DC flowing through 1/2 of them - so its a different scenario to using them as fixed inductors.

Since then I have had specially wound saturable reactors made for the job - I just used the PLA10's as proof of concept, and never paid much attention to finer things like temperature dependence - The parts I had made had a temperature sensor embedded so that active compensation could be applied.

FYI - All the above was not done exclusively for theremins - it was paid for by a project I was doing for a client using capacitive sensing in electric vehicles.. part of my "other life" where I am a sensor designer not a theremin designer - But not a lot happens in that life, as most calls I get are from the "defense" industry - in fact, almost all electronics work I am offered is from this "industry" - it seems they are the only ones who value over 50's - And the only ones looking for people with expierience in safety critical applications, LOL ;-)