Pitch Linearization Coil Sanity Check

Posted: 8/5/2012 2:15:50 AM
gnsmith116

From: Northern Virginia, USA

Joined: 5/10/2012

"With respect let me say: When people use the phrase 'can't see the forest for the trees' - it refers to people losing themselves in the details (trees) that you forget the larger picture or vision (forest). Some people get so stuck in the details that the details become the overwhelming focus that sometimes leads towards a different conclusion as you forget the original vision." -- Touchless

I couldn't agree more - all I really want is an instrument that is playable.  Like I had said earlier, it is time to stop trying to optimize the linearization when what I really want to concentrate on now is the waveform which includes coupling and pre-mixer waveform shaping.

That being said, this is the current LEV antenna status.

C1 to be 24" from the antenna because that was suggested earlier in this thread.  The purple and black squares are exactly the same frequency and stretch settings the only difference is the pitch adjust setting.

 

Linearity Plot

The first plot has shows the raw data.  The EPRO data I took from Peter Pringle's excellent comparison as accurately as I could for comparison.  At 900kHz I tried a lot of different spring stretches to find something that looked promising.  A stretch of 7 1/2 turns of a 24 thread per inch screw seemed pretty good.  I have not tried 7 1/4, 3/4 etc.  I figure the linearity will change when I add more intentional coupling and I would rather spend time zeroing in on the best frequency/stretch with that in place.  With the isolation walls I can pretty much make the speaker in the amp sound like a heart beating but that will change with the coupling.  In general I took points from C1 to C8.  I tried to set C1 to be 24" from the antenna because that was suggested earlier in this thread.  The purple and black squares are exactly the same frequency and stretch settings the only difference is the pitch adjust setting.  There is a lot of other information earlier in this thread on how I take these measurements, not repeated here.

Interval

This plot is simply the distance at C1 minus the distance at C2 and so on.  A perfectly linear response would be a horizontal line.  The yellow 900.2 T5 is a good example of what the Tmax linearity is with a standard tubing antenna for reference.

This is probably too busy but the EPro is still probably the best - I don't have enough experience to judge.  It pretty much has the same range over 3 octaves C3-C6 which is where it counts most I would assume.  The Tmax tuned for C1 at about 24 inches (pink) now is pretty good from C2-C5 which might make this a good "bass" theremin!  When tuned for C2 around 16", the Tmax has two two octave linear regions.

Anyway, there are lots more frequencies to try and stretches so the Tmax with LEV is at least as good as above (assuming the test technique is valid - below)

Test Setup

This is how I tested it.  Before I slid the can along a board but decided on the suspend on the string option.  The can slides along the orange string and a fiberglass measure runs along the bottom of the can so I can read its position.  The Tmax output goes to the keyboard amp and the line output of the keyboard amp goes to the tuner on the round table.  The can is grounded to the keyboard amp through the blue wire and the test lead.  For a test I slide the can until its bottom is 24" from the middle of the antenna and adjust the pitch until I get C1 (which is really hard to do - I have a 10 turn pot on order) or close.  Then I move the can a little until it is really at C1 when I am standing back behind the keyboard amp.  Then I walk up and write down the position of the can.  Same thing for the other Cs up to C8.  This may not be a valid way to make measurements but the measurements I took in this way with the standard 3/8" tubing antenna almost exactly matched what was predicted for a capacitive only model so I think it is basically valid.

 

Theremax

For those who actually have a Tmax - this is just a shot of what the inside of mine looks like now.  An LM317 board is in the lower right corner mostly out of sight.  The small board toward the bottom left helps keep the signal away from the volume comparitor noise.  (Previous post on "cleaning up" the Tmax noise) Notice that the oscillator is connected to the antenna via coax which isn't supposed to work with a LEV antenna but it works at least as good as the data above.

That's it for now - off to see the forest. 

Greg

 

 

 

 

 

Posted: 8/6/2012 1:38:50 PM
FredM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

Greg,

These results do look quite impressive - Particularly for a theremin which has no equalizing inductance.

I have some reservations due to the problems you are expieriencing [human-ground] as the capacitive relationships between ground / antenna are critical for accurate measurement of linearity - so at this stage I cannot be sure if these results are due to the LEV antenna or due to some other effects..

But they do look quite good.

What would be really interesting would be a comparison between the best LEV antenna result / settings, and if you then swap the LEV antenna for a tube giving the same capacitive area (as in, replace the LEV antenna with a tube which does not require re-tuning of the theremin.. perhaps a plastic tube with some aluminium tape round it, which you can peel to get the right capacitance)

But whatever - All real interesting..

Fred.

ps - I love your "lab"! - It fills me with envy ;-) .. I had about 2 sq metres in which to build and test my theremins - with final work being timed to co-incide with when my wife was out of the house, so I could use the kitchen.. and be damn sure everything was tidy and clean (as in, she didnt know I had used the kitchen) by the time she got home...

Posted: 8/6/2012 3:04:25 PM
Thierry

From: Colmar, France

Joined: 12/31/2007

Fred wrote:

ps - I love your "lab"! - It fills me with envy ;-) .. I had about 2 sq metres in which to build and test my theremins - with final work being timed to co-incide with when my wife was out of the house, so I could use the kitchen.. and be damn sure everything was tidy and clean (as in, she didnt know I had used the kitchen) by the time she got home...

Hell, Fred... are or were you married to a dragon???

Posted: 8/6/2012 5:26:32 PM
FredM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

As Christopher has linked to this page, I need to clarify my posiotion on this matter, and my opinion on the "Lev" antenna - It could otherwise perhaps appear that I am endorsing this antenna, which I am NOT! :

The only conclusion which can be drawn from this thread and the MANY other attempts at verifying the "Lev Antenna" is that THERE IS ABSOLUTELY NOTHING FROM ANY RESULTS which in ANY WAY confirms or verifies or even gives reason for any notion that the so-called "Lev antenna" confers ANY advantage or improvement in linearity.. This thread was just one of a few where the antenna was explored -

It also needs to be stated that the name "Lev Antenna" is utterly and entirely misleading - There is no record that Lev Theremin EVER used this antenna or any variation of this idea or any spiral or spring antenna of any kind..

A Spring / Spiral antenna used on a theremin is NOT a "Lev antenna" - Its a "Christopher Antenna" -

There is (as far as every engineer who has examined it can tell) no science behind its operation, and despite being explored and tested since at least 2005 by many independent constructors, there has not been ONE set of data produced which confirms Christophers claims, and there has not been a single well known precision thereminist who has endorsed it.

It might work - but I have never got it to work, Other than Christopher, I have only heard one person - a hobbyist who cannot play even a simple tune on the theremin endorse it.

I believe it highly likely that the so-called "Lev antenna" is a con, its as simple as that - I may be wrong, but my personal advice to anyone is this: Only explore the Lev antenna if you are really interested in exploring "mysteries" and have the time and patience to do so - and keep in mind that it may just be a fiction ... IMO, Dont bother with it if you just want a linear theremin to play - because I believe you are highly likely to be wasting your time and will end up dissapointed.

And If you do find that I am wrong, Please present this in detail here! - A full disclosure of measured pitch <-> distance and frequencies etc.. ie, the conditions by which you got the antenna to give linearity, would be great!

 

-----------------------------------------------------------------

 Fred.

You must be logged in to post a reply. Please log in or register for a new account.