Getting back to basics - tuning!

Hi,

Wife bought me a Moog Etherwave Standard for Christmas.  I found a small guitar amp (Line 6 Spider IV 15) for 30 bucks - no complaints.

As I'm neither a real musician nor an Electrical Engineer, I wonder if someone can put the tuning process into very dumbed-down layman's terms for me?  I can't help but wonder why "tuning" is necessary, or how it's even possible, when the slightest movement in body/hand position can affect the sound.  ???

I have read the Moog manual, and it helps, but as I eluded to above, something in very non-technical terms might help cement it in my brain a little bit better, and thus help make understanding the manual easier for me.

And if nothing else, one thing I have learned in 50-odd years is that if I'm thinking it, others are too!  ;-)

Thank you very much!

George P.

You have to tune the EWS because there are three interacting resonances on the pitch side:

1. The linearizing EQ inductor and antenna capacitance form an LC (inductor & capacitor) which acts something like an electrical pendulum - you can't tune this, it can change with antenna length or diameter changes, or environmental changes like temperature, humidity, large metal things nearby, etc.

2. The variable oscillator LC tank which drives and senses (1) - you can tune this.

3. The "local" oscillator which is used to "beat" against the variable oscillator (2) - you can tune this.

(1) & (2) interact in complex ways (two dissimilar pendulums tied together), the effect of (3) is much simpler and sets the null point.

After much frustration I gave up on the instructions in the EWS manual and instead do a rough tune of (2) by looking for maximum voltage swing near the antenna, and after this make only minor adjustments until it seems the most linear with my hand near the antenna (or I feel sanity slipping away, whichever comes first).  Thierry can probably tune an EWS in his sleep upside down and with one arm tied behind his back, but for the noob it can be a daunting and subtle experience, with many similar tunings an no clear winner.

I believe some older Theremins are tuned via a small variable capacitor from the antenna to ground, effectively tuning (1) which makes more sense to me but does tend to load things down.

I think that f360gtraces asked rather about tuning the instrument with the front panel controls, while dewater talked rather about the internal adjustment of the oscillators...

I will give some hints for the first topic:

Stand before the theremin which should be at belly height so that your left hand is above the volume loop while your left upper arm is pending vertically and your left forearm is forming a 90° angle with the upper arm. At the same time find a position so that you are standing at a little less than arm length from the pitch antenna. Now move your right hand at shoulder height from the pitch antenna to your right shoulder. You will hear a high pitch which goes lower, the more your right hand goes away from the pitch antenna. Sometimes the pitch will go to zero and then even go up again before your hand reaches your shoulder. Now adjust the pitch know in a way that the zero point is reached just at the moment when your right hand is near your right shoulder. That is the coarse pitch tuning.

Then, without changing your body position, try to play a defined interval, i.e. an octave in different registers(high, middle, low). Use the pitch knob again a little to optimize the pitch field so that this octave spacing will be more or less the same at the different arm positions. After that, check the position of the zero point again. It might now have moved before or behind your shoulder. Correct your body position and restart the game. With a little training, you'll find the optimal body-theremin distance and the optimal setting of the pitch knob for best equal tone spacing.

While this works, there is no need to tune the internal controls of the theremin. That is normally only needed after some years when component tolerances and aging made it impossible to adjust the pitch field with the front panel knob.

Thanks very much for both replies - they are helpful.  I think I accidentally stumbled upon Thierry's method, as I can now go from very low frequencies (where I can almost count the cycles) to very high tones as my hand gets right up to the pitch antenna.  I might have to tweak the pitch knob a tiny bit, as I sometimes have to lean back a bit before the low freq drops right off (what they call the "zero beat", yes?).

Similarly, I think the volume knob still requires some tweaking as I have a very narrow range of motion to get from no sound to max sound.

Having said that, what I think I'm now understanding is that the "tuning" process is actually adjusting the "size" of the fields (electrical?  electro-magnetic?  electro-something?) within which the players hands control the sound.  This is done for the Theremin to accommodate the different physical characteristics and playing styles of each individual player.  Close??

Thanks again!

gp

"Having said that, what I think I'm now understanding is that the "tuning" process is actually adjusting the "size" of the fields (electrical?  electro-magnetic?  electro-something?) within which the players hands control the sound.  This is done for the Theremin to accommodate the different physical characteristics and playing styles of each individual player.  Close??" - gp

Hi, 

As to tuning the EW, I think you are ok in the hands of Dewster and Thierry ;-) They are also able to answer the above, but its an area I like blabbing about and confusing everyone!  ;-)

Its not electro-magnetic, or magnetic, or any stuff like that.. its electrical or electrostatic or capacitive .. and what the player is effectively doing is being a grounded capacitor plate -

Capacitance is the effect one gets between two (or more) conductors - two "plates" will capacitively couple - If one has one plate and connects it to + Volts and connects the other to - Volts, these plates will charge up, current will flow, but the current wont flow across the plates.. its an unseen 'force' between the plates which are insulated from each other by a "dielectric"

The theremin antennas are "plates" and the players hands are "plates" connected or coupled to ground.. The Capacitance is determined by the overlapping area of the respective plates and the distance between them..

And in a theremin these plates form part of  tuned circuits, the frequency of these circuits being controlled primarily by the distance between the players "plates" (hands) and the theremins "plates (antennas).

Essentially its electrostatic, I think in radio terminology its refered to as electrical force.. But if you want more depth search capacitance here, or google for it and for LC resonance or AC Circuit theory - theres loads ;-) .. Electrostatic as a search word will bring up more about high voltage discharge effects and the like which is a lot less relevant.

I will perhaps post some links in a ps on this posting later.

Fred.

This: http://en.wikipedia.org/wiki/LC_circuit is a much better description than what follows

-||- is a capacitor, and on a theremin its formed by player and antenna..

A simple visual analogy to capacitance would be a pipe with a flexible membrane across it so that water cannot pass .. one can still get "flow" or movement of water if both ends are filled and go to some pump which "pushes" water from one direction, causing the membrane to stretch, then reverses direction and allows the energy from the stretched membrane to be recovered into pushing the flow in the other direction, again stretching the membrane, but at the opposite polarity.. I dont wish to 'stretch' this analogy too far, but if the membrane was the dielectric (air) between the antenna and the hand, then altering its thickness (distance) changes the rate at which the system oscillates....

The above is NOT whats happening, but it gives perhaps a basic visual "analogy" - the "pump" would be the coil and oscillator circuit, which provides a little extra oomph to keep things going and compensate natures losses .. like pushing a pendulum or swing...

"A simple visual analogy to capacitance would be a pipe with a flexible membrane across it so that water cannot pass...."  - FredM

One can visualize quite a bit of electrical goings-on via plumbing analogies: voltage is the water pressure (height of a reservoir holding water), current is the water flow rate, resistance is pipe diameter or a constriction in the pipe, etc. 

I usually think of a capacitor as a balloon full of water, but your membrane construct is much better as it allows for oscillation.  What would an inductor be in the land of hydraulics?

Just found this: http://en.wikipedia.org/wiki/Hydraulic_analogy#Component_equivalents

An inductor is a paddle wheel with mass.  Yes, coupled to a diaphragm that would oscillate.

Yeah, the paddle wheel and membrane work well..

I had one good electronics tutor at the University of Cape Town, oh, back in the early '70s.. It was an all male class, and he was obscenely rude (I was a raving christian at the time, and found it quite offensive!  LOL ;-).. But his analogies stuck in my head more than anything else I learned..

I constructed "my" membrane analogy based on his condom analogy - and in the interests of decency I wont go into his analogies for oscillator drive circuits or inductors ;-) LOL.

He and I became good friends when I dumped christianity - he got me my first real job as an assistant at Fort Hare university in the Western Cape (Where Nelson Mandela studied) , and signed me up with the ANC and SA Communist party - all of which were illegal groups.. He was arrested and vanished, and I was placed effectively under house arrest.. but thats not a tale for here..

I will always think about stretched membranes when I think about capacitors... And apart from that, he is often in my mind even now even though I know he will be deceased long ago. It is interesting that the components for these analogies are now formalised - but none will have the impact of the analogies I learned!

;-)

Fred.