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My theremin is full of eels.

Discussion in 'Electronic Design' started by dave vanhorn, Jul 13, 2004.

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  1. dave vanhorn

    dave vanhorn Guest

    At least, that's what it sounds like..

    If this ends up as a duplicate post I apologize, I sent one last night
    from google and it still hasn't appeared.

    Here's the schematic I'm working from:

    http://www.paia.com/theresch.htm

    Note, the mixers on the real PCB only use two diodes, not four.

    Four hartley oscillators, which are mixed in pairs to generate pitch
    and volume control hetrodynes.

    I'd like to fix it without a total re-design, if practical.

    I'm running it at the moment, from a lab power supply.
    The board for the circuit above, is mounted on 1" metal standoffs,
    above a ground plane of PCB material, where the pots all are mounted.
    No antennas are connected yet, so everything I'm talking about is
    independent of any antenna effects.

    Problem #1: Drifting. I added 0.01uF from the base control voltage
    point to ground, which got rid of the "hand effect" on the trim pots,
    but still it drifts significantly with temperature. I can detect
    blowing on it from 3' away.

    Problem #2: Sudden jumps of 100-400 Hz. These aren't rapid drifts so
    much as almost a step function. Can't see what's causing it.

    Problem #3: Very "twitchy" tuning. The coils are mouser parts, and
    they have just a breath of usable range. I'd like to expand this to
    some reasonable fraction of a turn.

    Problem #4: Odd behaviour near zero-beat. Waveform distortion, and
    worse, a large low frequency square wave impressed on the pitch audio.
    This comes and goes for no apparent reason. I can email a scope
    capture of this to anyone who is interested.

    All four oscillators were built with identical components, NPO caps,
    metal film resistors. I even used matched pair diodes. The power
    supply seems stable enough, though I am tempted to increase the zener
    current, or dump that entirely and go with a 7808 or 7810 regulator.

    I'm thinking of adding diodes in series with the control voltage pots
    to help the drift, but this will only help a little as there wouldn't
    be much thermal coupling between them and the oscillator transistors.
     
  2. sPoNiX

    sPoNiX Guest

    Isn't it *supposed* to sound like a bucket full of eels?

    ;)

    sPoNiX
     
  3. Dbowey

    Dbowey Guest

    dvanhorn posted, in part:

    << If this ends up as a duplicate post I apologize, I sent one last night from
    google and it still hasn't appeared.

    Here's the schematic I'm working from:

    http://www.paia.com/theresch.htm

    Note, the mixers on the real PCB only use two diodes, not four.
    Your other post received many good comments, but I don't recall your note about
    the diodes in the other post. Any configuration of only two diodes is a poor
    design, resulting in over-coupling between the oscillators. This can account
    for frequency jumping and general instability.

    For best results there should be a buffer on each oscillator output, but you do
    not wish to do a major re-design.

    So, my suggestion is to replace each two-diode mixer with a double-balanced
    modulator. This will give you major improvements.

    Don
     
  4. dave vanhorn

    dave vanhorn Guest

    Not at all, though it certainly can. Like a stradavarius can sound
    like a dead cat stretched over a box. :)

    Listen to anything by Clara Rockmore.
     
  5. dave vanhorn

    dave vanhorn Guest

    Isn't it *supposed* to sound like a bucket full of eels?
    Not really. It was never intended that way, though it's certainly capable.

    A stradavarius can also sound like a dead cat stretched over a wood box. :)

    Listen to anything by Clara Rockmore, or most anything by Lydia Kavina.
     
  6. dave vanhorn

    dave vanhorn Guest

    Isn't it *supposed* to sound like a bucket full of eels?
    Not really. It was never intended that way, though it's certainly capable.

    A stradavarius can also sound like a dead cat stretched over a wood box. :)

    Listen to anything by Clara Rockmore, or most anything by Lydia Kavina.
     
  7. dave vanhorn

    dave vanhorn Guest

    So, my suggestion is to replace each two-diode mixer with a double-balanced
    Interestingly, the original theremins used only a pair of hartley
    oscillators, and a single tube for a mixer. No buffers, almost an
    empty box! :)
     
  8. Dbowey

    Dbowey Guest

    dvanhorn posted:
    Interestingly, the original theremins used only a pair of hartley
    oscillators, and a single tube for a mixer. No buffers, almost an
    empty box! :)
    None-the-less, that is the first problem you should fix. Once done, you may
    not have any other serious problems.

    If you want to compare circuits, post the tube one. A tube can provide
    excellent isolation between oscillators; it just depends on how well it is
    designed.

    Don
     
  9. dave vanhorn

    dave vanhorn Guest

    None-the-less, that is the first problem you should fix. Once done, you may
    Agreed, I just have to work out the practicals of getting a solution
    in there.

    Interesting note: I dropped the emitter resistors to 22k, and have
    much better results. Stability seems better, still twitchy to tune,
    but some of the following circuit issues appear to be resolved.
    Apparently not enough osc level was one problem.

    Schematic drawn by Bob Moog: (I wish I could have been on that service
    call!)
    http://www.maxiespages.com/Articles/Theremin_tech/Circuit_diagrams_Vintage_theremins

    There are many others here as well.

    Bob Moog's comments on the instrument
    http://www.thereminvox.com/article/articleview/28/1/2/
     
  10. Jim Meyer

    Jim Meyer Guest

    (dave vanhorn) wrote in message
    Interesting...... Your Theremin is full of eels and a moose once
    bit my sister. Small world, isn't it?

    Jim
     
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