About Paia's Theremax...

[Harry Bissell]

There was a review (i think in Keyboard magazine or maybe Electronic
Musician) regarding PAiA... with a statement (something like) "the large
number of engineering careers launched by building a PAiA kit.... and how
many of those engineers were no longer in the engineering field.
    The moral is this... PAiA kits are a good springboard for the entry
into real design. I learned more making my PAiA 2720 WORK than I learned
building the kits... What components were "left out" to meet price
targets... what designs didn't work in the real world.
    I'm not flaming PAiA. My 2720 VCA's are STILL in my modular system.
The filters, oscillators, and env generators are long gone... but the
noise source is still in (albeit modified).  For those will entry-level
skills the kits are a real good price/performance ratio... If you are
looking for a truly PRO level instrument... keep looking.
    I'm tempted to this day to try another PAiA kit.

Harry (sorry i have an etherwave and it's bulletproof) Bissell  :^)

P.S. I'm one of the engineers who stayed in engineering

Tom May wrote:

> "Matti E. Leino" <mle at nic.fi> writes:
>
> > Hi !
> >
> > I have followed this now for few months, and seems like there's
> > quite smart people there. Now, my question is, has anyone of you
> > built this Paia's Theremax theremin...?
>
> I did and it sucks.  I never got mine to work very well, although I
> tweaked every subcircuit except the oscillators because pretty much
> everything needed fixing.  (And yes, I had PAIA's mod for better
> volume control or whatever it was.)  In many cases the decription of
> how the circuit works does not relate to reality -- it would be better
> if the circuit worked as described, not as it was built.  I gave up
> working on mine over a year ago, but off the top of my head the
> following things were wrong or strange with it:
>
> 1. The input and/or output from the ring modulator were attached to
>    the wrong points of the diode ring.  There was huge feedthrough of
>    the oscillator frequencies, and substantial filtering was thrown at
>    the result in an attempt to leave just the difference frequency.  I
>    rearranged mine to get much better oscillator rejection and backed
>    off on the filtering.  I was careful to keep minimal interaction
>    between the oscillators when I changed the ring modulator
>    attachment points by using strategic isolation resistors.
>
> 2. The design analysis (http://www.paia.com/theresch.htm) says "The
>    output of the ring modulators is boosted in level by discrete
>    transistor amps which also provide a second pole of low pass
>    filtering because of the the feedback capacitors from collector to
>    base."  In reality, these filters don't create a second pole, they
>    just shift the first pole (due to C22) to a lower frequency: C27
>    causes a Miller effect capacitance from the base of Q8 to ground
>    which is in parallel with C22.  One pole, not two.  In my redesign,
>    only one pole of filtering is necessary, but the original design
>    sure could use two poles.
>
> 3. The CV outputs (and this includes the CV that is controlling the
>    built-in VCA that determines the volume) use a bizarre frequency to
>    voltage / filtering / buffering circuit that sort of works, but not
>    at all like the circuit description.  There is still a lot of audio
>    frequency feed-through in the CV outputs.  The design analysis
>    says: "The fully filtered signal is converted to a square wave by a
>    Schmitt trigger, then differentiated to produce narrow pulses which
>    are integrated to produce control voltages."  The circuit actually
>    transfers charge with each pulse into C24 through C28 in a totally
>    non-linear way.
>
> 4. I also did some tweaking on the Schmitt triggers . . . for some
>    mysterious reason, the pitch and volume Schmitt triggers use
>    differing amounts of feedback.  Maybe to cause enough hysteresis to
>    ignore the oscillator frequencies leaking through . . .
>
> 5. It was difficult to get much range from the pitch antenna, and/or
>    the pitch vs. distance curve didn't seem very usable.  But your
>    mileage may vary.
>
> > Because, there's four
> > oscillator coils, 796 kHz, and it seems very difficult to find them
> > here in Finland,
>
> Just get the kit from PAIA if you really want to build one.
>
> fTom.