AETHERPHON, global theremin family

Here is another sound file. The high frequency stuff is still in there, 
but the volume response is a LOT better.

http://home.comcast.net/~ka4koe/keppinger-09-09-2007.mp3

Philzy

This instrument's voice is well... stunning.

Sounds like the volume side is interfering with the pitch side (I have  
experienced the same effect on my Theremax).

No doubt, once you work out the interference issue, you will have an  
instrument you can be proud of.

The tone really approaches the magic quality of Clara Rockmore's  
theremin... in fact, one can hear the notes morph from "ohs" to "ahs"  
.. particularly on the high note in "Amazing Grace".

Very exciting to witness the birth of a very exciting instrument.

-- Kevin


Quoting kmg5443 <kmg5443@...>:

> Here is another sound file. The high frequency stuff is still in there,
> but the volume response is a LOT better.
>
> http://home.comcast.net/~ka4koe/keppinger-09-09-2007.mp3
>
> Philzy
>
>
>
>
>
>
>
> AETHERPHON, the glocal thereminist community
>
> To contact the moderator, e-mail porphyrous@...
>
>
>
> Yahoo! Groups Links
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What frequencies are the oscillators running at?
I'd try to separate them by at least 30-40kHz to avoid audible mixing.

--- In aetherphon@yahoogroups.com, "David VanHorn" <microbrix@...> 
wrote:
>
> What frequencies are the oscillators running at?
> I'd try to separate them by at least 30-40kHz to avoid audible mixing.
>

The tone oscillators are around 190 KHz, the volume circuit is running 
in the neighborhood of 400 KHz. I haven't checked recently. I almost 
threw this thing in the lake Saturday. The oscillators were literally 
driving me crazy. They would stop working if you touched the antennas 
or even looked at it crooked.

Sunday afternoon I found a Navy technical text on the internet that 
dealt with this sort of thing with Hartley style oscillator 
instabilities and I just changed a couple of biasing resistor values, 
and VOILA, the thing WANTS to work now. I was VERY concerned that if I 
made (make) major modifications to the circuitry of Mark's design to 
make it work it will not sound right. 

I don't have a LOT of experience about how a theremin is supposed to 
sound but Kevin your statement means a LOT to me. All this frustration 
and hard labor has not been for naught.

Thanks

Philip

> The tone oscillators are around 190 KHz, the volume circuit is running
> in the neighborhood of 400 KHz. I haven't checked recently. I almost
> threw this thing in the lake Saturday. The oscillators were literally
> driving me crazy. They would stop working if you touched the antennas
> or even looked at it crooked.

Ok, you may see second-harmonic energy from the tone osc getting into
the volume osc..  That could cause some interesting effects.  On the
schematics I've seen, the tone osc is normally the higher one.
Would it be really hard to move the volume circuit to say 300kHz?
I'm thinking to put them in places where they don't have any direct
integer harmonic relationships.

Decoupling the power supplies is really important too, and star-grounding.
I'd use R-C decoupling in series with each oscillator's supply, and
try hard not to share bias voltage supplies, or anything else.
Basically, make the oscillators three independent (or four if that's
the design) modules that have only B+, Ground, and output connections.

> Sunday afternoon I found a Navy technical text on the internet that
> dealt with this sort of thing with Hartley style oscillator
> instabilities and I just changed a couple of biasing resistor values,
> and VOILA, the thing WANTS to work now. I was VERY concerned that if I
> made (make) major modifications to the circuitry of Mark's design to
> make it work it will not sound right.

It's possible, the oscillator waveshapes do come through in the mixing product.


> I don't have a LOT of experience about how a theremin is supposed to
> sound but Kevin your statement means a LOT to me. All this frustration
> and hard labor has not been for naught.

:)

Quoting David VanHorn <microbrix@...>:

>> The tone oscillators are around 190 KHz, the volume circuit is running
>> in the neighborhood of 400 KHz. I haven't checked recently. I almost
>> threw this thing in the lake Saturday. The oscillators were literally
>> driving me crazy. They would stop working if you touched the antennas
>> or even looked at it crooked.
>
> Ok, you may see second-harmonic energy from the tone osc getting into
> the volume osc..  That could cause some interesting effects.  On the
> schematics I've seen, the tone osc is normally the higher one.
> Would it be really hard to move the volume circuit to say 300kHz?
> I'm thinking to put them in places where they don't have any direct
> integer harmonic relationships.
>
> Decoupling the power supplies is really important too, and star-grounding.
> I'd use R-C decoupling in series with each oscillator's supply, and
> try hard not to share bias voltage supplies, or anything else.
> Basically, make the oscillators three independent (or four if that's
> the design) modules that have only B+, Ground, and output connections.
>

Yes, however Philip keep careful notes and make each change one at a time.

The timbre modulation is likely the result of interference / coupling  
tendency between the oscillators.  So, if you completely isolated the  
pitch and volume oscillators (if such is possible) you might also  
loose the timbre modulation. A subtle tweak to the frequency of the  
volume oscillator might be enough to do the trick.

There is very little energy above 7000hz in a theremin tone.  You  
might consider a low pass filter after after the amplifier.  I know  
that is kind of a "cheap and dirty" solution however it might get the  
job done.

Alas, I don't know much about tube circuits -- however my ears are  
telling me that you are on the right track.

I've messed around with my Paia Tube Preamp and I am convinced that  
the "magical timbre change" must be related to interference/coupling  
tendency between the oscillators.  The sound is different than a basic  
"overdriven tube" sound.

Hope you can find a way to get rid of the audible interference (I  
agree -- it is likely a harmonic of the (fixed?) pitch oscillator  
interacting with the volume (variable, perhaps?).  Perhaps you could  
low pass filter one of the oscillators (before it goes into the  
mixer/detector)??

Well, if you can preserve the nice timbral modulation while  
eliminating the audible interference well... what an instrument it  
will be!

-- Kevin

> Yes, however Philip keep careful notes and make each change one at a time.

Absolutely!

> The timbre modulation is likely the result of interference / coupling
> tendency between the oscillators.  So, if you completely isolated the
> pitch and volume oscillators (if such is possible) you might also
> loose the timbre modulation. A subtle tweak to the frequency of the
> volume oscillator might be enough to do the trick.

Probably.. My slant on these things, is not to try for that "magic"
config, but to make the front end very repeatable, and then introduce
the proper waveshaping on the output.

Quoting kkissinger@...:
>
> Perhaps you could low pass
> filter one of the oscillators (before it goes into the mixer/detector)??
>

I am writing this with a bit of "egg on my face"  :)

After I made the comment (quoted above) it occurred to me that to LP  
filter the oscillators makes no sense because, while it may diminish  
some harmonics into the detector, it wouldn't change the waveform (and  
the interference pattern) of the oscillator.

I agree with the idea to experiment with different frequencies for  
your volume oscillators.  Look forward to hearing the next progress  
report.  And, you may be able to do a LP filter after the amplifier  
stage.

-- Kevin

I am just sitting here tonight playing with it on my workbench. I love 
its sound...it is wonderful. I think the harmonics can be dealth with 
easily, BUT I am going to record the current frequencies prior to 
changing any settings. I am going to get some chassis mount caps to 
make adjustments easier after buttoning up this thing.

The brass antennas Rudd made for me made a world of difference in 
playability and behaviour.

Philip