9505 Theremax, tone w/Scott's 3)100pF, 1)6800ohm mods

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9505 Theremax, tone w/Scott's 3)100pF, 1)6800ohm mods

Post by PAiA-Scott »

Hello Franco.

Thank you for contacting PAiA with an interest in the Theremax. The complete Theremax as presented in the assembly manual is the 9505K and 9505C items, the electronics kit and the case kit. Since the front-panel and antennae items are a part of the case kit and useful items to those that would prefer building their own case, we make another accessory to the kit, the 9505FPA front-panel and antennae set. Then you have all items to build a unit that can be spread out on a table-top, piece of wood, or cardboard and tested, tuned, and operated. Then you can fit this to a box, cabinet, case, etc...

I'm including some tips and suggestions I've prepared for Theremax. There are some parts changes to make the waves available with the adjustment of the Timbre control more like the intended sine and square and with the sine setting, it is more like the theremin sound that is like a womans voice. When set towards the square wave, it is more like the violin sound. I have recorded an example of the tone recorded direct into the computer without any effects or additional tracks. About halfway through, I change the Timbre from sine to square. It is just the tone, I could not hear what was being played, but was only moving my hand to change it.

theremax sound 031008.mp3
(496.33 KiB) Downloaded 3518 times
Thank you.
Sincerely, Scott Lee

Franco Guiulfo wrote:
> Hi, so I was just wondering.... the theremin theremax kit is all I need to build a theremin??? then building my own case and thats it???
> can the sound be modified? because ive heard the samples you have on the website and they sound quite scifi and im looking for a classic theremin sound.hope you can help me.
> Thank You.


PAiA 9505 Theremax
Wave-, Volume-control-response- modifications

These changes affect the waveshapes available from the Theremax. The ccw and cw settings of the Timbre control will then be more sinusoidal and square, respectively. When the control is set ccw and sinusoidal, it is more like a woman's voice. You'll soon be sounding out the old Star Trek song. At the cw end it is with more harmonics and buzzier, like a violin.

The changes are subs of 100pF (101) for the 0.005/0.0047uF (502/472) ones at capacitors C39 and C44, and, the addition of a 6800ohm resistor to the Timbre control R81-3 to R81-2.

These open the frequency response of the VCA/output section and pad down the sine wave. These items were the way are to help hide leakage of the volume heterodyne as a weaker, secondary tone associated with the hand to volume antenna control. But, tuning can eliminate this volume 'tone' so the changes are OK.

Following is an alternate tuning method that keys on this volume 'tone' and prevents it. Note the volume control response can be altered/improved by changing capacitor C37 to be a 100pF too (when changing C39 and C44, you might as well put one here too).


A quick tuning method (no test lead jump used): Back all four tuning slugs fully out (ccw). Turn L1 and L4 one turn in. Connect the audio output to an amp/speaker and set the Volume control R83 to about 3:00. Set the Velocity to max. Slowly adjust L3 inwards through it's range listening for a faint wheep-wheep as it goes through its audio heterodyne frequencies. Now, as you go back and forth through this point, the Gate/Trigger LED should be lighting indicating the Volume CV is increasing and decreasing and the velocity cv is causing a trigger. For Traditional volume control response, leave L3 set just before the onset of the first audio band encountered with the inward adjustment. This causes a volume CV output and audio output that decreases with inward hand movement. Adjust L2 inwards for the pitch heterodyne tone (the 3:00 R83 Volume setting may be a bit high now as the pitch tone will come through loud and clear in comparison to the volume het).

For controller response (opposite of traditional), adjust L3 to null, or, to keep the volume heterodyne at a post-audio frequency (for no faint background volume 'pitch') adjust it further inwards to be just beyond the second volume (volume cv) peak.

Note the front panel Volume Trim control is optimized for tweaking the volume pair to be at null. For these post audio heterodyne settings, start with it centered and then you'll notice it will work to bring the pair into or out of audio range. Also it won't make very much change in volume in comparison to the way it did in varying the pair about null.
Note the unit can be tuned/tested with the antennae detached. I often work on board and panel assemblies just spread out on a piece of wood or cardboard. It can sometimes make the difference having the antennae or their cables not attached if there is trouble with the proximity of the antenna (or its mount) to a ground or a conductor to the floor. A near short between the shield and the internal wire can adversely load the attached oscillators one or four.

Moving the initial setting of L4 more inwards than the specified 1 turn in can provide more output if there is something loading it more than usual. The f-v circuits must have a normal strength heterodyne (no adverse loading of either oscillator in the pair) to produce the volume CV which varies the amplitude of 'tone' to the output; whereas, the 'tone' simply must be there--it being weak or not isn't doesn't have the consequences the volume pair does.

If available, an oscilloscope can be useful to confirm the oscillator transistor collector has a normal 10-12v pp oscillation. Then, preceding the volume f-v comparator stage IC1:B, the next spot to check for the volume heterodyne is on the collector of the amp Q9. There should be a 0.5 to 1v audio range wave when the pair is tuned at either side of null.

Scott Lee