PAiA 9505 Theremax, "classic RCA theremin tone"
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Author:  PAiA-Scott [ Mon Jun 08, 2009 4:53 pm ]
Post subject:  PAiA 9505 Theremax, "classic RCA theremin tone"

Ethan Ladd wrote:
> Hi, PAIA! I have a couple of question regarding the Theremax kit.
> 1) For the tone antenna, can I use an actually telescoping antenna (like a car antenna of sorts)?
> 2) In the part talking about the "classic RCA theremin sound" where would one go about purchasing a 10k ohm resistor? And what is the general difference between the stock Theremax sound and the "classic RCA sound"
> 3) Finally, does the kit require soldiering, or is it already done?
> Thanks!
> Ethan Ladd

Hello Ethan.

Yes, it is possible to use a telescopic antenna with Theremax but it is likely to not be so convenient as might be expected. It is a variation in the load to the radio-frequency oscillation which results in a varying pitch of the tone. Mainly, it is the hand moving about this field that works a node to the load of earth ground to the rf fluctuations, but the proximity of the antenna to the floor or nearby conductors in contact with the floor has an effect too. So as the unit is moved or positioned in differing situations and locations, there will be some variation in the pitch. A telescopic antenna length and positioning would introduce more variables. The slugs in the oscillator coils allow for big changes in tuning and the panel Pitch and Volume Trim controls allow for slight changes in tuning, but this is with the expected 'fixed' antennae for the kit. It is likely the substitution of a tele-antenna would not prevent the tuning, just complicate it, or necessitate more frequent adjustment of the slugs or trims. I always like to suggest starting out with your kit or project, as-is, and then taking it from there.

There is a note on the Theremin FAQ section of our site explaining the use of a 10k resistor to give Theremax a "classic RCA theremin sound", and this sound is probably most recognizable in the older black&white science-fiction/mystery/thriller movies. A current video, Theremin: An Electronic Odyssey, has many examples of this sound. In some instances it is almost like a violin and in others, more like a female opera singer. For the violin-like sound, the waves are more sawtooth or pulse sounding, buzzier tone and for the woman's voice it is more sinusoidal, a smoother, purer tone. Theremax has a Timbre control which allows for adjustment from a smooth to buzzy tone. With the 10k resistor added, two things occur, the oscillators have some increased coupling so they tend to want to stay at the same frequency and the wave distorts as they pull to each other, but it also weakens some. There are drawbacks to these side affects: the lowest frequency range is sacrificed and the lower signal level can make undesired noise or interference more apparent. In a trouble-shooting tips and suggestions guide I've prepared for Theremax, I list some capacitor changes and a resistor addition that work to make the buzzy sound buzzier and the smooth sound smoother without compromising lows or signal level: 100pFs as Cs 39 & 44 and a 6800ohm added to R81 terminals 2 and 3 (while 'in there' a 100pF at C37 gives improved hand-to-volume-antenna-control-response). Electronics component retailers are likely to have a 10k resistor, or the 3)100pFs and 1)6800ohm, but I am happy to send them in an envelope for you to try too.

Assembly of the kit involves soldering the parts to the board, mounting controls to the front-panel, and wiring panel controls and connectors. There are instructions for the various procedures with spots to check off each step along the way. Illustrations and diagrams are provided with the instructions along with a design analysis, testing, using, and troubleshooting sections. Attached is a pdf with general kit aasembly and soldering tips.

Thank you. Sincerely, Scott Lee

KitAssemblyTips.pdf [211.33 KiB]
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