Theremax in a custom case

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Theremax in a custom case

Post by PAiA-Scott »

Hello scott, now I have built the theremax, and it works. But there are some radio background noise, espcially when I´m close to the pitch antenna and play a high note.
I have built it in a fairly small case, maby some of the wires are too close?
Do you have a suggestion of thinks to tweak to get rid of that background noise?

Hello Mats.

It is good to hear you have got it together now! The wiring can affect the RF oscillation amplitude so maybe there is not as much there as usual and you're having to compensate with a higher Output level control setting and/or audio-amp/speaker volume.

Check through the attached tips and suggestion and you'll see an image showing the best way to arrange the wires. I like the sound when the 3)100pf and 1)6800 ohm parts are in place. You might like the results better when using the QuickTune alternative tuning method. Here is a link to a YouTube movie of the procedure.

If there is metal near the antennae, it will work to couple them to each other, or, to ground if the metal has a connection with the circuit board point G (0vdc/ground/circuit-common). If you are concerned the installation might be trouble, apply power and tune and test the operation, with the circuit-board assembly spread-out on a piece of wood, cardboard, or plastic. Even without antennae it should tune and respond to your hands moving to or from the ends of the antenna cables. Then, installing it in a case referencing the assembly instruction manual lectern case illustrations with wood panels, a metal front-panel and an aluminum base and with the antennae attached in their clamps it should tune and operate as intended.

In our kit, the board is placed up off a metal (aluminum) case-bottom via the #4-40 mounting screws/nuts and 3/16" spacer (about 4mm). A solder eyelet is under the nut at one corner of the board with the power supply minus pole and wire G (and the panel LED ground wire) which puts this whole bottom at zero potential or ground/circuit-common. This works as a local load to the four oscillator circuits and helps stabilize them somewhat. The metal front-panel being grounded or not is not so important, but circuit board wire SG goes to the panel connectors and controls joined by a solid-bare wire. The case bottom and panel are keep away from the antennae and mounts. The case bottom is closest at about 3" or 7-8cm.

If a custom case's panel with the antennae mounts is metal, the antenna lead must pass through a hole that is about 1/4" or 5mm and then the closest the clamp or antenna can be to the metal is about 1" or 2-3cm.

The material for the antennae is not so critical. We have used copper for them, but now use brass. I have seen units in for service where the builder has elected to use aluminum rods and these were OK--also plumbing fittings.

The loop for the volume antenna is to concentrate the field and the affect the hand has on the field. It is better to not have to move the volume hand so much. Then, it is easier to hold a steady pitch with the other hand. A plate or a spiral can be used too.

I built mine into a red, plastic pencil-box. I used a tray to mount the board and controls, pushed the hot iron through the box to allow the control shafts to extend through, mounted a 9pinD connector for the power, controller, and audio connections, and RCA phono connectors for the antenna mounts. These connectors didn't turn out to work so well as the antennae twist in the connector with metal rods in them, but they're OK with the lengths of red insulated wire I have in place now. They're each formed into spirals.

Scott Lee