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Theremax Builders FAQ

MODS and More

What is the range of the theremax. One octave? Two octaves?

The simple answer would be that the range is greater than 12 octaves from below the lowest pitch you can hear to above the highest. But in actual use it's very difficult to control more than two or three octaves with any pitch accuracy. Tuning can be tweaked so that the center of this optimum control range is within a few octaves of middle C.

How hard is it to build my own case? Does the bottom have to be metal?

There is nothing magic about the Tmax case - why does it look the way it does? Why not? A lectern case is easy to put on a table top and theoretically you can put sheet music on top. The pieces are easy to cut. But the Tmax electronics can go into just about anything that's bigger than the 5-3/4" X 4-1/4" dimensions of the circuit board. PAiA Tech Scott Lee, for example, has one built into a sandwich box hardly bigger than the circuit board. At the other extreme, Tmax boards have been built into interactive art works. In between, an unimaginable array of cases have been reported to us from custom transparent plastic ones with the ICs painted fluorescent and lit with black light on stage (wish that picture had turned out) to antique shoe-shine kits and vintage wine cases. We saw one for sale on eBay built into a Sesame Street lunchbox.

Metal cases can be used but may be problematic since the antennae will need to pass through the case without making electrical contact (rubber grommets would work here) and/or mechanically attached to the case, also without making electrical contact (shoulder washers come to mind here).

With any case there's one precaution: The PAiA lectern case has a metal bottom and the Tmax circuitry benefits from having a conductive grounded plane underneath the circuit board. If you're not up on your metal working skills, don't despair; aluminum foil can be used. Simply cut a suitably sized piece of heavy-duty foil and fit it to the surface where the Tmax electronics board will be mounted. Mounting hardware for the circuit board should be arranged so that the foil is connected to ground, the solder lug approach used in the Tmax manual will work well even if the board is secured with wood screws. Be sure to space the board above the foil to avoid short circuits.

Neither is there anything magic about the antennae. We use heavy gauge copper bus bar primarily because it's easy to work with, inexpensive and looks OK (copper is easily electroplated, so chrome is an option but sorry; we can't supply them that way). You can use coat hangers, you can use chrome plated plumbing fixtures. You can use a wire wrapped around a doorframe or conductive plates under a Plexiglass cover to keep tiny hands from picking at the works. In most cases the exact configuration of the antennae will affect tuning of the oscillator circuits, but not beyond range of calibration available. If even more range is needed, inexpensive parts substitutions provide it.

Can I use the PAiA MIDIBrain with my Theremax?

Yes, but ... the MIDI Fader configuration of the MIDI Brain outputs Control Change data and not Note-on/off messages. You can play a note on a MIDI keyboard or sequencer and use the Tmax pitch, volume and velocity outputs to control the note's volume, pitch bend or other parameters, but you must also have some controller that sends Note-on/off. There is enough room in the Lectern Case to add the Brain and a single power supply can be used for both circuits.

My oscillators drift and I have to keep tweaking the tuning ...

The Pitch Trim of your Tmax may be need to be nulled from time to time during a performance, and particularly during the first few minutes of operation, and because of this you may think that the Tmax oscillators are less stable than other designs. This is not actually the case - all LC oscillators will drift slightly over time and the Tmax circuits are no less stable than any others.

In order to keep a difference tone from appearing as the oscillators drift, it is common practice to "couple" them slightly so that below some minimum difference frequency they "lock" to one another. The down-side of this is that coupling sets a floor for the lowest pitch possible. If the coupling is such that the oscillators lock when the difference frequency is less than 130Hz (for example) then the lowest pitch possible will be C3, the C below middle C. In an unmodified state, Tmax is capable of pitches well below C0 - which, in retrospect, may be taking a good thing too far.

The cost of this super low end is that you have to tweak the Pitch Trim control from time to time. If any more than this is required (adjusting the coils, for instance) then something is wrong and the unit is not working up to spec. Maybe the Zener is whacked. Maybe NPO capacitors were not used for C8 and C10, which are the capacitors in the LC Tank circuits.

If you want to add or increase coupling, a simple "gimmick" capacitor between the two oscillators will do the job. For the gimmick, solder a 2-1/2" length of insulated wire to the "top" of R4 (the end that joins with e of Q1) and another to the "top" of R9. Twist these two pieces of wire together. Twisting tighter will produce a little more coupling, looser a little less. Here's what that looks like:

Theremax Gimmik Capacitor

This mod will set Tmax's lowest pitch to something between C0 and C1 and will keep oscillator drift from producing a pitch that slowly increases over time. If you add coupling, you should also read the following comments about maximizing sensitivity and repeatability.

Can I get a tone like the classic RCA theremins?

The string-like tone of the RCA theremins was in part the result of coupling between the pitch heterodyne oscillators, here's a slightly long winded explanation.

Here's a letter from Howard Mossman:

I own two RCA theremins, a MOOG ETHERWAVE instrument, a MOOG MELODIA theremin and an old SWTP unit, so I am quite a mavin on what a theremin should sound like and how it should respond to the players hand positions.

The (Theremax) Volume circuit is superb and ... does not produce a chirp when the volume loop is touched; as does (another theremin). The Pitch circuit also works well with a remarkable range - the problem I found with the Theremax is ... it does not sound anything like the RCA theremin.

But here is the good news! The Tmax can be easily modified to produce the classical theremin sound by makeing the following simple addition: ... in the (illustration above) ... if you connect a 10k ohm resistor (brown-black-orange) to these same circuit locations, instead of the "gimmick", the resulting change in sound is spectacular ...

The sensitivity of my Pitch Antenna seems to change over time and the same hand position doesn't always produce the same pitch.

On any theremin, particularly with coupling as discussed above, pitch sensitivity and repeatability of pitch to hand position will be optimum when the oscillators are nulled so that they are just short of unlocking with the sensing oscillator at a lower frequency than the reference oscillator.

Say what ..?

Well, look at it this way - as you bring your hand up to the Pitch Antenna, the sensing oscillator goes down in frequency (the pitch goes up because the difference in frequency between the two increases). If the theremin is nulled with the natural frequency of the sensing oscillator higher than the reference, then some hand motion will be required to bring the sensing oscillator down though the lock range until it so much lower than the reference that they unlock. Nulling with the natural frequency of the sensing oscillator lower than the reference increases sensitivity because the "wasted" hand motion is eliminated. Nulling to the same point at the bottom of the null band each time optimizes repeatability of pitch to hand position.

You don't really have to run all this through your head when getting ready to play, just turn Tmax's Pitch Trim control fully Clockwise (to "+") and then back the other way until the tone just drops to zero. On a Tmax, this will yield the greatest sensitivy and repeatablity. Other theremin's Pitch Null controls may work backward from this, but the same idea applies.

It is worth repeating: If you experience more oscillator drift than can be compensated for by the Pitch Trim panel control, something is wrong.

The audio output from my Tmax is low level and noisy...

Properly operating Tmaxs are not noisy. If you've heard rumors of a whistle accompanying Volume changes, it was true; but an early upgrade removed it and all kits now have this change.

But even a properly operating Tmax can be noisy if not properly tuned. Here are some very useful remarks that were recently sent by a builder:

I also built the PAiA kit and really didn't have any troubles. It worked right out of the box, so to speak, though I noticed a weird thing that if it's endemic of all Tmax'es probably should be added to the instruction manual. (It was - JS)

When I first tried following the tuning instructions where you shunt the volume control and only work on pitch control, I tuned the heterdyne tone as instructed and found for some weird reason that the pitch and volume antennas were switched! I looked for assembly errors, didn't find any and tried to tune it again.

It turns out that in the process of finding the heterodyne tone, on my unit you have to go through a "local maximum" - you go over a little mountain (a fake) if you go past that silence, then you get to a "big mountain" - ,uch stronger heterdyne tone. If you do the nulling procedure on the big mountain everything works fine. Weird, huh?

Have fun thereminning, everyone! :-)

Dave

***

These false tones are settings where the Pitch sensing oscillator is weakly heterodyning with one of the Volume oscillators. It is easy telling this from the much stronger true null of the Pitch oscillators, but the original instructions did not stress strongly enough that the first time you null the pitch oscillators the L2 inductor core should be adjusted fully from one end to the other while listening for the strongest heterodyne tone.

The apparent transposition of the pitch and volume antennae is only one of the many truly strange problems that can result from this kind of mis-tuning. It may also produce a generally noisy output (hiss and and high frequecy whines) and can sound like Radio Frequency Interference from radio stations.

There doesn't seem to be much change in tone color as I rotate the Timbre control ...

Agreed. You can remove the .005uF ceramic disk capacitor C44, which is connected between the collectors of Q10 and Q11. Simply clip it's leads off flush with the circuit board. This will make the square wave timbre very reedy.

My CV outputs don't seem to have much Voltage.

A properly tuned Tmax will have CV outputs that go from 0Volts to about 4 to 5 V. This is an appropriate range for most applications. For example, 5V. is the typical maximum Control Voltage input to many CV to MIDI converters such as our MIDI Brain and represents 5 octaves of pitch with the typical analog synth. This range of control is beyond the abilities of even the most accomplished thereminist. Clara Rockmore's theremin had not quite 3 octaves of pitch range.

Tmax CV booster ampA Tmax will give best results as a CV controller when tuned for Controller Mode as outlined in the manual. Correct oscillator nulling is very important for full control voltage range. The Volume Trim control on the front panel may not seem to having much effect on Audio Output level and still produce significant changes in the Volume CV.

For applications that require higher control voltages, the CV Booster shown below increases Outputs to greater than 10V.

In this circuit a single LM324 op-amp package provides 3 CV booster amps and an alternative to the Velocity sensitive Gate signal of a stock Tmax. The new Volume Gate is based on the volume hand's position, not it's change in position, an easier skill to master.

A kit for this mod including circuit board, all parts, wire and detailed instructions is available from PAiA. Order 9801K Tmax CV Booster .... US$11.25 plus 3.00 s&h.

Is the Gate supposed to act like this ?

Probably so, the Velocity sensitive Gate takes some practice to master. For best results the Tmax should be tuned to Controller Mode, as detailed in its manual. Remember that this Gate responds to changes in Volume hand position. Sensitivity is adjustable with the front panel Velocity control. At lower Velocity settings your hand must approach the Volume antenna more quickly to turn on the Gate, at higher settings very slow motions produce results. Constant motion toward the antenna is necessary to hold the gate on, it turns off shortly after motions stops or if you begin to withdraw your hand. If you get to close to the Volume antenna the Gate turns off

Tmax Gate ModHere's a quick mod that lets you switch between the normal Velocity Gate and a new Volume Gate that responds to constant hand position rather than change in position. This is very predictable and easy to control - once set, the Gate stays set until you rmove your hand.

The illustration is largely self-explanatory. Disconnect the end of R45 closest to pad "L" and solder a piece of wire between the free end of the resistor and the middle lug of a SPDT switch. Wire the top and bottom lugs of the switch to lug 2 of R83 and lug 2 of R84 respectively. If you're not concerned about switching back and forth, simply run the wire from the free end of R45 to the end of R70 closest to pad "S" (do not disconnect R70).

 


Can I process a keyboard with Theremax? How about hearing the pitch before I bring the volume up?

Theremax Aux Input

This cue circuit allows listening to a preview of the pitch being produced before bringing up the volume on a note. Tapping the audio path at the location shown in the drawing provides a constant level output to drive a small headphone amp such as our 9605 Headphone Buffer Amp.

The same jack can also be used as an auxiliary input to pass external signals through Tmax's VCA. When the added switch is closed, external inputs are mixed with Tmax's internal tone circuitry. With the switch open the internal tone circuitry is replaced with the external signal.



Will I need exotic test equipment to tune the Tmax?

Like all of our products Tmax was designed and the calibration/tuning procedure written so that no test equipment other than your ears are necessary. It's easier to follow when the Tmax and illustrations from the assembly manual are in front of you, but here's the procedure in detail.

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