Theremax question

I too noticed some distortion on my Theremax. I discovered that it was some bleed thru from the volume oscillators to the pitch oscillators.

I was able to eliminate the distortion by tuning the volume coils such that their frequencies stopped producing audible distortion.

Using the jumper cable to force the VCA "open" (as described on page 20 of the Tmax manual) simply tune a pitch (by turning the pitch knob or positioning your right hand near the antenna. While listenning to the pitch, tune one of the volume coils (L3 and L4) and then the other and you may notice a spot where the pitched sound becomes rough or distorted. My solution was to experiment with different pitch coil settings and to see if there was a volume coil setting that would allow a clean signal throughout the pitch range. I don't have a scope or an accurate frequency counter thus I can't provide you the exact frequencies that I used... however, about 30 minutes of effort paid off in getting a clean, undistorted tone throughout the Tmax's range (The Tmax has a (playable) range of 6 1/2 octaves).

While I wasn't getting audible hum when plugging the Tmax into my board I was getting a rough sound. I plugged the Tmax into an EbTech Hum Eliminator and the roughness disappeared.

The Gate LED pulls the pitch down nearly a quarter tone and I have been playing the Tmax with the gate control fully counterclockwise (which kills the gate signal). The LED is very bright -- I am going to substitute a larger resistor value in series with the LED and hopefully will produce a dimmer brightness and not pull current from the rest of the circuit.

I haven't modded the Tmax though the square wave output is a bit bright for my taste and I am considering use of a low-pass filter to tame the sound a bit. I would like to brighten up the basic heterodyne signal and I understand that one can substitute capacitor values to accomplish. (Paia's website has some suggestions for different capacitor values).

I would add that Scott at Paia was extremely helpful -- we did quite a bit of troubleshooting over the phone.

Good luck with your Tmax!

Hi,

You might try using the Lev antenna. I put two on my Theremax (one for volume and one for pitch) and I can tune out the interferrence. You still have to mess with the oscillator coils to get it running right though. Try using an AM radio to listen to the oscillators. My Theremax runs at about 796 kilohertz. Your Tmax may run differently... You can tune the pitch and the volume different from each other to keep them from interferring with each other.

Look for the Lev antenna at this site...
http://www.oldtemecula.com/theremin/

The Lev antennas will NEVER come off my Theremax! Except for transporting it that is...

Hope this helps,

Grasshopper

Oh, one more thing. Be sure to get your Theremax up and running as well as you can BEFORE you try the Lev antenna. That will give you a good starting place.

Grasshopper

Grasshopper, well-put explanation about tuning the pitch and volume oscillators such that they don't interfere with each other.

I have power lines near my house... in fact, it prevents me from listening to A.M. radio. Today I had some more of that pesky roughness in the sound... in desparation I connected the Tmax's ground to a wriststrap and the roughness went away. I may try my theremin out in another location to see if the roughness disappears.

On the topic of tone quality:

The PAIA website suggests a couple of techniques to increase the coupling on the pitch oscillator pair. I tried using the 10K resistor between the pitch coils (I won't go into detail ... you can find it at Paia's website) and the tone certainly improved (albeit, at the expense of losing some range in bass).

I am working with the square wave tone now and substituting values for C44 in hopes of taming the square wave tone a bit and I will post the result when finished.

I used the "gimmick" as described on the Paia site to incease coupling between the oscillators. I used much shorter wires after trying the longer ones as recommended on the site. Just a little coupling knocked the very low frequency off and made ithe theremax not so prone to drift and make low frequency sound all by itself. Before the "gimmick" it could go down to about 20 hz. I was fearing ripping my speakers to pieces.... And it was not that pleasant of a sound.

Grasshopper

Here are some more technical issues/mods that I have made to the Theremax.

~~ Oscillator coupling ~~

I went with the 10K resistor coupling to add some brightness to the tenor (and lower) registers. Grasshopper, does the gimmick affect the tone quality as well as increasing the coupling?

~~ the square wave signal ~~

To smooth out the square wave signal I soldered a .001 mf ceramic capacitor to pin 1 of R81 (the timbre pot), a 22K resistor to pin 2 of R81, and connected the free capacitor and resistor leads together. Thus, some of the high-frequency content of the square wave is diverted to ground. The resistor prevented an overly large signal loss.

~~ grounding, electrical interference ~~

I am still stuggling with the grounding / electrical interference problems. The cleanest sound here is to use the Ebtech Hum Eliminator AND to connect a wriststrap to the ground on the Tmax. If I don't use the Hum Eliminator, then the wrist strap makes no difference. If there is any "good news" the roughness is not severe and is audible in the lowest register of the instrument only.

From the best I can tell, the oscillators are all tuned ok and I think I am just getting some interference. If I tune (mis-tune!) the volume oscillators I can at times hear a radio broadcast. I was able to work around the problem by choosing a frequency away from both the radio station AND the pitch oscillators. I might add, that there is a lot of electrical interference in my location.

~~ The gate circuit and pitch drop ~~

In a previous post I erroneously said that the pitch was dropping a few "semitones" when the Gate light went on... I meant "cents" (100 cents = 1 semitone (half-step).

Anyway, when the gate goes "on" the pitch drops a few cents. In the treble, the pitch drop is barely noticeable however the drop becomes more noticible on low notes. I tried disconnecting the LED light however I still got the drop (my theory that the LED was pulling down the circuit turned out to be false). I connected the pitch VC output to my synth and noted that the problem didn't increase however the pitch CV is also dropping whenever the GATE is on.

I don't think that this will be a problem in actual playing however noticed this when doing excercise #2 in the Clara Rockmore Method (where she requires the student to hold a steady pitch with the right hand while fading the volume in and out with the left).

~~ delayed volume response ~~

This was occurring when I positioned the "Velocity" control other than the full ccw position. Regardless of how fast I pulled away from the volume antenna the volume would fade in gradually. I simply disconnected R54 and the sluggish response went away. Additionally, the Gating is much more responsive now. R54 is supposed to inject a differential into the VCA to produce some non-symmetry (even harmonics) into the tone at moments of higher velocity (that is, when the volume is increase rapidly). With the oscillator coupling in place (as stated above) the effect of this was no longer audible so I didn't really loose anything with this mod.

~~ volume antenna chirp ~~

I noticed that, with the jumper in place to force the VCA open, that at certain settings of L4 the pitch should change when I touched the volume antenna. When tuning the volume oscillators, I chose a frequency on L3-L4 that didn't cause such a pitch shift. I am not sure if this would be noticable in actual playing because when one touches the volume antenna, the VCA is fully "off" and no sound should get through anyway. If the volume oscillators are not nulled properly, then a chirp could occur when touching the volume antenna.

~~ summary of mods ~~

. connect a 10K resistor between R6 and R11 (see Paia website) to acheive oscillator coupling and non-symmetry in the heterodyne signal.

. connect capacitor and resistor in series across pins 1

sorry... my previous post had a typo...

I wrote:

"~~ volume antenna chirp ~~

I noticed that, with the jumper in place to force the VCA open, that at certain settings of L4 the pitch should change "

I meant to say "the pitch would change"

kkissinger,

The gimmick seems to affect the lower notes most.

I put the Lev antennas in pvc pipe so they can only be contacted by touching the screws that hold the springs stretched. Since the antennas can't be touched without really really trying, chirp is pretty much a non issue.

My latest antenna incarnation has a long Lev type antenna that goes almost to the pitch antenna and is a little over 30 inches long. So, it sticks out about as much as the origional loop antenna did.... Why would I do this you might ask? In this configuration the Theremax can be played either with one hand or with both hands. It is a win win situation. Improved volume ontrol with both hands and improved volume control using one hand. Or, actually using both hands for volume control. It responds to whichever hand is closest to the volume antenna.

My Tmax is grounded through the amp by the connecting cord. Are you using an ungrounded amp? If so.....

Grasshopper

I am able to get a clean and smooth tone out of the Theremax by grounding the TMAX seperately and isolating the grounds by means of the hum eliminator. Doing it this way eliminates the need to ground myself to the Theremax.

Since I was connecting the Tmax to my mixing board via a patch panel, I am suspecting, as Grasshopper mentioned, that my shield connection to the patch panel isn't an ideal grounding point.

Anyway, to return to the Theremax topic... it will play cleanly as long as it is properly grounded. Since the Theremax does not have a ground connection via a grounded plug, it must be grounded through the shield. If the shield to the amp (or mixing console) is ineffective, another solution is to ground the Tmax separately and isolate the shield from the Tmax with a ground lifter (in my case the Hum Eliminator).

Just wanted to add that I am quite happy with the Tmax's performance as a controller for my analog synth.

The only problem that I encountered was that the pitch CV response was sluggish. I removed C24 from the circuit and the CV response improved (though it still lags slightly behind the Theremax, it is playable now).

I noticed that the volume CV does not lag and it too, has a corresponding capacitor (C25) in its circuit -- thus there may be a better way of solving the problem.