January 2010

PAIA Theremax Theremin

Because of a lack of new content I’ve resorted to posting old projects. This is one of the first builds I completed somewhere around 1995, a PAIA Theremax Theremin. PAIA has been around since 1959 and has been making DIY kits for musicians for decades. In addition to several analog modular synthesizers, Reverbs, EQs, a vocoder they also have their twist on the Theremin, the Theremax.

From PAIA’s website:

…In addition to the traditional mellow sine-like tone, Theremax’s Timbre control can mix in a harmonic rich square wave to produce a signal that’s ideal for use with external filters and processors. And to control outboard units, there are Pitch and Volume Control Voltage outputs.


But the features that really set Tmax apart as a gestural controller are it’s Velocity Control Voltage (proportional to how fast you increase the Volume) and Gate/Trigger outputs. A convenient foot switch input allows muting the internal tone source without disabling the CVs…

Panel controls:

  • Pitch & Volume Trim – these control provide a vernier null of the reference and variable heterodyneing oscillators in the Pitch and Volume control circuitry.
  • Pitch CV – controls the range of control voltage produced in response to hand proximity to the vertical pitch antenna on the right side of the instrument. Clockwise rotation increases range up to a maximum of 6 Volts. This control does not effect the sensitivity of the internal pitch circuitry.
  • Timbre – controls the brightness of the internal tone generator. At full counter-clockwise rotation the output is pure sine wave. At full clockwise rotation the output is a square wave. Intermediate settings produce a mix of the two.
  • Velocity – controls sensitivity to rate of change of volume level. At min, there will be no velocity response no matter how fast volume is changed. Rotation toward max produces increasing response. At max, the Velocity Control Voltage output will produce a 4 Volt change in response to the quickest possible hand gestures. This CV is a transient with a time constant on the order of a second. Internally, this CV biases the VCA for asymmetrical distortion, with no distortion at the min setting.(asymmetrical, transient distortions are responsible for some of the perceived “punch” in natural instrument) This control also sets Gate/Trigger sensitivity, at max sensitivity the gate will respond to very slow movements of the Volume hand.
  • Volume – adjusts the sensitivity of both the internal VCA and external Volume CV output to hand movement relative to the Volume antenna. Output level is adjusted here, but more importantly this control is capable of driving the VCA into symmetrical distortion. (symmetrical distortions are responsible for much of the “loudness” of instrument when played fortissimo)

Jacks

  • Mute – a foot switch, or S-trigger to this jack mutes the internal pitch producing circuitry. CVs are not effected.
  • Pitch CV – Control Voltage proportional to hand proximity to the Pitch antenna as ranged by Pitch CV control.
  • Volume CV – Control Voltage proportional to hand proximity to the Volume antenna as ranged by Volume CV control.
  • Velo. CV – Control Voltage proportional to how fast a hand is moved in proximity to the Volume antenna as ranged by Pitch CV control.
  • Gate/Trigger – This TRS jack provides a 5 V. gate signal at the “tip” when Velocity exceeds a threshold. The “ring” connection is an S-Trigger compatible closure to ground. The LED lights to indicate a triggered condition. (note that the gate responds to change in volume, i.e. how quickly the volume control hand is moved, not the static location of the hand)
  • Audio Out – Output level is adjustable with the volume control but is nominally -10 dB.

I didn’t have the cash for the nice wooden desktop enclosure so I built my own out of spare wood and used metal shish kabob skewers for the antennae (not pictured). The Theremin still works well to this day although I’ve always had a little bit of a problem with the volume control. The 9v DC power supply is hard wired into the unit which was a poor decision on my part. An easy fix if I ever get to it. I’ve used the gate output to trigger an old Alesis D4 with good effect. The voltage control outputs are perfect for controlling various parameters of an analog modular synth.

If you’re comfortable around a soldering iron this is a great kit, an easy build, and for just the electronic kit it’s only $115. Money well spent considering what you’re getting.

MC303 Mod-Blue LEDs & Burr Brown Outs

I remember I though this thing was so rad when I first bought it back in 97-98ish. These days it pretty much collects dust except for the occasional use of its arpeggiator. Nonetheless I thought it would still benefit from some blue LEDs under the keys. What can’t be seen is the addition of two Burr Brown OPA132 Op Amps to the output section. They won’t do much for the timbre of the lousy PCM samples but they do make the outputs sound brilliantly crisp.