introduction

While the Moog Etherwave Pro® is commonly known among thereminists as one of the best actual theremins, many players complain its non-optimal volume response which is often described as “snappy”. That means that even delicate hand movements above the volume loop don't allow to play continuous crescendi/decrescendi from/to zero volume but silence is followed by a jump in volume from zero to a starting level of about -40dB. The consequences are not only a reduced dynamic range but also the impracticality of sophisticated expression at very low volume.

The individual player may be more or less aware of this problem, depending on the settings of the Volume Tuning and Volume Response knobs, the player's technique and the style of music. Technically this bug is immanent. That's why Dr. Robert Moog himself suggested already a modification of the pre-VCA circuitry, replacing one resistor (R67 = 3.3kΩ) by a 3 resistor and 2 diodes network, loosely soldered together onto and above the volume circuit board. Later production runs of the Moog Etherwave Pro® were shipped with this modification already done in the same way. It seems that the circuit board layout has never been changed for this part of the circuitry. Dr. Moog's modification lowers the volume starting level by about 17dB while the maximum volume is lowered by only 3dB, thus obtaining a dynamic range gain of 14dB.

This modification is widely seen as an important improvement compared to the original state. But there persists a sour note according to some Etherwave Pro® owners:

• Once the modification is soldered in, one is fixed to this “new” circuit response. Further changes require further opening of the case and soldering.
• The so called “snappiness” does not disappear totally but is still perceptible and sometimes perturbing.

These statements led the author of these lines to the development of the EPVM1345 module. It is shipped with 2 additional parts as shown:

   
A is a connector which will be soldered onto the volume board in the place of R67 or a previous modification. B is the EPVM1345 module. It has two connectors: B1 connects it to the volume circuit board and fixes it at a specific distance. B2 allows to select one of 4 different available response curves as described below, either by setting a jumper C or by connecting an optionally available pre-wired rotary switch (no picture available yet) for those who want quickly change the volume response and do not fear mounting such a component into the beautiful award-winning cabinet of the Etherwave Pro®.

what it does

The EPVM1345 module comprises 4 different circuits, each giving another volume response characteristic:

1. The original circuit, snappy, giving a limited dynamic range of ~40dB
2. The modified circuit developed by Dr. R. Moog, less snappy, giving a dynamic range of ~54dB
3. A circuit developed by the author of these lines, no perceptible snappiness, giving a dynamic range of ~61dB. The subjective impression which I had when testing the prototype was a volume response similar to that of my very well tuned and adjusted Etherwave Standard®.
4. Another circuit developed by me, with a still smoother response, giving a dynamic range of ~67dB. It allows a very sophisticated expression at extremely low volume but exceeds important left hand movement when using the whole dynamic range.

The following graphics do not show the over all volume response of the Etherwave Pro®, but only the behavior of the different circuits. One may deduct their impact on the volume response from this:

The first figure shows the different circuits' characteristics on a linear scale. It allows to see the increasing “smoothing” at the beginning of the curves.

The second figure shows the different circuits' characteristics on a logarithmic scale, corresponding to how we perceive volume or loudness. The beginning of the curves shows the decreasing minimum volume and thus the dynamic range extension.