Until recently, it never occurred to me to use a ring modulator as a VCA. I typically have sizable systems with dedicated VCAs. But if I think about it, why wouldn’t it work? A ring modulator multiplies the X input signal by the Y or carrier signal. Essentially, this is how a VCA works, but the ring mod does not have a CV or gain control. Those with a small system already know this trick and save themselves space in their rack.

But what about a vacuum tube ring modulator? It’s a very different design than a solid state ring mod, even though they both perform a multiplication of the X and Y signals. The module uses tubes and transformers in a 1930s circuit design originally purposed for sending multiple telephone calls down one telephone line.

To test the properties of the Vacuum Tube Ring Modulator, I compared the Dual Lin/Log VCA alongside the Vacuum Tube Ring Modulator. Using the Dual Lin/Log VCA in logarithmic mode, the amplitude is what we expect. The note opens and closes the audio correctly when using the ADSR as a CV source. Very clean and crisp. There is no audio leakage as long as the gain of the VCA is turned down.

For fun, I switched the VCA to “linear mode,” which is intended for CV. In this case, there is a change in the CV response to the audio signal. The precision is reduced, and there is more CV “leakage,” so to speak. Again, this is expected behavior. I found that switching the CV modes on the Dual Lin/Log VCA is helpful for variations in performance.

Next, I patched in the Vacuum Tube Ring Modulator. The audio was plugged into the X input, and the ADSR was plugged into the Y input. I found a noticeable sonic difference between the traditional VCA and the Vacuum Tube Ring Modulator. The tubes and transformers are certainly having an effect on how the signal is amplified. I would dare to say that the audio signal itself is warmer than what was heard through the Dual VCA module. 

However, there was a problem: audio was still leaking through when the envelope was closed entirely. In situations of long, ambient sounds, the leakage will likely not be an issue. But in shorter, blippy sounds, the Vacuum Tube Ring Modulator alone will not work well.

To clean up the audio leakage, I think adding an additional CV source is best. I patched the CV Source and ADSR into a Multiple. I then took the combined signals back into the Y input of the ring modulator. I made a few adjustments to the CV source and significantly reduced the audio leakage. 

In conclusion, you must remember that the Vacuum Tube Ring Modulator was not intended to be used as a VCA. Doing so is an exploitation of its original function. However, the ring modulator will suffice as a VCA. The leakage may not be appropriate for every application, but the tone might be more desirable. Personally, I like the warm tones the tubes bring. Oftentimes, I find that my patches sound harsh. As a result, I have to EQ the track to eliminate the harshness. The tubes seem to warm the sound by shaving off the high frequencies.