Friday, July 8, 2016

My King VOX Ampliphonic Octavoice II

In a discussion thread on my original wind controller post, I got a question from DBaylies about how to sample a brass instrument's sound at the mouthpiece. That reminded me that I had an old "octave divider" effects box that my brother Kevin had handed down to me at some point.

The octave divider, which was part of the Vox guitar amplifier line manufactured by the Thomas organ company, allowed a trumpet player to lower the pitch of his/her instrument by one or two octaves. It was called the Octavoice, and you can read more about it here:

To use the Octavoice, the trumpet player drilled a hole in the mouthpiece, and screwed in an adapter that allowed the Octavoice's transducer fitting to be attached to the mouthpiece. Since I'm a trombonist, this picture shows the fitting on a trombone mouthpiece.

And when the transducer is attached to the mouthpiece, it looks like this.

This device was produced in the late 1960s or early 1970s, so there is no digital signal processing going on. Inside the box are a bunch of transistors, resistors, capacitors, and inductors - not even any IC op amds. I'm a little stymied on how this box could produce an octave shift down, but I am a computer engineer, not an electrical engineer. Here are some photos of the inside of the box.

Whether or not I understand how it works, it does work. I used it to record the "tuba" track for an entry in the contest to be the song on the flexi-disc included with the Billy And The Boingers Bootleg collection of Bloom County comics by Berkeley Breathed. We didn't win, since we weren't so heavy metal, but I still do sort of like the song we made.

So, for DBaylies: if anyone can offer suggestions on how to build something similar to how build a mouthpiece-attached transducer, post here. The goal is to get an audio signal into the input of an analog-digital converter for processing in the digital domain.


  1. Cool! My my does that circuitry look messy though. But it works!

    My project is coming along - the Teensy works well, although the 1024 point FFT doesn't allow for great frequency resolution in the lower register. I'm currently looking into improving this resolution, as well as exploring other methods of transduction altogether.

    Hope you're well!

  2. hi there, octave down effects in analog domain are never done with fft or the like, and are therefore very fast. if you understand how it is done in analog, you can code a very fast version too.

    basically the signal gets squared and then run through a flip-flop (an arrangement of two transistors that activates either one or the other when an input goes high) nowadays there are flip-flop ic's. if you just "read" one of the flip flops output, you get....yes, half the frequency of the input, or octave down. so you take this squarewave and lowpass it again heavily to make it sound not so synthie and distorted. if you want to get fancy you read the envelope of the input and reapply it to the octave down signal. and if you want to get really fancy, you should stitch the original audio together based on the ovtave down squarewave. that is what is done in the oc-2 octaver by boss (all analog). here is a good link of a diy project that uses a similar approach to get very smooth octave down tones:


    1. Lokki, thanks for explaining how the analog octave-splitting works!

  3. hope it helps... based on your breath controller entries i ordered an mpxv4006 to use with my ribbon midi bass controller (, let's see how that goes. so thank you for this blog!