​Some considerations and findings

A project log for Dalek / Cybermen voice changer

A simple ring-modulator circuit which can change your voice to sound like a Dalek or Cybermen from Doctor Who

SproketSproket 11/20/2020 at 10:420 Comments

The LT700 transformer is readily available, relatively cheap, and can be used in a ring modulator circuit. Many people including myself have used it successfully to create a Dalek / Cyberman voice changer. However, it is also less than ideal. The transformer is primarily designed to be used as an output stage for driving a speaker. Which is kinda okay for the output half of the ring modulator, however, the input half is effectively using the transformer backwards. Driving a signal into the very low impedance output of the transformer (especially with a large step-up in turns ratio) is a bit tricky. The ever popular LM386 power-amp IC is supposed to drive loads of at least 4R, unfortunately the transformers single-winding has a resistance of nearer zero (ignoring transformer load relationships for simplicity). Ideally an audio isolation transformer with a turns ratio of 1:1 should be used. From the usual retailers these can easily cost £10 GBP or $13 USD single quantity, so an online 'auction' website may be the way to go for ~half the price.

TLDR: The LT700 should be replaced with an audio isolation transformer with ratio of 1:1 so that the LM386 can drive a more suitable load.

UPDATE: I have managed to find a suitable 1:1 600R isolation transformer with a centre tap for £4 single quantity. Shall hopefully have some Vigortronix VTX-101-1604 transformers arrive next week for testing.

Shame.. I have a bag full of LT700 transformers and now don't know what to do with them.

I very quickly designed a PCB for the ring modulator using Easy EDA, unfortunately the footprint I used didn't have large plated through holes for mechanical support. This is being corrected and will be designed in KiCAD with a custom footprint as one doesn't seem to exist.

Ring modulator PCB top
Mechanical through-holes need to be added to the next revision, but looks neat.
Ring modulator PCB bottom
Soldering is a mess due to a dirty board. The copper idents turned out nicely though.