Voltage Controlled FM Radio (CV Radio)
For a long time I have been toying with the idea of creating a Control Voltage tuned Radio that I can DIY and add to my Eurorack Modular Synth setup.
There are a ton of uses I can see in having easy access to the radio waves: random noise, strange sounds, fuzz and white noise.
But being able to control the tuning with CV so I can jump around the dial, or slew my way from one spot to another I think would be really awesome. If for no other reason than I like the idea of programming up a sequence and then every time it runs it will be different since the sound source is constantly evolving and changing.
So research began by looking at how radios work, and things didn’t look good to start off. It seems that radio tuning is all about capacitance, not voltage.
The simplest tuner consists of an inductor and capacitor connected in parallel. The capacitor is usually made to be variable (although the inductor can made variable it requires a more complex mechanism and is rarely used). This creates a resonant circuit which responds to an alternating current of one frequency
This means “most” of the time the tuning pot is not a pot but a variable capacitor type thing, which means you can’t just swap out the pot with a CV source (which was my initial hope, ignorance is bliss).
All seemed at a lose until I found out that some radios use what is called “varactor diode tuning” this is a diode that in essence replaces the capacitor bit, but the neat bit about the diode is that it’s voltage controlled. This means that the tuning pot will be handling voltage and suddenly we have somewhere we can bolt our CV into.
The next snippet of info, was that Ramsey (the makers of great electronics DIY kits) make a series of kits that all use Varactor Diode Tuning, include a $40 FM receiver kit.
So later part of 2010 I ordered one up, and it sat on a shelf for a few months whilst I tried to finish off other “higher priority” projects.
Then NAMM 2011 came around, and I was intrigued to see that Buchla, the makers of VERY high end modular synths was showcasing a new module the 272e which had 4 FM receivers that could all be referenced via CV. Now their concept is very different to mine, they have digital tuners that you tune and they then allow you to control the outage from each using complex envelopes, there is no CV control of the tuning – that said they do look really very nice indeed.
Buchla patch Model 272e through the 296e Spectral Processor. from Richard Devine on Vimeo.
But if nothing else it spurred me back into life. So earlier this week I soldered up the Ramsey kit.
With some awesome help over on the muffwiggler forums (MANY THANKS DAVERJ!)
I tapped into a couple of places on the PCB and hooked it up to a very low frequency Oscillator I had running a sine wave, and low and behold I had CV controlled tuning.
Now the tricky bit. I doesn’t work great. It doesn’t work with CV only with the Oscillator, so it needs a stack load more tweaking and testing. But my hope is in the next week to get it hooked up to Silent Way and have a fulling CV tunable radio incorporated into my modular synth.
