a simple ringmodulator-ish FX pedal

Demo Video

Introduction

Earlier this year I wrote a post about how to build a simple shortwave receiver with nothing but widely available hobbyist electronics parts, i.e. 74HCxx ICs, standard transistors and op-amps. Half-jokingly I called it “a 74xx defined radio“. The heart of the receiver is a 74HC4051 analog switch that I use as a (slightly unorthodox but cheap and easy to use) HF mixer.

Now I’m going to present you almost exactly the same circuit (or at least a part of it), working in the exact same way as in the radio, but instead of an antenna, we’re going to plug in an electric guitar. The goal is to make the guitar sound weird. Something like a ring modulator effect.

The PCB you can see in the video was actually designed for the 74xx receiver but now has only half the components assembled :). Of course you can build the circuit just as well on normal protoboard.

Schematic

If you compare this with the schematic of the shortware receiver, you will find out that I threw threw away the second half of the circuit (IF filter and demodulator), keeping only the frequency mixer and the local oscillator. However I changed some part values:

  • C13 from 47p to 10n
  • R16 from 10k to 100k

This makes the local oscillator slower, bringing it from the RF range (3-30 MHz) down to the audio range (20Hz-20kHz). We want to create beat frequencies between the guitar signal and the oscillator, thus both signals need to be in a similar frequency domain.

Furthermore I changed

  • R1/R2 from 22k to 220k

This gives me a larger input impedance (circa 100k), which is easier to drive with a weak source, such as an electric guitar.

If you use the older CMOS 4000 series chips 4051 and 4046 (aka MOS40xx, CD40xx), i.e. without the 74HC in front, then you can even power your circuit directly from 9V DC from your pedal board, instead of requiring an extra 5V supply.

How it works

Of course I can advise you to read the previous article about the shortwave receiver (“a 74xx defined radio“). But maybe that is a bit “too much detail”. Here is the gist:

  • The 4046 is a PLL IC including a voltage controlled oscillator. We just use the oscillator.
  • The potentiometer (RV2) is wired as a voltage divider, creating a control voltage that the determines the oscillator frequency. Instead of the poti you could feed in a CV signal from a modular synthesizer if you happen to own one.
  • The mixer is basically a device for inverting the input signal. If the incoming audio is inverted and un-inverted continuously in the rhythm of the oscillator, then you hear a lot of weird tones which are beat frequencies between guitar and oscillator signal.
  • If the guitar is silent, the output is also silent. If the input signal is zero, also the inverted signal is zero, thus no output.

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