Wishful Coding

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Bucket Brigade Delay

So far in my quest to build a synthesizer, I’ve controlled a Game Boy synth with a linear potentionmeter and made a pressure sensing circuit for the SoftPot. Today I want to make an analog delay using a bucket brigade device.

To do this, I used a BL3207 bucket brigade device, and a BL3102 clock generator, which are clones of the MN3207 and MN3102, respectively. The datasheets for the former are a single page and not of much use, but since they are clones, the original datasheets provide more useful information. Most importantly, this reference circuit.

MN3207 reference schematic

I did not build that exact circuit, but kind of used it as a hookup guide for the two chips. The MN3102 basically uses the RC filter to generate two opposing clock signals. It also generates a special voltage that’s 15/16 of Vdd for some reason. The MN3207 then takes an analog input, and with some pull-down resistors, produces a delayed output signal. In the above circuit there is a lot of filtering going on, while my circuit uses a simple first order RC filter for now. Both the delayed output and the input signal are fed into an opamp adder to make a nice feedback loop. In the above video I’m simply turning the feedback gain.

ball of wires

To be honest, the current circuit sounds quite bad and is not very flexible. It just adds a fixed delay and loops it back on itself, going from inaudible to metallic to oscillating in the blink of an eye. The best sound I got out of it is the click track when it’s almost oscillating and becomes this funny tone by itself. Maybe all of the filtering in the reference circuit make it a lot better, but I have some other ideas.

I have like 5 of these delay chips, so it’d be fun to chain them together for a longer delay. The other thing is the clock generator: You can disconnect the RC and drive the oscillator externally. I’m thinking I could create an external clock, and modulate that with an LFO to create a vibrato effect.

Update: Vibrato works like a charm. I simply drove the clock with an Arduino as follows. Then I played a sine wave and some acoustic guitar into it for demonstration.

void setup() {
}

void loop() {
  int i;
  for(i=0; i<2000; i+=30) {
    tone(2, 40000+i);
    delay(1);
  }
  for(i=2000; i>0; i-=30) {
    tone(2, 40000+i);
    delay(1);
  }
}
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