ISD1820 Compressor / Fuzz
The ISD1820 chip is cheap cool way to make lofi loopers, but did you know it also has a feedthrough feature, allowing you to bypass the recording/playback section and send audio straight through the chip. The guys at Bastl realised you can still use the chip's heavy low pass filter while feedthrough is activated.
An overlooked feature is the analog gain control ("AGC", pin 6), essentially a compressor. A 4.7uf capacitor to ground from this pin gets you nice clean consistent samples from your lofi looper (see the Katzenjammer schematic). But let's try and get a traditional compressor pedal going using the feedthrough mode...
The first challenge is to make the input stable. Just sending audio in like in the Bastl schematic causes nasty clipping. You can ground the mic ref (pin 5) for a stable input, but the AGC doesn't seem to want to work with this set up. Interestingly if you connect the mic ref to 5v you get a kind of saw wave oscillator where the Filter knob controls the pitch, could be useful.
The trick is to recreate the balanced microphone signal that the chip expects. Here I have a transistor inverting my input signal as well as buffering a non-inverted signal. Sending one to the chip's input and the other to the reference pin gives you nice clean audio and a working AGC, an acceptable lofi compressor!
If you send the AGC pin straight to ground you get a decent fuzz, which works well with the filter. So I have a switch to toggle between compression and fuzz. Here's the schematic for that so far, including the Bastl LPF, slightly modified.
I tried pots / resistors in all the obvious places to get some controls for the amount of compression with no luck.
The amplified (compressed) ground hum, along with the overall compression and filter, reminds me of a knackered VHS tape. You could make a cool VHS-type pedal with some added pitch wobble, say a PT2399 or use the two outputs for a phase shift vibrato. (To be fair the hum could just be from my desk / pickups!)
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