Koogo Analog Delay schematic (Donner Yellow Fall)

    I've been experimenting with controlling analog effects pedals, like this chorus. I bought a cheap "Koogo Analog Delay" to try to repeat my success. Turns out that it's not analog at all. It's the same digital PT2399 chip that seems to be in every delay. It does have a leg up by adding a sa571 compander though. Turns out this is the same design as the Donner Yellow Fall pedal; It's just been rebranded.

    I decided to draw a schematic in case there was anything to learn from it. I didn't add any capacitor values since they're not marked, and would have to be desoldered and measured. I did put in a few speculative values though.

    The PT2399 lives on a daughterboard and follows the datasheet design pretty closely. So closely that I just copied and pasted the last schematic I made from a PT2399 board.

PT2399 daughterboard

    

    The main board isn't too interesting either. It uses variations on the compressor and expander from the SA571 datasheet.

    

    An extra lowpass filter has been added before the expander. This is probably to help knock down some of the aliasing noise that would throw off the envelope detector of the SA571.

    A bypass FET has been put in the feedback loop so that you'll still get "tails" when turning off the circuit. This means that the delayed signal won't be immediately cut off; The last repeat will ring out, but no additional repeats will be created.

    The input buffer adds pre-emphasis, boosting the high frequencies. The summing amp is the complement to it, doing de-emphasis duty. These cancel each other out, and have no real effect on the dry signal. The benefit is to the signal-to-noise ratio of the delay line. By de-emphasizing (low passing) the output of the delay line, the high frequency noise can be greatly reduced.

Mainboard schematic

    I think these additions were inspired by actual analog pedals, despite the delay line being digital. You can see the pre/de-emphasis, lowpass filter, compander, and feedback bypass in the Boss DM-3

Boss DM-3