My friend at DIYEffects was kind enough to send me a PCB he had constructed for a delay pedal. His is based on a PT2399, which is a very popular (and extremely cheap) integrated circuit that will take an audio signal and produce an echo of up to 500-600 milliseconds. It does all this in 44k of digital memory. That’s nothing at all. Oh and they cost about $2 at the time of writing. Seems too good to be true?
Maybe. There are compromises of course. Mainly in what amounts to fidelity of the repeating signal. With only 44k of memory there’s no way this will be a sparklingly clean 16bit/44 kHz digital delay that you might expect from Lexicon. As is fairly typical in the world of guitar effects, it turns out nobody actually wants a crystal clear delayed signal. If you ever get to try it out you’ll find that the repeats tend to drown out the dry signal as a hi fidelity ‘wet’ signal will have all the same frequencies as the dry signal so it kind of stomps all over it. The ‘low fidelity’ wet signal of the 2399 has somewhat muted high frequencies, so it sits under the dry signal quite nicely.
The biggest issue I had with this circuit was that it would only work intermittently. In fact, it worked perfectly on first power-up, and I was able to play it for an hour or so with no issues. But when I came back to it this year ( I had populated the board and got it running a couple of years ago) and fired it up, there were no echoes. None at all.
I always blame power first, so I did some measurements to ensure that the op-amp had some power, and the 2399 chip itself had things in the right place. It looked like the 5v regulator wasn’t quite doing what it should, so I replaced that and made sure the pinouts were going to the right sockets. This helped as now I had a steady 4.8v at pin 1 (Vcc) of the 2399.
Still no echoes though. I read a post (here) that talked about “latching problems”:
First, I’ve had the PT2399 ‘lock up’ on me two or three times now. When this happens the circuit passes signal but you get no delay. The regulator inside also gets hot (and could cut out I guess). The effect can come back to life with the same PT2399 after disconnecting it from the power supply for a spell.
This sounds exactly like the issue I’m having. So this post lead to one from Merlin himself:
Some devices appear to latch up or fail to function properly if digital ground (pin 4) is left unconnected. This pin should usually be connected to analog ground (pin 3).
So I made a simple jumper from some scrap bits of stranded copper wire, and soldered it across pin 3 and 4. Problem solved.