I'm trying to simulate the filtering in various PT2399 based delays, and the LPF pins on the pt2399 confuse me a bit.
Using the Rebote 2.5 Delay schematic,
http://tonepad.com/getFileInfo.asp?id=98 , I can see that pins 16 and 13 are the inverting pins for an opamp, and pins 15 and 14 are outputs, with 15 being the input to the delay portion. I want to assume that the "positive inputs" to the built in opamps are grounded, but i'm not too sure....
I breadboarded a similar circuit portion from "delay out" to pin 14 (from the rebote schematic still) using a jrc4558. So it was 2 passive low pass filters (making up a single 2 pole filter) feeding into another funky active low pass filter, with a bump in gain. That's what it sounded like, simple enough.
Then I breadboarded the same circuit, just using pins 13 and 14 of the pt2399 for the filter. I feel like I properly powered/filtered all the necessary pins to the pt2399, not leaving anything floating. The result was a lot more gain, very distorted.
I tried to hook up a quick and dirty delay on my breadboard, using input/output stages of the echo base, omitting the passive low pass filters, and omitting feedback. The result was still loud and distorted. Might be a mistake in my wiring, or maybe there's too much gain from those active LPF's without using the passive filters to bring the signal down a bit....
So my question (sorry for the lengthiness...) is how are the LPF pins set up in the pt2399? Are they simply inputs/outputs of basic inverting opamps with the positive terminal being tied to ground internally?