Thanks for all the feedback, some good ideas for mods. A wet feedback loop should just be as simple as breaking the connection between lug 2 of the feedback pot and the 20k resistor and splicing your sent/return jacks in there. So long as whatever pedal you put in the loop has a high input impedance I don't think you'll need a buffer.
BTW, I had to do a bit of a double take, but you have the terms "open" and "closed" with regards to switching switched around. "Closed" is generally understood to mean that the circuit is closed, i.e. completed. "Open" means the circuit is broken.
Slaps head! yeah you're right I got it the wrong way round, unfortunately I can't edit the post, hope it doesn't confuse too many folks.
I have been thinking about PT2399 delays a lot lately. A couple questions keep coming up.
First, the PT2399 chip runs off 5V so the signal fed into it should be kept in a range of 0 to 5V. The PT 80 delay uses a NE570 to compand the signal. But the Rebote and others usually use an op-amp with a gain of about 1 as the input buffer. My Rebote sounds pretty good. Is there a chance of clipping? Would it be better to use an input buffer with a gain of something like 1/2 and recover gain at the output buffer?
I think if you reduced the signal going in and then boosted it afterwards there would be a risk of increased noise. I've never heard anyone complaining about distorting a PT2399 based circuit so perhaps it's not a problem. I guess if the signal was loud enough to cause distortion it would already be quite dirty.
Second, most of the examples I see use inverting stages for the input and output buffer. An inverting stage at the input is more prone to noise. This is due to the need for large input and feedback resistors. Larger resistors create more noise but are required to keep the input impedance high. Then again my Rebote sounds good. Maybe the metal film resistors produce less noise, or any noise they create adds "mojo" to the sound.
I wondered about that, because I've read about noise caused by high value resistors, but never really understood what it meant.
The reason for using an inverting input buffer in these sort of things is that the easiest way to mix the wet and dry signals is with an inverting opamp mixer. To keep the dry signal at the output in phase relative to the input you then need an inverting input buffer. I did use a non inverting buffer originally but I figured as the dry signal would always be going through the pedal it was best to keep it in phase, so I switched to an inverting input buffer.
I can't hear any noise caused by the pedal, and it doesn't appear to colour the sound when bypassed so I'm happy with it.
Hopefully someone with a bit more knowledge can explain the noise issue and whether it's something to be worried about. Or maybe there's a way to bootstrap an inverting stage or something so you can get a high impedance without big resistors?
Just a though, but have you considered adding the plug detector as a voltage input for the delay time??
Interesting, I think you could do this or an envelope follower. Basically if you get the voltages right you could feed whatever CV signal you wanted into the base of the PNP transistor to control the delay time. I've thought about adding an expression pedal/CV input but I don't think I've got room in the current box.
I'll try and do some soundclips of the more subtle modulation effects, I think I went into "Demo mode" a bit with the original ones. Hopefully the schematic should be ready tomorrow.
