As a few people have mentioned distortion problems recently I decided to take some measurements off mine to see if I can pinpoint what the problem might be. I fed the input with a 1Khz sine wave and adjusted the volume until I got visible distortion of the wave on my scope. The test was done with all the controls turned down, so it doesn't take into account the intentional clipping caused by the diodes.
I measured at the output first, to see when the input and output buffers would clip, and the signal was clean up until about 5 volts peak to peak, after this it clipped harshly. To me this says the buffers have plenty of headroom, so if you're getting distortion when the pedal is bypassed there's either something wrong with it or you just need to turn down whatever is in front of it.
Like most stompboxes it's not designed to go in effects loops though, so depending on the level of your loop I suppose the signal could be too hot for it.
Using a different opamp in place of the TL072 might give it more headroom but as 5 volts is the same as the PT2399's supply voltage it is going to clip at that point anyway.
Next I measured the signal at pin 14 of the PT2399, this is the output of the chip. The signal stayed clean with an input of up to about 400mV then it clipped. So I guess this confirms what people have said that hot pickups or a loud boost, overdrive, fuzz or whatever will cause distortion. I guess I must just have wimpy pickups and not run my pedals very loud because I've never heard any distortion from mine.
I'd try the suggestion from thehoj and others and reduce the value of the 47k resistors, maybe change them to 22k - 30k. To stop this reducing the level of the delay you need to also decrease the 10k resistor after the level pot by a similar proportion.
