Abductor II hiss / white noise issue

[iefes]

Hi everybody,

after working on a tube amp the last months, I got back to my effects pedals a few days ago and noticed some serious hiss being produced by the abductor delay in my signal chain (the hiss was also present the last time I tried it, I just didn't care too much about it). Despite the fact that this is an absolutely great delay pedal that I really love, the hiss is something I'd be happy to sort as it would be even more usable for me.

I have googled a bit and found this thread over at madbean https://www.diystompboxes.com/smfforum/index.php?topic=111625.20 . This guy had a similar issue so I tried some of the suggestions, without any luck so far.

I probed the signal path using an audio probe and found out that the hiss is only very subtle on the input stage (IC5 / pins 3,2,1) and on pin 6, but is very strong on the output of the output-buffer stage - pin 7. Most of the hiss goes away when I pull the 78L05 voltage regulator supplying the PT2399s. When doing these tests I turned down the volume-controls of all the delays to ensure no PT2399 noise gets fed to the mixing/output-stage.

After I noticed that the regulator gets quite hot, I tried a LM317 in its place, without an improvement. Additionally I moved the diode D1 before C1 to ensure the resistance of the diode forms a HPF with the capacitor to shunt power supply noise. No improvement. Then I tried a 100n cap from several points in the power supply to gnd and from pin 5 to gnd without any success. I tried a different opamp (OP275) instead of the TL072, no success. I also tried two different power supplies without success.

The only thing that helped was to put a large-ish cap in the feedback path of the mixing stage. When I used a 10n cap the hiss was at the same level like on pin 6, however this did also result in some treble roll-off which is not very convenient.

That's all I could think of up to now. Are there any other things that could try? Or is this something I have to live with? I've build loads of pedals but never had such a strong hiss going on. I have to admit, however, that's one of the most complicated builds so far.

Attached is a photograph of the finished build and the schematic I used to build this beast.

Thanks heaps!