I am building a guitar effects pedal from a schematic I found online. Unfortunately the schematic is poor quality and hand-drawn, but there is a diagram of it constructed on veroboard (this is generally the way it is done in the world of effects pedal building.

It uses a PT2399 delay chip. From the creator's original post:

The chip's input stage is set up as a comparator with the aid of the Vref pin, so that gives us an ultra-amplified, squared-off waveform straight away.

The comparator means the input is quite heavily gated, so I only get sound out of it if I play my guitar very hard, or amplifying the signal before it gets to the pedal with an external device. Basically, it's not very responsive or sensitive.

I would like to make it so the required input volume is lower, and I can play the guitar more gently and still be able to trigger the comparator. The schematic:

enter image description here

And the veroboard diagram:

enter image description here

So the vref pin is pin 2 I believe (pinout here) and it is connected to pin 16 (the input) via R2. The audio input goes into pin 16 via a 47nF capacitor (C4) but I'm not too familiar with comparators, so I don't know which values to change. I've experimented but I don't think I'm on the right lines and I'm not getting any change in sensitivity. Can anyone help?

  • 1
    \$\begingroup\$ First step is to get hold of the documentation princeton.com.tw/Portals/0/Product/PT2399_1.pdf \$\endgroup\$ Oct 30, 2016 at 17:18
  • \$\begingroup\$ If that is supposed to be an echo effect, then you might want to try the schematic given in the data sheet for echos. \$\endgroup\$
    – JRE
    Oct 30, 2016 at 17:32
  • \$\begingroup\$ It's 'sort of' an echo effect, but quite a specialist one and not just any echo. \$\endgroup\$
    – TCassa
    Oct 30, 2016 at 17:55
  • \$\begingroup\$ Please spend the 7805 two 100nF capacitors between Vin and GND (5a-5b) and Vout and GND (4b-4c). The 7805 itself wants the first one for running stable, the PT2399 wants the second one —I think mostly to have the shift register work flawlessly. \$\endgroup\$
    – Janka
    Oct 30, 2016 at 20:32

1 Answer 1


Well, this circuit left out the input and feedback resistors, and feedback capacitor.

Try Rin of 100k, in front of pin 16.

Between pins 15 and 16, try 150k and 47pF. Actually, use a 500k pot for this, so you can adjust it. After you found a sensitivity that you like, replace the pot with a fixed resistor.

  • \$\begingroup\$ I was up to suggesting the same, then thought: maybe the 100k between pins 2 and 16 are only 10k or 1k by mistake, and that's why one had to hit the guitar hard. \$\endgroup\$
    – Janka
    Oct 30, 2016 at 20:24
  • \$\begingroup\$ @Janka I have just checked, and I've got a 330R resistor as R2 instead of a 100k! So that's quite a difference. I will check this out and get back to you. \$\endgroup\$
    – TCassa
    Oct 30, 2016 at 21:39
  • \$\begingroup\$ @RichS, Should those resistor and capacitors be in serial between 15 and 16 or parallel? \$\endgroup\$
    – TCassa
    Oct 30, 2016 at 21:40
  • \$\begingroup\$ @Tcassa: The + input of the comparator is internally tied to Vdd/2, and that resistor ties the - input also to Vdd/2, so that comparator can be flipped by the smallest input change – given the input signal is strong enough to feed current into that resistor to Vdd/2. So it was an informed guess. Okay, still a lucky one. \$\endgroup\$
    – Janka
    Oct 30, 2016 at 21:48

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