Alternate Analog Filter for PT2399 Delay

I am using a MAX7401 switchcap filter in my simple delay pedal, but this chip is expensive and not a common junk box part people have on hand. So here is an alterante choice – its only four pole (not 8 pole like the MAX7401) but it should work fine. I use an OPA1652 op amp but others could be used. The op amp needs to be rail to rail because of the 5 volt supply used. A TL072 will not work! I think using the PT2399 for the 1/2 supply ref and sharing the 5Volt supply is a good idea with regard to digital noise.


Filter Plot:


12 thoughts on “Alternate Analog Filter for PT2399 Delay

    • I have simulated this filter. I have also built this filter with a slightly different cuttoff freq and it was used for a different pt2399 circuit. So no I have not used this filter in place of the switch cap filter in this exact circuit, but I am very confident it will work well. Based on my previous experience – you will notice long repeats are grainy especially with very high repeat level and full mix level. Having said that, the other 90% of possible settings will probably sound the same as the switch cap filter. It is still a step up from other designs.

  1. Ray this is very interesting, could a none rail to rail opamp be used, it if is run from a bipolar supply? – I am asking because I am trying to get multiple Pt2399s to work in series for longer delays and I thought that this filter may fit nicely between the individual delays. The rest of the circuitry around the delay sections is running at +/- 12V currently.


    • Absolutely – because I wanted simple I did not optimize my signal chain and filters to use a bipolar supply…….therefore, I hadto get the widest swing op amp possible(that would run on five volts) , but if you are going for a more studio quality design – your approach would be preferable. Interestingly, I am going to put a post in the next couple days about a great little IC, I use to generate dual supplies form single supplies all the time.

  2. Hi Ray sorry to be a pest. I recognize the filter topography to be Sallen-Key 4 poles but what response is it?. I have been playing around with Texas Instruments Filter Pro software and using SLOA093 ( Filter Design in 30 seconds) to investigate various designs and I was using your filter as a reference point. If I am not mistaken your filter has stop frequency of 1.5kHz to 1.6kHz. Am I close?


    • no problem

      In the circuit salad post – I gave a plot of the performance -check there to see the curve. The way to read the curve is the point where the signal gain is down about -3dB(from zero) is the stop or corner frequency- which is about 2KHz. If you start with this and its not enough or its too much – ou can tweak the resistor values to suit your desired response. My goal was to match the response of my original design as close as possible.

  3. Thanks for this Rring. Could I simply put two of this circuit in series to increase this to a 8-pole filter or would that give me phase problems, bad high frequency dampening and hell on earth?

  4. Hi I’m trying to build your circuit.i put a ne5532 op amp but all I get is a cut off sound which goes and comes.
    What do I do.
    Will a Opa2134 be of any use as I do not have the Max Ic.

    • So your op amp should work but your issue could be anywhere or anything…so not exactly sure where to start. I would simplify the circuit as much as possible and test each sub circuit. So build your input stage and see if something comes out when you put something in..etc and then do the same with the filter section..good luck!

      • Finally got it working.excellent circuit.i bought the max chip..thank you very much..

      • Glad to hear that…Its frustrating to me when people have trouble because I know the circuits work but I can only give advice from a distance. That’s why I also try to post design ideas that are not overly complicated!

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