Pt2399 feedback

[VolksWilliam]

So I've got this little pt2399 lofi delay pedal. I like it but the feedback knob is very touchy. I want self oscillation but it will only do that when the knob is just right around 2 I clock. Anything more and it just gets to loud and crazy. If I put a different ohm pot in there will that change it? I also want to add a footswitch that kicks it to that spot where it feeds back. Any suggestions?

[FUZZZZzzzz]

Do you have a schematic available for this pedal? You can probably change the potmeter value of the feedback knob.. For the 2nd question: You can add a 'momentary' dpdt switch to switch between the potmeter and a fixed trimpot/resistor with the sweet spot value. Thats how I should try and do it. hope this helps

[samhay]

+1 for a schematic.
There is probably a resistor from the wiper (lug 2) of the feedback pot to pin 16 of the PT2399. If you make this bigger, then you will get less feedback.
Another trick is to connect an LED from pin 7 to ground. This keeps the overload/clipping from sounding too gross.

[Mark Hammer]

If the OP is asking that question, I'm assuming they didn't build it.  And if that is correct, the odds are pretty good that it is a commercial pedal which uses SMD components.  If so, mods are going to be very tricky for a novice.

[VolksWilliam]

If the OP is asking that question, I'm assuming they didn't build it.  And if that is correct, the odds are pretty good that it is a commercial pedal which uses SMD components.  If so, mods are going to be very tricky for a novice.

It's a kit. I did not build it. But it is a standard throigh hole pcb so mods shouldn't be too difficult. I mainly just wasn't sure wether to use a larger or smaller pot. And how to wire a footswitch with a certain feedback amount. I may be able to find the schematic but I'm not sure what schem the Builder used.

[Mark Hammer]

I did not see that coming. 

There will often be a resistor between the wiper of the feedback pot and the return point where it gets mixed in with what's being fed to the delay chip.  What I like to do is split that resistor, replacing it with two in series that add up to the same value, and run a cap to ground from their junction.  Depending on your cap choice, it can trim off top end from the repeats, in a progressive way, a little more each repeat.  This can have the effect of keeping runaway feedback on a leash.  And since it is a progressive "treble-shave", that means that the first couple repeats won't sound that much different, with the effect really only coming down hard after 4-5 repeats.

You will still get something that approaches runaway feedback, but it will simply seem to linger in the background as a dull roar, rather than being a harsh caterwaul.

[VolksWilliam]

I did not see that coming. 

There will often be a resistor between the wiper of the feedback pot and the return point where it gets mixed in with what's being fed to the delay chip.  What I like to do is split that resistor, replacing it with two in series that add up to the same value, and run a cap to ground from their junction.  Depending on your cap choice, it can trim off top end from the repeats, in a progressive way, a little more each repeat.  This can have the effect of keeping runaway feedback on a leash.  And since it is a progressive "treble-shave", that means that the first couple repeats won't sound that much different, with the effect really only coming down hard after 4-5 repeats.

You will still get something that approaches runaway feedback, but it will simply seem to linger in the background as a dull roar, rather than being a harsh caterwaul.

That sounds good. What size cap? And would a larger resistor cause more or less feedback? I also like the led idea.

[Mark Hammer]

I don't know what you built, but I'll use the Rebote 2.5 as an example, since I just installed this mod on one the other day.

The build document from Tonepad shows a 15k resistor coming off the "Repeats" wiper, and notes that reducing it to 10k would be useful for runaway feedback.  I compromised and replaced the 15k with a 6k8 and 4k7 (in that order), and used a 3-position on-off-on toggle to run either a 22nf or 39nf to ground from the resistor junction for either no rolloff or one of two different rolloffs.  In the rolloff settings, you can dime the Repeats control and it will certainly linger, but it won't get that increasing buildup of loud noise.

I've actually been doing a mod very similar to that for over 30 years, since back when I was using table-top analog delay boxes.  Rolling off some treble from subsequent repeats has a nice way of making repeats move a little more into the background so that you get the ambience effect but without distracting and cluttering up your sound.  It also has a nice way of moving delays closer to the reverb end of the spectrum, becoming more of a "wash", and less in your face.  Worth doing even if you aren't trying to tame runaway feedback.

[FUZZZZzzzz]

can you post a link to the kit?

[VolksWilliam]

can you post a link to the kit?

I was told it was from synthrotek. But they don't make it anymore. I believe this is it. But I'm not positive.

http://www.synthrotek.com/wp-content/uploads/2014/02/pt2399_delay_v04.pdf

[samhay]

The 'Feedback' pot in the schematic looks like it should be a trimmer that sets the max feedback - any chance you have a trimmer on your board? If so, try fiddling with it.

[VolksWilliam]

I don't know what you built, but I'll use the Rebote 2.5 as an example, since I just installed this mod on one the other day.

The build document from Tonepad shows a 15k resistor coming off the "Repeats" wiper, and notes that reducing it to 10k would be useful for runaway feedback.  I compromised and replaced the 15k with a 6k8 and 4k7 (in that order), and used a 3-position on-off-on toggle to run either a 22nf or 39nf to ground from the resistor junction for either no rolloff or one of two different rolloffs.  In the rolloff settings, you can dime the Repeats control and it will certainly linger, but it won't get that increasing buildup of loud noise.

I've actually been doing a mod very similar to that for over 30 years, since back when I was using table-top analog delay boxes.  Rolling off some treble from subsequent repeats has a nice way of making repeats move a little more into the background so that you get the ambience effect but without distracting and cluttering up your sound.  It also has a nice way of moving delays closer to the reverb end of the spectrum, becoming more of a "wash", and less in your face.  Worth doing eveb if you aren't trying to tame runaway feedback.

So it looks like there's already a cap to ground on the feedback wiper. Would you still add another resistor between the feedback and the cap? Maybe a larger cap to cut some highs?

[Mark Hammer]

The schematic PDF you linked to does not show that.  What it does show is a pot in series with a 3k3 resistor, and an electrolytic cap in series after that.  The electro keeps DC and hum out of the feedback, but does nothing for top end.  Assuming the schematic IS what you built (and some aspects about it just seem plain wrong), the best place for what I suggested would be running a cap to ground from the junction of the 3k3 and feedback pot.

R18/C18 already forms a lowpass filter for the entire delay signal (not just repeats), but is set around 2.8khz, which is simply to keep clock noise and zippering out, and not dull the treble.  Doing what I described in my earlier post requires additional lowpass filtering at a lower rolloff.

[VolksWilliam]

The schematic PDF you linked to does not show that.  What it does show is a pot in series with a 3k3 resistor, and an electrolytic cap in series after that.  The electro keeps DC and hum out of the feedback, but does nothing for top end.  Assuming the schematic IS what you built (and some aspects about it just seem plain wrong), the best place for what I suggested would be running a cap to ground from the junction of the 3k3 and feedback pot.

Ok that sounds like a plan. Ya like I mentioned. I didn't build it just ended up with it in a trade of some pedals and that's what the previous owner said it was. And I thought it sounded cool so I went for it. I just want a little less crazy noisy feedback when I crank it. Thanks for your help!

[blearyeyes]

Set the pot to what you want to happen and measure the resistance.  Then you will have a general idea what to aim for as far as switching out the pot. Like someone else mentioned there usually is a trimmer to adjust the feedback control on the circuitboard..

[blearyeyes]

Socket R17 and try a higher resistance until you get what you want with the pot wide open.  The crazy Warmoth mod is just put a momentary switch that places a jumper from before R17 to after the feedback pot eliminating all resistance. Use a normally Off then when you push it down you bypass R17 and the Pot .....feedback craziness..

Another approach would be to find the general resistance and then put a trimmer that is a higher resistance than what is close replacing R17,  attaching one one outside pin and the center wiper pin to replace R17 and then dial in the perfect setting.

[deaftone]

Hello guys,
i'm having kind of the same problem where the feedback is too harsh and unpleasant to use as a background noise or atmospheric pad.
Can you please tell what i can try to tame it in a certain pedal? I'm attaching the schematics. Much appreciated!!