Effects loop in PT2399 echo feedback loop

[Ksander]

Dear all,

I have previously built a PT2399 echo using the schematic from the datasheet along with the anti latch-up mod explained on electrosmash. This effect works nicely. Now I would like to modify the circuit to add effects to the delayed signal, such that effects compound with each repetition of the original input signal.

I'm not very knowledgeable about electronics and have only just started building effects. I think the way to go is to open up the circuit at the wiper of the 50k pot, or before this pot, but after pin 14; connect this to an output jack via a buffer, and then reconnect the circuit with an input jack via a buffer somewhere at or before the 4.7uF electrolytic capacitor. I'd like to use a TL072 for the buffers. I am, however, confused by the orientation of the capacitors: coming from pin 14, the + of the 10uF cap is oriented towards the chip, but so is the + of the 4.7uF cap connecting to pin 16.

Can anyone tell me how to wire in the buffers and additional jacks?

[antonis]

Hi & Welcome..

Both pin 14 10μF and pin 15/16 4.7μF caps are oriented with their negative leg to GND, via 50k pot & 10k resistor respectively..
(same stands for 4.7μF Input cap, also..)

Can't say about in-out buffers interconnection 'cause I'm not able to get what you wish for..

[Ksander]

Hi & Welcome..

Both pin 14 10μF and pin 15/16 4.7μF caps are oriented with their negative leg to GND, via 50k pot & 10k resistor respectively..
(same stands for 4.7μF Input cap, also..)

Can't say about in-out buffers interconnection 'cause I'm not able to get what you wish for..

Thanks for the quick reply! What I want is to add effects to the feedback loop. Maybe I'm wrong, but I thought that the signal coming from pin 14 is the delayed signal, which is fed back to pin 16. So if I'd add effects in between, I'd achieve my goal?

It looks like both 14 and 16 going to ground however is not compatible with this being a feedback loop?

[anotherjim]

An effects loop starts with deciding if it's for line-level gadgets such as studio outboard or other guitar pedals. Let's assume it's the latter.
The signal level in the delay can peak around 3v which is hotter than guitar generally so a volume pot for the send is wise. You already have one in the 50k so its wiper feeds the effect send jack. Nothing else is needed for that.
The effects return can make use of the volume control of the last thing in the external FX chain so it only needs to feed the 10k that heads back to pin 16.
Being clever, you can use switched jack sockets so with nothing plugged in, the 50k pot wiper connects to the 10k as intended.

[antonis]

It looks like both 14 and 16 going to ground however is not compatible with this being a feedback loop?

Compatible enough 'cause 50k pot wiper feeds a proportion of pin 14 out to pin 16..
(50k - x / 50k, where x = resistance between wiper and grounded lug..)

edit: Jim is faster..

[Ksander]

An effects loop starts with deciding if it's for line-level gadgets such as studio outboard or other guitar pedals. Let's assume it's the latter.

Yes, that's what I want to do

The signal level in the delay can peak around 3v which is hotter than guitar generally so a volume pot for the send is wise. You already have one in the 50k so its wiper feeds the effect send jack. Nothing else is needed for that.
The effects return can make use of the volume control of the last thing in the external FX chain so it only needs to feed the 10k that heads back to pin 16.

I tried doing it like this (in between the wiper and the following 10K pot), and then I connected a "Howie's Metal Simplex" fuzz pedal to the send/receive part. Unfortunately, this only produced noise and some very high pitch squeal. While troubleshooting, I read somewhere that these simple fuzz pedals have some issue with the impedance, which I understood as them sucking up too much signal. Also, I read on 'some other forum', in a thread on putting an effects loop into any effects pedal, to use buffers in both the send and receive parts.

Using the TL072 seemed a clean solution for this, but I got confused in implementing it: what to use as the Vref -the ref pin from the PT2399?, and which wire goes to the non-inverting input and the output of buffer 1 for the send? And which wire goes to and from which pin on buffer 2 for the receive? The cause of this confusion was that the orientation of the capacitors seems to be inconsistent with the path that the signal takes (unless the audio signal can travel through a capacitor from "in = +" to "out = -" as well as vice versa - does that make any sense?)

Are the buffers unnecessary? Should it work without buffers for any effect inserted in the feedback loop?

Being clever, you can use switched jack sockets so with nothing plugged in, the 50k pot wiper connects to the 10k as intended.

This would be really nice. I've got one of those jacks lying around, so I will definitely implement that - once the main thing does what I want

Thanks for the help!

[antonis]

You can get some ideas about buffers and Vrefs here: https://www.diystompboxes.com/smfforum/index.php?topic=122859.0

P.S.
Tom is a very willing guy as long as you play along with his liking..

[Ksander]

Thanks for the suggestion, but I don't really understand what to look for in that topic. Could you give a more specific pointer?

Aside, I just probed the signal path to see from where to where the signal goes. It appears that the 10K resistor is where the delayed signal is coming from, instead of where it is going towards. So I'll go from here...

[antonis]

Could you give a more specific pointer?

Just a general idea..



P.S.
Just make sure RETURN buffer output has a DC blocking cap (like the 4.7μF one)..

Warning: Possibly popping prone configuration..

[anotherjim]

The basic method I outlined requires the pedal feeding the return to be capable of driving a low impedance. That often means basic things like vintage fuzzes are out. The problem is the 10k resistor mixing the echo back to the PT2399 input is too low and loads down the fuzz circuit. It really needs a buffer but some pedal outputs do include a buffered output as Boss do.
However, a fuzz also amplifies a lot and since it's going to recirculate through the delay it will just build up to a mash of noise!
You really must only use fx with normal unity gain like phasers/flangers/eq etc in the loop. There is nothing to stop you from treating the fx send as a separate delay-only output which is easily tried if your amp has a second input. It won't recirculate but you will be able to smash it to death with a fuzz.