Summing Signals?

[Ripthorn]

I'm dinking around with a couple of ideas, one of which involves cascading two Deep Blue Delays. I breadboarded this, where the output of the first one goes to the mix pot and feeds the input of the second delay. The second delay's output also goes to the mix pot, which naturally introduces a feedback loop. If I want to sum these two signals without creating the feedback loop, will I need to buffer each output? Is there something easier? Will a single opamp summer work? If you have any insight on this, I would love to hear it. Thanks!

[antonis]

It depends on particular Deep Blue Delay version..
(there are simplified versions with inverted output whereas original version isn't inverted..)

[Ripthorn]

I'm using the schematic from pedalpcb.com, https://www.pedalpcb.com/product/seabed/

This uses the inverting input to the output buffer. I'll try to make a quick block diagram of what I'm doing.

[PRR]

"Phase" does not matter around a "delay". (Any useful delay is multiple in/out/in/out-phases.)

Yes, a block diagram!

[Ripthorn]

Had a little time to breadboard. I used some simple BJT budgets to split the signal from the first pt2399; one to the mix knob, the other to the second pt2399. The output of the second pt2399 went through another buffer to the mix knob. Much better, but I was getting some distortion and such, which I suspect may be due to the bjt buffers. I'll try with opamps tomorrow and see if it gets better.

I sat down and drew a basic block diagram, but can't post it from my phone. I'll post it when I get back to my laptop.

[Ripthorn]

Alright, so here are the simple block diagrams. First one is what I had, where there was a clear feedback loop on the second PT2399. The second one was what I did last night. I get some distortion with the second one, though, and I'm not sure if it's due to impedance issues or what, but for the BJT buffers I am using exactly what Jack Orman has on the AMZ page (http://www.muzique.com/lab/buffers.htm). I plan to try opamp buffers next, but any advice would be greatly appreciated. Thanks!

[ElectricDruid]

Use a op-amp mixer circuit to mix the two signals together and then you won't have the feedback problem and you won't need all those buffers cluttering the thing up. It'll look like your first diagram, except without the feedback around the second delay

The op-amp mixer has the advantage that the signals don't "leak out" back up the wires to where the other inputs came from. The benefits of virtual earth mixing!

[PRR]

[Ripthorn]

Use a op-amp mixer circuit to mix the two signals together and then you won't have the feedback problem and you won't need all those buffers cluttering the thing up. It'll look like your first diagram, except without the feedback around the second delay

The op-amp mixer has the advantage that the signals don't "leak out" back up the wires to where the other inputs came from. The benefits of virtual earth mixing!

That is exactly what I was looking for! I wasn't stoked about all the buffers, but I didn't know if the opamp mixer would "leak out" the signal, so it wasn't my first go. But I'll implement know and report back! Thanks all!

[Ripthorn]

This seems to be doing the trick. Thank you, fine people!