Two delays in parallel

[Guzz]

Hi guys,

I've recently been toying with the idea of having two delays running in parallel. One would be footswitchable, as shown on the far right on the schematic.

The idea being that I would have one delay on short repeat time and the other one on longer repeat time for some whacky/noisy/drony effect.

Would the following schematic work or would I need additional splitting/mixing ?

https://imgur.com/wfcbmnA

Thanks.

[induction]

The splitting is probably ok, though there may be some cross-bleed with the output of each feedback pot feeding the input of both delay chips.

The mixing probably needs some work. The last opamp is a differential amplifier, designed for two inputs. The volume of the individual signals is determined by resistor values. You may want to rework it to properly mix three signals.

It might work fine as drawn though. It's worth a try on the broadboard.

[Guzz]

Would grounding the input of the second delay when it's switched off cure the potential cross-bleed ?

I guess I have some reading to do about differential amplifier then.

[induction]

It'll help when the second one is off, but when they are both on, you may get delays in series as well as parallel.

Hard to say how much impact it will have, though. Try it and see.

[Guzz]

after a little more digging I found this: https://imgur.com/zK4M7Mr (taken from https://www.diystompboxes.com/smfforum/index.php?topic=99196.0)

Is he using the switching Jfets to avoid that bleeding problem when in parallel or is it completely unrelated ?

I'm starting to think this design is a bad idea and I should instead use two fully independent circuits and mix them together with a separate splitter/mixer, something like this: http://www.runoffgroove.com/splitter-blend.html

[Sphere80]

Take a look at the vfe Mobius Strip project at the Madbean site.

[induction]

I'm not familiar with the switching arrangement in the schematic you linked, but on initial inspection it doesn't look like it would solve the (possible) bleed-through. I could easily be wrong, though, as I haven't looked at the rest of the circuit that closely.

The simple solution to bleed-through would be to replicate the first op-amp stage to provide an isolated input for use by the second delay path. The feedback signal won't go backwards through the op-amp, so the other delay chip will never see it.

I want to stress the importance of breadboarding this before building it or following my advice. At this point, we don't even know if there is any audible bleed-through at all. I only emphasized it because isolated delays in parallel is awesome, but uncorellated delays in series with feedback turns into a racket pretty quickly. I'd want to err on the side of caution, myself. You may feel differently. Especially if you're looking for a noisemaker.

Guzz wrote

I'm starting to think this design is a bad idea and I should instead use two fully independent circuits and mix them together with a separate splitter/mixer, something like this: http://www.runoffgroove.com/splitter-blend.html

Your general approach is excellent, and I've been planning a similar build for quite some time.

As far as building two, fully independent circuits, that's essentially what this is. How to isolate the inputs, and properly mix the outputs is all we're working out. A delay pedal in general is nothing but a splitter-mixer with a series delay circuit in one if the paths. Adding a third path with another delay circuit is simpler than building another nested splitter layer around the whole thing.

By the way, on closer inspection, your output stage is not a diff-amp after all. It's just a passive resistor mixer feeding the output amplifier. You may like it just fine as it is. If not, you can replace it with a simple three way mixer as necessary. The breadboard will help you decide.

[Guzz]

Thanks Sphere, I'll take a look.

And thank you too Induction, your help is really appreciated. From an idea standpoint I know this could be a really fun little box, as I plan to add reverb after the two delays too. I'd like it to be tamable at first, providing some really "ethereal" and atmospheric sounds, but I'll definitely try to make it a little noisy/drony too.

[Guzz]

Ok now I'm lost again.

On the Mobius schematic, which is a parallel delay, we can clearly see that the output of one chip still has a path to the input of the other (through the FDBK pots). Unless I'm going mad which might very well be the case at this point.

http://www.madbeanpedals.com/projects/_folders/VFE/schematics/VFE_MobiusStrip.gif

So I guess that means the inputs don't have to be isolated ? Because as far as I can see on that schematic they are not.

[induction]

Guzz wrote

So I guess that means the inputs don't have to be isolated ? Because as far as I can see on that schematic they are not.

You're getting ahead of yourself. I haven't said that the inputs have to be isolated. I said it was something to check for. If you want to know for sure, just breadboard it.

I think maybe you've been taking my comments differently than I've intended them. All I've been saying is that there is some potential for bleed-through here. What to do about it depends on how big of a problem you actually find when you breadboard it. If you don't hear any bleed-through, you might choose to accept the design as it is.

I have no idea whether the Mobius has good isolation or not. Madbean has good designs, but I haven't built it so I can't say.

Again, I'm not saying your design is bad or needs to be changed. I'm just pointing out some things to check when you breadboard it.