clean blend that doubles as optional buffered bypass?

[thunderaxe]

hi all,

i'm interested in the idea of adding a clean blend to an existing distortion effect, and i'm wondering if it's possible to also use it as a switchable buffered bypass option.

with the help of this very useful tutorial by @jesuscrisp:

J

TUTORIAL Thread 'Why Your Clean Blends Suck ... or "How to (Clean) Blend"'

Feb 16, 2024

Since there have been a couple of threads on the topic of "I want to do this pedal but with a clean blend" and "I want to blend two circuits" and I don't want to repeat myself all the time, here is a bit about clean blends. I have zero electronics background, am a self-taught pedal nerd and won't be able to answer ALL technical questions they arise. I am more of a f*ck-around-and-figure-it-out guy who loves to keep things reasonably simple, but also loves if things are engineered nicely.

So that being said, this is how to blend...

1. Buff n Blend

The...

• jesuscrisp
• Replies: 11
• Forum: The Test Kitchen

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i feel pretty certain that i'm going to go with a dual op-amp buffer and active blend setup, though i'm not sure yet if i'll be doing one that goes from 100% dry to 100% wet version, or just the purely additive one. i do have some questions for the forum though:
1) are there particular advantages and disadvantages to placing the input buffer before or after the split? should i buffer the input right off the bat and then split passively into clean and distorted paths, or split first and then have the clean path alone have a unity gain op amp buffer? the first stage in the distortion circuit is a non-inverting op amp boost of +10.4dB with a 15hz high pass, which should serve as the buffer for that path.

2) the way i envision doing the buffered bypass -- and if there's a better way of doing this please let me know -- is to have the 3PDT stomp switch still switch between on and bypassed, but a second toggle switch changes whether the bypass path is true bypass, or send the signal back into the effect, and then out the end of the clean blend path before the clean/distorted mixing section. the question for me is whether it's possible to get an isolated clean signal to the output through this method without the distorted signal "leaking" backwards through the mixer and into the clean signal. i've attached some crude concept diagrams of the switching scheme that will hopefully make my idea clear.

 

[Feral Feline]

The "ON, TBP" and "BP, TBP" diagrams are the same.
The "ON, BBP" and "BP, BBP" diagrams are the same, save the additional text regarding leakage.

What's being bypassed?


[EDIT] Ahhh, nevermind. I finally clicked on one of the screenshots and saw the full pic.
I would say that in the last diagram, there's no way around having the buffered bypass with the clean&distortion separate — you have to have some way of separating the two if you want just clean on buffered-bypass.

The following was written before the lightbulb came on, but all still holds true...


Please post a schematic, whether complete or just the dirt-circuit.

I'm not seeing what the advantage is of sending the "buffered" bypass back into the circuit's "clean" path.

Why wouldn't you just have a bypass that either sends the signal through the dirt-circuit (with added clean-blend) ie engaged,
or bypassed one of two ways:
1) Buffered,
2) Unbuffered.

What could be cleaner than completely bypassed (whether buffered/unbuffered)?


I'm working on a boost with buffer, that can be true-bypassed or bypassed while retaining the buffer in the signal path.
Aion's Cygnus (Cornish G2) is the closest I've found so far to that bypass topology, yet uses a much more complicated switching method than what I'm intending to use on my boost-buffer build.






Maybe the Cygnus' bypass has something to it that may in turn help you.

 

[thunderaxe]

i'll look into the cygnus switching and see if i can wrap my head around it, but in the meantime:

if i was really committed to the switchable true/buffered bypass, obviously the easiest thing would be to just give it its own switchable buffer op amp dedicated to the bypass path. but honestly i don't care enough about that feature to do so, i just think that if the clean buffer can double as a bypass buffer, it would be a cool bonus feature to throw in there. so my idea was, when the pedal is in buffered bypass state, to route the signal through the clean path buffer and then out before the clean signal gets mixed in with the distortion (the green/blue switch).

however, since the signal is still also going in parallel through the distortion path, and then downstream getting mixed together with the clean signal, the question then is, will the signal from the distorted path "leak" back through the mixer into the clean path when it's in that state? are you saying it's unavoidable the way i have it set up?

 

[thunderaxe]

okay, i traced the paths, it's pretty much the same idea as mine except with two differences:

1) the switch between buffered and true bypass is a 4PDT, and the other two poles switch in the two resistors at the bottom when it's in buffered bypass mode. i don't really understand what those do and why they're necessary, any help would be appreciated.

2) there isn't a clean blend path for the distorted signal to potentially "leak" back through, so that isn't a concern here.

 

[Feral Feline]

Wow, great work.

I was trying to wrap my head round the extra couple switching-bits — being for the resistors all makes sense now, thanks for working all that out, your latest screenshots really helped me better wrap my brain around the Cygnus' switching.

R38 and R39 are setting the output impedance of the buffer when it is used in BP-BBP mode.
Electrosmash explains that resistor pair in the analysis of the Microamp.
The analysis says they also limit the output current and protect the op-amp that precedes them.





If you use some isolating resistors on the outputs of both the CLEAN and DIRT, then you shouldn't get bleed-through. (I think/hope — depends on what the rest of the circuit looks like / is doing).
Here's an example of the isolating resistors, as at the end of this modded ROG SplitnBlend (100Ω to 1k for R1 and R2 is suggested as being sufficient), from this thread:

Clean blend with clean volume/boost...Can someone look at this schematic

Clean blend with clean volume/boost...Can someone look at this schematic

www.diystompboxes.com

I don't know if there's unavoidable leakage or not, I'd need to look at the full schematic of your project,
and even then, nothing beats breadboarding it and finding out definitively.

 

[thunderaxe]

the pedal i'm looking to do this to is the sour grape (keeley rotten apple) but i don't think that actually matters so much as the way i'm planning on wrapping a clean blend around it, so my plan was to have an op amp buffer up front:


though, as mentioned in the first post i don't know if it's better to put it right up front, and then split into the clean and dirt paths, or split first and then put the buffer only in the clean path.

and then in back, an active op amp mixer, either R.G. keen's panner that is equal volume 100% clean to 100% wet:



or else, if i decide i don't ever need to turn down the distorted signal relative to the clean, i might go with a purely additive mixer at the end:



in both cases i'm pretty sure it's only the ratio of the resistors that matters, so if it would help, you can add one or even two zeros to their values.