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

[jesuscrisp]

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 DIY classic, the one that is always mentioned first whenever people ask. What it actually is, is a JFET buffer that splits into 2 paths into a blend pot. As pictured here, some versions use a single JFET, others do 2 parallel ones. From my experience it doesn't matter whether it's a single JFET or 2. Linear pots usually work better, ideally even using a smaller value, else you lose a bunch of volume towards the middle portion. You could theoretically also go for any other buffer you prefer, whether it's an op amp, BJT or MOSFET transistor. Regardless which version we're looking at, I personally am not a huge fan as it really is just a quick and dirty less than ideal solution that you should only resort to if you want to drop in a small footprint clean blend for whatever reason.

Why it sucks?

• A direct clean blend will at best only reach unity volume as nothing is boosting it, hence it will be easily overpowered by whatever else you're blending it with.
• It won't work with phase-inverting circuits (right-away) as it will put your clean signal out of phase.
• You don't have a master volume so you'll always need to adjust your blend AND volume knobs.

What can you do to improve it?

• More volume: boost your clean path with a small clean boost circuit BUT be aware of phase issues! In case you're using a parallel circuit that doesn't flip phase you would need something like a NON-inverting op amp to boost.
• Phase-inversion: putting a boost in parallel with a phase-inverting dirt circuit can kill two birds with one stone. Basically speaking, any single-transistor boost (LPB-1, SHO, ... ) or an inverting op amp gain stage can take care of flipping phase AND getting you more volume on the clean path.
• Master volume: If the dirt circuit you're blending with is stupidly low volume itself, you can also just skip the volume control on the dirt circuit, put a boost after the blend knob and go with a volume knob at the end. The issue is with any circuit that ISN'T low volume, since the blended clean signal is only unity gain when maxed out and will be overpowered by the parallel circuit.

Truth is though, since now we're already using 2 transistors or a dual op amp and a bunch of caps and resistors to solve the inherent drawbacks of this simple circuit, why don't we go with something that gives you better results right away?


2. Op Amp Panning Mixer


Just like the Split n Blend, this is a "panning" clean blend. It goes from completely clean to completely dirty, which depending on your application may be desirable. These snippets are taken from the PedalPCB Bootleg. As much as I like to trash JHS, this blend design is an actually smart one (mostly, NE5532 isn't a great input stage for a high-impedance buffer, there was zero need to also keep the BJT buffers of the TS and the volume is also BEFORE the blend knob).
What this one needs is a buffer, no matter if BJT, JFET, MOSFET or op amp, but using a dual IC is usually the most practical (unless your circuit has a leftover IC stage anyway for example). From the input buffer (left picture), you go with a capacitor for each path and end up at the blend control (right picture, don't forget your caps that aren't picture here!).

Why is it better?

• The summing op amp stage will provide a volume boost to the respective side of the blend control while also actually turning the other side down, not just putting resistance in the respective signal path.
• You can give your circuit more volume by increasing the feedback resistor of the summing op amp (R21 in the picture) without needing another stage, plus also tame some high end with the feedback capacitor (C12 here).

When is it not ideal?

• Very loud circuits might be hard to balance (and worst case could make the mixing stage distort), so you might want to mess with the resistor values on either side of the blend knob.
• Blending phase-inverting signals might still be an issue, same solutions apply as in the Split n Blend, although you can get away with an inverting op amp buffer or just "unity gain" transistor boost.

3. "Additive" Op Amp Blend


My personal favorite solution because honestly, I don't ever feel like I need less overdriven than clean signal when putting a clean blend in a circuit. This snippet is from my thread about simplifying the Gladio/Scenario SC, find it HERE.
Like the previous blend circuit, this needs a buffer to split off the clean signal and a capacitor to the clean blend knob, as well as an op amp mixing stage. What we're doing here is basically just turning up and amplifying the clean signal and mixing it with the dirty path. The other side of the pedal stays the same volume throughout, you're just adding back clean signal and thereby potentially some clarity and dynamics. It's basically what the Klon is doing, just seperated from the gain knob here and simplified. Add a master volume at the end.

Why I like it?

• Very simple, very effective, probably the best choice for most things that aren't EXTREME fuzz and distortions like Muffs and HM-2s. Put this on your favourite low to medium gain overdrive and get back some dynamics and sparkle or crank it up!
• It's pretty low parts count all things considered.
• You can also use it to amplify low volume dirt circuits (adjust R9/R10).

When is it not ideal?

• Yet again, it doesn't help if the circuit in parallel is out-of phase.

4. ROG Splitter-Blend


Another DIY classic, this one is more aimed at blending 2 FX loops as opposed to just a clean signal with your favorite dirt pedal that you wish had a clean blend.
IMO it is quite bloated, but if you want a tool that helps you come up with circuit combinations to blend, it's probably useful. The phase inverter might also be useful if you're building a circuit with a clean/fx loop for people to plug their pedals into.
Other than that it really is just an elaborate Split n Blend that gives you the built-in phase inversion option and also buffers both paths at the output.

5. Multi-Channel Op Amp Mixer

This is basically the elaborate version of the "Additive" blend, only that instead of only just giving the clean signal a volume control, you're adding one to each path, which could be 2 or more. This is a good option if you want seperate clean and dirty volume controls or you're running 2 dirt circuits in parallel that you want to control the level of individually. Each of the inputs in the picture would be the output of your respective path and after the summing up amp you can also add a master volume. This one is more for the sake of completeness really, but there's certainly use for it other than just building a 3 channel mixer.

For most guitar effects, I feel like the volume pots would be better off as 10K ones and be aware that the schematic pictured here is running off a bipolar supply, so all the ground connections would be Vref (half supply voltage) instead using a standard power supply in your circuit.


6. General Tips

• Be aware of phase inversions! As a general rule, inverting op amp stages and transistor stages where the signal goes through the Drain/Collector flip your signal. Non-inverting op amp stages and transistor "buffers" where the signal goes through the Emitter/Source do not flip phase. If there's an even number of phase inversions, it's NOT flipped, if it's an uneven number it IS flipped.
• As mentioned, a single-transistor boost or spare IC stage can help to flip the phase back if needed.
• If the circuit you want to add a clean blend to already has an input buffer, by all means just use that input buffer for signal splitting or replace it, there's no need to go buffer into buffer (like the Bootleg/Moonshine).
• JFETs and JFET-input op amps are generally preferred for signal splits/input buffers due to the high input impedance they can have, but there's nothing wrong with using BJTs or BJT-input op amps either.
• If you're designing a pedal from scratch, DON'T just blindly go for the Split n Blend circuit.
• Clean-blending vintage-style fuzzes will require you to make adjustments to the circuit or add a pickup simulator transformer AFTER the buffer in the path of the fuzz. Else you have the effect of a buffer in front of a fuzz pedal, which generally is a no-no.

There's also more elaborate but also stupid blend designs out there but I think these here cover most of the ground.
Please feel free to ask stuff and also correct any of my mistakes, preconceptions, generalizations or inaccuracies.

 

[phi1]

awesome write-up!

My understanding is JHS didn’t design that blend, I think it originated from an RG Keen article (with the 15K Rs and 10k pot). It’s possible he pulled it from somewhere, but he talks about the theory of choosing 15k and 10k, not just blindly copying.

PPCB uses it in the fv-1 projects here, and I’ve used it in some designs, since I think it works well (controlling for the caveats you mentioned).

 

[bluedmc777]

Suprised there wasn’t any other users on here sharing their thoughts. I’ve always been interested in blending. I’ve probably got 20 different schematics saved. Nothing on adding a blend to a completed circuit though.

Thanks for sharing.

 

[jesuscrisp]

Suprised there wasn’t any other users on here sharing their thoughts. I’ve always been interested in blending. I’ve probably got 20 different schematics saved. Nothing on adding a blend to a completed circuit though.

Thanks for sharing.

Adding a blend to a complete circuit is often sort of an afterthought and the buff n blend is perfectly ok for that. But if you're designing a circuit from scratch, there's better options depending on what you want to achieve, just like I outlined when and where I'd use what sort of circuit. Ultimately, none of this is rocket science once you figure it out, but there certainly is a bit of nuance and decisions you need to make. I usually go for the most economical but effective solutions.

 

[fontoponto]

Thank you for this information!
I ended up using this one which is easy to use and has some information for clean boost as well as a phase inversion:

http://www.rutgerockhorst.com/nucleonfx/documentation/fusion.pdf

There was another version on the madbean forum but it's not available right now

https://www.madbeanpedals.com/forum/index.php?topic=26242.0

 

[jesuscrisp]

Well the Fusion one is just an IC based buff n blend...

 

[bluedmc777]

The fusion reminds me of the xotic fx x blend. Not sure if they’re the same though.

 

[jrhevron]

Anyone know where to get the nucleon FD boards?

 

[jrhevron]

I never done perf…how hard would it be to make an additive op amp blend like the nucleon fix one (starting from scratch)?

 

[thunderaxe]

i'm interested in adding a clean blend to an existing distortion effect. pretty certain i'm going to go with a dual op-amp version, but i'm not sure if i'll be doing the 100% dry to 100% wet version or the purely additive one. i had a couple of questions 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) it would be particularly appealing to me if i could also use the clean audio path as a buffered bypass path, so that you can choose between either true or buffered bypass. the way i envision this (and if there's a better way of doing this please let me know) the 3PDT stomp switch still switches between on and bypassed, but a second toggle switch changes whether the bypass path is true bypass or back into the effect and then out the clean blend path before the chosen/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]

Further to phase-considerations...

It should be noted that many circuits SLIGHTLY SHIFT the phase (via caps, topology, whatceteraver), without necessarily flipping it 180º.

It's when anything gets close to 180º that the problems ensue; yet flipping the phase of one signal may not end all your worries — depends on how the signals are lining up with each other, the fundamentals and partials.

https://www.talkbass.com/threads/question-about-blend-pedal-w-phase-inversion.435436/

Bongomania*: "...there is no guarantee that any one of your pedals will sound better or worse with the phase inverted, as each of them will change the wave somewhat, changing the alignment and thus changing the interaction with the parallel signal. ..."
*If unfamiliar with Bongomania, he is a knowledgable compressor-junky over on Talkbass and has sold his own comps for a spell and has a great resource of compressor reviews at Ovnilabs.

If you need a phase inverter, there are options as posted in the most excellent OP. Ideally you'd have phase-shifters, ie adjustable inverters that can tweak the phase from 20º to 180º — great on a workbench with a scope, not so handy on a gig.

Bongomania continued from above:
"If the effected signal was either perfectly in phase or perfectly out of phase with the clean signal, then flipping the phase would cause exactly as many problems as before. Frequencies that had been cut too much would now be boosted too much, and vice-versa. It's only because the phase variances are infinite that phase inversion can sound good at all- you get wave boosts and cuts that are different from what you had before, but not "extremely" so, and may sometimes sound better. Typically the boosts and cuts that occur from flipping these infinitely-variable phase differences are less drastic, and thus more musical and useful, than flipping perfectly-180-degrees would be."


Things get further complicated by the fact that you may be adding or taking out pedals from your chain mid-song and depending on which pedals are inverting or not will affect the rest of the chain. So flipping the phase in one of your clean-blended pedals may not help you overall, it'll just ensure that one pedal doesn't sound like garbage.

How does a pedal reverse phase?

How does a pedal reverse phase?

www.diystompboxes.com

KESH
"You can set up a stage (transistor or op-amp) to invert or not invert, by using the appropriate components and connections around them.

Transistors generally invert when you need voltage gain, as in many gain type pedals, and usually non-invert when you need current/impedance gain, often at the buffers at input or output. Typical inverting circuit is called a common emitter (or common source). Non-inverting is called an emitter (source) follower or common collector (drain).

Op-amps are more flexible and you get more choice over whether to invert or not.

Whether the pedal as a whole does or doesn't invert is down to how many inverting stages there are."

From the same DIYSB thread above,
PRR: "If you want *both* voltage gain and current gain, ALL one-device amplifer stages invert. Tubes, BJTs, FETs whether J- or MOS-."
The exception, he notes, involves transformers, and some discussion of mics etc. Check the thread if interested.


Some interesting info from our own Chuck D Bones on a circuit that crosses the inverting-line back and forth:

Post in thread 'Sweepable mid values with Maleficent Mids'

Dec 13, 2023

We'll set the HI/LO switch to LO and MIDS at 10. Doesn't matter where SWEEP is set. With MIDS at 10, C3 is connected to IC1-1. Under these conditions, the signal at IC2-3 is +7dB above the level at IC1-1 and shifted 180° at resonance. IC2.2 subtracts the signal at IC1-1 from the signal at IC2-3. Since those two signals are 180° out of phase, they are effectively added. The gain at IC2-7 is therefore 14dB at resonance.

Now let's look at the opposite case. Set MIDS to 0. Now C3 connects to IC2-7. At resonance, the signal at IC2-3 is 7dB greater than the signal at IC2-7, and it's...

• Chuck D. Bones

If you search for it, on Talkbass or TGP and possibly elsewhere, you will find lists of pedals that invert phase.


If you need to invert phase...
Check GGG's Dual Booster – Polarity Switcher (Invert/Non-Invert as from the GEOFEX Site) pdf.
RG Keen's method utilises uses a single opamp (5 resistors 2 caps), whereas the
Parasit's Phase Inverter-Buffer uses a dual opamp (6 resistors 2 caps).

From Parasit's Frederik on his utility vero-board from the link above: "Here is a small utility board - a simple buffer followed by a phase inverter. You can take the output from either stage. It can be very useful if you are adding a clean blend to a circuit that is inverting the phase.

I recommend putting this in front of your main effect, but keep in mind that not all effects will respond the same with a buffer in front. I didn't include polarity protection, DC filtering and an output capacitor since this is meant to be used together with another effect (not stand alone)."




SIDE-NOTE

Spoiler: Inversion Tester

I have just started searching for a way to test if a pedal inverts or not. I want a box that you plug any given pedal into and an LED will light up RED if it inverts and GREEN if it does not.

It is for existing PEDALS, not for looking at a schematic or breadboard and counting how many times the circuit inverts phase (even number it won't, odd number it does). Also, NO! — I don't want to plug the pedal-in-question into the signal chain and listen for phase-cancellation / signal-degradation — the whole point of the Phase-Detector-Box is to determine in advance whether I have to stick a scum-sucking-inverting-infilltraitor into my chain via a separate inverting looper-switcher pedal.

This is not for the scope of this thread, interested parties in developing such a Phase-Philanderer-Phinder can PM me and if there's enough interest I'll start a separate thread. If there's not enough interest, then fine, I'll work out my own Phase-Philanderer-Phinder — with Blackjack and Hookers.

 

[Feral Feline]

I never done perf…how hard would it be to make an additive op amp blend like the nucleon fix one (starting from scratch)?

Download DIYLC (Do It Yourself Layout Creator), a donate-something-but-if-you-don't-it's-free program.
I don't have a direct link to download Bancika's software, but you should be able to find it without too much trouble

If I can learn how to use it, then anybody can.

Try laying out some simple one-transistor circuits first, then an op-amp dirt-circuit... build up to layout out the blend, and then it use it with the dirt circuits you've laid out!