Adding a blend knob to a circuit design, not to an already finished pedal

[Tuxedo3]

Hey all, I've been designing an Acapolco gold with Gain, Volume, and Tone knobs. Here's the question how do I add a clean blend to my schematic? I know that the "clean blend" question is brought up all the time but I hope I'm being clear that I'm not trying to add a totally separate PCB to my pedal that I splice my in and out to, I'm trying to incorporate the clean blend in my actual PCB design. Make sense?

[samhay]

>Make sense?
Yes.

But the design, if I have looked at the correct schematic (perhaps you could clarify?), does not lend itself to an easy mod to add a clean blend. You will need to add an input buffer and you will probably also want an output mixer, so it may be easier to go with a second board of known design.

[Tuxedo3]

But the design, if I have looked at the correct schematic (perhaps you could clarify?), does not lend itself to an easy mod to add a clean blend. You will need to add an input buffer and you will probably also want an output mixer, so it may be easier to go with a second board of known design.

good observation, I was thinking the same thing. It's an odd circuit with absurd amounts of volume, so I was wondering how that would factor in to the blend. My issue with implementing a pre-made board is depending on how this one goes I'm interested in producing a lot of these. I guess I could cross that bridge when I get to it.
file:///C:/Users/dgroh/Downloads/Blend%20Pedal.pdf this is a really cool blend pedal I found on this site, just for info.

[midwayfair]

In general:

You need a clean signal to tap and a clean output stage to re-inject the signal, with the additional condition that your polarity should remain the same when you tap the signal.

Places where you can tap a clean signal without a polarity flip are:

1) the input itself provided it can be buffered to prevent feedback when you reinject the signal.
2) On transistor designs, the emiter or source of the first transistor, PROVIDED it is not bypassed by a capacitor.
3) An any design, an input buffer if it's there.
4) Some designs will have intentionally clean but amplified first stages.

What you can't do is just do an end run around a circuit. You'll either get positive feedback (squealing noises) or negative feedback (silence) or just load your guitar signal into oblivion (when it sees a 10K volume pot or something).

This circuit is just a slammed LM386. You have no clean spots in the circuit except the input, so you have no internal active devices with a clean signal to repurpose for a clean signal, so you need to add a buffer to the input. Then you need to figure out some way to make the wimpy clean signal even remotely as loud as the dirt signal.

You aren't doing this without another circuit, and a single buffer probably won't cut it. An op amp might. Then you might as well build a proper blender-splitter.

Do things the right way, especially if you're going to sell them.

[Tuxedo3]

You aren't doing this without another circuit, and a single buffer probably won't cut it. An op amp might. Then you might as well build a proper blender-splitter.

Do things the right way, especially if you're going to sell them.

I don't think I explained this very well, forgive me.

I'd love to use a circuit to add a blend knob, even one that requires an opamp because an input buffer is necessary. I just want to incorporate the blend circuit into the PCB i'm making, not have to splice in a second PCB that only functions as a blend. Bad explanation on my part. Jon thanks for the thorough explanation.

[Rockhorst]

I don't understand your question. You're designing a PCB, you know of clean blend solutions so...what's stopping you from adding the extra parts to your PCB design? That is how I read your story.

By the way, even if the effected out is phase reversed from the input there's ways to fix that with and extra transistor stage. It might be a good idea to have a gain trimpot on the clean signal so you can boost (without distorting) it a little bit to better match the effected signal.

I've designed PCBs for all these potentialities in the past. If I wanted to add them to a new PCB as a standard feature I'd just copy/paste the parts and route signal wires on the PCB instead of external wires when using a breakout board. So: input to the board, split signal in two, one part remains clean the other is send through the effect, add a mixer/blend stage at the end, output brom board.

[Tuxedo3]

I don't understand your question. You're designing a PCB, you know of clean blend solutions so...what's stopping you from adding the extra parts to your PCB design? That is how I read your story.

Didn't mean to ask this in a cryptic way, I should've taken my time to ask it in a more detailed way. This is exactly what I want to do, I want to just add the extra parts to my PCB design. My question REALLY is, how do I structure the blend circuit with the Acapolco Gold? The AG has a volume pot at the end, does that mean the volume pot goes right before it gets sent to the blend knob? In all reality i'd love the volume pot to control the volume of both circuits while the blend controls the ratio between the two.

It might be a good idea to have a gain trimpot on the clean signal so you can boost (without distorting) it a little bit to better match the effected signal.

This is a great idea and answers another question I had about the volume difference between the AG and just a buffered clean signal.

So: input to the board, split signal in two, one part remains clean the other is send through the effect, add a mixer/blend stage at the end, output brom board.

This makes so much more sense as to how that link I posted works. So it's just a buffered split at the beginning (does this have the potential to effect the AG circuit since buffers can mess with heavy distortion and fuzz?).

[storyboardist]

Here's an example of what I think you're trying to achieve. The guy who designed this used JFETs for the blend circuit.



When I implement a clean blend into a design I generally use an opamp, but the concept is the same. Something along these lines: http://www.grindcustomsfx.com/wp-content/uploads/2017/05/Schooner.pdf

[Tuxedo3]

Here's an example of what I think you're trying to achieve. The guy who designed this used JFETs for the blend circuit.

Correct me if I'm wrong but it looks like the split is happening before the buffer in that schematic, what am I missing?

[Tuxedo3]

Here's what I've got so far. It's a fuzzdog Sunny T with a tone stack from AMZ plus a blend control.

[Rockhorst]

The signal is split AT the buffer basically. You should look at it more like that.

[Rockhorst]

This should help you out. Verified and tested, just insert your effect between FXI and FXO. It's a modification of the Schooner by Jubal/Grind that was previously mentioned.

[midwayfair]

Here's what I've got so far. It's a fuzzdog Sunny T with a tone stack from AMZ plus a blend control.

How much isolation between signals do you think 10K at the output can provide?

There's a reason that some of the blend circuits linked to have more complicated setups. They're to deal with the problem that the two signals can have wildly different levels and that there are a limited number of ways a single potentiometer can provide complete isolation. I highly suggest reading the entire Geofex article on this.

[Tuxedo3]

This should help you out. Verified and tested, just insert your effect between FXI and FXO. It's a modification of the Schooner by Jubal/Grind that was previously mentioned.

This is awesome, thank you for this! Will that "level" pot help me with balancing the clean vs. effect volume difference?