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DIY Stompboxes => Building your own stompbox => Topic started by: Elijah-Baley on December 07, 2024, 04:04:16 AM

Title: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on December 07, 2024, 04:04:16 AM
Hello!
After an AB box to plug two guitars, a friend of mine, would like an... ABCD box! Four guitars, at least. Better if there is a stereo output, I still don't know if I can do it isolated or not.

He want a footswitch version and I'm searching some schematics.

I found this one:

(https://brightonion.co.uk/cdn/shop/files/ABCDInputSwitcher.jpg?v=1705838348&width=713)

It's ok, but maybe not so smart as I wanted. I still have to figured out the diagram about it.

I would like to have a schematic with one footswitch with a sequential selection of the 4 input (or 5 input!), of course with one LED for each input.

Then I found this one:
https://www.mikehillservices.com/webshop/switching-pedal-shop/a-b-c-d-guitar-switcher-input-switching-pedal-4-in-1-out/

I need a larger box, but could be nice.

Do you have some good projects?
Title: Re: Searching an intelligent 4 input selector box
Post by: ElectricDruid on December 07, 2024, 06:47:26 AM
At the point at which you get to four inputs, you're looking at either solid-state or relay-based switching. The footswitches become simple SPST buttons which act as controls, rather than being part of the switching path themselves. You could do the control circuit with something like a 4017 decade counter. You keep pressing the button, and it moves to the next input, turning off the previous input. That'd be good for up to ten inputs!

The circuit wouldn't need to be too complicated either - just a debounced footswitch to clock the 4017, the 4017 for the control, and some relay drivers.

If you want more flexibility, you start getting into Arduino or some other microprocessor to do the control. Then you can have whatever control system you can dream up.
Title: Re: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on December 07, 2024, 12:33:03 PM
Thank you, ElectricDruid.
I guess something like 4017 could be enough. But is kind of new for me, I have to find some good schematic for my purpose.
Title: Re: Searching an intelligent 4 input selector box
Post by: RickL on December 07, 2024, 01:09:12 PM
It can be done completely mechanically. Divide your inputs into two groups (A/B and C/D). Switch 1 selects either A/B or C/D. Switch 2 selects either A or B and switch 3 selects either C or D. LEDs can indicate which group has been selected and which input within that group has been selected.

Using the same principle this could be expanded to 8 or 16 inputs. Or, with less efficiency, 5, 6 or 7 inputs. For the 5 input example you would have 3 sub-groups (A/B, C/D and E). Switch 1 selects between the group of A/B and C/D or just E. Switch 2 selects either A/B or C/D, switch 3 selects either A or B, switch 4 selects either C or D. In this case you would presumably use input E as the input you used most often because it could always be accessed by a single button press.
Title: Re: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on December 07, 2024, 01:20:00 PM
Yes, mechanically can be done, somehow. For example, like you just said.
Meanwhile, I was looking at this: https://paulinthelab.blogspot.com/2014/05/1-of-4-guitar-instrument-selector.html

It could be the one I'm searching. Just one footswitch to pass in order from Input A to input D.
I just need to understand if I can add one input more, what happen if I don't plug one or two guitars, if I can set it un-switched, somehow, looking at the datasheet of the CD4017 and the CD4066.
Title: Re: Searching an intelligent 4 input selector box
Post by: ElectricDruid on December 07, 2024, 07:54:44 PM
Quote from: Elijah-Baley on December 07, 2024, 01:20:00 PMMeanwhile, I was looking at this: https://paulinthelab.blogspot.com/2014/05/1-of-4-guitar-instrument-selector.html

It could be the one I'm searching. Just one footswitch to pass in order from Input A to input D.
I just need to understand if I can add one input more, what happen if I don't plug one or two guitars, if I can set it un-switched, somehow, looking at the datasheet of the CD4017 and the CD4066.

You'll notice that on that schematic the Q4 output of the 4017 is connected back to the Reset input, so the counter cycles around 0-1-2-3-0-1-2-3-etc. You can move that reset to any of the ten outputs, so you can have it cycle through as many or as few as you need. There's no way to blank all of the outputs of the chip, so you can't turn everything off without adding something else - one of the outputs will always be on / one of the channels will always be selected. You could add further 4066 chips to add more inputs if required. There's four SPST switches on each 4066, so two chips gives you up to eight inputs.

I don't like that schematic much, I'm afraid. There's no DC conditions set for the 4066 at all. The inputs and outputs are both connected via DC blocking caps, but the chip doesn't have any DC level set either. You should read RG's page about CMOS switching to improve that:

http://www.geofex.com/Article_Folders/cd4053/cd4053.htm

The page deals with the 4053 triple SPDT switch, but the 4066 quad SPST is basically the same thing. The important part here is the details about getting the DC biasing of the switches right, and coupling the signals in and out with capacitors (the schematic you found does that part right, at least).

HTH
Title: Re: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on December 09, 2024, 01:20:32 PM
Let me see if I got it. 8)
In this schematic I could add two 1M (or 2.2M) one across the ground and one across 9v after (or before?) the input cap for any "channel"?
And... a 1M across the ground at the output of the buffer?

(https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg6KT1FKlMGsVI7MP2hR0V7CwGk-QGUDRXPHFo6Qg4vVBPOBbrEG25go6GKCPE4y85fwsr1xQBGjIkCERXGifaL8XgDzxPybTtuNp_A9_zpRl5ksx5Yr7z0NrsAcAHMyzcCTKUNLju0aGIT/s1600/Guitar+Instrument+Selector+Schematic.jpg)
Title: Re: Searching an intelligent 4 input selector box
Post by: R.G. on December 09, 2024, 06:48:17 PM
As Tom says, you will want to bias the signal pins of the CD4066 to half its power supply. From my own experience, it will almost certainly pop when you change signals.

I strongly recommend that you make a 10K/10K bias divider with a filter cap to ground, then run a 1M or so resistor from 4066 pins 1, 2, 3, 8, and 10 to the bias voltage, and also from pin 2 of the TL081.

The 10M/10M bias resistors on the TL081 will add excess thermal noise.
Title: Re: Searching an intelligent 4 input selector box
Post by: ElectricDruid on December 09, 2024, 07:56:05 PM
Quote from: R.G. on December 09, 2024, 06:48:17 PMalso from pin 2 of the TL081.
You mean pin 3, RG. I know you know that.

But yes, otherwise...Do what the man says! Set up a nice bias supply with a 10K/10K voltage divider and (say) 22uF to Ground. 1M resistors from the 4066 inputs to ground, between the chip and the caps. Same again on the output, 1M to bias. Remove the two 10M resistors on the TL081 and replace with another 1M to the bias.

For bonus points / to save a few parts, you might be able to connect the 4066 output directly to pin 3 of the op-amp (no cap) so they only need one 1M to the bias supply. Try it and see.

It might also be possible to drop D1/R1/C6 and just connect Q4 direct to the Reset input. I've certainly seen sequencer circuits that do this, so it should be ok. The way it works is that the Q4 output goes high, so then the Reset goes high too, the counter resets to zero, at which point Q4 goes low again. And this is all over in nanoseconds!

HTH
Title: Re: Searching an intelligent 4 input selector box
Post by: Rob Strand on December 09, 2024, 08:45:58 PM
Quote from: ElectricDruid on December 09, 2024, 07:56:05 PMBut yes, otherwise...Do what the man says! Set up a nice bias supply with a 10K/10K voltage divider and (say) 22uF to Ground. 1M resistors from the 4066 inputs to ground, between the chip and the caps. Same again on the output, 1M to bias. Remove the two 10M resistors on the TL081 and replace with another 1M to the bias.

For bonus points / to save a few parts, you might be able to connect the 4066 output directly to pin 3 of the op-amp (no cap) so they only need one 1M to the bias supply. Try it and see.
A single Vref like that is the best way to avoid pops.

Quote from: ElectricDruid on December 09, 2024, 07:56:05 PMIt might also be possible to drop D1/R1/C6 and just connect Q4 direct to the Reset input. I've certainly seen sequencer circuits that do this, so it should be ok. The way it works is that the Q4 output goes high, so then the Reset goes high too, the counter resets to zero, at which point Q4 goes low again. And this is all over in nanoseconds!

As is the circuit does two things:  It resets off Q4 however it is using R1 and C6 to provide a reset pulse at power up so it doesn't start in a random state.

The thing that catches my eye is the 4017 is powering the LEDs and the controlling 4066. Good idea to check of the output swings high enough for the 4066.
Title: Re: Searching an intelligent 4 input selector box
Post by: R.G. on December 10, 2024, 12:26:11 AM
Quote from: ElectricDruid on December 09, 2024, 07:56:05 PMYou mean pin 3, RG. I know you know that.
DOH!!! Yeah, what he said!

Quote from: Rob Strand on December 09, 2024, 08:45:58 PMThe thing that catches my eye is the 4017 is powering the LEDs and the controlling 4066. Good idea to check of the output swings high enough for the 4066.
Yeah. The LED is a big load for an unbuffered CMOS gate. It might be possible to eagle-eye the datasheet for the 4017 and a high efficiency LED and get away with it. Even simpler conceptually to just hang a CD4049 on the outputs for LED driving. The TI sheet for the 4017 seems to imply that something over 1.5ma will start dropping off the output voltage.
Title: Re: Searching an intelligent 4 input selector box
Post by: ElectricDruid on December 10, 2024, 07:29:24 AM
Ok, I think I've made all the tweaks everyone has come up with. It's turned into a very collaborative project!

(https://electricdruid.net/wp-content/uploads/2024/12/SequentialSwitch.jpg) (https://electricdruid.net/wp-content/uploads/2024/12/SequentialSwitch.jpg)

If anyone spots anything else, please shout.
Title: Re: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on December 12, 2024, 09:16:39 AM
Thanks for your answers.
I will try to follow all your little tips!

But I had to work on veroboard, I didn't want anything too large. Is the CD4069 really necessary?

Meanwhile, I'm working on the layout based on the paul in the lab's one with this mod.
Title: Re: Searching an intelligent 4 input selector box
Post by: ElectricDruid on December 12, 2024, 04:00:07 PM
Quote from: Elijah-Baley on December 12, 2024, 09:16:39 AMIs the CD4069 really necessary?
That depends how much LED current you draw if you connect the LEDs direct to the 4017 outputs like Paul-in-the-lab does. If you draw too much, the voltage will get pulled down so much the 4066 won't switch properly, or you'll fry the 4017 output stages. They weren't really designed to drive LEDs. It'd be sensible to read the datasheet for the particular 4017 chip you're using and see what output current it can provide (mA), and what power dissipation the chip can tolerate without burning up (mW).

That said, if you use modern LEDs (which give lot of light for not a lot of current) you can probably drop it. Alternatively, you could use four transistors to drive the four LEDs instead. But that doesn't seem like much space gain to me.

Good luck with it.
Title: Re: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on December 13, 2024, 01:39:07 PM
I'd like to know something more about the current draw, the risk, and see if I can use it in a safe way.

I'm finding a lot of schematics, projects, PCBs with just a 4017 and LEDs.

https://www.ti.com/lit/ds/symlink/cd4017b.pdf?ts=1733953480648&ref_url=https%253A%252F%252Fwww.google.com%252F

Is it good to know this?
«DC INPUT CURRENT, ANY ONE INPUT..............±10mA»
Title: Re: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on January 03, 2025, 04:05:48 AM
Hello,
I drawn a veroboard layout, starting from the original paulinthelab one, but heavily based on the ElectricDruid's schematic. :)
The layout (UNVERIFIED and still to check it out well!) is 22 x 18.

So, looking at the ElectricDruid's schematic, there's just some changes. I kept the input caps 1uF and the 10uF output, to avoid any low end cut. I reduced even R4 560R to 100R, for a little bit more transparent signal.
I guess all these changes make a tiny difference.
Another change is about the biasing of the TL071, not for my choice, but due to the layout design. Two 2.2M resistors on the 9v and on the ground. I think that is ok (or even better?).

Next challenge is try to set the reset of the CD4017. I'm not sure if that is possibile, but I'd like to make it automatically using switched jack.
Of course if you need this box you have to use at least two guitars, and I want to make something to alternate only the first two input if you use two of them, or three input if your use three, or four input if you use all four.
Title: Re: Searching an intelligent 4 input selector box
Post by: FiveseveN on January 03, 2025, 05:28:09 AM
Quote from: Elijah-Baley on January 03, 2025, 04:05:48 AMI'm not sure if that is possibile, but I'd like to make it automatically using switched jack.
Yes but you'll need a couple of AND gates and the two spare inverters. And it will only work if you plug the guitars in sequentially. I.e. if you only use two inputs but they're A and C it will act as if you're using 3 inputs.
At some point a microcontroller makes more sense than all of this discrete logic, particularly if your goal is to keep it small. It could for example identify which inputs are used as well as how many, and only switch between those. It could recall the last state at power up, you can reprogram it to behave differently without changing the hardware etc.
Title: Re: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on January 03, 2025, 06:42:10 AM
Ok, thanks.
This thing is becoming a bit complicated if I need a lot of stuff.

What about a microcontroller? I don't know if that is out of my skill. What I need?
Title: Re: Searching an intelligent 4 input selector box
Post by: FiveseveN on January 03, 2025, 07:41:06 AM
Do you have any experience at all with writing code (ideally C)? The Arduino landscape is aimed at beginners and there are a lot of resources for all levels. But how fast you can get where you're headed depends of course on where you're starting from.
Title: Re: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on January 03, 2025, 08:41:18 AM
Non experience with programming.
I still like to build analogue stuff and I don't have watched anything about arduino, but one of this days...  ;D
Title: Re: Searching an intelligent 4 input selector box
Post by: PRR on January 03, 2025, 03:53:07 PM
Parallax BASIC Stamp is a teeny controller for NON-programmers. Yes you have to tell it what to do. No, it isn't traditional programming, more like a "How To" for cooking or stage setup.

OTOH, if you can figure how your switched jack can tickle the CLK input of the 4017 when it hits an unused hole, it may be simple hardware. (I can see it but only with much more complicated jacks than we use.)
Title: Re: Searching an intelligent 4 input selector box
Post by: ElectricDruid on January 03, 2025, 06:33:18 PM
Quote from: Elijah-Baley on January 03, 2025, 04:05:48 AMSo, looking at the ElectricDruid's schematic, there's just some changes. I kept the input caps 1uF and the 10uF output, to avoid any low end cut. I reduced even R4 560R to 100R, for a little bit more transparent signal.
The resistor in line with the output is to help the op-amp maintain stability when there's a decent amount of cable (and therefore capacitance) plugged into the output. Reducing it reduces that margin, but unless you plug a long cable in, you'll *probably* never notice.

QuoteI guess all these changes make a tiny difference.
They do. The original way was done that way for a reason.

QuoteAnother change is about the biasing of the TL071, not for my choice, but due to the layout design. Two 2.2M resistors on the 9v and on the ground. I think that is ok (or even better?).
It's almost certainly not better. Think about what happens to power supply noise with the new way - exactly half of it is coupled directly into that input. The other way with the voltage divider includes a capacitor that filters this power supply noise and provides a low impedance path to ground. Here's a reference:

https://www.analog.com/en/resources/app-notes/an-581.html

Will it work with the 2M2s? Yes, it'll work. But in my view these changes are not really "improvements", sorry. I understand the need to get things to work on a stripboard layout, and I know from experience that that isn't easy, so perhaps a few compromises have to be accepted. You choose how many - it's your project.

HTH
Title: Re: Searching an intelligent 4 input selector box
Post by: R.G. on January 03, 2025, 06:39:17 PM
Only mildly more complicated is Great Cow Basic ("GCB"). It's a true compiler, and accepts more-or-less standard Basic syntax.

I mostly quit using anything else for PICs because its just so fast and easy. I once programmed an eight loop programmable any-order FX switcher in GCB. For stuff like sensing foot switches, it's plenty fast enough.

The programming for this function is incredibly simple for everything except switch debouncing. Mostly, if you can write down how you want the footswitches to act, in English, the Basic code just falls out - with one exception, that being switch debouncing. I used a variant of a vertical adder routine to sense up to eight inputs simultaneously, then debounce them all simultaneously, but independently.

If I was writing this app, I'd pick a 14 pin PIC, like perhaps the 16F15323, about US$1.50 in one. Then:

====
' initialize a bunch of PIC specific stuff; this is chip dependent
' initialize a bunch of variables used in the program; here is where any memory of previous state would be raised from the dead
' set up a 10mS interrupt on a timer, so that the program has to sample footswitches every 10mS

MAIN:
IF footswitch_state was changed by the switch debouncing THEN change the actual output pins, move the previous state to a variable so we can tell it changed

GOTO Main:

ON_INTERRUPT:
' 10mS tick happened
' read the input pins
' use input pins' state this interrupt to compute any changes
' IF any switch was debounced to a new state, write it into the variable that MAIN checks for changes
' update previous states so we can tell something changes from now
RETURN
Title: Re: Searching an intelligent 4 input selector box
Post by: PRR on January 03, 2025, 09:40:54 PM
> Great Cow Basic

Good stuff if you know what a PIC is. The STAMP series pre-solves many detail that confuse beginners (even me, when not working on PC class machinery). You pay more for board and cord but it is plug and play. To each her own, of course.
Title: Re: Searching an intelligent 4 input selector box
Post by: PRR on January 04, 2025, 01:22:46 AM
> ....It's a true compiler, and accepts more-or-less standard Basic syntax.

For jobs like my blower-control, and Elijah-Baley's switch-fitting, most BASIC syntax is over-kill, and thus frustrating to the first-timer. GOTO is NOT "harmful" if you only have a dozen lines and don't have experience in thinking IF..THEN..ELSE. Compiled or interpreted is purely an implementation detail which should not be left to the ignorant.

Liam posted a deep-thought piece:
https://www.theregister.com/2025/01/03/reevaluating_basics_legacy/
I don't agree with half his thesis-- he may not have gone far enuff. And today I don't care which company to blame for BASIC's supposed downfall. But there is food for thought.

(Yes, I think FORTRAN 58 is, after machine-code, the one true language; why do you ask?)
Title: Re: Searching an intelligent 4 input selector box
Post by: FiveseveN on January 04, 2025, 05:30:14 AM
While we're still on the subject, there are "visual programming" platforms for Arduino (and others) that let you bypass the coding entirely:
https://xod.io/
https://www.visuino.com/
http://www.visualino.net/
https://mblock.cc/
https://github.com/BlocklyDuino/BlocklyDuino-v2
Title: Re: Searching an intelligent 4 input selector box
Post by: R.G. on January 05, 2025, 04:18:35 PM
Quote from: PRR on January 03, 2025, 09:40:54 PMGood stuff if you know what a PIC is. The STAMP series pre-solves many detail that confuse beginners (even me, when not working on PC class machinery). You pay more for board and cord but it is plug and play. To each her own, of course.
I had not looked at the BASIC Stamp for a long time. The real ones are expensive ($13 to $29) but the clones are down at about $6. The Arduino Nano is about the same, with clones down at about $5 or so. Arduino stuff generally needs programming in what amounts to C++, but much of this is hidden inside the Arduino IDE.

Over the years, I've become language agnostic. I write down what I want to do in kind-of-English statements that mimic the programming statements I'm used to, like GOTO, GOSUB, IF-THEN-ELSE and so on, and then get out the specific language manual and start transcoding.

The joy, and at the same time curse, of BASIC to me is its simplicity. Need a new variable? Just type in a new variable name and assign a value. The curse of that is that it allows you to subtly hose up something you did elsewhere in the program.

A whole lot of the "advances" in newer languages is designed to force the programmer to not make subtle mistakes by making the compiler refuse to play along. Declaring variables and types is actually good programming hygene, even if it's infuriating. I cursed at Arduino/C++ scope declarations a lot, but finally figured out that this was freeing in many ways.

If you even want to have fun reading about computer languages, look up the "Whitespace" language, which has all of the language syntax in non-printing characters.  :icon_lol:
Title: Re: Searching an intelligent 4 input selector box
Post by: ElectricDruid on January 05, 2025, 04:58:29 PM
Quote from: R.G. on January 05, 2025, 04:18:35 PMA whole lot of the "advances" in newer languages is designed to force the programmer to not make subtle mistakes by making the compiler refuse to play along. Declaring variables and types is actually good programming hygene, even if it's infuriating. I cursed at Arduino/C++ scope declarations a lot, but finally figured out that this was freeing in many ways.
I agree, but I've still never been able to get on with C's "header" files. That just seems like it's *you* doing work for the compiler's benefit. Like "I'll tell you what I'm going to do, so you don't have to work it out". It's a damn computer! Working stuff out is what it's *for*!! Do another pass of the code if you want to see what variables and functions I've declared. Grrr. Other languages don't seem to need this "ahead of time" declaration of every bleedin' thing, so it's not "necessary" in that sense. Bugs the hell out of me. Plus you finish up having to do a load more pointless statements so as not to include the same header files multiple times if several files in your project all need the same stuff. It's really the worst way to try and "organise" things. Dozens of files, all with another file that says what's in them, and then a ton of dependencies between them all. Nightmare.

Ok, sorry...please feel free to get back on topic and I'll shut up!!  ::)
Title: Re: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on January 26, 2025, 07:21:42 AM
Quote from: ElectricDruid on December 10, 2024, 07:29:24 AM(https://electricdruid.net/wp-content/uploads/2024/12/SequentialSwitch.jpg) (https://electricdruid.net/wp-content/uploads/2024/12/SequentialSwitch.jpg)

I'll stay on this schematic.

I'm thinking about to add a rotary switch to connect the positive of the 1N4148 to the common pole and set the reset connect the other pole to (Not Q0 and Q1) Q2, Q3, Q4 or Q5. In this way, when you switch on Q2 and plug just one guitar on that channel you have in the loop just Q0 input (A), Q1 (nothing plugged) getting mute signal, or at least this is what I wish, and it starts again.
Then, if you plug two guitars, input A and B and the switch is on Q3, you got Q0, Q1 and Q2 mute and it starts again. And go on until getting four guitars, input A, B, C, D and mute.

I hope this is clear, and it is possible to do.
Title: Re: Searching an intelligent 4 input selector box
Post by: amptramp on January 26, 2025, 08:07:50 AM
Do you have any plans for noiseless switching in this project?  A lot of effort is spent in some stompboxes to ensure switch pops are kept to a minimum level.  There are several ways of doing this:

1. Use of JFET switches that can be slowed down so the switching takes place at a lower frequency than the audio signal.  Only possible with a CD4007 in CMOS.

2. Delaying the switching until the original signal and the new signal are at the same voltage so you are switching from one voltage to the same voltage on the next selected channel.

Switch pops are not always a problem.  If the stompbox is on or off for the entire song, the only switching is between songs and that is usually not a problem.  But if you are switching an effect during the song, you might want to consider reducing switching noise.
Title: Re: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on January 26, 2025, 09:06:15 AM
I thought the momentary footswitch (normally open) wired like in the ElectricDruid's schematic, as in the paulinthelab on, was just fine.

This device is just to change the guitar between a song an another, I hoped there's no bump or pop with the footswitch.

And now I hope there's no pop changing the reset position with the rotary switch, I never worked with this circuit, I still have order all the materials. But, thinking about it, once you select the number of inputs with this rotary switch before start to play, it remains for all the session, you don't need to change it.
Title: Re: Searching an intelligent 4 input selector box
Post by: ElectricDruid on January 26, 2025, 04:46:14 PM
Ron, you'll see from the schematic that the inputs and outputs of the 4066 switch are biased to the same level, so unless you switch while there's signal going through it, the two sides of the switch should be at similar voltages. It's true that it won't neceesarily be silent if you have a signal on either the channel you switch from or to - that would require the kind of soft-fade you're describing. But for changing guitars between tracks it should be fine.

The reset switch shouldn't affect the audio path at all. It's purely part of the control circuit. The exception to this is if you happen to be on "Input 4" (say) when you switch to "Only 3 inputs", in which case the circuit would change input - but that's the same as you switching it yourself, as above.

HTH
Title: Re: Searching an intelligent 4 input selector box
Post by: Elijah-Baley on January 27, 2025, 04:19:57 AM
Thank you guys.

I have another question now. My friend has these four guitars, of course with different output level, so it could be useful have a kind of Volume or gain control.
I'm planning to turn the final buffer stage in a boost. The rotary switch has two poles, so I want to use the second to switch the pots in the feedback loop, one for every channel. I just did once with an AB box without any issue, so I assume do it with four channel should be fine, too.
And maybe add a Volume Master at the end of the circuit.

I'd prefer have four boosted circuit with a Volume pot for every channel to boost and cut the output, and a single Master Volume. But I guess I need more switching part.
I was trying to draw a schematic with a four boost in front, then I thought to use an unique boost with the four output, and switchable gain pot, in front to go into the CD4066. But I'm not sure if was ok push that.

What's a cool solution to control output for every single channel?