Help needed designing an effects loop box

[john1056]

I’ve been thinking for a while about building an effects loop sort of box that would have an input, output and 2 send/returns.  I would like the box to be switchable so that the effects in the loops could be combined as A B, B A, or A and B in parallel.  Seems like it might be cool to be able to mix two different distortions together etc.   I’ve working out how to do the switching with a 3 position, 4 pole switch, but what I don’t know is what I need to do to split and combine the input signal for the parallel case (split from the input to the A and B sends, and combine the A and B returns to output).   I think I would need to amplify the input signal to send it to two outputs.  Is this correct?  Is there a simple circuit that demonstrates this?  For the A and B returns I think I can just combine them.  I was thinking I could vary the mix of A and B return before combining them by connecting them to the outer lugs of a 1M pot (return A to lug1, return B to lug 3 ) and grounding the center lug.  Center would be 50% both signals.  Anyone know if this will work?

Thanks!
John

[yano]

You should check this out:

http://www.generalguitargadgets.com/v2/index.php?option=displaypage&Itemid=122&op=page&SubMenu=

Also:

[B Tremblay]

The Splitter-Blend at runoffgroove.com was developed to mix two parallel effect loops.  However, there is no capability for switching the order of the loops.

[Transmogrifox]

The schematic above shows the + terminal of the opamp connected to "V-" supply of the op amp (ground).  The + terminal should actually be connected to a "V/2" reference.  

The circuit as shown above will create a nasty fizzley ratty sound.  Some op amps would oscillate in this configuration.


2 sends, 2 returns, or mix both together...
This isn't as easy as it sounds:

it's probably best to think of a switching matrix where each input has a switch between each output and the mixer, the same, treated as another effect.  This way of thinking about it requires the use of relays or CMOS switchs and a state logic machine to guarantee the correct states can be accessed and illegal states cannot be accessed.

The function you want can still be done without this level of complexity if you're only doing a 2 send, 2 return box.  I came up with the more complicated idea as I have been toying with the idea of a 10+ FX loop patch bay using relays and a digital microcontroller switching brain.  It would be more like a multi-FX processor using external pedals.

Anyway, I hope the first idea gave you some ideas of how to approach the problem ... take care.

[yano]

Yeah, I realized my mistake before you even posted...Its' been corrected.

[john1056]

Thanks for the responses!  This forum is awesome.  I should I have just asked where I could find a schematic!

[john1056]

The Splitter-Blend at runoffgroove.com was developed to mix two parallel effect loops.  However, there is no capability for switching the order of the loops.

What (if any) effect on tone is there if the looped effects are bypassed?  In other words, do the opamps color the tone any appreciable sense?

Thanks,
John

[B Tremblay]

What (if any) effect on tone is there if the looped effects are bypassed?  In other words, do the opamps color the tone any appreciable sense?

They're just acting as buffers, so there should be no impact on the tone.

[john1056]

The Splitter-Blend at runoffgroove.com was developed to mix two parallel effect loops.  However, there is no capability for switching the order of the loops.

The switching scheme I worked out still works (I think) to allow Red->Green, Green->Red ,and Green || Red.  Referring to the Splitter-Blend diagram, break the connection after the input coupling cap (220n) and 1M resister with Pins 3 and 5 of the TL072.   Imagine the Green Send opamp input labeled “Pin 5”, and the Red Send opamp labeled “Pin 3”.  Also the return jack for Green would be labeled “Jack Green Return”, and “Jack Red Return”.  The Jacks would be disconnection from the Red/Green Returns.  The switch would arranged as follows (4P3T):

Poles: In | In | Jack Green Return | Jack Red Return

The three throws would be set up as follows (NC = no connection):

Position 1: Green->Red

Pin 5 | NC | Pin 3 | Red Return

Position 2: Red->Green

NC | Pin 3 | Green Return | Pin 5

Position 3: Red || Green

Pin 5 | Pin 3 | Green Return | Red Return

So for example, the switch would connect the following for Position 1, Green->Red

Pole 1: In->Pin 5
Pole 2: In->NC
Pole 3: Jack Green Return->Pin 3
Pole 4: Jack Red Return->Red Return

So basically Input goes to Send Green, Return Green goes to Red Send (via Pin 3), and Red Return goes to Red Return which goes to the output.

In the Postion 3 || Red Green you would have

Pole 1: In->Pin 5
Pole 2: In->Pin 3
Pole 3: Jack Green Return to Green Return
Pole 4: Jack Red Return to Red Return

The part I have in mind is mouser 10WW043 cost $1.57+knob.

[ryanscissorhands]

This might be a cop out but in DIY'ing the best way to remain effecient is to know which things to build and which to buy. The blend knob is a gooder, but if you just need a/b through/return, I would seriously consider just buying the Boss LS-2. Simple, it works, less frustration and wasted effort. Unless you just want practice doing builds like that. It's your call, but consider this pedal as one of the best candidates for "Screw it, where's my wallet."

[john1056]

The LS-2 is where I got the idea for this pedal.  I am new at this stuff so I do want to build it for the hell of it.  But I also want to be able to bypass each effect seperately, and I want true bypass as well.