Parallel loop phase inverting switch?

[canman]

Hey guys,

I'm building a parallel looper pedal so I can run effects in parallel with my dry signal.  However, it's become apparent that some effects have phase cancellation issues (due to inverting/non-inverting inputs, I assume?) so when the blend knob is in the middle, volume gets pretty low. 

I've been looking all over for some kind of phase inverting circuit on a switch, but I haven't seen anything.  Is this possible?  If not, I'm thinking the best solution will be to put a clean boost after everything and just crank up the volume when the cancellation is occurring.

What do you guys say?

[B Tremblay]

http://runoffgroove.com/splitter-blend.html

[canman]

I wish I'd seen this before I built |v|ark's Split'n'Blend...if I can't work out some sort of phase inverting switch I'll just build that thing up for sure.  What is the logic behind the phase switch on that schematic?  When the switch is open does the circuit just not pass through the + input?

[B Tremblay]

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

[mth5044]

The old one - two. ROG then GEO, straight from the horses mouth 

[canman]

Yup...I've been doing lots of reading.  It's starting to sink in.  Looks like something I'll have to just build and see what happens...till then, I'm thinking the clean boost after the blend to compensate for volume loss will work for now. 

(I need to stop doing projects that get me in over my head...)

[canman]

OK I've been looking at this more and more, but I'm not sure how exactly you'd calculate what values you need for all the resistors and the cap.  Since it's not really amplifying anything (or is it?) how crucial are the values?

[Kipper4]

Look at the values used in the schematic from reply #1
Breadboard it maybe?

[canman]

Genius!!  Gah, I wish I wasn't so narrow minded...that's such a simple solution.  Checking the first link would more than likely give me the values I need.  Thanks for pointing that out to me, I appreciate it!

OK I just looked at the first link...it looks like the opamp for the phase switch is the second part of a dual opamp...could I get away with a single opamp here?

[mth5044]

You can just go with the schematic on GEOFEX using the values from ROG, the inverting stage only uses 1 of the 4 opamps in the ROG project. The other 3 are used for splitting and blending purposes. May or may not need that transistor stage in front, but you want U2A and it's surrounding bits (like on GEOFEX).

[canman]

So is +V at pin 8 9v?  I'd assume so, since it looks like pins 5 and 6 have a voltage divide, which I assume would be 4.5v?

Also, I'm trying to decipher that last post...haha...let me ask this: if I use a TL072, for example, and I don't use pins 1-4, would it still work?

[mth5044]

Sorry for being unclear, I took for granted some opamp info. The entire opamp chip will need power, so pin 8 will get the 9V and you have to have pin 4 go to ground. The correct way to terminate an unused opamp is to short the negative input to the output, then connect the positive input to a voltage between the power supply rails. If you have a VRef somewhere in your circuit, tie it in there (you will have one, check what the 1M and switch connect to in the ROG circuit!).

Essentially figure 3a from the page below. They are creating a half supply voltage off the positive input, but you'll just take yours from the VRef you have for the polarity circuit.

http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Amplifiers/Properly_terminating_an_unused_op_amp.aspx

Or you could just leave pins 1,2 and 3 floating, but it's bad practice and it's easy enough to terminate it correctly since you already have the 4.5V

[canman]

Very interesting...I didn't know you had to terminate unused pins.  I'll for sure give this a go tomorrow.  Thanks for the explanation!  I will probably have more questions, but hopefully I can pull it off!

[canman]

I drew up this quick stripboard layout for the switching portion...I was hoping someone could take a look and let me know if I did this correctly?



I do have cuts under all four strips where the opamp is, and I have a cut under the horizontal 220k resistor.  I'm also just going to pull Vref from the clean boost portion of the circuit (Microamp).  I think everything else looks OK, based on the top right portion of the schematic as posted earlier: http://runoffgroove.com/splitter-blend.html

Thanks for everything guys, I hope I did this right!

[B Tremblay]

The FET buffer at the input of the Green return section maintains high input impedance in both phase settings.

[canman]

Huh...well, I gave it a go without the FET buffer...switch worked no problem!  I don't know if I put it in the right spot in the blender I built, but it certainly makes a difference.  I put it after the return jack, and before the blend pot (same as the Split'n'Blend I think). 

So without the high input impedance, does that mean putting pedals AFTER this blender may not sound good?