3 parallel fx loops, buffered and polarity switches

[Wounded Paw]

So here's a new project along the lines of the GGG Parrallelyzer.  I'm buffering and splitting a signal into 4 channels, 3 effects loops and a clean channel then recombining all 4 to one output.  The effects loops consist of an effect pedal followed by a volume pedal.  Each effects loop return also has a polarity switch on it, from R.G. Keen's article.  I'm assuming there's no problem with using the polarity reversing circuit he describes as a buffer at the same time.  I found with a previous project that it it necessary to buffer all returns before mixing them.  Some of the component values are still a bit arbitrary but here's my version 1.0

[B Tremblay]

The Splitter-Blend incorporates some of these features, albeit with only two loops.

[Pushtone]

That looks a lot like what I'm doing with the whole shebang in one box.

Here is my schem
http://www.diystompboxes.com/smfforum/index.php?topic=66172.msg532132#msg532132

I have found that RG polarity reverse needs a buffer in front of it to maintain unity gain.


I was hoping to save a few parts and skip the pol rev on the direct channel but it seems I need it to keep all four channels/loops sounding the same.
I fine that the "clean or direct" channel sounds different without the pol rev circuit in-line.

I don't buffer my mixer channel returns and I don't see why you would need to.
Your not going to plug in a passive guitar pickup at the effect returns (mixer inputs).
Only active effect circuits with a low imp output.
At least my prototype with the GGG simple mixer (passive/resistive mixer inputs) worked fine mixing the output of my pedals.

[Wounded Paw]

I know the ROG Splitter-Blend does much the same thing, it was one of my influences for this circuit and my previous attempt at something similar a year ago.  Are you adding Q1 before the polarity switcher because as Pushtone says, it needs a buffer in front of it?  Do you find that the channel without the polarity switcher on it sounds different that one that does?  I mean with the effect off or the loop jumpered.
My previous version didn't work properly until I buffered all the returns before mixing so I'm definitely doing that this time.  It might be a bit of overkill but it's really just one more TL074 IC and a few more resistors.

[Wounded Paw]

I've added the JFET to the polarity switcher section like in the ROG Splitter-Blend because it would appear this is necessary to keep unity gain?
The input buffer section is similar to the AMZ Quad Buffer. 

I'm using a lot of 10uf caps because I ordered a pile of axial caps from Small Bear by accident, I wanted radial.  So my plan for this project is to keep it nice and flat, all ICs, resistors and axial caps sitting flat on the pcb.  Maybe this will come in handy when I try to stick it in a 1590BB enclosure with the 8 1/4" jacks, 9v dc jack, clean volume pot and 3 polarity switches.  If there's anywhere that the 10uf cap will cause problems, please let me know.

I should also mention that although I will make one for myself the first use this will go to is for someone who wants it for electric piano, not guitar.

[Pushtone]

Hey Paw,

Whats with that power supply bit in the lower right corner?

I used a voltage divider to get the Vref the pol rev circuit needs but I don't see one in in your schem.

How are you generating Vref with an opamp output?

Dave

[Wounded Paw]

There was a small mistake there but I fixed it.  It's still a voltage divider it just goes through the op-amp too.  I got that from the "4 Channel Buffered Mixer" by Ben Milner that I seem to have downloaded at some point, probably from around here somewhere.

[Pushtone]

I got that from the "4 Channel Buffered Mixer" by Ben Milner that I seem to have downloaded at some point, probably from around here somewhere.

Yeah, I remember that one from a mixer thread, I downloaded it too.
It was an important schematic for me and a big boost. Lots of good bits in there.

Thanks Ben for posting that.