Four loop Splitter Blend + Mini Mixer, did i get it right ?

[NoFi]

Hi everyone, i opened PCB express for the first time yesterday and tried to draw the schemo of a four-looper based on the ROG Splitter Blend and GGG mini mixer. In fact it's pretty much like JD Sleep's "Parallelyzer" on GGG, but there is no proper schematic for that one yet that's why i'm trying to make one.   
I don't really have any knowledge in electronics, i'm just guessing the Splitter Blend can be expanded using quad op-amps and can be powered with 18V without modifications, but i'm not sure of anything.
Here is my schematic (sorry it's probably not drawn the academic way, especially the groundings but it's my first one lol ) :
http://thebin.free.fr/schematics/DualSplitterBlend+MiniMixer.gif

Did i get the quad op amp Splitter Blend right ?
Do i have to change anything because i'm powering it with 18 V instead of 9 ? Will my Ref V be correct ?
Same for the mini mixer, can it work on 18V, do i have to change anything, (the resistance before the LED maybe ?)
Can i just connect the splitter blend  and mini mixer the way i did ?
If i wan to switch the loops, can i just put DPDT's before the first op amps ? Or would it be better to use 3PDTs ?
Would it be possible to use two of those devices in series to have an "octolooper" or would i lack transparency ?   
I'd be very grateful to anyone who can help and correct me on this one.
Thanks.

Greg

Schematics for the mini mixer and splitter blend :
http://generalguitargadgets.com/diagrams/mixer_sc.gif
http://www.runoffgroove.com/splitter-blend.html

9V to 18V :
http://www.geofex.com/circuits/+9_to_33.htm

[R.G.]

You have the concept down pretty well. There are some details that could be changed.

If you are using a charge pump inverter, why not make -9V instead of +18? That simplifies the circuit quite a bit.

I'm not sure why there's a JFET buffer in front of the polarity inverter. An opamp buffer is simpler and has performance very near or better than the JFET, but that's not your doing. If you don't absolutely need the polarity inverting, you can just use a single opamp for recovery and buffering of the return.

You can use DPDTs at the loops to bypass the loops. You can use either DPDT or 3PDT, or a DPDT plus Millenium bypass for indicator LED on the loop.

You can use two of these, although you may need a second charge pump for enough current to drive the power supply.

[B Tremblay]

I'm not sure why there's a JFET buffer in front of the polarity inverter. An opamp buffer is simpler and has performance very near or better than the JFET, but that's not your doing.

When I decided to upgrade the original, mashed-together Splitter-Blend, I asked Gez for his take on the new version.  Here's an excerpt from his e-mail to me, suggesting the buffer:

My only concern is that U2A has lower input resistance than U2B.  Although there's a nice big 1M bias resistor to Vref there, the amp itself has low input impedance due to it being wired up in inverting mode - 10k in parallel with 1M with the switch open and 5k in parallel with 1M with it closed (bit on the low side).  U2B's input has 1M input resistance due to it being wired up in non-inverting mode.

If it works, then it works - end of story!  If you're concerned about the impedance difference then you could buffer U2A's input with a JFET to bring it up to 1M too, but then you might as well stick to two JFET buffers as in the original design and then use a single op-amp mixer along the lines of the thing I sent you.  Alternatively, to keep the parts count down, you could do away with the JFET at the output and use IT to buffer U2A's input instead.  If you're not getting any problems with doing the mixing passively followed by the buffer, then you shouldn't without it as the op-amps have really low output impedance.  You might want to up the value of the blend pot if you do this though (100k?).

You'll never catch me claiming to know anything about electronics theory, so I always try to ask someone smarter than me!  That's what happened with the Splitter-Blend 2 (and with many other 'groove circuits).

[seanm]

I don't claim to know anything either (and no I did not leave the "about electronics" out by mistake ), so I have a couple of questions about the splitter-blend:

Why does the jfet buffer have it's own Vref rather than using the one used by the opamps? Is there some sort of feedback problem?

The 1 meg pulldowns seem awfully big at the outputs.

Now about the four loop version:

I agree with R.G., if you use the charge pump you do not need Vref. Except for the JFETS, but they already provide their own Vref.

If you think about it, you only need the polarity reverser on two channels. Just a though if you want to save some components. Plus then you could use a third quad buffer and replace the JFETs with opamps.

You also probably don't need a second Vref for the mixer.

[NoFi]

Thanks a lot for the comments ! Especially the -9V trick. I did another schematic with the charge pump delivering +9 / -9V but i'm not sure i understand the whole Vref thing. Do you mean i can wire the -9V all over the place instead of Vref ?

Anyway here is the improved schematic, i have another question about the switching...
http://thebin.free.fr/schematics/DualSplitterBlend+MiniMixer2SWITCH.gif
What is the best way to switch each loop individually (the letters are on the schem.):
- Just a swith in position A or B ?
- DPDT switching in B and C or even in A and D maybe ? lol
I guess the buffers prevent any tone sucking anyway ?

As for the Jfets, i've been thinking of the layout, and i'll probably keep them because they're so small and convenient to fit in separate places on the board. That's also why i swapped around half of each TL074 on the scheme, i guess it also works that way.
And your absolutely right Sean, i was thinking or removing the polarity switching on loop 2, it's useless and again it makes the layout much simpler. Loop0 will be for the clean sound most of the time that's why its zero lol.
I should be able to fit two of those devices in a hammond 1411w enclosure. 
++
Greg

[seanm]

Thanks a lot for the comments ! Especially the -9V trick. I did another schematic with the charge pump delivering +9 / -9V but i'm not sure i understand the whole Vref thing. Do you mean i can wire the -9V all over the place instead of Vref ?

No, just use ground everywhere you had Vref. Vref is basically 1/2 the supply voltage. So for a single ended supply, like the original splitter-blend, you need to provide 4.5V for Vref. With a dual supply, +/-9V, half is 0V or ground.

So the Vref components R10, R11, C3, and C4 go away. As do R38, R39, C25, and C26.

I would also drop R40 and D3 since they are just an "on" led.

So on the TL072, pins 3 and 5 go to ground. And all the lines marked Vref go to ground.

[NoFi]

Okay that makes sense, i was reading things about voltage references and looking at op amps datasheets and saying to myself connecting those to -9V was probably silly. 
Thanks a lot, my work is cut out !

Edit :
So that should be it ? 
http://thebin.free.fr/schematics/DualSplitterBlend-FINAL.gif

I think next time, i'll be able to recognise the Vref voltage divider lol. 

Greg