Splitter-Blend Question (Series/Parallel switching?)

[fattcamp]

Hey guys. I'm wanting the Xotic X-Blender (http://ow.ly/SRyKzi but it's fairly expensive... and don't really need the tone circuit. It seems like I should be able to handle the Splitter-Blend circuit (http://ow.ly/SRz0Q and pretty much do the same thing.

I can't find an affordable looper/blender pedal that has the series/parralel or "blend" switch on it like the X-Blender. I just want one or two loops, a blend knob, and a series/parralel switch on it with led's. I do care about buffers and phasing which is why the splitter-blend seems to make sense. Am I missing something or is it a simple add-on for the splitter-blend?

-Dave

[blackieNYC]

I've built something like this but mine was a little different. (see profile picture. complete overdesign) Looking at the ROG drawing, switches are needed of course.  Switches are choices. 
the output of the Ret Red circuit, just before the blend pot.  That point has a choice to make:  shall I connect to the blend pot, or to Send Green?  There's the first pole of the switch.
At the same time, Send Green (right where the 10u cap and the 1M resistor meet, the connection point of the input jack of the Green pedal group) needs to make a choice: shall I connect to the output of the Send Green op amp (right after the 10u cap, again) or shall I connect to (this is tricky ) the outer pin on the 1st pole switch that is selected when Red Ret is not looking at the blend pot.  The  output of Red is not going to the pot, Send Green is not seeing signal from its input op amp U1A. the output of the return red op amp, thru its 10u cap, is connected to the Green Send, all by itself.
Draw this out.  I think in either position of this two pole switch, audio is passing, nothing is double loaded.  I have no paper or pen right now.  I'll draw it out if you are still confused.
When in series, it appears your blend pot will be a volume pot for the entire chain. 
You can switch Green and Red above if you wish to.  the polarity won't matter much if you are not blending.  Do you need an LED to tell you?  a 3 pole switch then.  A stomp switch. Or will a great big toggle switch with no LED do?  That's all I did.  More of a studio feature than a live performance option IMHO. for me anyway.

[fattcamp]

Thanks for the reply. Unfortunately I can't quite follow the instructions though. I'd definitely want led's for when the pedal is on/off and when it's in series/parallel.... but I could just have 1 led for series/parallel and one stomp for the same purpose. I don't care about true bypassing the pedal if it's clean... But yeah, essentially I want the features of the x-blender (could care less about the tone circuit but think it's a cool addition). Anything else you could help out with to be able to get this on a breadboard or something. Any pictures of the circuit you're talking about? Hard to visualize with just text. Hope to hear back from ya!

-Dave

[slacker]

If you look in my gallery in the layouts gallery you should find a diagram to add a series/parallel switch to the splitter blend. I'll post it later if you can't find it.

[blackieNYC]

As shown, it is in the series config. the hollow circles are the jacks of course.
here's some help with the LED http://www.generalguitargadgets.com/effects-projects/boosters/effects-loop-switch-boxes/

[slacker]

Her's the switching I did, it's the same as what blackieNYC drew.

[fattcamp]

Thanks guys. Did you use that switch with the splitter/blender or a different design?

[slacker]

I never used it, I just drew it for someone who wanted to add a series/parallel switch to a splitter/blender.

[blackieNYC]

I did build it into my splitter/blender/ mixer. I seem to be using so many effects that I haven't made much use of the series mode, but I think it's an important option to build. I could see myself using it in series someday. I think I said mine is a toggle, not a stomp. Because I have too many. I have a stomp that lets in the signal from the output of the Loop A devices, a stomp for Loop B, a third stomp for the clean input only (which has its own volume control so it's a 3 ch mixer). And a fourth stomp that engages an output boost. The boost I don't use. There are many silly switches I don't use. I drilled 30 holes in that thing. Newbie mistake.  At least it came out quiet and reliable. What I should have done was make a third send, just for a tuner output.

[fattcamp]

Thanks again guys. Finally getting around to making this thing. As it turns out, I have all but the 4 x 220n caps (in the process of getting them) and I'll build the pedal with the series/parallel switch and show 'er when she's done.

[Granny Gremlin]

Figured I'd tack on to this instead of starting a new thread.  I am also looking at using this or a similar circuit in a project and had some questions.

If U2A is providing the buffering for the green return, as well as (along with SW1 and related parts; the 220ks in the feedback loop and that 1M to vref) phase reversal, what is Q1 for?  Q1 appears to be a PNP wired as common base, so it cannot invert phase so I don't understand why it's on the green return but not on the red (shouldn't it be symetrical other than the phase bits, or does the phase rev circuit have some losses that need to be made up for vs the red return's path)?

Was also thinking whether this could be simplified to using 2 transistors only, but that might be too complicated as regards matching gains (even if both returns use same transistor, one has to be switchable to common  emitter for phase rev, and so the gains won't be the same even if both transistors are the same.... right?)

[slacker]

Q1 is a JFET acting as a buffer, this is to give the Green channel the same 1 Meg input impedance as the Red channel. They added this because the input impedance of U2A changes between about 1Meg in non inverting mode and 100k in inverting mode, for some pedals 100k might be too low causing volume loss or cutting low end, most will be fine so you could do away with the buffer.

[Granny Gremlin]

Thanks for the explanation! 

And apparently I didn't see the little legend on the bottom left re what to use for Q1 and U1/2 - doi.  Any thoughts regarding whether Q1 can be a J201?  I've heard they're not always compatible with MPF102, but they are supposed to be pretty interchangeable with 2N5457s.

[PRR]

> what is Q1 for?

I have wondered that myself.

Ian says what it does with that topology.

But it bothers me that U1A could drive 2 or 20 loads without strain; so what is U1B for?

Make U1A drive both Sends.

Use the freed-up (from duals/quads) U1B to buffer the variable-impedance phase flipper.

With the better (than a JFET) buffer we can also pick inverter resistors for lower hiss.

[B Tremblay]

I've been following both of the recent S-B topics and it's great to see builders interested in it. I've wanted to improve upon the circuit for some time and Paul's simplification is exactly what it needs - thanks, Paul! I also wanted to add in a better blending arrangement like RG's panning approach to keep the output level consistent.

[Granny Gremlin]

it bothers me that U1A could drive 2 or 20 loads without strain; so what is U1B for?

Make U1A drive both Sends.

Isn't there any advantage to the 2 sends having seperate buffers? 

Does the phase section have a gain of 0 (not to familiar with op amps in inverting mode), cause otherwise replacing the transistor on the green return (which is in a rather low gain mode... no wait I dunno about JFETs, that's when I thought it was a PNP; not sure if that's the same) with the now-spare op amp section increase the gain on that channel too much (vs the red return - would skew the 50/50 point of the blend pot).

[PRR]

> Isn't there any advantage to the 2 sends having seperate buffers? 

Your town has 50,000 lamps fed from one generator (or substation). Just Y-ed together, tapped off the street wires. Audio is just electric power, though we are a little more fussy about waveform and sag.

Pro-Audio often drives 8 or 16 loads with one amplifier.

The only real trick is the individual series resistors so a short on one does not kill signal to others.

16 "600 Ohm" loads could be a very heavy load; the amps are like small loudspeaker amplifiers.

Here we have just two loads. And we can be 99% sure they will be 50K or higher impedance. So the combined load is "nothing" to an opamp. Another 2K in series is "no" (0.3dB) loss. And if say Green is shorted (oops), the opamp can easily drive that shorted 2K isolation resistor and the 50K working load (1.9K).

If the possibility of 0.3dB "loss" bothers you, wire U1A for 0.5dB gain. (They never complain about gain.)

> the transistor on the green return (which is in a rather low gain mode... no wait I dunno about JFETs

In old days, we called this a Cathode Follower. By 1956 an Emitter Follower was the same idea. When JFETs got affordable in the 1960s, the Source Follower was the same idea in a new name.

Voltage gain is typically 0.90-0.98. Input impedance may be high or very-high.

The U2A gimmick works as a Voltage Follower one way, and a Voltage Inverter the other way. The 220K (or 22K) resistors above it are picked equal so the inverter has gain of negative 1.0, same as the positive 0.999 gain of the follower only upside down. The 220K (or 22K) lower-left of U2A should be same-as the upper resistors: then the input impedance is *same* both ways (a point I had not noticed).

220K is just large enough to add hiss above guitar or TL072-etc opamp. Maybe not more hiss than most effects, and certainly many fine guitar-chains have had hiss-resistance in this range. But there are players who pluck soft and let the note ring-out; they will be flirting with hiss. Going to 22K or 10K hiss resistance gets us down to the hiss-resistance of the Volume pot which is in every guitar; also the hiss-level of the TL072 (or 12AX7) which seems to be in every guitar chain.

That cross-fade does "a right thing" if loaded with infinite impedance. It gives half of Grn and half of Red which nominally adds-up to "unity". If loaded with <62K, the volume clearly drops in the middle. And when adding two "different" signals/effects, 1/2 + 1/2 does not always sound like "1". Disco mixers have struggled with many alternate cross-fade schemes. I dunno that there is any all-purpose solution.

BTW, in classic terminology "panning" is one signal to left or right. Two into one is cross-fade. Agree that terminology drifts and varies in different fields. No use pointing out that amplifier "Tremolo" usually isn't.

[Granny Gremlin]

Really appreciate the time you take to explain these things.  Thanks.

[Granny Gremlin]

So, I  thought through my project a bit more.  I'm going to need to split 1:3 and then mix 3:1 back.  The split part is easy - add another 2.2K to the above revised ROG split circuit for a 3rd send (I am choosing to not listen to the deep dark fears in my head telling me that these splits should be isolated from each other by more than a resistor), but then how to mix back (I realize I lose the option for a single blend control; I'm cool with 3 separate pots - thinking of using lit sliders actually)?  I was thinking of just using passive summing because all the signals being mixed are already buffered.  The signals are fixed source so either I will need the phase reverse or I won't (def won't need it switchable and likely can remove it all together but should test to confirm - that's a long way out the way this is going and considering what all else is on my bench).  One of the sources has a 100k output vol pot so was thinking it would make sense to do something like this :

FX1 -> 0.1uf cap -> 100k pot -> 10k resistor ->
Dry -> 0.1uf cap -> 100k pot -> 10k resistor ->  output
FX2 -> 0.1uf cap -> 100k pot -> 10k resistor -> 

the cap is to keep things from getting weird if one of the pots is cranked.

Given the nature of my project is there any benefit to all those additional active stages in the ROG blend?  Basically it'll be 2 seperate fx borads (modded and somewhat stripped down) + bufferred dry from the revised ROG split above, all in one box.

[PRR]

Reposting image:



EDIT- the pin-numbers in the image are cut/pasted wrong. Use the chip pin-out and your own common sense.