Passive blend/balance box

[CounterfeitGentleman]

Hi all, I've just begun working on my own pedal projects (hear the sigh from the veteran members...) but have been poking around inside guitars for a decade so I'm pretty comfortable with passive (and very basic active) circuits, wiring stuff up, troubleshooting etc.

I've just completed my first pedal, very basic, completely passive. My effects chain is split into two signals, I wanted something that could blend the two signals back together for a mono input into my amp. I had been using a Boss RC-20 loop station as it has 2 inputs with individual volume controls but it's a pain to carry around if I'm not using the loop function, and, well, I wanted to have something I made myself.



The footswitch is a little closer to the lower volume control than I'd like, but my tools and resources are limited so I had to go with a Radio Shack project box, which has all sorts of inconvenient inner structures that make it hard to place components where I'd like...

So far so simple, 2 inputs, 2 500k volume pots, a switch to kill one input signal and leave the other active, one output. Does more or less what I need it to. However, the two volume controls affect each other in the same way the volumes controls of, say, a Les Paul do when the pickup switch is in the middle position. It's a lot more noticeable than that though, as there are two very tonally distinct signals so the volume controls only have a very narrow range (between, I would say, the 8 and 9 positions of a knob numbered up to 10) where they are effective; otherwise one signal is totally dominant and the other inaudible.

Is there a quick fix for this? I feel like there should be a pretty simple component that I can put between each volume pot and the output that will just shunt the signal in one direction and stop it feeding back into opposing volume controls, but I don't know exactly what component that would be (my high school electronics theory thinks maybe a diode?) or what specific value of said component I would need to use.

Any suggestions/advice?

[eatyourguitar]

well look at a 3 lug pot vol pot when its turned down. the output is tied to ground. when you have two vol pots that have lug 2 tied together, when one of those pots is turned down it ties 1 & 2 together with ground. so you get lug 1 & 2 on both pots touching ground. thats why a passive mixer will have 100k resistors after each pot. the resistors get tied together for the output. the resistors are doing the mixing. on your pedal you can just add a 100k for each pot on lug 2. then the resistors connect and thats your out.

I would actually do this a little different. I would use a 50K or 100k linear blend pot and put a volume pot after it. this makes much more sense. you dial in the sound you want on one knob. the one volume knob makes it easy to change volume without changing your mix.

[CounterfeitGentleman]

This is exactly the kind of advice I need. Thanks! So a 100k resistor between each pot and the output? I'll try this first, definitely looking to alter the mod down the line anyway.

I think I was turned off the idea of using a blend pot from when I tried using one on a strat mod a few years back that wasn't so successful, but looking back I think it was more down to my skills (or lack of) than problems with the mod or components...

Just out of interest, what is the advantage of using a 50k or 100k blend pot as opposed to, say, a 250k or 500k blend pot? I'm still getting my head around the difference these different values have on sound/performance.

[McSpunckle]

Are the pots also wired like a Les Paul? Input on the outer lug, and output on the center lug?

Try switching it. Wire the output to the outer lug and input to the center lug. That's how it's done in a Jazz Bass.

Different values depend on what the input is.

Guitars use 250K or 500K because their output impedance is really high. Basically, that means the circuit looks like there's a high resistance in series with it-- so if you have a 10K output impedance on the pickup, a 10K pot would basically always appear to be half way down. Then, the inductance in the pickup also creates a low pass filter with the low resistance and you lose highs.

But, there is a problem with just using higher value pots. As you turn them down, the output impedance increases-- which creates a low pass filter with the capacitance in your cable. That's why the guitar gets muddy when rolling down your volume control. That's why Fenders don't get as muddy as Gibsons-- Fender tends to use 250K pots, while Gibson uses 500K. 500K will be brighter when the control is all the way up, but turn it down and that advantage goes away quickly. 1M pots are even worse.

If the signal to your blender comes from a pedal, the output impedance could be anywhere from just 10s of ohms to very high. But there's no inductance, so 100K is usually plenty.

[CounterfeitGentleman]

Cool, thanks for that explanation.

[eatyourguitar]

I would just add resistors and dont flip the pots around. if you flip the pots around it can get muddy as you turn it down.

to answer your question about why not a 500k pot, the bigger the pot, the bigger the resistor in the signal path. too big and it changes the impedance usually causing it to sound muddy. a 1M pot would be so extreme that it would be quiet as soon as you start turning it down a little. blend pots dont go to ground so you can use 50k instead of 100k. most pedals have an output impedance about 1k but it varies.

[McSpunckle]

Most passive blend pots are just two volume controls stuck together... unless there's a circuit I don't know about.

Pedals don't have as low of an output impedance as you might think. Take a Big Muff-- the output impedance without the volume control is 10K, then it goes up as you turn down the volume control. That's about the same as fairly hot guitar pickup.

Pedals with buffered outputs do have pretty low output impedance, though. But you can't assume that's all the blender will be seeing.

[eatyourguitar]

Look at the runoff groove splitter blend. The passive blend control is the last thing in the circuit and its not a dual pot. The outside lugs are the inputs. The middle lug is the output. This works best when the output impedance of the effect is less than 50k