Sunday, August 17, 2008

Two-channel passive mini-mixer

Hello all,
Well, I didn't want to just stop after posting about the guitar. I have been known to dabble in other little projects after all. So don't be surprised if those other projects make it on here for time to time.
Today I'd like to show you the little two-channel passive mixer I made. I needed something that would let me plug two guitars into the same amplifier (and both be used at the same time). So, if someone were to come round to my house, we could both plug a guitar into the same amp. You can't just use a y-connector, since that's just amateur, and changing the volume on one guitar would affect the other. I'm pretty sure it could be unhealthy for certain amps too. Also, I wanted to be able to adjust the level of each guitar so that if one were more powerful than the other, this could be balanced out. Lastly, I wanted to make it as portable as possible, so that I could even stick it in my guitar case.
So, I searched for the simplest two-channel or multi-channel mixer schematics I could find and then came up with this:

Note that I don't claim to be the inventor of that circuit. Five minutes on Google and I'm sure you'll find a similar circuit posted on other sites. I have, however, drawn a nice clear circuit diagram for your convenience.

As you can see, the circuit is incredibly simple. Just three quarter-inch jack sockets, two 10k logarithmic pots, two 10k resistors and a box to stick them in (see notes just below).

NOTE: I've noticed several forums pointing to this post, and a lot of debate about the 10k values of both the pots and resistors as being too low (it has been suggested that 500k or even 1M pots would be better, for example). I've used this mixer a lot, as has a friend of mine who I made one for, and both of us have found that it does a perfectly good job, especially at home. My advice is to just give it a go and see how it works for you, rather that wasting too much time in advance theorising about how it might sound. If you find that it doesn't sound good to you, then by all means try different pot/resistor values, and report back in the comments below.

NOTE 2: See the comments at the bottom of this post for more discussion about this. 250Kohm pots and resistors seems to be a good choice by the looks of it.

I couldn't find a suitably-sized metal enclosure at the time of making this, so I settled on a plastic box with a metal bottom. My thinking was that I could always line it with tin foil (aluminum foil) if it picked up interference, but to be honest, it has never been an issue.
Here's a pic of how it looks inside:

And a closer pic of the resistors, etc.:

As you can see, it only just fitted in there. It may have been a better idea to put the two input jacks to the outside of the pots, but to be honest, I don't think it matters either way.
So, here's how it looks from the outside (front and back):

I've been able to use this little device on a few occasions, and it's been fantastic. The fact that it can fit into your pocket makes it incredibly handy too.
I haven't been in this position yet, but it could even save your ass if you were at a small venue with another guitarist and one of the amps stopped working.
And the cost? Pfft... maybe 200 Taiwanese dollars, which works out at about US$6.

Also, just one more comment from me - I ended up re-boxing this mixer. I never had any problem with interference, but I decided to re-box it in an aluminium enclosure anyway, since I found a size that was just perfect, and I liked the idea of it being more robust. Here's the re-boxed mixer:


Rich said...

Your mini mixer looks great. Do you sell these? Or, where could I find a commercially available equivalent? Thanks.

Anonymous said...

Hey, thanks for this. I was looking for a schematic and pictures of how to do this, and behold, I came across this. Nicely detailed and I'm sure I'll be able to build myself one. Thanks again.

Aron Cruce said...

thank you this was extremely helpful since I was trying to figure how to basically get something like this projects results, but from aby schematics. One question all I had were 500k pot which i noticed you updated that might be a better choice, but do you use the same 10k resistor or match the pots ohms?

stu said...

Hi Aron, yes bumping the resistors up to something in the same ballpark as the pots would be wise. You would probably get away with them being a bit lower (say half the value), but 50 times smaller would end up giving some weird results probably. Just to explain, there's a compromise going on here. What you want to do is avoid sucking too much volume away, but also have enough of a barrier between the two guitars that adjusting the volume of one wouldn't affect the volume of the other too much. Probably the best compromise would be something in the middle like 250k (logarithmic/audio) pots and something in the same area, or slightly lower for the resistors - say somewhere between 100k and 250k ohms). To be honest, though, you'll probably be surprised how forgiving this circuit is.

Anonymous said...

Where can I buy a morrow minimix?

stu said...

You can't! You have to make it yourself.

Unknown said...

Hi, would this work with 2 distorted guitars?

stu said...

Hi Pablo,
That should work fine.

blackpizza said...

Made mine today. Love it. I use it for mic and guitar to one effekt chain into a looper and to the amp. Made a second to go into 2 amps some time. Both directions work great.

Check out the pics.

Thanks for sharing…

Cheers pizza

stu said...

Hey pizza, that's great. Thanks for letting us know it worked. So it looks like you used 250k pots and 240k resistors. Is that right?

blackpizza said...

Jeah thats right. I read it works so i took the 250k pots. The lowest ohms i could get at my favourite guitar shop :-)
I will label the boxes these days. So check out the link above to get to know the blackpizza design ;-)
Are the words on your box printed on foil?

I like the mirrowed mor…


stu said...

Thanks for replying, pizza. On the original plastic enclosure, it was just white Letraset (very amateur, I know). On the metal enclosure, I printed onto a transparent plastic sheet with an adhesive back.

blackpizza said...

Hey, i tried to solder the ground on the pot as you did. But that did not work. How did yyou do that. Could it be a different material of the pots back in my case? Seemed like there is some kind of varnish on it that melts with heat.

I had the same idea with the adhesive plastic and prints. Maybe i will use a stancil. Should work too.
Njce thing you did.

stu said...

Hi pizza. Yes, pots can be difficult to solder to. Best idea is to scrape the back of it first, and even add some flux if you have some. A pretty hot soldering iron helps too (at least 40-60 W), but watch out--you can ruin pots if you hold heat on them for too long. Instead of soldering to the back of the pots, you can just choose the ground connection on one of the jacks. That'll work fine. Also remember that if you're using a metal enclosure, the jacks are all grounded already.

blackpizza said...

Hi stu, yes i used one of tbe jacks for the ground connection. But, es you say its not needed? I can wire it the easy way next time in a metal enclosure?
Without special ground connection.

But you did that too?!

stu said...

Hey pizza. Actually, since I'd already made it for a plastic enclosure, I just moved the parts over to the metal one, so I didn't change the wiring. However, if I did a new one in a metal enclosure, I would just miss out all of the ground connections for the jacks, and for the ground connections for the pots, I would just connect these to one of the jack's ground connector (if doesn't really matter which jack).

blackpizza said...

Ah ok. Understand.
Thanks for explaning all that to a newbee like me :-D

Did you ever do a booster?
I am planing to make one. So maybe you can tell me how to wire that?

stu said...

You're welcome.

Yes, two boosters. One is a clone of an MXR Micro Amp:

And the other is a sort of Brian May-style treble booster (though you can experiment a bit to get the tone boost you want):

blackpizza said...

Jeah great. Your my new web heroe :-D
Will try that. Hope i can make this.
But you explained well so i think i can do this. Otherwise i will ask you every single detail

stu said...

Ha, no problem.

ZanBon said...

Hey, here's my version! I added a switch for the b channel, 2 leds and a 3v battery. Things got a little tight but it works like a charm! Thanks for the idea! Here are some pics!

stu said...

Nice one, ZaniusBonius! Thanks for the pics.

Unknown said...

This is just what I need. If I wanted 4 inputs would anything change in terms of resistor/pot values or is it simply a case of putting more stuff in the box?
Thanks a lot.

Unknown said...

Well I answered my own question. I used 4 10k pots and 4 10k resistors. Totally works although the top end disapears a bit. I just needed something to use in the teaching studio so it does what it needs to.

Brad Granath said...

I'd like to use a similar design, but for a different application. Couple of questions:

1. Why have the pots? Isn't there already a pot just before the output built into the guitar? Is there any disadvantage to using the volume control on the source (other than flexibility)?

2. My 2 inputs are at headphone level, not instrument level. Does that change my resistor values?

For background: I'm trying to mix a twoway radio's headphone output with a cellphone headphone output, in as cheap and simple a manner possible.

Unknown said...

Any chance of some help?
I am trying to build a mini mixer: 2 or 3 xlr in to volume pots to 1 xlr out.
A. Do I need resisters? if so what size and where.
B. Would A250K pots be okay.
C. Have you a drawing? even better, drawing/schematics & photos.
Kind regards.

nuno gouveia said...

Great work. Thanks.
However I was trying to find a different model: a single fader to mix between 100%A up to 100%b, like a DJ fader. And maybe include a dB cut in one channel to compensate for differential input loads (say an active pickup...)
Can you show me where I can find a schematic for this? Thanks.

Bernard said...

i wanna make this using xlr, can you send schematics? email me

chrisfromiowa said...

Excellent. I've been looking for something like this for a couple of years. Asking on several forums to much ridicule and derision because I wanted to keep it simple and not active, no power requirements. Anyway, my variation will be to use a stereo volume pedal and switch the dual pot with a blend pot 100->0+0<-100 with a 250k blend pot. Thank you for posting this marvel of simplicity. Regards BS

Boobs said...

Awesome! I know the is very old, but two questions: 1. Can I build this circuit without pots? 2. Where did you get that aluminum enclosure?

Tai said...

Ola amigo eu queria fazer um pedal desses para fazer um serie e paralelo dos meu dois pedais de delay alguém sabe como adicionar uma chave para alternar de serie/paralelo?