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: