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:

Some more pics of the finished guitar

Well, I got asked if I could put up some more pictures of the finished guitar (and specifically the neck joint), so here's what I've got (apologies for the dust). I thought I had a pic of the wiring too, but I can't seem to find it. Next time I change the strings I'll remove the scratchplate and take some.

Enjoy.

(EDIT: The wiring pics have now been added below)

   
And the wiring pics...

Putting it all together

Well, considering I had already put everything together before lacquering, this step was dead easy. In fact, the hardest part was having the patience to wait for the lacquer to totally dry. I’d recommend waiting several days, or even a week or two if you can bear it.

Be very, VERY careful with screws, screwdrivers, or any other item which might scratch your nice new finish.

It was a wonderful experience making this guitar myself. I highly recommend it. You’ll learn a lot and you’ll get an amazing feeling of self-satisfaction when you put the whole thing together and play it.

You should also take some time to set the guitar up right. Intonation, string height and neck tension will all make a huge difference to how your guitar plays. If you can’t set it up yourself, then consider taking it to your local guitar shop and paying a small fee for them to do it. I recommend learning to do it yourself though, but do be patient. It’s a learning process, so don’t expect to get it right first time.

Here are a few more pics of the finished guitar from various angles. Enjoy.

Please feel free to leave any comments or questions. I’ll be happy to reply.

Finishing the neck

It can be a frustrating time waiting for the lacquer to dry on your cool new guitar body, so this is the perfect time to do some work on the neck.

If you buy the neck (which I did), then there is very little work to do really, other than finishing it off. In fact, here is a list of work you will need to do:

1. Make sure that the machine heads fit in the holes, and if not, enlarge the holes (remember to remove the machine heads after making sure they fit).
2. Do any final sanding, etc.
3. Apply any decals/transfers to the head.
4. Lacquer the front of the head.
5. Lacquer the back and sides of the head and the neck.
6. Fit machine heads, string retainers and a nut.

At the top of this post, you can see a picture of the neck after applying decals and a few layers of lacquer. The packs of letters I used for the logo are there too. They're just standard Letraset (although the brand I used is actually called "DECAdry"). This neck has a rosewood fretboard, which does not require lacquering, so it needs to be masked off. After this stage, the masking paper is removed totally and the neck is then laid face down while lacquer is applied to the back and sides. I found that this method worked really well.

Here is a nice picture of the head of the guitar after all the hardware had been added. I'm pretty happy with it. My surname is Morrow, so I tried to play with this word a little. The upside-down R means that whatever way up the logo is, it looks the same. It's very satisfying to be able to put your own name on the head of a guitar, I have to say.

The finish (part two)

Before you spray your guitar, you should really apply some sort of sealant, otherwise the lacquer (or whatever) will get sucked into the wood like you wouldn't believe. This will make life much more difficult for you and can even affect the tone.
So after applying some stain, that's what I did. Just one layer of sealing varnish, followed by a bit of light sanding.

Next, it was a case of applying layer after layer of spray-on lacquer. This will probably take longer than you think. I lost count of the number of layers I added, and this was probably due to my not applying the sealing varnish correctly. The wood just kept sucking that lacquer in.

Every few layers, you should do some very light wet sanding to try to keep the surface as perfect as possible. If you manage to remove some of the stain by accident, just re-apply it. It's actually not as serious a problem as you might think (or at least it wasn't in my case).

Here's a picture of the guitar with several layers of lacquer applied (as well as some preparation work for the electrics installation, which I'll discuss later):


Even after many, many layers, you can see how the wood is sucking that lacquer. After this photo, I removed the metal parts and kept going with the lacquer until I managed to get a nice smooth finish.

Here are a couple of pictures of the body after the lacquering was completely finished:

The finish (part one)

Finishing a guitar is pretty tricky. I suppose if you just want to paint it, then that would be a lot easier, but I wanted to stain mine, then lacquer over the top. This is how I did that.

Firstly, I had to construct some sort of holder for the guitar. I did this with an old broken wood plane I had lying around, which fitted the neck joint nicely. I then made a wooden handle for it and drilled a big hole down it so that I could mount it on a spindle. Badly as this photo turned out, you can see the stand holding the guitar up. It is sitting on a spindle poking out of the top of a big block of wood, held down by a vise. The theory was that I could freely rotate the guitar body this way, and even lift it off the spindle and hold it by the handle.


The first thing I did was to add some water soluble stain. This is just wiped on with a cloth and immediately adds a lot of character to the wood. Here it is after a layer or two of the stain. You can add as many layers as you want. Each one will progressively make the guitar look darker.

Checking that it fits together

At this point I decided to assemble the guitar, just to check that everything fitted together correctly. Obviously I would have to take it all apart again to add the finish. I had only acquired one pickup by this stage, but that’s OK. Here are a few pics of the assembled guitar.


I plugged it in and it sounded absolutely fantastic. Really, really beautiful sound.