Building a mixer

[yano]

This isn't exactly a stomp box, but its related. I'm looking at the possiblility of building a simple mixer.

Essentially the only requirements I have is that i need to be able to combine at least 6 channels of audio into one for recording purposes or what have you. So each channel would just consist of an input connected to an audio taper pot (for a level knob) which would then be connected to the output.

Am I going to run into problems if I just connect the outputs of all the pots to the lug of the output jack?

I'd also probably have a master level knob.

Any suggestions are welcome.

[Somicide]

wow, that sounds like a great project.  my only regret is i have no advice, im sorry.  If this turns out, lemme know, i have a similar situation.

[ExpAnonColin]

I assume not.  If I were you I'd basically just make 6 simple level knobs (input to 1, middle to output pot, side to ground) and then have one output level pot, with all 6 of the input's pots connected to it, I'd imagine you'd have no problems.  However, you might want to consider using op amps for active level control for less tone cutoff.

-Colin

[Ansil]

I assume not.  If I were you I'd basically just make 6 simple level knobs (input to 1, middle to output pot, side to ground) and then have one output level pot, with all 6 of the input's pots connected to it, I'd imagine you'd have no problems.  However, you might want to consider using op amps for active level control for less tone cutoff.

-Colin

you could also use a single cmos 4049  to provide the same thing.  it has six inverters.

[brett]

I made a similar thing for a SS amp I built (ok, I know SS sounds crap when overdriven, but I don't overdrive it coz it puts out 400W!).  I used 4 channels, thru 500k pots into a non-inverting op-amp with about 3dB of gain.  In hindsight, I realise I should have used a jfet buffer (see the AMZ site for buffer designs) for each input, coz 500k isn't really enough impedance for direct input from a guitar pickup.  

Probably the simplest (and one of the best) mixers you can make is a bunch of parallel jfet buffers (1 per channel) feeding volume pots (1 per channel), through a low-gain single jfet stage (1) feeding a master pot (1).  Has high input impedance, low noise and low output impedance.

[Mike Burgundy]

Buffering after pots (especially if there's a lot of them) is a good thing: if you don't use this the pots will be interactive as h*ll and there might be impedance issues.
Actually, if you want the inputs to be totally independant, you'll have to buffer *each* volume pot. A simple one-transistor or JFet will do.
Tie all these outputs into a mixer amp, into master volume and ta-dah.

[Somicide]

yano - when you get this all built lemme know how it turns out, this sounds really really cool.

[R.G.]

Mike B. is correct - if you don't do something to keep them from interacting, the knobs will be dramatically interactive.

The stock way to do this is to do the front end just as you've said, one audio taper volume pot per input. Then you take a series resistor from each pot wiper and join the free end of all the resistors. That point connects to the inverting input of an opamp, with a single resistor from the output of the opamp to the inverting input. The inverting input of an opamp is a "virtual ground", so all the inputs mix at zero volts and do not interact at the pot settings. The ratio of the feedback resistor to the individual series input resistors can give you gain, and in fact a different gain per input if you want that.

Jack Orman has a drawing of a four input mixer on his site. I did one back in about '94 for the first internet schematics site. The circuit was decades old even then.

If you're into this, get a copy of "Recording for Musicians" or something similar by Craig Anderton. It's a single book that details how to build a real, no fooling recording mixer in modules, and anything simpler along the way.

Active mixers are vastly better in performance than passive mixers.

[smoguzbenjamin]

http://www.generalguitargadgets.com/v2/index.php?option=displaypage&Itemid=123&op=page&SubMenu=

Check the schemmo, you see 4x a 10k pot, and a 100k resistor  + 0.1 uF cap. Just add 2 more of those and you have 6 channels. I added a 50k pot to mine as a master volume. Works great for me

[Mark Hammer]

Since the nature of the input sources will vary, many mixers provide a variety of input stage options.  Many of the mixer projects I've seen in back issues of hobby mags often provide a set of inputs for vocal mics, another set for line-level sources like keyboards or cassette decks, and others for "instrument-level" sources like guitar and bass.

These various sources have different output levels (hence different additional gain requirements) and different output impedances (hence different input impedance requirements).  If you need the mixer to combine such widely different sources, expecting a one-size-fits-all solution to work may be disappointing.

In which case, what you often see is something like this.  The level pots are connected in a fashion identical to what RG describes, but each pot is preceded by an op-amp based input stage with a suitable amount of gain (or padding/attenuation) and a suitable input impedance.  In some instances, you also have an additional buffer stage so that you can invert the signal if needed.  The basic idea is that:
a) nothing loses any bandwidth because of compromises,
b) level-pot settings each mean the same thing, visually
c) nothing inadvertently cancels out when mixed tomono because of phase relationships.

When all of this is translated into a feasible design, essentially what you end up with is this:
a) line-level or otherwise high-output low output impedance signals come in through medium value passive pots (10k-100k) and then onto the op-amp based mixing/output stage
b) similar to slightly lower level high output impedance signals (e.g., guitar direct) come in through a high input-impedance buffer stage and then onto the same value pots as above,
c) mics come in through an op-amp stage optimized for a modest input impedance (an NE5534 is close to ideal) that has adjustable gain and maybe low-end rolloff built into it, then onto the pot as above,
d) stereo line-level sources come in through passive pots but  the pots are dual ganged.

There are fancier ways to do it, but that represents the bare-bones minimum way of giving a broad range of inputs a fighting chance alongside each other.  You can see a nice sampler of this and more at:
http://www.discovercircuits.com/A/au-mixer.htm

[kroushl]

Here's a really in-depth mixer project:

http://sound.westhost.com/project30.htm

It's way too complicated for this application, but I thought some readers might be interested.

Enjoy,
Brad

[Greg Moss]

I found this site while looking for simple mixer schemes.  It's informal, has several mixer circuit blocks (input, eq, output) and some other interesting stuff:

http://www.all-electric.com/b&c.html

[yano]

right on, thanks for all the links. Now to digest. I'll be back with my progress after a while (and it might be a while, i'm busy with school and work)

[Brian Marshall]

mixers can get extreemly complex, and usually require lots of buffering. sounds like you only want a single buss mixer so that should keep it relatively simple.  

One thing that i have never really looked into (because i have never built a mixer) is how to deal with balanced inputs.  I know that there are two hots and a cold, but dont really know how it works.  i think one hot is basically dead, and just measures noise, and then is subtracted from the 'real' hot.  

I know craig anderton has a few mixer/preamp projects.

if each channel isnt active you will have big problems.

Brian

[yano]

It looks like I'll probably be building the mixer shown here:
http://www.generalguitargadgets.com/v2/index.php?option=displaypage&Itemid=123&op=page&SubMenu=

I'm not sure if I should just go ahead and get the PCB and stick with 4 channels or if it would be worth it to build the whole thing on perfboard so I can have 6 or more channels.

Also, how easy would it be to add a diode that lights up when the signal is being clipped? I found a schematic of a clipping detector circuit on this page:
http://sound.westhost.com/project30a.htm (schematic of clipping circuit here:  http://sound.westhost.com/p30a_f8.gif)
It may not be worth my time to implement that particular peak detector.

Another thing I'm interested in adding would be "flow through" jacks for each channel. This way, at least for recording, the instruments and their possible effect chains will all be before the mixer, and then go from the mixer to the recorder, but it could allow each channel to go to their own monitor if the musicians so choose.

If I don't implement the "flow through" jacks, I'd like two output jacks.

For both of these jack-related issues, would it suffice to simply wire the jacks together. Could the input jack and the flow through jack for a channel simply be wired together? Same for the output jacks, can they just be connected to each other and then the circuits output? I'm not sure, but it seems like I might run into impedance issues.

Thanks for the help.

[aron]

This site is hard to read, but here you go.

http://www.all-electric.com/schematic/simp_mix.htm

BTW: R.G. Used to have a really simple mixer like the one above. I built it in a day and used it at a gig.

[yano]

the bill of materials and the schematic call for a resistor of value 8k2 what does this mean? 8200 F?

Alright, so my multiple mixed output question is answered, how about the individual channel outputs. Can I just run a line from after the input buffer/pot to the output jack, or should I have some electronics before the output (a resistor/cap pair like is on the mixed outputs)?

[aron]

8.2K resistor.

I'm not sure what you mean by channel outputs. You could make insert jacks or have a switch to output each channel directly. This would be gettting complicated.

[Mark Hammer]

Yano,

Note the difference between a simple *level* detector, and a "clipping" detector.

A one-LED detector circuit can show you a glow that corresponds in brightness to signal level, but it will need to be harnessed to some sort of comparator (designed with information about the circuit itself) in order to identify when the signal has exceeded some critical level.

That's not just a nitpicky bit of semantics, and that's also not to say that you couldn't eventually learn to associate the LED brightness with certain sonic characteristics.  However, often unless you have dedicated mic preamp chips like the SSM2011, which actually have outputs for single LED clipping indicators (i.e., it only glows when the input level falls within some window), most simple one-led level meters are there to twll you if there is signal at all and if it is as cranked as you can get the mixer/channel to be.  That's helpful, to be sure, but it is not failsafe.

[kroushl]

mixers can get extreemly complex, and usually require lots of buffering. sounds like you only want a single buss mixer so that should keep it relatively simple.  

One thing that i have never really looked into (because i have never built a mixer) is how to deal with balanced inputs.  I know that there are two hots and a cold, but dont really know how it works.  i think one hot is basically dead, and just measures noise, and then is subtracted from the 'real' hot.  

I know craig anderton has a few mixer/preamp projects.

if each channel isnt active you will have big problems.

Brian

The link I gave earlier addresses balanced inputs.

Brad