Tips for DIY audio mixer

[grogantech]

Hi all,
I am fairly new to audio electronics, however I have decided to take on a rather ambitious project. I am looking into building a 4 channel analog audio mixer. These are the rough specs for now:

• 4 mono channels
• 3 band EQ per channel
• Mic preamp on each channel
• Fade and pan
• Stereo VU meter

I am not going to bother with phantom power supply as I am unlikely to use condenser microphones, neither will I include auxiliary busses.
I am interested in the opinion of any sound/electrical engineer is who have done similar projects to this and if they may have tips for a beginner. Let me know what you think of the specs and if you have any recommended parts, circuits etc.
Any help will be much appreciated.
Thanks,
James

[calexanian]

What style of construction do you want and what level of presentation do you want to achieve? Is this just a throw together for learning purposes or would you like to make something professional looking as a project sort of thing? Would you like to lay out a PCB and 3d print a nice case or vero board flying wires and a scrap aluminum panel sort of thing? Or no front panel and make everything tiny on a PCB and silk screen everything onto that?

[grogantech]

 I am looking for a relatively well presented project and I will be getting custom PCBs and will make the case with sheet metal by hand.

[law_freak]

I've used a few of the circuits here to good effect.

I think you will find most of what you need there.
mixer
3-band eq
pre

[grogantech]

I've used a few of the circuits here to good effect.

I think you will find most of what you need there.
mixer
3-band eq

Thanks that should come in real handy [emoji2]

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[law_freak]

Have you seen ?

[grogantech]

Yeah of course. Thanks for recommending though

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[KhronX]

May i ask what exactly is the purpose of this exercise?

Just thinking that if a 4-channel mixer's all you need/want, you could save yourself a whole lotta cash and time, and just browse through the used market 
I'd understand if you needed some interesting / custom features included, or if it was a more "esoteric" project, but... I dunno, just thinking out loud here 

[alexanderbrevig]

My vote would be to get a copy of Small Signal Audio Design by Douglas Self. Someone on here tipped me about that book a while ago on here, and it's time I pay it forward.

Good luck on your project!
PS: I'm also curious why you want to build it yourself.

[magetoo]

I second the suggestion for Small Signal Audio Design. (site here)  It basically describes all the sub-circuits that go into a mixing console; you should at least check local libraries and read through it for inspiration if they have it.

Edit: Here's a sample - Op amps in small-signal audio design - Part 1 2 3 4

[Buriedcode]

I have been meaning to build a 4-8 channel mixer for recording drums/guitar (multiple mics) and if that is your purpose, then buying an off the shelf 'product' is cheaper and of course easier.  However!  As you wish to learn, then you can break it into a modular system, after all, once you've designed one channel.. you just add more.

Before getting custom PCB's though, you'll have to do a fair bit of bread boarding to find test out each part, and of course, learning along the way.  From a solderless breadboard (which will be unshielded = noise) to perhaps a veroboard prototype, again just one channel.  Getting 10 PCB's is probably about the same cost as getting one or two, so you'll have some spare.  I would advise, if space allows, adding extra footprints for parts that have different packages, lots of test points, and perhaps even trimmers if you wanted very accurate gain/EQ centres (probably not worth it though).  Not trying to make life more difficult but I always thought a latching 'mute' button would be handy as well.

The backplane can have the PSU, master volume, VU meter (with clipping indicator) etc.. At least that is how I would do it.

[Noize]

Here is a page of app notes that you may find helpful. They also do free samples if you ask nicely(honestly).

http://www.thatcorp.com/Design_Notes.shtml

[Richard Crowley]

http://sound.whsites.net/index2.html is a first-class source of information about circuits, perhaps the majority of them audio-related.

http://sound.whsites.net/project30.htm  specifically is a complete set of articles about a full-feature audio mixer.  With circuit descriptions, schematic diagrams, PC board layouts, and many of Mr. Elliot's projects have PC boards available for sale as well.

[Buriedcode]

I'll second the Elliott Sound Products website, it was invaluable to me waay back in 2003 when I was playing about with mic preamps for acoustic guitar.  Good explanations, and 'common sense' when it comes to parts selection - something almost rare in the audio world.

[DaJMasta]

The elements of everything should be pretty straightforward, the nice thing about understanding the way a mixer works and routes things is that the same understanding basically gives you a very modular block diagram for your own build.

Just some considerations:
Adding phantom power actually isn't too tough, either with taps from your transformer or a boost converter, it's not a big circuit and it could be neat to work with

Adding a compressor or compander to a couple of channels could also be potentially useful if you're intending this for more than just a project.  Especially using integrated solutions they're not complicated to add.

There's no need to go nuts with high performance components, the NE5532, for example, is a standard, available, high performance audio op amp which should have a noise and THD floor well under any potential issues with the layout of construction of a project mixer and even if you've laid things out carefully... it should be under audible levels of noise and distortion.  No need to drop a crazy amount of money on fancy ones when you'll likely be using lots.

When you finalize your signal path, pay special attention to power supply noise (reducing as much as possible), ground loops (don't have them, star ground when possible), and a little physical distance between signal traces or cables to reduce crosstalk.  A lot of this stuff doesn't show in the prototyping phase, but when it comes to the final design it can matter a lot.

Also consider in advance what sort of controls you're going to use.  Sliders or turning pots?  What kind of switches for switching in and out signals?  How will they be arranged to make sense to the user and how will they get the structural support they need to last a long time?  Using poorer quality bits can mean noise while adjusting or switching or premature wearing out.  There's also some circuits that will show you how to keep from getting a zero crossing and a whole bunch of noise when switching things - there are a number of ways to get around it, but it's not always an obvious consideration and it will make a big difference in usability of the final product.  Nothing like a big pop when switching a channel or a scratch when adjusting a level to spoil your sound.


Good luck!  It's a big project but it's probably a fun one.

[grogantech]

Thanks Guys for all your input I really appreciate it.
The project is for my GCSE coursework (bit ambitious I know) so that's why I am avoiding pre built modules.
Also the PCB manufacturing will take place at large quantity when the whole class has finished their projects. Thanks again
James [emoji2]

[cvanc]

Good luck with your project.

Another book to consider is "The IC Op-Amp Cookbook" by Walt Jung.  It's an oldie but a goodie that thoroughly covers all the audio blocks you would need.

https://www.amazon.com/Walter-G.-Jung/e/B001IOHAIW/ref=dp_byline_cont_book_1

[Messtechniker]

Here another source to keep you busy:

Just look around for the series of articles of the Eighties by
Steve Dove "Designing a Professional Mixing Console".

Perhaps from here:
https://www.yousendit.com/download/UVJoR0lYQzNRR2RBSXRVag

Yours Messtechniker

[grogantech]

Here another source to keep you busy:

Just look around for the series of articles of the Eighties by
Steve Dove "Designing a Professional Mixing Console".

Perhaps from here:
https://www.yousendit.com/download/UVJoR0lYQzNRR2RBSXRVag

Yours Messtechniker

I can't thank you enough this will be invaluable to me [emoji3]


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[Richard Crowley]

Yes, the Steve Dove series is a real tour-de-force.  The discussions of requirements and problems to be solved and block diagrams and circuit topology, etc, are timeless.  However the actual methodology of solving the problems (the practical circuit design) was "state-of-the-art" back in 1980. Technology has progressed significantly in 30~35 years.

If you want an example of a more contemporary implementation of most of those circuits, suggest studying something like this...
http://schems.com/bmampscom/mackie/Mackie_1604-VLZ_16_channel_mixer.pdf

[grogantech]

I am just wondering how I would implement a latching mute switch. I can't seem to find the hardware that is small enough and illuminated for this task. Does anyone know of where I could find one?

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[Richard Crowley]

I am just wondering how I would implement a latching mute switch. I can't seem to find the hardware that is small enough and illuminated for this task. Does anyone know of where I could find one?

I would consider using something like this DPDT, latching pushbutton switch.
Use one pole for muting, and the other for controlling an adjacent LED indicator.
Alas, small, integrated, illuminated pushbutton switches are terribly expensive

http://www.taydaelectronics.com/electromechanical/switches-key-pad/push-button/push-button-switch-dc-30v-0-1a-dpdt-7x7mm.html