Mixer design question

Hello, I stumbled onto this schematic while browsing the web. I am currently building a studio / practice space in my basement, and I was actually planning to buy a JamHub. But I thought this design looked interesting. Because I could use this as a JamHub and a monitor mixer in the studio.


I am planning to build it with 8 mono inputs and two stereo inputs, and a master volume. But I also thought it would be cool to add a panpot / balance pot to the channels, and maybe an AUX send for the mono channels, and a AUX return on the master channel.

My thought was to use a buffer for the AUX send, and a simple passive panpot.

I stumbled onto this:


I am pretty new to electronics, and allthough I get Ohm's law pretty good, impedance on the other hand has me stumped... and asside from a couple of booster pedals and guitar effect loops, I haven't built that much yet. But I am eager to learn.

Can I use that as a panpot? Do I need to alter it in any way to get matching output / input impedance to the other circuit?

The summing resistors, do I need to alter their value to sum more channels?

All channels will be fed from my main console's outputs, this project does not need to work as mic or guitar preamp.




2008-01-08 11:51 pm
Hi, docZ

This is a difficult post to answer.

I Have been a guitarist for over 40 years, I have been interested in electronics for just as long and I have been involved in electronics professionally for over 20 years.

The mixer circuit you have posted will undoubtedly work, although it is not how I would choose to design a mixer. This is also true of the pan control.

googling 'mixer schematic' I found:

Simple Mixer Schematics

Audio Amplifiers Circuits - Mixers - free electronic circuits - Links

Audio Mixer Circuit - 3 Channels

Mackie CR-1604 VLZ Mixer Schematic

The last is a schematic of a complete Mackie professional mixer. The others are just a representative sample of many mixer circuits. There are circuits for very many purposes on the web, all that is necessary to find them is to pick the right search argument.

Electronics is a difficult subject. Even once the principles are understood, there are many other considerations. For example there are probably a dozen ways of making a capacitor, all giving the same value of capacitance, but some good for audio, and some not.

Of course, anything that is rewarding can be expected to present some difficulties.

If you want to build a mixer, I would suggest finding a complete circuit with details of component types, voltage tolerances, accuracy tolerances, power handling and other such details as you may find problematic if you try to build a less well-specified circuit such as this, which will require two different types of capacitors.

By all means experiment, read about electronics, take a course even, but when you want a piece of equipment for performance or practise, be careful that you do not allow yourself to become sidetracked or held up by complications. In many instances the best option for a keen musician is to buy an off-the-shelf piece of equipment.

Of course, you, like me, may end up as an electronics professional.

Whatever you decide to do, good luck.

Appreciating waki's good post, I would observe: knowing electronics relates to creating circuits the way knowing notes relates to creating music. There is a lot more to it.

Do you want to build a mixer because you need a mixer, or because you want a fun project? If you just want a fun project, one you can even use, then great, go for it. But if your main goal is to have a useful mixer, may I suggest looking in the market for commercially available products. Especially used gear, a lot can be had for not a lot of money.

To make a mixer there is the circuit, then there is the layout, important to reduce hum, noise, and crosstalk, then there is the mechanical assembly, and so on. And if you value your time... The circuits you linked are nice enough circuits but they are just little pieces of the whole thing. A nice clean power supply would be the first requirment, for example. Going to want phantom for your mics at all? Will you be using any mic inputs or just line level?

And while there are interesting project circuits on the internet, if you want a functioning piece of gear, you could also find a commercial mixer with the basic set of features you want, and copy their circuit.
Thank you for your replies.

Just so we are clear, I am not trying to clone a "JamHub" just to save a buck. It is more of challenging myself with a somewhat difficult fun project, and if it turned out to be a usable piece of gear then that would be a big bonus - but not the primary goal. The goal was actually to try to understand how these circuits work.

I have completed some projects, one very challenging one was restoring my dads old champ type tube amp from the 50s. I found it while clearing out my parents basement. It didn't work, so I studied the schematics and managed over the course of 6 months to get it working in good condition. It is now my main practice amp, and I learned a lot about valve circuits and how they work.

So I was contemplating that this could be a similar project to learn more about op-amps and their application. I could offcourse just buy a JamHub and be done with it. But I have a little fantasy of creating these wall panels with jacks and pots to make an in-wall monitoring system. Maybe I am in way over my head?

I do not want phantom power or mic pre's this is going to be run off the AUX outputs of my Mackie mixer, and the LINE outputs of my Focusrite soundcard.

Hello, I stumbled onto this schematic while browsing the web. I am currently building a studio / practice space in my basement, and I was actually planning to buy a JamHub. But I thought this design looked interesting. Because I could use this as a JamHub and a monitor mixer in the studio.


There is an issue with that design. There is no DC bias for the op-amp non-inverting input. He may get away with it by using a FET input op-amp, but it's going to have poor RF rejection and possibly suffer all sorts of funny symptoms.


2008-01-08 11:51 pm
Yeah, ritchie's right about that design, there's nothing to set the bias on the opamps. Easy to fix, but indicative of the kind of issues you're likely to encounter. You'd need a couple of large value identical resistors (100k?) from the + opamp inputs after the 10uF caps to both supply rails.

Ok, so I had a go at drawing this thing up... LTSpice didn't have a pot symbol, so I just put two resistors together to make one.

Not sure if I got it right, but I basically just put the two schematics together.

But I am wondering, should the channel volume come before the pan pot? If it is after I guess I'm going to need a dual-gang pot to avoid having to use two pots.

You think this thing will work?



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