Simple Mono Mixer Design Help!

Hi all,

I'm wanting to design a simple mono mixer for live PA use with an acoustic type band - cajon, vocals, guitar etc. The focus is on simplicity of design and build, quality and reliability. I would like the input channels to be modular, so I can build them individually and stack as many as I need into the master section (within reason!)

Simple Mono Mixer Design

Mic Input Channels x5

XLR input socket (mics will all likely be dynamic 57/58s)
Low cut switch (to remove rumble and low end from channels that don't need it)
Phase switch (i've read this can help with eliminating feedback but happy to leave out if this is BS)
Mute switch (convenient way to kill a channel)
Tone control pot (a simple guitar pedal style treble cut/boost)
Channel volume pot

Master Section

Power switch (I may run the mixer from an external wall wart or laptop style supply)
Aux 3.5mm stereo jack input w/vol (for music from my phone where needed or using a backing track)
Master output mute switch (a "KILL" switch :) )
Master 2 band EQ (treble and bass cut/boost)
Master volume pot
LED VU meter (a visual indication of the output being at line level - if there's a simpler way, great!)
Phono / 1/4" jack outputs (standard line level)

There are plenty of simple summing preamp/mixer designs kicking about the web, but nothing that uses NE5532 opamps (which is what I would like to use) and incorporates all of the features I want. Some have an opamp on every input channel and then another one on the master channel, and some are passive into the master channel. I am presuming that active channels are better so would like to go with that. I am also aware that the unit may need additional PSU filtering to get rid of any noise from a switchmode power supply unit.

So, if I've covered the pertinent details, is there anyone interested in helping me? :)


Any inverting amplifier will perform good as summing amplifier, from a high mu triode to the best op amp. Only need a kind of supply (single or dual), a couple of resistors and pots for each input, and a only one resistor from output to input. The ratio between them will give overall gain.


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So use 5532 if you like. Op amps are op amps basically. You will read threads about who prefers what type, but thousands and thousands of mixers are on the market based upon basic TL072 or 45xx series op amps. If you see a dual op amp of say the OPA series, the circuit would also work with a TL072 or 5532.

You are proposing a MAJOR project. HAve you built anything close to this before?

Here is a basic small mixer, if you have not built one before, try duplicating something like this to get a feel for it:
Behringer Xenyx 502 Mixer | Sweetwater
I don't know where you live, but the US is drowning in defective mixers.
I bought what you want, dead, for $75 including shipping for 120 miles. 5532's on input, XLR and 1/4" phone in, RCA jack out line level. Even a 75 W 8 ohm or 4 ohm output on screw jacks out. Peavey MMR-875T. Mine had 5 input cards. Slots for up to 8.
Two bad solder joints means the unit had hardly been used since 199?. So the pots were not worn out. Rail caps were dried up, $1 parts.
Cruise e-bay musical instruments for "mixer for parts or repair". Then get out the DVM, download a schematic from Peavey or elsewhere (they are the best at support) and start tracing. A unit with good solder joints might need some pots or connectors. Chicken feed price parts. You might need an analog VOM to trace the music in if the problems are in the music feed instead of the power supplies as mine was.
Try to get something from the 80's or 90's, they have leaded parts repairable by a $75 soldering iron. Even from the 60's and 70's for strictly monaural output. I got a working 60's mono Ampeg for $40, 8 channels, big enough to use as a keyboard bench. Needed new PS caps, not urgently. Had a spring reverb!
In fact here is the Peavey MMA-875t schematic. The inputs are on page 4. If you want to replicate all the machine work to build the case and rack, and get all the boards etched and loaded, have at it. I found I spent about a year getting hum & hiss out of an inferior mixer, the Herald RA-88A (pseudonymn Olson). Peavey had already done that work, just not soldering a couple of joints correctly. If you're better than the Peavey workers, have at it: I'm not.


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Hi all,

I'm wanting to design a simple mono mixer for live PA use with an acoustic type band - cajon, vocals, guitar etc. The focus is on simplicity of design and build, quality and reliability. I would like the input channels to be modular, so I can build them individually and stack as many as I need into the master section (within reason!)

Have you seen Rod Elliot's project design for this?
He uses 5534 on the inputs and 5532 for master.
Lots of options presented in his design and it is modular.
Audio Mixing Console - Part 1
Look around your area, I have several Behringer mixers (PMP3000, PMP5000, others) each with some defect, but still quite usable. I am not going to bother to fix them, and am looking to sell them dirt cheap locally. I bet there are similar opportunities near you.

(Defects like powered mixer with one power amp not working, or another with DFX bad)
Hi all,

Thank you so much for the replies!

To answer a couple of things... I dont really want to buy and fix up an existing unit. I'm looking at this as more of a project that I can take my time with and learn a lot as I go. I appreciate that I could learn a lot from fixing something, but i think designing from scratch would help me to learn skills that are more useful going forwards. I know troubleshooting can be a pain, and I'd like to keep it "fun".

To answer those that think the project is quite a task - you make a fair point. I have a half decent beginner's skillset and have made guitar pedals etc before. I can follow simple schematics and do stripboard layouts etc, as well as making chassis and mounting pots, soldering etc. So by no means an expert, but I believe I have the skills to get it done in time, and with a little help. My main goal is to design something that works, and to UNDERSTAND why.

So, what if we start with the mic pre first? That's assuming that a standalone mic pre circuit with XLR input and line level jack output can essentially be connected directly to a summing/master section without serious modification later on? So, get the mic pre working, then make a master section, then join the dots...?

So we start with the preamp (which I can build and test in its entirety before moving forwards):

XLR input socket
Low cut switch (passive RC hi-pass network before the opamp gain stage?)
Phase switch (Simple DPDT)
Mute switch (Simple switch)
Tone control pot (treble boost/cut - how to implement this? filter in the feedback loop)
Channel volume pot
Output jack

So, start with something like THIS (but use a pot instead of switched resistors to set gain) and go from there?

Copying is an education. Peavey put the high level tone controls & low cut next to the hum inducing transformer. The switches and pots were on the front up there. The 75 W amp was up there too. The low level high gain mike amps were in the back of the case, 14" away, behind two steel walls. There was a level set pot on each preamp, shaft out the back wall. I found on the RA-88a I had to banish the AC transformer & power switch from the case to eliminate the hum. Went to a wall transformer, as most vendors of mixer without a power amp have done now. There were other tricks I installed to reduce hum coming from input cables, like separating the jack rings from the case ground. Also RF management- you don't want sports talk AM radio in your mix. Amplication and tone control are only half the job of designing a useful appliance.
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Doug Self's "Small Signal Audio Design" covers a lot of stuff about mixers in detail, it would be a really useful reference for this project.

And it explains where you'd prefer a TL072 to an NE5532 and vice versa (opamps are not just opamps, basically, they vary in many parameters and you choose the best tool for the job - sometimes low bias current matters, sometimes load driving matters, sometimes voltage noise matters, sometimes current noise matters).