Help with active two channel stereo mixer design

[isophase]

Hi all,

I would like to build a simple x2 channel stereo mixer for use with my pedal board.

I'm using two effect chains and both end with a stereo effect, so i would like to combine the two stereo channels to end up with only 2 outputs.

I first thought of building a simple passive mixer, but in the end i decided i want to build something more flexible and adequate > an active mixer with level control on each channel.

I did some research online on pedal mixers and mini mixers but there isn't very much stuff out there that would fit my needs.

So I did a small diagram about what i would like to do. It's very simple and flexible.
The mixer could be used either full stereo are mono or a combination, and also has the option of summing the stereo output with a toggle switch.
i don't know what i should use as input buffer and output driver? there is a lot of options available, any suggestion or advise is very much welcome! I had a look at the AMZ buffers and preamps, they look very nice and i think they would be ideal, but it would turn out a bit too expensive so i would rather build something from scratch or use some available "naked" PCBs.

I would like the mixer to be the most transparent possible. What type of preamp design would be best to use at the input and at the output respectively? Bipolar or MOSFET or JFET or ? the mixer will be placed at the end of the signal chain between the pedalboard and the amplifiers (or sound card)

Can someone help me with a schematic for inspiration or share a link to some available PCBs on the market to do this?
Also is there some similar type of pedal mixers available on the market that could do this in a compact package? please share.
Thank you!
Jonathan

[11-90-an]

Hello

I have a "simple" design here, uses basic inverting op-amp topologies. Basically, the incoming signal gets boosted 10x, but it becomes inverted. It gets mixed with 4.7k resistors, (I put 100k, but when I thought about it, a lower value makes more sense...), then gets inverted again. The trimmers are probably best to be 20k... (schematic error again... ). The use of the trimmers is for the final gain adjustment.



Another schematic error.. forgot to define VREF... just connect 2 10k resistors in series from +9v to ground... then get the VREF from the middle...

I don't think this would work in reality, though... 

[isophase]

Thank you for your help and schematic!
What type of opamp is a good choice? Is it better to use single or dual opamp for each stereo channel?

[antonis]

In case of you don't need null offset adjustment, I can't see any reason for not using dual op-amps..

As for input/output type (Bipolar, Mosfet, JFET, etc) 11-90-an should be more qualifed for answer according to his circuitry proposal.. 

[quad4]

Here's the schematic for the Rolls MiniMix which is a 6 channel mono/stereo mixer in a format that is workable for a pedalboard.  You can go higher end than this, but more than adequate for a pedalboard.

You can pare it down to 4 channels and remove the headphone opamp (which I believe they deviated from the schematic and used 4560 instead of 4566) if you like. Note the mono doubling to stereo if only the right channel is inserted. Comes in handy.

https://rolls.com/doc/manuals/manual_MX28.pdf

[PRR]

The Rolls is a very heavy load for some guitar-cord devices.

The hi-Z inverting inputs in reply #1 are going to hiss at guitar levels.

"Virtual Earth" mixer is usually NOT the best or intuitive action in small stage mixers. A little interaction between controls is not fatal, and often does a good thing (keeps the sum average about constant instead of adding-up).

[isophase]

Thank you for your help!
Yes, I had a look at the rolls mixers and actually downloaded this manual, but haven't had the time to look at it, how great that the schematic is shown in the manual! I thought this was a lost practice.
This unit is one of the very few tiny mixers commercially available on the market. I also had a look at the small Behringer mixer but unfortunately it's mono only. Electro-Harmonix also has a pedal with 3 fx loops but mono... I'm sure there is other stuff but just to say, I'm a bit surprised that when I searched for « mini stereo mixers » there is not a lot of stuff. Hundreds and hundreds of splitters/buffers/boosts/stereo pedals but not any simple tiny mixer? Especially now that pedalboards have become an extremely popular thing. I'm just surprised not a single pedal company is proposing such a very useful tool.
Anyway, the good thing is I will get to learn some more electronics building this thing

I will try to drawn a schematic and post it here.
For sure there will be more questions... can't wait to strat this project!
Thanks

[isophase]

Hi,

I bought some parts for the mixer, i'm going to try the design you posted PRR.
I did a bit of reading on opamp design theory and i have a few questions.

- Should i add an input resistor to ground between input and the first 10Kohm resitor (R4) ?
- What is the input impedance of this non inverting circuit? is it set by the 1Mohm bias resistor? or is the input impedance reflected from the opamp itself > very high impedance?
- Can i use the bias circuit proposed by 11-90-an up the conversation: x2 10Kohm resistors in series to ground and tap the bias from the middle? should i add a big capacitor in parallel to ground? 220uF or 470uF?
-I'm trying to understand the input coupling cap 470nF (C3). Since there is no resistor from input to ground, how can i calculate the cut off frequency or bandwidth of the input? or what is the limit value of the input coupling cap?

I will use TL072 instead of TL071, building on veroboard and trying to fit everything in the smallest possible box. Looks like i should be able to fit everything in a 1590BB enclosure.
For now, I can only fit x1 9V battery (still working on the layout) and also planning to add a DC jack input.
Can i power this circuit from 18VDC for max headroom, and if needed, still be able to run the mixer off a 9V battery? will the bias circuit adapt itself if fed 9VDC or 18VDC?
Sorry for very basic and beginner questions 
Thank you for your help!

[antonis]

- Should i add an input resistor to ground between input and the first 10Kohm resitor (R4) ?

No need for doing it..

- What is the input impedance of this non inverting circuit? is it set by the 1Mohm bias resistor? or is the input impedance reflected from the opamp itself > very high impedance?

Both but they are set in parallel so it's actually is the 1M resistor value..
(to be more presice, slightly lower than 1M..)

- Can i use the bias circuit proposed by 11-90-an up the conversation: x2 10Kohm resistors in series to ground and tap the bias from the middle? should i add a big capacitor in parallel to ground? 220uF or 470uF?

Yes to both of the above..
(capacitor value may be lower than 470μF - it has a dual role: it forms a LPF with upper 10k resistor and also prevents Vbias ripple..)

-I'm trying to understand the input coupling cap 470nF (C3). Since there is no resistor from input to ground, how can i calculate the cut off frequency or bandwidth of the input? or what is the limit value of the input coupling cap?

C3 forms a HPF with R3 (bias resistor) 'cause R3 is effectively connected to AC ground (due to Vbias cap discussed above..)
on the other hand, in the absence of Vbias cap it should also be connected to ground via the parallel combination of 10k voltage divider resistors - one of them going straight to ground and the other going to AC power supply ground..)

[11-90-an]

Can i power this circuit from 18VDC for max headroom, and if needed, still be able to run the mixer off a 9V battery? will the bias circuit adapt itself if fed 9VDC or 18VDC?

Yes, you can. The resistors in this circuit automatically bias the voltage you get from the middle to be 1/2 of the positive voltage.

[quad4]

On the higher voltage, just make sure your capacitors are rated higher than the voltage they will see. Be convervative and choose parts that are rated at twice the expected voltage.

[isophase]

Thank you!
Working on the layout now.
One more question. Gain in both stages (input + output) is set by the two resistors R6 and R5 both 1Kohm and this gives a gain of 1, there is no amplification. Is this correct?
What can I do to the circuit in order to have unity gain when the pots are roughly positioned at 12 o clock? Should I make the final stage amplify by 2? Or what resistor combination should I use to make the second stage to have a bit of gain? is it a good idea to add a 10k trimmer in series with the feedback resistor so I can set unity gain with the level pots positioned at 12o clock? Or is it really not necessary to amplify the signal?
Thank you again for all the help and advice

[antonis]

No, gain is 2 'cause it's a non-inverting configuration..
(1 + R6/R5)

To result into unity gain, you'll have to set R6 zero ohms or make R5 infinite (open circuit)..

[11-90-an]

To result into unity gain, you'll have to set R6 zero ohms or make R5 infinite (open circuit)..

Which is... gasp! A buffer!

A little gain will be fine...

[isophase]

Thanks,
Sure a little gain should be ok, that's what I'm thinking.
So as is, with the level pots at full clockwise position the gain is x4 from input to output, since there are two stages with a x2 gain.
So this means that in order to have unity gain from input to output, i will have to lower the level pot a little bit, correct? That's exactly what I want!
Ok back to work!

[PRR]

2:1 loss in passive mix network.

This is not my first audio design.

[isophase]

2:1 loss in passive mix network.

This is not my first audio design.

I believe you sir 
I mean it's not the first time you help me with a schematic or advice with audio electronics. Thanks a lot PRR for sharing your design! I'm sure it's perfect and exactly what I need!

1:2 in the first stage, 2:1 loss in the passive network, and 1:2 in the final/output stage. So overall gain of the circuit is 1:2 = +3dB. Perfect.

Almost done with the layout, It's quite tight to fit everything in 1590BB box but it's going to fit!
I did some mechanics today, I drilled the box to fit the connectors, pots and switch. I decided to use a very nice (and big!) "Cats eye" push button instead of the more conventional toggle switch (not a big fan of toggle switches) The switch is taking a lot of space but looks like everything fits!
This thing is going to turn out nice! I will  send some photos when the unit is finished.

Honestly, someone with the skills should make this design fit in the smallest possible enclosure and sell it at a correct price and make money. This thing is so practical, useful and flexible I still can't believe no pedal companie is proposing such a useful tool in there catalog. Or maybe I haven't searched enough. Please if you know of any equivalent or similar tiny mixer (apart from the Rolls stuff) please share some links or brands/models. I'm very curious to see other designs and « inventions »

PRR, again thanks a lot maestro!

[PRR]

> I'm sure it's perfect and exactly what I need!

I am sure it is NOT.

But it is a pretty-close plan which you can use to focus your needs. Unity gain (if that even matters) will happen with pot at part-turn, giving also boost and cut. It should be clean and low-hiss.

[isophase]

Hello,

The stereo mixer is finished !
Thanks again to all for your help with extra thank you to PRR for sharing his design mith me.
Build was not very easy but a real pleasure to do, and it worked on first power up! 
Sounding very good indeed, transparent and absolutely quiet. I've been playing with it for a couple of days and its really perfect  (sorry PRR  ) and exactly what i needed!
Cheers,
Jonathan


A few photos of the build