? about building stereo active mixer

[e1999]

Hi there,
Hoping that someone here might be able to help me out. I am attempting to build a stereo mixer, ie take a mono input(s) and pan between left and right outputs.

I've used syntherjack's 4sum mixer
https://syntherjack.net/wp-content/uploads/2018/09/4SUM-portable-mixer-schematic-syntherjack.png
and
the info/schematics from allelectrickitchen
http://www.all-electric.com/schematic/simp_mix.htm
to base the design off of.

When building a mono channel everything functions well. But when I duplicate the channel to create stereo, using TL072 op amps it seems that there is a phasing issue and the two channels when played together sound distorted and cancel parts of each other's signal out.

*edit - i think this is actually a power issue as the the distortion and cancellation happens even when the two signals are not both outputting sound - that is it happens as long as both op amps are receiving power. As soon as I cut power to one amp, the other performs as it should. also, trying out different decoupling capacitors altered the interference between the two op amps.

Any suggestions / ideas / help would be very much appreciated!

[PHOBoS]

I suspect it might have to do with the fake/virtual GND, assuming you aren't using a bipolar supply, but I'd have to know exactly
what you build. It seems a bit weird that it works for mono but not for stereo but that might have to do with the panning control.

[e1999]

Thank you for the response:)
It turned out to be something quite stupid after all!
I should have waited to post the question but it was really doing my head in after several days of trying to figure it out!

I needed to move one of the op amps to a second breadboard (some kind of interference) and I hadn't yet implemented the panning control of the circuit and was just sending the same signal into left and right channels.
Now that I correct these two things its working.

That said, I'm curious about a few aspects of this design. I'm really new to this - I'd greatly appreciate if you could give any insight about the circuit in general or about any of the following, though I know some of these questions might be very novice!

I attached a drawing of what I'm doing

- I've read that adding higher value resistors introduces noise into a mixer design. right now I'm using 22k with a 220k on the 1st inverter of the op amp to add quite a lot of gain if needed. Would there be any advantage or disadvantage to using for example 10k and 100k?

- I wanted originally to use a 12v dc supply, but using the supply without voltage regulator added too much electrical noise into the circuit. So I used 78l06 to produce 6v. Do you think it's worth it for the extra headroom to use instead a voltage regulator rated for 12v? and if so could I still use 12v power supply or would I need to use something larger like 15 or 18v?

- finally I've been reading about virtual earth. but still feel like my understanding about the advantages and disadvantages of various methods is lacking. here I used voltage divider with (2) 22k resistors and a 1uf capacitor. can you say anything about possible issues with this solution?
or how it compares to for example the solution I saw in mutable instruments filter designs where they use an LT1054 into 7805 and 7905 voltage regulators?

also in that design they use something like 16 bypass capacitors, when typically I see people using just 2 or so, do you know what might be the reason for that? is this because it is receiving signals from a cpu?

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[PHOBoS]

good to hear it works now

[e1999]

ok thank you
lots of v useful info here
i will try the op amp configured as a voltage follower

would you use more than one of these op amp voltage followers if you have a circuit that is multiplying in complexity with many ic's / parts in the design?

also in such a design with many parts (on a single supply), would it be in any way helpful to make an op amp voltage follower for the +V power as well?

[Grumble]

You could also try this: http://www.falstad.com/circuit/circuitjs.html
Here you can enter components and see immediately what happens when you change a resistor value etc.
And as a side-note: You have a 6volt regulator, why don't you use that as the virtual ground and power the opamps with +12 volt and GND? If you are worried about the noise, you should add some capacitors close to the power connections of the opamps (10uF parallel with 100NF), and maybe a 10uH coil in series in the +12 volt power line.

[PHOBoS]

[JovianPyx]

The use of virtual grounds is a design choice that should be made with care and complete information. There are a few scenarios where they work well, but many more where they don't. Small designs that use low currents are most likely to tolerate virtual grounds. When you are designing something, it is always a good idea to start by using a good quality bench power supply that is already dual voltage and more than beefy enough for the current needs.

Virtual grounds are likely to cause problems in circuits where the design is not solidified and is growing. For these, the bench supply will not introduce new problems related to virtual grounds so that the development works properly (functionally as well as troubleshooting) until the idea has been completed. Once that is finished, if there is truly a need for virtual ground, then the bench supply can be replaced with a single supply and virtual ground and then regression tested. However, as the design grows it becomes less and less friendly to the virtual ground. For battery operated designs, consider making a dual supply with two 9 volt batteries as opposed to one 9v battery and a virtual ground. The +9/-9 supply will give more headroom comfort and will have a much more stable ground than the virtual ground method. If you begin a design with a virtual ground, once the current needs grow beyond the capacity of a virtual ground you will experience problems, but they are weird problems that will make you chase your tail mucking with things that were actually quite well designed. Failure of a virtual ground can show up in the oddest places and will flumox the designer. This is what happened to Cynosure with the CMOnSter project (somewhere here on EM). It was fine until a certain point where adding things just wouldn't work. Once he replaced the virtual ground supply with a real dual supply, it all worked as designed.

As PHOBoS pointed out, the problem with virtual ground is current. While it may be used as a voltage reference, there is still current draw. Virtual grounds can be made more stable (as PHOBoS points out) with an opamp voltage following buffer, but only to a limited degree. Most opamps are comfortable sourcing or sinking around 10 mA or so. If more than that is needed, the voltage of the virtual ground can shift. That injects noise and that noise will go everywhere the virtual ground is used.

In my own design world, I would never want to use a virtual ground for any mixer, but that's just me.

[AlanP]

Not to be a downer, but is there any reason you need to use single rail supply?

If the circuit as a whole is relatively low current, you can use chips like the MAX1044 (or better) to supply low current circuits on the negative rail, respecting the limitations of the chip.

[e1999]

Thanks to everyone for all the great info & advise!

I am going to try all of the suggestions:)

Just as an experiment, I tried voltage follower in several parts of the circuit for the + power rail and it got rid of all the noise that the caps to ground hadn't been able to handle although it reduced the + rail to something like 11.77 instead of 12v. Still better than the 6v from the 6v voltage regulator I happened to have. Anyways, I don't think I'll use that as a final solution-at least for the + rail, but good to know about regardless.

Also I get what you all are saying about dual supply vs virtual ground. And it totally makes sense that using dual supply is objectively better, especially for sound applications.

However, I'm new to this and I've read a lot of warnings about jumping straight into building a dual power supply, without the requisite knowledge and understanding. As well there are a few practical/logistical reasons (ie travel a lot and dc supply is smaller, lighter, easier to replace if broken, no step up/down converter etc...) that make a DC supply(if possible to use) in some ways preferable for the current project I'm working on.

As a side note, I've noticed that there are quite a few commercial synths now being made that run off DC supply, such as Moog Mother 32 or Dfam for instance. At the same time I've heard that traditional AC dual supply is still more reliable for modular synths and mixers.

Does anybody have any insight or experience on dual supply created from single supplies for synths?

Is a dual supply that is created from a switching psu such as doepfer psu-3 also using a virtual ground?

[PHOBoS]

[e1999]

Hey thats smart.
Thanks for sharing your schematic!

Have to put this project on hold for the next week bc other things, but will try this out when I resume!