Stereo mixer choice : active/passive, pot/switch, hum risk ?

[Djano]

My goal is to mix multiple stereo signals to send a single signal to my preamplifier. Three questions concern me:

1. Should I prefer an active or passive mixer? I know that a passive mixer will reduce the overall volume, which is not a problem because I have a sufficient gain reserve. I was hoping that the passive mixer would avoid adding any noise or distortion, unlike an active one, but I would like your technical opinion on this.
2. If a passive mixer is acceptable, is there any advantage to giving up potentiometers that allow me to balance the inputs on the fly in a centralized location in favor of a model with simple ON/OFF buttons?
3. Could I risk having "hum" issues for any reason? I don't see how a ground loop could happen here, but this seems a complex topic. The devices connected to the mixer would be:
   1. Analog DAC output (powered and fed via PC USB)
   2. Guitar effects processor (powered and controlled via PC USB, fed by guitar jack)
   3. Microphone return, useful for online gaming sessions (powered via PC USB, fed by VModa Boom Pro)
   4. Echo Auto (powered via PC USB)

For your information, the mixer I have spotted and like the most is the Tenealay X41: compact, inexpensive, and made of aluminum.

 

[AnalogSteph]

Should I prefer an active or passive mixer?

A passive mixer is awfully primitive and nonideal in many ways (including leaking some signal to the other inputs), but may still get the job done. You can't really build a "proper" mixer without an inverting summing amplifier.

Could I risk having "hum" issues for any reason?

Yes, or unwanted noises in any case. You've got new ground loop running through the USB power ground return of your DAC and back through the USB power ground return of additional devices. Worst-case you may need 3 independent USB chargers to supply power, though you might get away with less.

Microphone return, useful for online gaming sessions (powered via PC USB, fed by VModa Boom Pro)

What kind of device exactly is that?

 

[Djano]

Thanks for your answer

A passive mixer is awfully primitive and nonideal in many ways (including leaking some signal to the other inputs), but may still get the job done. You can't really build a "proper" mixer without an inverting summing amplifier.

Would audio leaking be a real problem, as the purpose of the mixer is to hear every device simultaneously ? But it is true that, as one of the devices (n° 2) is ULTRA noisy, I would like to not hear anything at all about him when the potentiometer is turned down.
And about sound quality (not volume), would you prefer to go active (with probably cheap components) or passive ? I found and active one based on NJM4556. Form factor is less nice, but if this allows for better sound, why not...

Yes, or unwanted noises in any case. You've got new ground loop running through the USB power ground return of your DAC and back through the USB power ground return of additional devices. Worst-case you may need 3 independent USB chargers to supply power, though you might get away with less.

I do not understand this problem, as everything is connected to the same ground point (computer plug, only one wall socket). Would a single USB power supply with triple slots cause the same issue ?

What kind of device exactly is that?

I use it as a condenser mic amplifier with an ADC to transmit the signal via USB (analog is possible too)
The device does more than that (DAC, headphone amp, 3 channel mixer), but I only use it for this function. Its name is "Tactical Audio Controller" (lol), from Turtle Beach. It can be bought used for dirt cheap. Here is what I think about it:
DAC-wise, it should be OK. Problem is the preset-EQs it applies, you can not bypass them (neutral is not neutral)
Headphone amp, it is terribly noisy after 80%
As a mixer, it caused no problem at all
A nice function is it lists as two devices on Windows, so you can output different apps on each device and manage their respective balance

 

[AnalogSteph]

But it is true that, as one of the devices (n° 2) is ULTRA noisy, I would like to not hear anything at all about him when the potentiometer is turned down.

That wouldn't be an issue.

And about sound quality (not volume), would you prefer to go active (with probably cheap components) or passive ? I found and active one based on NJM4556. Form factor is less nice, but if this allows for better sound, why not...

It kind of depends. You obviously don't want lots of crappy ceramic capacitors in the audio path, not enough supply voltage to support the output level needed, or severe construction issues (I don't know, substantial hum introduced by poor PSRR and such). But really, it's a pretty simple circuit, there's not too much to screw up.

NJM4556A is an interesting choice, I guess the output will also drive headphones then? You wouldn't want to do that with a passive mixer...

I do not understand this problem, as everything is connected to the same ground point (computer plug, only one wall socket).

Yes, but between the common ground at the PC and the audio ground of each device there's a piece of conductor carrying each device's supply current back, with a finite resistance. This is going to be different for each of them. Now since we know that the USB ground potential at the PC and audio ground potential at the mixer are the same for all devices, these differences have got to end up somewhere. There must be differences between the ground potentials of each device then. Those are effectively in series with the respective output signal.

This is pretty much the same kind of problem you get when using some of these small PC speakers with a 3.5 mm cable for audio and USB for power supply, and plugging both into the same computer.

I mean, you can always try it first and see whether the interference level bothers you. There should be an improvement with all 3 devices on one charger and further improvements as the number of independent chargers is increased. The problem may become insignificant somewhere along the way.

I use it as a condenser mic amplifier with an ADC to transmit the signal via USB (analog is possible too)

But you would just be using the USB ADC + preamp function, correct? There should not be a problem with that then. Now the analog output, that would be a different story...

 

[DVDdoug]

I would also recommend active and as long as the mixer is "decent" I wouldn't anticipate any audio degradation.

The "theoretical advantages" of passive are outweighed by the actual-practical advantages of an active circuit. And, line-level signals are "easy".

 

[Djano]

NJM4556A is an interesting choice, I guess the output will also drive headphones then? You wouldn't want to do that with a passive mixer...

The purpose is to send the output of mixer into a preamp, then into a 2.1 amp and a headphone amp. So I have plenty of gain reserve to accomodate for volume loss.

Yes, but between the common ground at the PC and the audio ground of each device there's a piece of conductor carrying each device's supply current back, with a finite resistance. This is going to be different for each of them. Now since we know that the USB ground potential at the PC and audio ground potential at the mixer are the same for all devices, these differences have got to end up somewhere. There must be differences between the ground potentials of each device then. Those are effectively in series with the respective output signal.

I struggle to understand this. Not because of your explanation, but because of my ignorance. I will have to do some readings to understand these mechanisms better.

But you would just be using the USB ADC + preamp function, correct? There should not be a problem with that then. Now the analog output, that would be a different story...

Mic preamp + USB ADC would be used to send the microphone to the PC, so that I can communicate with friends online.
But I would also use the analog output for the "mic monitor" function. The goal is not to speak too loudly during the most intense moments. It happens quickly, to get caught up in the heat of the action... Having a feedback of my own voice allows me to avoid this, so as not to disturb my wife sleeping in the next room

I would also recommend active and as long as the mixer is "decent" I wouldn't anticipate any audio degradation.

The "theoretical advantages" of passive are outweighed by the actual-practical advantages of an active circuit. And, line-level signals are "easy".

What are the actual advantages of an active circuit, apart from avoiding volume loss and channel leaking into each other, as mentionned by AnalogSteph ?

____
So, to have the best chances to avoid hum, everything should be powered via independent usb chargers. This can be done for my preamp, the Echo Auto, but not for the guitar effect processor (USB is used to program it), and not for the microphone thing (USB is used to send the mic signal to the PC)
However, i can these to USB that have to be connected to the PC could be separated: one in the motherboard USB, and one in my PCI-E USB expansion card. Would that improve anything ?

Thanks to both of you for your help

 

[AnalogSteph]

However, i can these to USB that have to be connected to the PC could be separated: one in the motherboard USB, and one in my PCI-E USB expansion card. Would that improve anything ?

I don't think so... in fact, it might be even worse.

So, to have the best chances to avoid hum, everything should be powered via independent usb chargers. This can be done for my preamp, the Echo Auto, but not for the guitar effect processor (USB is used to program it), and not for the microphone thing (USB is used to send the mic signal to the PC)

Then we've got the Echo Auto sorted.

Both the guitar FX processor and microphone thing have mono output (or at least you don't need more than one channel for them), right? Then you might get away with one Behringer HD400 as a line isolator for both, using just one output channel each (e.g. left channel). You'd just need to split mono to stereo again afterwards. Otherwise you'll probably need two of the things, plus assorted cabling (1/8" stereo to 2x 1/4" TS and such).

 

[frabor]

I would say active.

For inspiration or full pcb, take a look at Elliott Sound Products, this particular pcb is a basic preamp mixer, just add pots, jacks and power supply.

 

[Djano]

Thanks for your help. Sorry for the time I take to answer, as this is a complex problem to me!

I have been thinking about a way to manage the necessary USB that can not be unplugged from the computer. Such devices are:
- E30 (data usb only, power can be plugged in 5V charger)
- Microphone thing
- Guitar FX processor

Are the cheap "USB isolators" a real thing ? Amir tested an expensive one and it was BS. But I do not know about the rest. Here is an example

(EDIT: Behringer HD400 would indeed require me to do a lot of stereo/mono conversion. Guitar FX processor outputs stereo, for example. So this solution was a good idea, but my particular devices will make it hard)

 

[mhardy6647]

... a classic passive mixer from Sony

(borrowed image -- I was actually once offered one of these, but couldn't rationalize what I'd do with it!)

How does the Sony 6 channel mixer MX-8 work? It has no power supply. There is no phantom power from the items I connect to it yet it work...

Answer (1 of 6): The MX-8 is a passive mixer. Here’s a wiring diagram for a simple one, which mixes three inputs into two identical outputs. There’s no need for power other than the input signals themselves — presumably dynamic mic outputs. In fact, just the 1K resistors together constitutes a pa...

www.quora.com

 

[AnalogSteph]

Are the cheap "USB isolators" a real thing ? Amir tested an expensive one and it was BS. But I do not know about the rest. Here is an example

As long as you do not expect more than USB 1.1 speeds (12 Mbit/s max) and probably a good bit less than 500 mA worth of current (possibly only 100 mA), they're generally OK, AFAIK. Some will have a connector for external power to augment the internal isolated DC/DC power supply.

 

[Djano]

So, I guess I will manage the hum problems with cheap USB and 3.5mm isolators. This will only apply to the sources for which I do not care too much (so it will not apply to the main source, which is the E30 dac)

I finally found a mixer that has the perfect functions for what I want to do (active, as you told me I should get, 4 inputs, 2 outputs):
It is said to use a BA3121 to eliminate noise, but I do not understand if all problems will be gone or if I will need the isolators previously mentionned. Here is the BA3121 datasheeet

Do you confirm the two outputs potentiometers would provide almost the same benefit as a switch ?
Outputs would be headphone amp and speakers amp.
Currently, I simply have an Y RCA to feed both. I can hear a slight hiss with ultra-max gain on the headphone amp, which disappears when I unplug the RCA cables of the speaker amp. I guess replacing the Y/splitter with a switch would get rid of this, and was hoping that the potentiometers would work almost the same.


For future people who might be looking for mixers and/or distributors, here are the references I found:
- AU 401 (two chassis available): active, 3.5mm, 4 in, 1 out
- AU 402 RCA : active, 4 in, 1 out
- AU 204 : active, 3,5mm, 2 in, 4 out
- Tenealay X41 : passive, 3,5mm, 4 in, 1 out
- Tenealay X21: passive, 3,5mm, 2 in, 1 out
- Tenealay R41: passive, RCA, 4 in, 1 out