Need advice on audio mixer/volume control

Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,333
So I need to connect 3 audio sources with standard line out, to one output with the addition of volume control. (digital switching)

So my questions are...

1. Is this solution viable?
2. What are the resistance values needed?
3. Wattage ratings of the pots?
4. Do I need to enclose and shield?

Yea...audio was never my thing...

The MUX are analog and I only show 2 switches, but there will be 9. (70Ω or so per channel X2)

A_MUX.jpg
 

sghioto

Joined Dec 31, 2017
1,556
I don't understand your drawing. You mixed the three audio sources before the mux then show two volume controls feeding another mux. Why is that?
 

Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,333
I'm mixing the audio sources first then sending to the MUXs for volume control so I can select different volumes.

The MUXs will allow me to chose different pots for volume control using binary.
 

crutschow

Joined Mar 14, 2008
25,660
Not the the peak plus and minus audio signals must be on larger than the plus and minus supply voltages to the Muxes.
If you use a single supply voltage then you need to offset the audio so the negative peak does not go below ground.

Do you want individual volume control for each of the inputs?
 

Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,333
Not the the peak plus and minus audio signals must be on larger than the plus and minus supply voltages to the Muxes.
If you use a single supply voltage then you need to offset the audio so the negative peak does not go below ground.

Do you want individual volume control for each of the inputs?
The line out will share the same ground with the MUXs so I don't think they will go below ground...or will they?

No, I don't need individual control for the inputs.
 

crutschow

Joined Mar 14, 2008
25,660
The line out will share the same ground with the MUXs so I don't think they will go below ground...or will they?
A typical audio signal in an AC signal referenced to ground so he signal voltage will go equally above and below ground.
An oscilloscope set to DC input will verify whether that's true in your case.
 

KeithWalker

Joined Jul 10, 2017
1,299
I read your explanation but don't understand what you are trying to do. It would be much easier to understand if we knew what the application is.
Do you have 9 mono inputs, each composed of three mono inputs resistively mixed together, or just one?
If you only have one mixed mono input, do you want to switch it to the input of any one of 9 potentiometers?
What do you want to do with the outputs from the 9 potentiometers? Do you want to just select one of them to connect to the output?
From the look of it, it would be much simpler to use one digital potentiometer, and set that with your binary input.
I may be able to offer some suggestions once I know exactly what you are trying to achieve.
Regards,
Keith
 

Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,333
I read your explanation but don't understand what you are trying to do. It would be much easier to understand if we knew what the application is.
Do you have 9 mono inputs, each composed of three mono inputs resistively mixed together, or just one?
No

If you only have one mixed mono input, do you want to switch it to the input of any one of 9 potentiometers?
Yes

What do you want to do with the outputs from the 9 potentiometers? Do you want to just select one of them to connect to the output?
Yes, sending this to an amp.

From the look of it, it would be much simpler to use one digital potentiometer, and set that with your binary input.
I may be able to offer some suggestions once I know exactly what you are trying to achieve.
Regards,
Keith
 

AnalogKid

Joined Aug 1, 2013
8,693
If you want to select any 1 of 9 physical pots for the combined audio signal, then I don't see the need for the left-hand mux (reference designators ! ! !). Sum the three sources resistively as shown, buffer the result with a unity gain voltage follower that drives the top of 9 pots in parallel, and mux the outputs from the wipers. Or combine the inputs with a true summing amplifier, which also fulfills the buffer requirement. The buffer amp presents a very low source impedance to the pots, which prevents interactions among them.

Note that if the pots are standard audio-taper parts, then when the knob is in the middle of its adjustment range the output signal amplitude will be 10% of the input (-20 dB). Consider adding a little gain to the buffer circuit.
https://en.wikipedia.org/wiki/Potentiometer#Logarithmic_potentiometer

ak
 
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crutschow

Joined Mar 14, 2008
25,660
so you're saying that the signal ground is not the same as the device ground?
No.
Having the audio signal referenced to ground does not mean the signal does not go below ground.
It means the signal average value is at ground potential, thus the plus side of the AC signal goes above ground and the minus side goes below ground (potential).

Make sense?
 

AnalogKid

Joined Aug 1, 2013
8,693
To use analog muxs with only one power rail, you can use one opamp can create a virtual GND that is 50% of Vcc. That plus 4 coupling capacitors and you're ok. Any high-quality dual opamp can cover the virtual GND and the buffer / gain stage.

What power supply voltages are available for this circuit?

ak
 
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Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,333
No.
Having the audio signal referenced to ground does not mean the signal does not go below ground.
It means the signal average value is at ground potential, thus the plus side of the AC signal goes above ground and the minus side goes below ground (potential).

Make sense?
Yup, glad you pointed that out, as I said audio is not my thing.

AK the source audio is from a 5 volt device...Adafruit Audio FX mini.
 
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