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I'm doing a project that the idea is to accept 3 audio inputs: 1 amplified input (27Vpp), a pre amplified input (6Vpp) and a microphone input (About 150mVpp).

Both amplified and pre amplified signals are balanced and came from a class D amplifier that has a 4 Ohms output impedance, for them I am thinking to use a passive attenuator, 'L' shaped to set the signal to about 2Vpp, bench testing apparently it worked fine (I don't really need studio quality). So, here is my first question: Is it the best option to attenuate the signal? Any other options? After attenuated, the signals go to the MUX (MC74HC4052ADWR2G). Do I need to be careful with the audio impedance matching with the mux input impedance?

I don't want to use a negative power supply, so I thought I'd give the signal a DC gain before going into the mux, because of the attenuator circuit i i thought i in using an AMPOP in the setup shown in the image: Ampop config Is it a good solution? What consequences does it have for the audio?

The output of the MUX will go to an ADC, i was looking at the ADC datasheet and saw that its input impedance is high (about 22k) - compared with the 4 Ohms of the inputs. Do I need to do something to match the impedances? Or does the ADC "accept" a range of input impedances because it has a higher impedance input?

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  • \$\begingroup\$ Impedance matching is irrelevant for audio-frequency signals because the frequency is very low. When you get to RF signals, impedance matching matters. The mux input impedance does matter however, because it acts like an extra resistor in your voltage divider. \$\endgroup\$ Commented Jul 11, 2022 at 12:41
  • \$\begingroup\$ Thanks for the answare! I hadn't realized that the MUX ON resistance would cause signal attenuation. The ON resistance is a maximum of 240 Ohms, I believe it is a considerable resistance. Any suggestions on how to get around this? \$\endgroup\$
    – ViniBC
    Commented Jul 11, 2022 at 13:11
  • \$\begingroup\$ oops I did mean the ADC input impedance not the mux (the 22k you specified). But yes the mux ON resistance matters too. Actually I had no idea it would be so high. Calculate how much of a difference a 22k/240 ohms voltage divider makes to your signal, then tell me whether it's actually a problem. \$\endgroup\$ Commented Jul 11, 2022 at 13:17
  • \$\begingroup\$ In fact, it won't be very significant at all! Thanks for your help! :) \$\endgroup\$
    – ViniBC
    Commented Jul 11, 2022 at 13:26

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Is it the best option to attenuate the signal? Any other options?

The cheapest method is to use resistor dividers to attenuate the signal. Make sure the resistor divider do not load the output of the PA. Remember that if this signal will be used somewhere else as an input then you do need to buffer it.

Do I need to be careful with the audio impedance matching with the mux input impedance?

There's no such thing as "mux input impedance" because a mux is basically a switch. But there's input capacitance, which can be an important parameter if the input signal has high frequency (e.g. at least 100 kHz) content. What you should worry about instead is the ON resistance, which can vary from a few Ohms to a few hundreds of Ohms. Depending on the input impedance of the following stage this ON resistance might be important as it may cause some extra, unwanted attenuation.

Is it a good solution? What consequences does it have for the audio?

It's basically an inverting buffer with DC offset. Don't forget to bring the input signal through a coupling capacitor (something between 4u7 and 10u may be sufficient) or else the DC offset (if the input signal has) will be amplified as well. If the offset is high enough the output may get clipped.

Do I need to do something to match the impedances? Or does the ADC "accept" a range of input impedances because it has a higher impedance input?

No. Provided that the on resistance of the MUX is low, that input impedance will cause no problems.

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  • \$\begingroup\$ Thanks for the clarifications! \$\endgroup\$
    – ViniBC
    Commented Jul 11, 2022 at 13:04
  • \$\begingroup\$ Yes, i forget to say, but i am bringing the input signal through a coupling capacitor of 10uF. I hadn't realized that the MUX ON resistance would cause signal attenuation. The ON resistance is a maximum of 240 Ohms, I believe it is a considerable resistance. Any suggestions on how to get around this? \$\endgroup\$
    – ViniBC
    Commented Jul 11, 2022 at 13:11
  • \$\begingroup\$ @ViniBC it's not that high because the input impedance of the ADC is a few tens of kiloohms. So there'll be no significant attenuation. If the input impedance of the following stage was something like 1k then the on resistance of the MUX would be a significant problem as it'd bring the 80% of the input amplitude. \$\endgroup\$ Commented Jul 11, 2022 at 13:15
  • \$\begingroup\$ Excellent! Thank you very much for your help \$\endgroup\$
    – ViniBC
    Commented Jul 11, 2022 at 13:18

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