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I have designed this 4 stereo channel TRS switch.

enter image description here

You have 4 channel stereo input and 1 stereo channel out. You press a button and you switch which channel you want to send to the output. Once the input is selected by the flip-flop and the MAX359 it is amplified by a IC TEA2025B.

The power supply works like this: it is a virtual ground. It receives +15VDC from an external adapter and converts it to +7.5VDC / 0V / -7.5VDC, needed by the switch in order to keep the signal on the middle of the rail.

It is working almost wonderfully but I have two problems:

  1. when I press the switch to select the input channel I hear a click/pop noise on the output. Like a transient.

  2. I have hum noise all over the place. I am already with the whole thing assembled on a breadboard. I have no shielding anywhere. What should I shield when I have it on a PCB or what can I change to remove this noise.

What can I do, change or improve to fix both problems?

Thanks in advance.

NOTE: The diagram shows +5V and -5V. The voltages are in fact 7.5VDC and -7.5VDC.

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  • \$\begingroup\$ What you've described is a qualitative description of the problem. What points on the schematic have you probed with an oscilloscope to measure the noise signal that is manifested as hum? \$\endgroup\$ – AlmostDone Mar 31 '18 at 15:12
  • \$\begingroup\$ I do not have an oscilloscope right now. Saving to buy one.. \$\endgroup\$ – SpaceDog Mar 31 '18 at 15:23
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    \$\begingroup\$ IRC the DC blocking caps can also cause some pop, maybe try putting a high value resistor to ground on their "floating" side? \$\endgroup\$ – Wesley Lee Mar 31 '18 at 15:41
  • \$\begingroup\$ what capacitors/resistors are you talking about? \$\endgroup\$ – SpaceDog Apr 1 '18 at 6:23
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The pop noise you experience when changing channels is most likely due to charge injection from the switch logic. Look at page 9 of the MAX359 datasheet for a discussion on this. This is a characteristic of all CMOS analog switches and multiplexers, but some are better than others on this particular spec.

Not having a scope, try to isolate which section is causing the noise by listening to the hum from the speaker. Start with shorting the input at the jack to ground. If that doesn't improve the problem, remove IC8 (MAX359) from its socket and short pins 8, 9, 15 together which shorts both outputs to ground. Moving on, short out the wiper of each gain pot to ground. If you still haven't narrowed it by now, then you need to take another look at your amplifier.

One other thing you can do is connect your DC power supply through a capacitor into your laptop mic input and observe if there's noise on it using your laptops mic check or skype or... Do this for both + and - supplies separately. Be sure to use the series capacitor!!

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  • \$\begingroup\$ so, there in no way to remove that? \$\endgroup\$ – SpaceDog Mar 31 '18 at 15:31
  • \$\begingroup\$ You can disable the audio for a brief moment during the transition from one channel to the next. You can also use a different mux chip that is better suited for the application. \$\endgroup\$ – AlmostDone Mar 31 '18 at 15:41
  • \$\begingroup\$ You can disable the audio for a brief moment during the transition... how? \$\endgroup\$ – SpaceDog Mar 31 '18 at 18:41

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