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I am using an arduino to monitor the voltage of the input lines to a stereo amplifier connected to speakers. When the arduino detects voltage changes on Chan1 or Chan2 it activates an IR LED to change the state of the stereo amplifier and switch channels, power on, off, etc.

Chan1 and Chan2 are different sources (CD player, ChromeCast) coming to the stereo amplifier on RCA cables. The circuit below has a voltage divider to shift the voltage +2.5V with a small cap (C3) to isolate any DC. The circuit behind the voltage divider is a class-a amplifier based on this schematic

I've put Y splitter in the left channel of both sources. One branch of the Y splitter connects into the stereo and the other connects to J1 (CD player) and J4 (ChromeCast). Pins 1 and 3 of J1 and J4 both connect to a common ground. The arduino monitors the voltage levels on CHAN1 and CHAN2 on the base of Q1 and Q2.

This all works fine on my test bench with two different audio sources (phone and computer) and no stereo amplifier, but fails miserably when attached to the stereo amplifier. CHAN1 and CHAN2 cross-talk through the ground. Both CHAN1 and CHAN2 are detected as being on when only one source is playing. Even worse, when two sources are playing at the same time the audio of the other channel bleeds through the ground connection.

I've looked at this post about isolating ground between different elements of a guitar amp, but I'm not sure that this is the same problem I'm having.

How can I effectively isolate CHAN1 and CHAN2 from each other on the ground connection?

Amplifier Circuit

circuit w/ stereo and arduino

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  • \$\begingroup\$ Please show the CD player and ChromeCast device connections. I find your description of the Y splitter connections confusing - it sounds like you may be shorting the two inputs in the Y splitter. Also, you only need one voltage divider and coupling capacitor in each channel. You should also have power supply bypass capacitors - perhaps 10 uF and 0.1 uF between supply and ground. \$\endgroup\$ – Peter Bennett Apr 11 at 19:48
  • \$\begingroup\$ @PeterBennett I've added to the diagram to show how the splitter is set up. Both the left and right channels of the CD and ChromeCast connect to the stereo as would be expected. The left channel of both also connect to GND and CHAN1 and CHAN 2 in the circuit. The problem probably exists with the fact that all the GND are common. Also, what is the function fo the bypass capacitors? \$\endgroup\$ – Aaron Ciuffo Apr 12 at 20:16
  • \$\begingroup\$ Bypass capacitors help to keep the supply voltage stable. Without them, the audio channels may be coupled via the power supply lines. Your added Y splitter drawing confuses me even more - perhaps something more like a block diagram would clarify what is intended to connect where. Do all the "CHAN1" symbols connect together? You may know what you are connecting to what, but I, and others who may be able to help, only know what you show us. \$\endgroup\$ – Peter Bennett Apr 12 at 21:06
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Try adding audio isolation transformers to both inputs. Should solve any ground loop problems.

This is the part i'm familiar with:

radioshack audio isolation transformer

Ebay link which will probably die soon

Digikey Search

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  • \$\begingroup\$ Can you explain what this does? I've read about these, but I don't quite know what's going on with them and how they would help. I'd like to build a pcb around this, so if I can get this as a component, that would be most helpful. \$\endgroup\$ – Aaron Ciuffo Apr 11 at 20:37
  • \$\begingroup\$ The transformer will add an isolation barrier, so that there is no DC connection between the inputs and your device. Check the digikey link for PCB mount versions, but I'd at least try it first. \$\endgroup\$ – Drew Apr 12 at 0:39
  • \$\begingroup\$ From reading at the link below, it sounds like transformers are good for isolating DC signals from a source, but in this case, isn't my problem an AC signal interfering with the source? Won't the transformer allow AC to flow in both directions? electronics-tutorials.ws/transformer/audio-transformer.htm \$\endgroup\$ – Aaron Ciuffo Apr 12 at 20:18
  • \$\begingroup\$ Any difference in the relative grounds will cause currents and noise in your audio. A transformer will eliminate that issue, even if there is an AC component to the ground difference (which there probably will be). \$\endgroup\$ – Drew Apr 12 at 22:01

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