I have a cheap 50W stereo amplifier PCB that receives its audio signal via Bluetooth. It is currently powered via a wall wart. (I could also power it with batteries.) I have one of the the board's outputs connected to a speaker and it works as expected. However, when I touch either the oscilloscope probe ground or probe tip to one of the 2 speaker leads, the sound stops. I haven't been able to imagine why. If I power cycle the board, then it will work again.
I've tried doing this with the oscilloscope turned off, and also with the oscilloscope not plugged into the wall (no earth ground). Same behavior. It doesn't happen if I touch one of the leads with my figure or use the probe while it is disconnected from the oscilloscope. Anyone have an idea of what is happening and why it would cause the sound to stop? Protection circuit on the PCB? Protection from what? (I may next be asking how I am supposed to see the audio signal on the scope.)
Also, I am seeing about 2 mV of signal noise on my (new, and new to me) oscilloscope when the connected probe is not probing anything. This seems like an awful lot. Is this normal?
I've tried doing this with the oscilloscope turned off, and also with the oscilloscope not plugged into the wall (no earth ground). Same behavior. It doesn't happen if I touch one of the leads with my figure or use the probe while it is disconnected from the oscilloscope. Anyone have an idea of what is happening and why it would cause the sound to stop? Protection circuit on the PCB? Protection from what? (I may next be asking how I am supposed to see the audio signal on the scope.)
Also, I am seeing about 2 mV of signal noise on my (new, and new to me) oscilloscope when the connected probe is not probing anything. This seems like an awful lot. Is this normal?