The Modulus-686 is a bridge-tied-load (BTL) amplifier and therefore can’t be measured in the normal manner using a scope probe and grounding lead to the +/- of the output, as that would cause a short circuit.
Leaving the scope probe grounding lead floating (i.e. not connected to anything) also causes issues with the measurements sometimes looking okay and other times causing malformed waveforms like below right.
This can lead to thinking that there is a problem when there isn’t.
The scopes probe grounding lead should be connected to the power GND of the amp, but for BTL we also need to use both scope leads for the +/- output and both scope grounding leads connected to the power GND.
One further step is required to show the actual signal that the speaker will exhibit. Use the scopes Math function to perform a CH1 – CH2 operation on the signals.
