2015 SGS stock head unit with RF flash. SD800.4 bridged, yellow baskets in the fairing. When setting amp gains , using a dmm. I set the output to 28v using 1k and 1.5k test tone playing through the USB port.
Now I use my phone connected BT and check the amp output volt with same test tones and same hu volume I get 9.4 v output.
I'm I actually still sending 28v when using BT or 9.4v that the dmm is measuring?
Thanks all

 

I'm a bit confused. You're saying you are using the same material (test tones) playing through 2 separate sources (USB and BT) and measuring using a DMM on the amp outputs?

You're not measuring what you're "sending" (inputting) but what the amp output is. An amp can only multiply (gain) the input signal to output. This is telling you your BT input is at a lower voltage than your USB. Have you got your phone volume turned up? It's always best to set gains for the "worst case" (highest input) anyway. If that's what your phone BT puts out so be it. Turn up the volume when you are playing BT as your source.

The challenge you have is measuring or manipulating the output of each processor in your HU (BT vs. USB), you can't with the stock HU.

 

Set gains using the source you’re going to use when listening to it and stick with that. If you always play via Bluetooth, that’s what you use.

Different sources will have a different output to the head unit but sub 10v to 28v seems a bit extreme. Did you have the phone volume all the way up when you checked via BT? When using a USB it doesn’t matter because it plays at a set level but with BT you can turn the volume up/down on your phone as we as the head unit and it will obviously make a significant difference.