Memphis 16-ST1500D

[Dr Zeus]

THis is the 3rd Memphis amp I've repaired in the past ~3 hours. This one is the biggest, Class-D most likely. The smaller ones needed some gain/switch cleaning, but this one seems to be OK in that area. The three were brought to me for repair and were likely installed all together.

This amp powers and plays fairly well, but it's output is just slightly dirty. This is what shows up on the scope at idle with no input source or load.

Is this normal for this amp? If so then this amp likely has no problems and was just brought in with the other two because they needed repair. The other two are ST1000D and ST2004. I'm gonna check the ST1000D for the same thing as this amp in a moment.

.5v/Div @ 50uS

 

[Dr Zeus]

Well the ST1000D has something slightly similar...

.2v/Div @ 20uS

I wonder if this will be another ghost chase. Maybe I need to work on better amplifiers

 

[Perry Babin]

Class D amps pass a fair bit of residual from the carrier. It's all well above audibility so it's not generally a problem. When you connect a speaker load to it, the level of this 'noise' generally drops.

The bottom one is a bit different. It appears to have a lower frequency (~22k if your scope is in calibration) mixed with the noise. It's likely inaudible but unexpected.

For most class D amps, I set the scope to 10v/div and 2mS. If the amp produce clean audio and the trace is relatively thin, it's OK.

 

[ComputerJLT]

my st1500 has the same thing; a kicker kx2500.1 has it even worse. you'll never hear it on a subwoofer. even on a full range speaker its hardly audible above the signal