Oscilloscope jagged waveform when setting gains

[Ovovo]

I have a Pioneer Gm-D9705 amplifier and I set the gains with multimeter for the first time when installed in my Corolla. There was slight RPM whine which increased when accelerated.

Bought an Oscilloscope model DSO153 and tried to reset the gains, the waveform was jagged (front and rear channels) even when gains are at minimum using 1kHz test tone from kicker. While subwoofer channel output was clean all the way to max 600w (34v) with 50hz test tone from kicker. Below image of front channels using 3.5mm to RCA cable with phone.

Output from Head Unit RCA preout are clean 4v. Even tested with 75% volume.

I did try following things to isolate the problem:

1- Chnaged the ground.
2- Grounded the HU.
3- Used 3.5mm to RCA cable with phone.
4- Tried at bench with a 20A power supply and phone as source, still same weird lines.


Considering the subwoofer channel is by default at LPF, I tried to change the front and rear channels to LPF and HPF and still same waves. Even when there's no input, there's a little movement (a few mV on oscilloscope) when amplifier is on.

When I increased the front and rear channels gain towards 80w+ (20v+) the waves becomes squared at top and bottom which clearly is clipping.

Need some input from experienced people, should this be ignored? The sound is clean to my inexperienced ears. Is that class D switching residual that is showing on oscilloscope? Is my amp faulty?

 

[amirm]

Is that class D switching residual that is showing on oscilloscope?

Likely that or other noise sources due to high bandwidth of oscilloscopes.

 

[solderdude]

I have a Pioneer Gm-D9705 amplifier and I set the gains with multimeter for the first time when installed in my Corolla. There was slight RPM whine which increased when accelerated.

This points to a grounding issue. A loop through the chassis and signal ground wires. Possibly because of 2 devices being connected to ground at differeny points on the chassis.

Bought an Oscilloscope model DSO153 and tried to reset the gains, the waveform was jagged (front and rear channels) even when gains are at minimum using 1kHz test tone from kicker. While subwoofer channel output was clean all the way to max 600w (34v) with 50hz test tone from kicker. Below image of front channels using 3.5mm to RCA cable with phone.

Output from Head Unit RCA preout are clean 4v. Even tested with 75% volume.


View attachment 464059

I did try following things to isolate the problem:

1- Chnaged the ground.
2- Grounded the HU.
3- Used 3.5mm to RCA cable with phone.
4- Tried at bench with a 20A power supply and phone as source, still same weird lines.


Considering the subwoofer channel is by default at LPF, I tried to change the front and rear channels to LPF and HPF and still same waves. Even when there's no input, there's a little movement (a few mV on oscilloscope) when amplifier is on.

When I increased the front and rear channels gain towards 80w+ (20v+) the waves becomes squared at top and bottom which clearly is clipping.

Need some input from experienced people, should this be ignored? The sound is clean to my inexperienced ears. Is that class D switching residual that is showing on oscilloscope? Is my amp faulty?

The 'noise' signal on the output of the amp is probably the RF of the class-D operation of the output stage and the switching RF not being sampled fast enough when measuring 1kHz.
Try to see with a faster timebase if the scope can see a continuous sine somewhere between 100kHz and 1MHz (most likely).

If that is the case don't worry about the amp... it is supposed to do that and well above the audible range.

 

[antcollinet]

Is that class D switching residual that is showing on oscilloscope?

It looks like it is only around 20kHz. Much too low for class D switching.


EDIT - unless, as pointed out by @solderdude , your scope sample rate is too low to capture it without aliasing.

 

[Ovovo]

This points to a grounding issue. A loop through the chassis and signal ground wires. Possibly because of 2 devices being connected to ground at differeny points on the chassis.

Should I ground the HU by removing the negative wire from harness to the same point where the amplifier is grounded? This will be 4.5 meter long since the amplifier is mounted in the trunk.

 

[raindance]

Doesn't your scope have a low pass filter?

 

[Ovovo]

Doesn't your scope have a low pass filter?

Don't know if it has, it's a cheap $25 DSO153 scope.

 

[Ovovo]

Update:

RPM whine noise went away after I changed the ground point from sheet metal to frame body, where seatbolts are. At least it hasn't came back in a week.

Amplifier sounds very clean, however those jagged lines still present, wither switching noise is 100kHz or oscilloscope unnecessarily picking it up.

 

[Andysu]

denon 8500h has high frequency issues of jagged edge