Odd looking 40hz wave on O-Scope

[djunified]

Hello all,

I have some spare LOC's laying around, and an Alpine CDE-100 which is now my test deck. I originally bought the LOC's to use with my stock Ford stereo system. They worked great, but at about half volume, the bass started to sound really crappy and distorted. It was nothing to do with the gains on both the amp and LOC, so i took the LOC's and chose a different route.

I hooked them up to my test deck today, and noticed something very interesting. With my o-scope attached, I played a 40Hz 0dB tone, and observed the wave. It looked like a normal sine wave until about volume 26. Then it turned into a wave that I've never seen before.

Here's the interesting part. This "odd" looking wave only showed up when playing the 40 Hz tone. When I played the 1 kHz tone, it looked like a normal sine wave, and started clipping normally at volume 30. The same applied to the 4 kHz tone.

Hopefully some of you audio guru's out there can identify this mysterious waveform. Pictures are below

View attachment 26530615View attachment 26530616

Click to read more...

 

[LosIsATool 5,000+ posts]

Looks like its picking up a ground loop

 

[AStovall 10+ year member]

What about below 40?

 

[keep_hope_alive Premium Member]

The second half of each cycle is missing...

 

[maylar 10+ year member]

That's some nasty looking distortion. Does it do that suddenly, or does it gradually change from sine wave to that? Is the HP filter on? It looks like something in the amp is going bonkers at that freq.

Yuck.

 

[ThatChevyGuy 5,000+ posts]

Dam son, never seen that.

 

[keep_hope_alive Premium Member]

Sounds like it is gradual, that at a certain volume this happens. obviously an internal problem. like a transistor is being pulled to ground. for it to happen at a specific signal level points to maybe a slight short, one where it takes a certain voltage to overcome resistance to ground. or a bad FET. hard to guess without knowing how that LOC is made. i assume this is an active LOC - that is is powered?

 

[djunified]

I never tried any frequencies below 40 hz. The next highest was 1 khz, and it was clean. The wave looked like a normal sine wave and then started turning into this odd looking thing. I guess this a lesson to never buy cheap parts when it comes to car audio

 

[kramer_212 10+ year member]

might be clipped.

 

[AStovall 10+ year member]

I never tried any frequencies below 40 hz. The next highest was 1 khz, and it was clean. The wave looked like a normal sine wave and then started turning into this odd looking thing. I guess this a lesson to never buy cheap parts when it comes to car audio

I wonder if it could just be a poor design. I don't have a lot of experience with this particularly. I work in the RF/EMI world and i have seen seen waveform like that when when you try and pump a signal threw something that it was never designed to handle it. 1KHz is pretty far away from 40Hz , it could just be that by 40Hz you've exceeded that LOC frequency of operation.