O-Scope Question

[nathanotis]

Hello, all. Brand new here. I apologize in advance for the long first post. I'm trying not to leave out any details.

Recently installed a new head unit, RCAs to a 75w 4ch amp powering 4 5.25" speakers. And a compact 8" powered sub.

I also purchased an inexpensive oscilloscope to check output from the head unit and to set gains on the amp and sub. The O-scope shows a clear, clean square wave when connected to it's own ground terminal, but the attached image shows the sine wave I'm getting from my head unit:



No matter what I do, (different connection points, car running, not running, etc), I can't get a clean sine wave. It's always jagged and spikey. Is it ground, or noise or something?

I was playing a 1kHz test tone. The O-scope is not tracking the frequency correctly. Also, the amplitude of the sine wave didn't change while I was turning up (or down) the volume on the head unit.The V/Div in the photo is also way off (50mV, I'm not sure why). I just pulled a very similar looking sine wave moments ago while V/Div was set to 2V. I went to "Hold" mode for a slightly clearer image.

I know (from testing with a speaker attached) that the head unit (which has a volume range of 0-35) goes to 30 without any audible distortion. I bought the O-scope in order to see the waveform and confirm what my ears were telling me. The plan was to set gains at the 4ch amp and the powered sub using the O-scope as well.

This is my first foray into car audio, and I've never used an oscilloscope before. I admit that I might be connecting it wrong.

It's been a frustrating couple of days trying to figure this out. That's why I'm here. At this point, I know enough to know that I'm flailing and nothing is getting accomplished. I hope someone here can give me some guidance.

I guess my first question is: How should I be connecting the oscilloscope to the head unit?

Click to read more...

 

[dragon.breath 10+ year member]

First question, what head unit are you using? Second question, are you measuring the rca output or the speaker output?
I can tell you that I have the exact same oscope. When I tested my rca outputs they were clean sine waves. So we know that model of scope is capable of reading rca outputs. ( we don’t know however that yours is functional)

 

[nathanotis]

First question, what head unit are you using?

Sorry, I added that when I registered, I thought it would show up like a signature or something

HU: Kenwood KMM-BT328U

Second question, are you measuring the rca output or the speaker output?

I tried connecting the alligator clips to the RCA outs (first with one jaw poked inside and the other clamped outside, then trying to clamp to just the outside), but got no signal. So I stripped an RCA cable and plugged that into the HU, then connected the O-Scope to the stripped wire. That produced the jagged sine wave.

I can tell you that I have the exact same oscope. When I tested my rca outputs they were clean sine waves. So we know that model of scope is capable of reading rca outputs. ( we don’t know however that yours is functional)

The only test to ensure the integrity of the scope is connecting to the ground on the scope itself. That shows a very clear square wave. Beyond that, I'm pretty clueless.

Should I be connecting both clips from the scope to the HU? Or does one go to the head unit and the other to ground somewhere? (see? Clueless.)

 

[dragon.breath 10+ year member]

Make sure the switch on the scope is in ac, not dc.

 

[nathanotis]

Make sure the switch on the scope is in ac, not dc.

I was very careful with that switch. It has been in AC mode during every stage of my testing.

 

[nathanotis]

I can tell you that I have the exact same oscope. When I tested my rca outputs they were clean sine waves. So we know that model of scope is capable of reading rca outputs. ( we don’t know however that yours is functional)

1. How did you connect to the RCA's?

2. What can I do to confirm my scope is functional?

(60hz is where a lot of noise lives. I'm getting a better power adapter to start. I think the junk one I found in my basement storage is likely part of the problem)

 

[dragon.breath 10+ year member]

I connected to the end of the rca cable. One clamp on the center pin and the other one to the outside ring.
I am using the 9v battery adapter that came with my scope. I have never even used it ac power adapter that was included.
you could try the speaker output on your head unit just to see if you can get a clear sine wave.

 

[dangerdan86]

@nathanotis
I'm using a similar scope (just different color) and a 9v adapter also.
Purchase one of these guys and save the headache of alligator clips:
https://www.amazon.com/gp/product/B006VK8IEI/?tag=caraudiocom-20

Looks like you are using a 60hz'ish test track? Use a 1khz 0db test tone from the kicker website for the full range and the 50hz 0db for the s/w channel.
https://www.kicker.com/test-tones

You also need to disable all hpf/lpf filters and any boosts, the sound curve should be completely flat on the eq when measuring. It looks like your voltage is super low, so you need to turn up the volume on the head unit while you are playing the test track and taking measurements.

Yours should look something like this (this is with a kenwood dmx1037s at 38 of 40 on the volume):

 

[nathanotis]

You guys are awesome. Thanks for all the help so far.

@dragon.breath I almost went for the 9v pigtail. Time will tell if I choose poorly.

@dangerdan86 I just ordered that bnc/rca connector. Thanks for the link. That 60hz'ish result does not accurately reflect the tones I was using at that time (which I downloaded from the Kicker site). The new 9v adapter I'm getting tomorrow will confirm what I suspect. That is noise from my crappy, universal power adapter. The voltage reading is another story. I'm not sure what I was doing at that exact moment, but I have reproduced the same jagged sine wave set to 2V -5V V/Div and .2 ms Sec/Div

Again, thank you both. I'll post back some results when I have the new connector and power adapter. Assuming this cleans up my results, I'm sure I'll have follow-up questions pertaining to setting gains on amplifiers and tuning a powered subwoofers.

 

[dangerdan86]

You guys are awesome. Thanks for all the help so far.

@dragon.breath I almost went for the 9v pigtail. Time will tell if I choose poorly.

@dangerdan86 I just ordered that bnc/rca connector. Thanks for the link. That 60hz'ish result does not accurately reflect the tones I was using at that time (which I downloaded from the Kicker site). The new 9v adapter I'm getting tomorrow will confirm what I suspect. That is noise from my crappy, universal power adapter. The voltage reading is another story. I'm not sure what I was doing at that exact moment, but I have reproduced the same jagged sine wave set to 2V -5V V/Div and .2 ms Sec/Div

Again, thank you both. I'll post back some results when I have the new connector and power adapter. Assuming this cleans up my results, I'm sure I'll have follow-up questions pertaining to setting gains on amplifiers and tuning a powered subwoofers.

What volume on the head unit were you using when you took the measurement?

 

[nathanotis]

What volume on the head unit were you using when you took the measurement?

Could have been 10, could have been full blast (35). The wave didn't visibly change regardless of the volume at the head unit.

 

[Firewalker]

Most people use 40hz for the subwoofer these days as 50-60hz,and the fronts use something higher as he said. I used a 1K just on the midbass iirc and going to use a 2.5K for the tweeters when I do them.

 

[dangerdan86]

Could have been 10, could have been full blast (35). The wave didn't visibly change regardless of the volume at the head unit.

Somethings off. It should be mostly flat at 0 volume (a little static, but flat), and as you turn up the volume, the amplitude of the sign wave will increase. You keep increasing until clip, and then you know your max volume thats safe.

This is what happends if i go above 38 on the volume of my head unit.

 

[dangerdan86]

Most people use 40hz for the subwoofer these days as 50-60hz,and the fronts use something higher as he said. I used a 1K just on the midbass iirc and going to use a 2.5K for the tweeters when I do them.

I guess it would really depend on how you have your crossovers set. I run my subs (10's) up to 90hz midbass (6") at 80-350, Midrange (3.5") 350-3500, and tweeter (1") 3500+

I guess you could run different tones for each different type of speaker, i never really thought about doing that. I just use 50hz and 1khz.

 

[Firewalker]

I run active on my setups and soon going to a Helix DSP. I cross my midbass over at 120-160 right now. I use multiple different tones when setting up the midbass to experiment. I might be wrong on the 1K for the midbass I honestly can't be 100% sure right now as its been a few weeks but I am going to re-do it since I am swapping out CDT 6.5" 2 ohm to a set of 6.5" Stevens Audio once I am feeling better from surgery.