using osciloscope to set amp gains

has anybody used an osciloscope to set amp gains?  i have a fluke scopemeter in my box and was trying to use it the other day but i couldn't get a sine wave.  i was checking both the + and - of the same pre-amp output ...... and then the + of a pre-amp output to chasis ground.  no matter what i did at any setting there was no wave. 

does anybody know what the test points are?

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brad

The test points are the output terminals. You most likely have your meter set to display DC, (it should be AC) or ranged wrong. That, or you have a bad set of leads.

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Even if it was on DC you would still get a wave, just an average of the original AC, there's definately a setting you need to play with such as it might be set to find ground point which would show you a straight line or your volts per division and seconds per division could be way off, and your trigger could be way off.

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2003 Chevy Avalanche,Eclipse CD7000,Morel Elate 5,Adire Extremis,Alpine PDX-4.150, 15" TC-3000, 2 Alpine PDX-1.1000, 470Amp HO Alt.

... you are playing back a test tone, right?

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yup ..... 0db certified db drags cd i use for my competitions..... i tried both AC and DC.  i know the signal is AC but i just wanted to check and see what i could get.  i know the leads are good kuz i can calibrate them on the o'scope.  if they were bad, it wouldn't let me do it ... so i know that isn't the prob.  i've tried every rate there is on my scope, 100ms - 20s ... and i've tried all the ranges ..... .2mv -15vrms AC.   and nothing shows up...... lol. 

so know i have a question for ya .... i thought the trigger was for when the signal generator on the scope was needed?  i bought and used the scope to test the 3 phase generators on the aircraft and all i had to do was hook the probes up and "bam" ... three phase power on my scope (if all the legs were good on the generator) and i didn't need to use or set up the trigger.  .... how does the trigger apply to reading a signal from an amp or head? (i'm not being a smart @zz... sounds like it, but i'm really asking out of curiousity )   oh oh oh ............... and how will a sound signal look on an o'scope?  just like an sine wave, or something a little different?  and i'm looking for a platue at the peaks correct (if i can ever get a freakin signal)?

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brad

Are your speakers connected while you do the test??

Yes you should see a simple sine wave with a test tone CD.

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Alexandre C.
2002 Nissan Maxima 6spd
Kenwood KDC-X891 HU
Kicker SX650.1 & SX400.2 amps w/SXRC Remote
Soundstream VGW-12 subs

ummmmmmmm .... not while i'm checking at the pre-outs.   kewl ..... now how the heck am i supposed to do that? .... lol

i'm trying to get the reading at the head unit to find out where my stereo can be loudest with a clean signal.  after i find that then i'll set my amp gains with my stereo at the loudest, cleanest reading.  sorry for the confusion.  but i can't get a signal and it's driving me mad. 

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brad

Connect the + lead of your oscope to the center pin of an RCA cable and the - lead to the outer shield of the cable.  If your scope has a separate ground connect this to the chassis.  Play your test tone and turn the volume up.  The voltage should be in the 2-8 volt range depending on your HU preamp output levels and yes, clipping is generally seen as a flattening of the sine peaks, sometimes accompanied by a noise spike at the leading edge of the wave.

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lol ...... I GOT IT !!  .... LOL ...... I GOT IT!!!   ... lol ...... you'll never believe what the problem was ... lol ...... lmfao ....

there is a zoom feature on the scope and it was zoomed in to a point where you couldn't see any part of the wave.  i zoomed out and "whoop!" there it was!!! sweet!!!

ok, so i tried the + and - probes like you said and it only works in meter mode.  i have to use the B&C probe with the ground terminal to the - side of the pre-out and REF turned off.  no trigger needed. 

the wave that i get is pretty dirty looking though.  even with the volume really low it almost looks like a sine wave within a sine wave ( if that makes sense to you)..... picture this .... there is the normal wave as if you were looking at AC voltage but the wave itself is peaking and falling (and i get this no matter what rate or range i measure with).  what the heck is that? ... lol .... almost like i'm looking at a $7000 etch-a-scetch.... anywhooo, when i crank up the volume the signal doesn't flatten out at the crests at all, the wave just gets really thick to the point of where it's almost unreadable.  but i do see the spike that you talked about right before the crest..... and when i say "spike" it looks like my jeep got struck by lightning .. lol .... holy cow!! that's too funny.... any idea about why i'm not seeing the crests flatten out (when it should be) and why the signal gets so "thick"?

lol ..... maybe i'll just get the RTA extension for my term-lab.... lol

thanx for the help again guys!!!  quick replies .... i love this place!!!  (other than the metra kits j/j)

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brad

thnx for the help DY, heamphyst, aleckxs, alpine guy.  if your ever in montana i'll get you guys a drink and a lift pass.

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brad

The crests not flattening out is because your preamp is simply not clipping.  It's possible the preamp has an anti-clipping circuit, especially if it goes up to a certain voltage and stays there even if the volume control has more gain to go.  The spike (and all the other noise you describe) is either because the head unit is producing a very dirty signal or because the source CD is poorly recorded.  The trace getting "thick" is actually lots of noise (distortion) and what you're seeing is signal instability (fluctuation around the fundamental frequency) caused by the electronics in the preamp becoming overloaded.

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strassehase3 wrote:

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thnx for the help DY, heamphyst, aleckxs, alpine guy.

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Glad we could get you fixed up! It's what we do!

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."