# Can I use a DMM to check RCA signal?



## bird333 (May 28, 2008)

My music went silent for awhile and I am trying to determine if it is the HU or my amps. I guess the amps could have gone into protect mode but to me it just seems unlikely that both would have at the same time. Can I use a DMM to check to see if the HU is sending the signal to the amps? Can you give me an idea of what measurements I should be seeing?

TIA


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## falkenbd (Aug 16, 2008)

ac voltage between the center pin and the outer ring.


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## bird333 (May 28, 2008)

falkenbd said:


> ac voltage between the center pin and the outer ring.


Are you sure it's AC and not DC? AC seems to wildly fluctuate.


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## gijoe (Mar 25, 2008)

bird333 said:


> Are you sure it's AC and not DC? AC seems to wildly fluctuate.


That means it's working. Music will fluctuate. If you get no movement, then you don't have any signal.


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## JJDH (Mar 8, 2009)

dc makes sense to me, if not , explain, thanks


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## sqshoestring (Jun 19, 2007)

Signal will be really small until you nearly max the HU. What might work best is a 60Hz tone any DMM will see that easily. Don't forget if you have HU xovers 60hz will not be in the highs depending. Cheap DMM do not always work well with higher frequency but usually work some.

AC is music, DC would be really bad your speaker would go out and stay there (or in and stay). Can also use a tiny radio or headphone speaker just don't blow it up.

If you see DC anywhere in the speaker wires or RCA, you have a problem. A beat old amp might have a little, but we are talking fraction of a volt.

You also should test for ground on the shield while you are there the HU should be grounding it. I had an old amp one day that would not work until I plugged RCA in and grounded the shields, but most amps are not like that still it could maybe cause a protection issue.


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## falkenbd (Aug 16, 2008)

JJDH said:


> dc makes sense to me, if not , explain, thanks


DC is the power to the head unit and amps.

All analog signal (music) is alternating current. The RCA's are just very low level music. The amplifier amplifies that signal (stays AC just higher voltage)


Why does DC make sense to you??? Do you have some sort of magical signal from your head unit to your amp?


You got AC voltage so it's not the problem.


HERE is what you check next:

(play music softly)
check your DC voltage at the battery with the car OFF and ON.
Compare that to the DC voltage at the AMP with the car OFF and ON.

If there is a voltage drop you have a power/ground wiring issue.

No voltage drop? Keep going:

Turn up the music and see if voltage drops at the amp.

Also if your amp cuts out again, immediately go see how hot it is.


Check out Basic Car Audio Electronics - you can find troubleshooting guides there.


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## JJDH (Mar 8, 2009)

i wasnt the op.. i was reading and didnt understand the answer to the op question. now i kno. thanks for the knowledge. also no magical hu signal


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## JJDH (Mar 8, 2009)

also , u are saying the amp gains put out ac current to the speaks? just making sure


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## sqshoestring (Jun 19, 2007)

JJDH, Yes all AC. The speaker is just a magnetic linear motor. It moves one way with current and when you reverse polarity it moves the other way. The faster you reverse the current the faster the cone goes in/out. The more voltage you apply the more the cone moves each way. The amp just takes the 12v and the power supply steps it up to ~20 or more volts depending on how big the amp is, then the outputs apply that voltage + or - to the speaker using the RCA signal as the input to do so...it copies that signal or, amplifies it. In simplified terms, it runs that RCA signal into the output transistors that are connected to the higher power supply voltage. As the RCA signal goes positive and negative, the outputs apply the same signal using the +/-20v. Otherwise your HU can only apply 12v, that would be +/-6v or about 18wrms it works out to. With no power supply to raise the voltage, that is all you can get out of 12v supply.

Remember Hertz is cycles per second, so a sub at 10Hz will go in and out 10 times a second. That is how fast the amplifier changes positive to negative voltage to the speaker, the volume is how much of the rail voltage it applies. 60Hz AC power in your house would sound just like a 60Hz tone played on your stereo if you could listen to it, and it is 110v which makes for a big amp. The RCA input will be the same signal as your speakers get, except if you have a 5v HU and turn it all the way up you might see 5v there and with a HU speaker out maybe 11v, and with a smaller amp maybe 40v (that is positive and negative combined). A large sub amp might have +/-50v for a 100v swing at max output, though things are not perfect so you never get 100% of rail voltage to the speaker in practice before it clips. Note that actual RMS voltage would be less than that, you would use RMS to figure out wattage produced to rate an amp in RMS output.


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## falkenbd (Aug 16, 2008)

Well the info I provided is for the OP. Since he is getting AC voltage on his RCA's he needs to start troubleshooting further.

It's always a good idea to make sure you are getting proper voltage on the electrical system in your car. Especially if you stress it with a big stereo.


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## JJDH (Mar 8, 2009)

sqshoestring said:


> JJDH, Yes all AC. The speaker is just a magnetic linear motor. It moves one way with current and when you reverse polarity it moves the other way. The faster you reverse the current the faster the cone goes in/out. The more voltage you apply the more the cone moves each way. The amp just takes the 12v and the power supply steps it up to ~20 or more volts depending on how big the amp is, then the outputs apply that voltage + or - to the speaker using the RCA signal as the input to do so...it copies that signal or, amplifies it. In simplified terms, it runs that RCA signal into the output transistors that are connected to the higher power supply voltage. As the RCA signal goes positive and negative, the outputs apply the same signal using the +/-20v. Otherwise your HU can only apply 12v, that would be +/-6v or about 18wrms it works out to. With no power supply to raise the voltage, that is all you can get out of 12v supply.
> 
> Remember Hertz is cycles per second, so a sub at 10Hz will go in and out 10 times a second. That is how fast the amplifier changes positive to negative voltage to the speaker, the volume is how much of the rail voltage it applies. 60Hz AC power in your house would sound just like a 60Hz tone played on your stereo if you could listen to it, and it is 110v which makes for a big amp. The RCA input will be the same signal as your speakers get, except if you have a 5v HU and turn it all the way up you might see 5v there and with a HU speaker out maybe 11v, and with a smaller amp maybe 40v (that is positive and negative combined). A large sub amp might have +/-50v for a 100v swing at max output, though things are not perfect so you never get 100% of rail voltage to the speaker in practice before it clips. Note that actual RMS voltage would be less than that, you would use RMS to figure out wattage produced to rate an amp in RMS output.


 i needed to kno this stuff. i have been reading on bcae quite often. much appreciated:snacks:


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## bird333 (May 28, 2008)

The power wire wasn't making a good connection at the distribution block. Thanks for all the good info.


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