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Thread: How to analyze a factory system

  1. #1

    How to analyze a factory system

    How do we determine the answers to the Big Three Questions?

    The first step, whenever possible, is to get the wiring diagram for the audio system.

    First place to look is in between the head unit and the amplifier (if there is an amplifier - if the speakers are all connected directly to the head unit, we know that we have a speaker-level signal).

    If there are (+) and (-) leads for each channel, we have a balanced signal or a lower-power-level speaker level signal. (Often, the diagram will show a shield around the balanced leads with a ground drain at one end).

    If there are (+) leads for each channel, and a single (-) lead, we have a common-ground output signal.

    That answers the signal type, and it also helps us with wire colors for the next part of our test - measuring the range and response.

    To do this, we need a real-time analyzer with a usable balanced line input. If you have a Windows laptop, the cheapest way to have this test setup is:

    - True RTA, $39.99 for the 1/3-octave version
    - An Icicle or MicMate XLR/USB adapter ($50-ish)
    - A 30dB XLR inline attenuator if you plan on measuring higher-voltage amplifier output signals ($20?)
    - A 6-foot XLR cable ($15?)
    - A pair of Radio Shack meter probes (I recommend these because they are $5 and come with both alligator clips and sharp probes).

    Hack the right end off of the XLR cable, strip it back.

    Hack the meter insert ends off of the probes. Splice the meter probes onto the (+) and (-) balanced lines of the XLR, and leave the ground capped off.

    Now you can measure the frequency response of an electrical signal!

    For a test signal, I recommend making a CD using the 20-minute pink noise track found here:

    http://www.rockfordfosgate.com/scrip...=2:faqs.faq_id

    And you're ready to go.

    Edited to add:

    Make sure to set any head unit tone controls to flat, any EQ settings to flat, and any processing (Logic 7, etc.) off.

    As is pointed out below, take measurements at various volumes, and save them for future interpretation (unless they are all flat as a pancake, in which case they are pretty easy to interpret). For more on how to interpret the results, read this thread.
    Last edited by VP Electricity; 08-23-2010 at 08:31 PM.

  2. #2
    Founding Member RickVS's Avatar
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    Thanks VP! I believe its important to note that measurements should be taken at several points along the volume scale, as many modern systems employ volume-dependent equalization.

    Would an M-Audio Mobilepre USB work in place of the inline attenuator?
    STI<>GTO

  3. #3
    I should have mentioned that here - I did it in the other post and forgot in this one. I have edited the original post.

    I'm not sure that you can - I need to find my MobilePre and test for that. The high voltages coming out of some factory amps scare me in terms of connecting them directly to my netbook.

  4. #4
    Big Daddy Chad's Avatar
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    do not hammer the mobile pre input with speaker level output.

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    Founding Member birdie2000's Avatar
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    Just bought an Icicle and made up one of these cables and wanted to clarify something before I start testing. The 30db attenuator is only if I'm measuring high-power outputs from a factory amp, right? So if I'm just measuring the speaker-level outputs from the radio itself (amplified, but only by the head unit itself) I should be fine without it, correct?

  6. #6
    I actually use it fairly often just so I don't have to change input sensitivity settings on my RTA. I leave it in and take it out when the signal is too low on the screen.
    "Of course, the laws of science contain no matter and have no energy either and therefore do not exist except in people's minds. It's best to be completely scientific about the whole thing and refuse to believe in either ghosts or the laws of science. That way you're safe. That doesn't leave you very much to believe in, but that's scientific too."

  7. #7
    Member Salami's Avatar
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    Quote Originally Posted by VP Electricity View Post
    Hack the right end off of the XLR cable, strip it back.

    Hack the meter insert ends off of the probes. Splice the meter probes onto the (+) and (-) balanced lines of the XLR, and leave the ground capped off.

    Now you can measure the frequency response of an electrical signal!
    Can anyone post a picture of what this should look like when done? I am having trouble visualizing the connections as I am not familiar with XLR cables.


    Also where exactly are the measurements being made? Am I simply connecting the alligator clips to the speaker out puts on my stock head unit?

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    Controller AL9000's Avatar
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    Quote Originally Posted by Salami View Post
    Also where exactly are the measurements being made? Am I simply connecting the alligator clips to the speaker out puts on my stock head unit?
    Hook it up at either the radio's speaker outputs, or the balanced line between it and the factory amp.

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  9. #9
    Big Daddy Chad's Avatar
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    you could use a good DI box that takes speaker level inputs also.

  10. #10
    Member Salami's Avatar
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    Instead of buying the MXL MicMate Classic and the 30db 30dB XLR inline attenuator can I use something like this instead? http://www.shure.com/americas/produc...dapter#details

    Will it allow me to attenuate the signal coming in when I check the speaker level outputs from a head unit or amp? Cost is slightly higher (~$20) then buying the other 2 items but it appears to be of much better quality.

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