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Thread: Measuring Amplifier Frequency Response Part 2

  1. #1
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    Measuring Amplifier Frequency Response Part 2

    Rather than threadjacking DS-21, I figured I would start my own thread to ask the ever important question: What is the best way to measure an amplifier's frequency response with TrueRTA?

    As of now, I own a USB Mic Mate and have a Countryman Type 85 Direct box on its way to me. I've been playing around with measuring Pink Noise by importing pink noise into ARTA, and it seems that there is never a true point where the Pink noise is perfectly flat. As a result, I am wondering how I can go about measuring the amplifier to see if it is perfectly flat or not at the outputs.

    Do I need to get something like the M-Audio MobilePre and have TrueRTA send the impulse to measure for flatness OR do I need to invoke some other mode in TrueRTA to test for flatness via a CD with Pink Noise? I know there are some instances where I will not be able to have the impulse and the measurement on the same clock, such as measuring factory HUs.

    Pardon me if I am asking in the wrong place as I am just trying to gain insight into how I can perform testing of the amplifier outputs as well as posting results after my Countryman gets here.

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    Devil's Advocate Adam_MSS's Avatar
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    Could you send a sweep to the amp inputs then measure the outputs using a voltage divider to feed a signal back into the micmate? I'm not familiar with TrueRTA but I'd think this would work on the other software I've used.

    VP described basically the same thing in this thread: http://www.mobilesoundscience.com/f2...ory-system-48/
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    Tester Extraordinaire ErinH's Avatar
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    What are you looking to measure? The preout of the headunit? Simple: rca/xlr cable into the mobile pre. You'll likely not need a voltage divider for this.
    Send pink noise into the headunit via the aux input (should share the same output characteristic as the other sources such as cd/ipod).

    Voltage divider on the output of the amps would be my choice. IIRC, VP Electricitiy used a 50dB attenuator on his xlr cable. I personally just go with a voltage divider.


    I wonder if putting a load would affect the FR??? Chad?
    Your ears: The best tools you have... and they're free, too!

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    I am planning on using the Countryman Type 85 in lieu of the voltage divider since it can handle a lot more voltage than most things on the market. The Countryman is traditionally used to go inline between instrument amplifiers and speakers while feeding a signal to the mixing console while running off of phantom power. I should be able to figure it out from here. I think...

    Thanks Adam!

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    Devil's Advocate Adam_MSS's Avatar
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    Just make sure you take into account the FR and Distortion of the Countryman when evaluating your results. There is a thread on PSW that shows FR and distortion for a number of DI devices. IIRC the Radial units performed the best.
    You don't use science to show that you're right, you use science to become right. - R.Munroe

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  6. #6
    I know this is a "how" thread...

    I use a 30dB XLR inline because that's the highest-value attenuator I have found, but remember, I'm not looking for frequency anomalies when I do it. I've already seen any freq anomalies on the INPUT side (from the OEM audio system) and now I'm looking to get the output of the aftermarket amp to pretty close to flat. The reason I'm not trying for closer than "pretty close" is that by the time I get there, I'm tuning for the acoustic response anwyay, so aiming for electrically flat is a waste of time after a certain point.

    Which I guess brings me to my question - why are you looking to measure FR of an amp, and why have you settled on TrueRTA?

    I would not use TrueRTA to do this (I own it). I haven't learned the RightMark SW yet, but I think it's more suited for this task...anybody?
    "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."

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    Quote Originally Posted by VP Electricity View Post
    I know this is a "how" thread...

    I use a 30dB XLR inline because that's the highest-value attenuator I have found, but remember, I'm not looking for frequency anomalies when I do it. I've already seen any freq anomalies on the INPUT side (from the OEM audio system) and now I'm looking to get the output of the aftermarket amp to pretty close to flat. The reason I'm not trying for closer than "pretty close" is that by the time I get there, I'm tuning for the acoustic response anwyay, so aiming for electrically flat is a waste of time after a certain point.

    Which I guess brings me to my question - why are you looking to measure FR of an amp, and why have you settled on TrueRTA?

    I would not use TrueRTA to do this (I own it). I haven't learned the RightMark SW yet, but I think it's more suited for this task...anybody?
    Yup, I think so too.
    Last edited by t3sn4f2; 10-23-2010 at 02:52 PM.

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    Quote Originally Posted by ErinH View Post
    I wonder if putting a load would affect the FR??? Chad?
    Yup. Well, it can. Especially for amps that use output inductors, zobels, etc.

    But the kind of impedances a voltage divider would be looking at? Nah. You'd want your voltage divider to be in the kilohm range, or thereabouts. Don't really even need a voltage divider, actually. You could just use a series pot. The input of your recording device probably has a reasonable resistance (on the order of k ohms).

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    I don't like RMAA. I have absolutely no hard evidence for saying that. I just seem to recall getting some funny numbers using that tool. Something didn't jive. Maybe the user's an idiot.

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