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Thread: Measuring Amplifier Frequency Response Part 2

  1. #11
    Tester Extraordinaire ErinH's Avatar
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    Quote Originally Posted by MarkZ View Post
    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).
    Well, the reason I ask is because of the same reason you'd want to load an amp down to set the clipping point: real world experiment.
    If a load does have an effect on measured data, then you'd want to be as close to real world as you can. If you don't load the amp down and take measurements, then (assuming we know that a load DOES effect the output) what have you really gained? You've not gotten results that really tell you what's going on once you load the amp down. So, would you want to use a dummy load? A real speaker? The same speaker you plan to use? Does it really matter at all?
    Your ears: The best tools you have... and they're free, too!

  2. #12
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    Quote Originally Posted by MarkZ View Post
    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.
    How so? I've had weird results when adding EQ or a crossover filter to the test signal when doing an all digital loop back test. Like when you play the test file from foobar and add some EQ work to see how the FR looks on the graph. I can't seem to get it to maintain a 0dB alignment across the full bandwidth. The curve offsets about .5dB from 0.

    Never had issues anywhere else though. It's also used on loaded headphone amp testing all the time.

    Here's one with an 8 ohm load also.

    Go to the specs tab and scroll to the buttom for the RMAA results link.
    The M Stereo Headphone Amplifier

  3. #13
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    Quote Originally Posted by VP Electricity View Post
    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 am looking to see of any of my remaining amplifiers have pre-equalization built into them, hence the reason I want to measure the frequency response of them.

    As for TrueRTA, I just wanted to use that as an example. I have ARTA and an older version of Smaart at my disposal too. I picked TrueRTA because I am more comfortable with it than I am with ARTA and Smaart. Every time I fire up ARTA I discover a new feature. Maybe I need to read the manuals again, lol.

    Quote Originally Posted by VP Electricity View Post
    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?
    Maybe one of the other programs at my disposal will be able to accomplish my goals. I would have no problems with purchasing RightMark either if that would be a better fit than what I have available.

  4. #14
    Tester Extraordinaire ErinH's Avatar
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    I personally don't see anything wrong with using truerta for this. I've been doing it for a couple years and I've not had any problems.
    I just tap into the channels I want and go.
    Your ears: The best tools you have... and they're free, too!

  5. #15
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    Quote Originally Posted by ChrisB View Post
    Maybe one of the other programs at my disposal will be able to accomplish my goals. I would have no problems with purchasing RightMark either if that would be a better fit than what I have available.
    It's completely free for the base version, which is all I've ever needed. I suggest finding an older version like 5.5. It performs each test much much faster then the newer version. Then once you have all the experimenting needed to get the best results, you run the final test run with the latest version to take advantage of the newer perks. Both are just as accurate by the way.

  6. #16
    Quote Originally Posted by ErinH View Post
    I personally don't see anything wrong with using truerta for this. I've been doing it for a couple years and I've not had any problems.
    I just tap into the channels I want and go.
    For the now-stated purpose, TrueRTA is fine. If you were attempting to measure the FR of an amplifier, I would use something else. But for testing for eq, this is obviously how I use it already, and for that, I would buy an XLR attenuator if I used anything. Markertek
    "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. #17
    Tester Extraordinaire ErinH's Avatar
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    I still don't see what the issue with testing FR out of anything with trueRTA is. Unless, of course, you use a low resolution version. But, with something like 1/24th, I think it's a great tool. Of course, it gets costly there, unless you know people. ;)
    Your ears: The best tools you have... and they're free, too!

  8. #18
    The way you say that, you don't sound like you're asking, but I'm a sucker and will answer anyway.

    You will find out NOTHING about amplifier sound with TrueRTA. Only the worst performing solid-state amps will fail to play pink noise into a resistor flat out from 20 to 20k. Most will look identical to this rudimentary and limited test. It will be as useful as testing aftermarket HU responses with it - it doesn't tell you much other than whether or not the HU has auto-loudness. I notice you didn't stay on that very long.

    If you really want to explore the performance differences between amplifiers (other than pre-emphasis-type curves in the preamp section, which I don't consider an amplification performance aspect), you would do better to use some SW with the ability to look at distortions and frequency response together and get some more relevant data with the same test process.
    "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."

  9. #19
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    Well, I received my well used Countryman Type 85 Direct Box yesterday, so I decided to play around with ARTA.

    This is what I get off the USB Mic Mate with the Countryman Type 85 connected:


    Here is my Orion HCCA 225 measured through the Countryman/Mic Mate with Pink Noise on a 2 ohm subwoofer load:


    Also, I always wanted to see what my "servo" unit did as far as equalization. This thing here:


    EDIT: It is supposed to function as an equalizer, feedback controller, and crossover.

    Here it is with my subwoofer built in the spec enclosure that it is supposed to control at 2 ohms:


    I was told that the servo was tuned for 4 ohms and I shouldn't use anything other than a 4 ohm load. So I decided to see how drastically different it was at 4 ohms with a dummy load:


    Not that drastic.

    Granted, I would need more equipment to test the feedback applied by the controller since technically it applies negative corrective feedback to the inputs of the amplifier to compensate for the less than ideal enclosure size.

    Regardless, testing stuff is FUN!
    Last edited by ChrisB; 10-26-2010 at 04:16 PM.

  10. #20
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    I also paralleled the test using True RTA.

    USB Mic Mate with nothing else going (i.e. just plugged into the computer):


    Orion HCCA 225 measured with straight up Pink Noise connected to a 2 ohm subwoofer:


    Orion HCCA 225/Servo measured with Pink Noise connected to a 2 ohm subwoofer:


    Orion HCCA 225/Servo measured with Pink Noise connected to a 4 ohm Dummy Load:

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