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Basic DIY amplifier measurements?

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  • #16
    Originally posted by TN Allen View Post
    Just out of curiosity, given the variety of test equipment different people posting test results on the forum are using, how consistent are the results likely to be? I can see where different amplifiers could significantly affect Omnimic and Holm results, but how much variation is likely to result from different sound cards, and CD or DVD players, rather than initiating the test signal via computer memory? Is there a practical way to quantify or allow for the range of variations when looking at response graphs, a percentage or plus or minus a given number of decibels?
    In reality the variations from most sound cards and amplifiers are minuscule compared to the speaker being measured. There will be perhaps +- a fraction of a DB in response and a fraction of a percent of distortion, but I consider that way within tolerance. You can get bigger variation simply re-measuring in the same spot with the same settings consecutively. Loop back mode solves that entirely. Sent from my SM-G960U using Tapatalk
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    • #17
      Originally posted by jhollander View Post
      Ben, thanks I get it. The cost of the 2-channel balanced sound card might kill this. Can I assume that all microphone audio inputs are balanced? So, something like the Behringer U-Phoria would work?
      Yes, that has a combo TRS/XLR input, so it should be a differential input. You'll likely want to use the 1/4" TRS input as that should be a "line" input, not a microphone input.


      • #18
        Here's a drawing of the measurement set up. I wasn't sure if a series load bank was correct or if I needed a voltage divider (L-pad) network. I can always check if I'm clipping in ARTA and the sound card.

        John H

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        • #19
          TI's PCM4222EVM might be useful in this, is an AD converter with differential input for balanced impedance interconnection, exhibits high signal to noise ratio, low THD thru 20kHz, and provides AES3 output. You would need a computer digital audio interface that accepts AES/EBU input. A proper AES/EBU input is transformer coupled, providing galvanic isolation between the digital audio interface attached to the computer and the AD converter board.
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          • #20
            Originally posted by 4thtry View Post
            I use your fall back procedure of A/B testing with different amps. This is how I found out that the little Lepai chip amp that I was planning to use for testing was messing up my low frequency measurements. .... They all give the same results, but the little Lepai amp rolls off the deep bass.

            I've analyzed a bunch of chip amps. All exhibit low frequency roll off as mentioned by 4thtry. It's inherent in the low cost, simplistically designed amp modules - the chips' target market .

            Here's a $20 BT 2.0 module based on the very respectable TI TPA3116 chip. Red is BT chip's FR to the received test signal - into an op-amp stage. Blue is after the op-amp stage at the chip amp's input. And green is the chip's power output response to the speakers (ignore the green response above 5K Hz - test equipment failure).

            Click image for larger version

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            • #21
              Originally posted by Millstonemike View Post

              ...(ignore the green response above 5K Hz - test equipment failure).
              Are you sure it was test equipment failure? I had a Sure class-D board that had its output filter resonance at 17 kHz. When testing with no load, it showed a response just like that as the output filter was un-damped.


              • #22
                Originally posted by 1100xxben View Post

                Are you sure it was test equipment failure? I had a Sure class-D board that had its output filter resonance at 17 kHz. When testing with no load, it showed a response just like that as the output filter was un-damped.
                Maybe not.

                I measured the two line level signals with my scope. Since I don't have a 10X probe, I couldn't use the scope for the full power out measurement - the sine wave's peak voltage exceeded the scope's range. So the power out was done with a DVM; a $5 Harbor Freight unit. I had done some comparisons between the scope and the cheapo HF unit and it was good down low. So I assumed it was the DVM for the anomaly up high. It may be. Or perhaps the old saying: "*** U and Me" applies here - except it's all on me this time.

                So you make a good point. In hind site, I should have measured at a lower power out - something within the scope's range. Alas, retesting is not on my radar, but if I ever do, I let you know.

                But the fact remains that many of these low cost Class D amp modules have some degree of loss in the low end. It can be corrected on modules whose "on/off" switch is really the chip's "On / Stand by" input (the board remains powered). On those modules you can increase the value of the interstage DC isolation caps to get the inherent LP filter down in the single Hz range. But the modules that actually use a switch to control DC power to the board - increasing the same capacitors results in on / off speaker pop.


                • #23
                  There is a multi-part series of articles on using sound cards for audio measurements at AudioXpress.

                  A great test and measurement article series by Stuart Yaniger, which started to be originally published in audioXpress, June 2015.
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