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  • Basic DIY amplifier measurements?

    I’m getting ready to build a few small chip amps, one of which I’d like to use for measuring speaker frequency response with my OmniMic.

    It occurred to me how do I know if the little amps are not distorting the signal? Meaning are there some basic amplifier tests I can do? May be something using OmniMic? I do not have an oscilloscope.

    My fall back plan is to do some A-B tests using different amps, same speaker, same close mic position and fixed voltage measurement at the speaker then compare FR graphs.
    John H

    Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

  • #2
    With a resistor network to pad down the output you should be able to do some measurements of the amplifier using Omnimic. The idea would be to pad down the speaker level output to a similar level expected for the mic input, no knowing exactly what that level is I can't give you values though.

    There is a significant problem with most typical chip amps though, they're not single ended meaning the negative output isn't fixed to ground and is typically BTL, bridged, differential, whatever you want to call it. That means you'll only be able to measure one of the two differential signals, which will be more distorted since in phase noise cancellation occurs between the + and - outputs.

    So in short...
    1. Make sure it's a single ended output amp, no BTL or bridging
    2. Determine your mic input voltage and build a voltage divider (L pad) to knock down your speaker output to match, be sure to include dummy load resistance
    3. Get crafty with the jig to hook it up
    4. Run Omnimic / ARTA / REW / Holm... whatever sweeps to get freq / phase / distortion results
    Electronics engineer, woofer enthusiast, and musician.
    Wogg Music

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    • #3
      I assume you mean: Source -> Load -> Input to processing/ program. The OmniMic test tracks are disconnected from the OmniMic program so there would not be closed loop. Then there's the issue of the USB microphone input. I'd need to figure out the pin out for the USB? The usb input value is going to be a head scratcher.
      John H

      Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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      • #4
        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. I now use either a KLH 100wpc integrated amp, a highly modified Dynaco ST400 (200wpc) or one of my re-built SWTP 207A power amps (60wpc). They all give the same results, but the little Lepai amp rolls off the deep bass.

        Another idea is to make plans with other builders to bring your OmniMic and some test speakers along to an event and then run FR tests during breaks in your hotel room, comparing results. This is how I found out that my laptop CD drive was messing up my OmniMic measurements. Craig brought his OmniMic to Iowa a few years ago and we tested several speakers with it, including a set of my speakers that I also tested at home with my equipment. As a result, I found out that I need to use a dedicated Sony CD player, using the original OmniMic test disc, to get comparable FR measurements.

        Bill


        Plumber's Delight: http://techtalk.parts-express.com/fo...notech-winners
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        • #5
          Originally posted by jhollander View Post
          I assume you mean: Source -> Load -> Input to processing/ program. The OmniMic test tracks are disconnected from the OmniMic program so there would not be closed loop. Then there's the issue of the USB microphone input. I'd need to figure out the pin out for the USB? The usb input value is going to be a head scratcher.
          Ahh... nope. USB mic will not work at all, there's an audio interface built into that that is needed. Look into Right Mark Audio Analyzer for an example, you'll basically need the amp in a loop back path from sound card output to input for the software to compare expected vs. actual. ARTA could do it with some crafty jig work and a good sound card / device, but it sounds like Omnimic would be more challenging.
          Electronics engineer, woofer enthusiast, and musician.
          Wogg Music

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          • #6
            Now that I'm home and can look at ARTA, I really wouldn't recommend using something like that for an amplifier test without an oscilloscope. You really need a scope to measure the actual output voltage and watch for exactly when clipping kicks in to do a proper amp test. ARTA and REW don't seem to have a scope display to watch for clipping.

            A scope + ARTA and loopback rig is on my future amplifier testing write up project list. Should be able to quantify true RMS power, frequency response and distortion measurements with the lower distortion limitation being your AD/DA converters in the sound card.
            Electronics engineer, woofer enthusiast, and musician.
            Wogg Music

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            • #7
              There's SOME kind of scope built into the latest DATS (although I've never used it - don't know how really). Could that work?
              Does ANYone know ANYthing about that?

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              • #8
                Originally posted by wogg View Post
                A scope + ARTA and loopback rig is on my future amplifier testing write up project list. Should be able to quantify true RMS power, frequency response and distortion measurements with the lower distortion limitation being your AD/DA converters in the sound card.
                I'd be interested to see that write up. I'd be happy contribute to your buy of a low cost external sound card if you go that way. I assume the o-scope would be to watch how and where the waveform starts to distort.
                John H

                Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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                • #9
                  Originally posted by jhollander View Post

                  I'd be interested to see that write up. I'd be happy contribute to your buy of a low cost external sound card if you go that way. I assume the o-scope would be to watch how and where the waveform starts to distort.
                  Hey JH - you can get a DSO138 pretty cheap. Might serve you well in this endeavor:
                  1X Soldered DSO138 2.4" TFT Digital Oscilloscope. Adjustable trigger level position, and with instructions. After your item dispachted. After you win the item OR place a new order. Order information will be updated when. | eBay!

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                  • #10
                    Thanks Don. Maybe if I knew how to use it I could tell if it was decent...
                    John H

                    Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

                    Comment


                    • #11
                      Originally posted by jhollander View Post
                      I’m getting ready to build a few small chip amps, one of which I’d like to use for measuring speaker frequency response with my OmniMic.

                      It occurred to me how do I know if the little amps are not distorting the signal? Meaning are there some basic amplifier tests I can do? May be something using OmniMic? I do not have an oscilloscope.

                      My fall back plan is to do some A-B tests using different amps, same speaker, same close mic position and fixed voltage measurement at the speaker then compare FR graphs.
                      Jon, I don't have time right this second to put out a full write-up, but I'll give you some key information and I'm sure others will chime in. I might have some time this weekend to do more of a full write-up with detailed wiring diagrams and instructions.

                      The basic setup is just sound card output into your test amplifier and output of the amplifier into sound card input (possibly using an attenuator). You'll also want to terminate the output of the amplifier with a resistive dummy load (typically 4 or 8 ohms).

                      Ideally, you're going to want a soundcard with balanced, differential inputs for a couple reasons. The first reason is that most pro-audio USB audio interfaces will be able to naturally accept higher input levels, and you may not even need any kind of attenuator. I use a Focusrite Scarlett 6i6 and it can take something like 20 V-peak before the input of the soundcard clips. This would allow you to test your amplifiers up to an output level of about 24 watts (into 8 ohms). If you want to go higher than that, you can make a really simple resistor divider. I've done this before so I can measure the output of amplifiers with 200 V-peak output levels. The Scarlett 2i2 would also probably be a good option, and is quite a bit cheaper.

                      The second reason for using a differential input, and this is a BIG one, is so you can measure the output of bridged amplifiers. When you hook up the amplifier output to the soundcard input, you will have 2 connection points. If you have a soundcard with sindle-ended inputs, then you'll have to connect one side of your amplifier output to the ground of your soundcard. If your amplifier is single-ended, then is not a big deal. However, if the output of your amplifier is bridged, then you're going to be shorting out one side of your amplifier, through the input cable of your soundcard, which is very likely to cause problems, of which the worst case is destroying your amplifier output and/or the input to your soundcard.

                      For software, ARTA is a great choice using STEPS. You don't actually need a scope to "see" the output of the amplifier, assuming it was built correctly. It is actually easier to see the onset of clipping using a spectrum analyser, which ARTA has. You will essentially run a frequency sweep and look at the THD. When you get to the point of clipping, your odd-order distortion will rise VERY rapidly. If you take a look at any of the datasheets for amplifier chips, they should have graphs that show THD+N vs output power. Take a look at figure 19 on this LM3886 datasheet: http://www.ti.com/lit/ds/symlink/lm3886.pdf . The sharp knee in the curve is where the amplifier begins to clip. ARTA also will tell you the signal level coming into the sound card after running each measurement. When you see distortion rise in your measurement, make sure that you're not clipping the input to the sound card first, before you assume it is amplifier clipping.

                      Ideally, you should also have a low-pass RC filter on your sound card input to help reduce some of the switching noise from the amplifier, but a lot of sound cards input anti-aliasing filters will also take care of this for you.

                      I hope that's not too much info too fast. Again, I'll see if I can't find some time to make a little write-up with more specific wiring.

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                      • #12
                        Originally posted by 1100xxben View Post

                        Jon, I don't have time right this second to put out a full write-up, but I'll give you some key information and I'm sure others will chime in. I might have some time this weekend to do more of a full write-up with detailed wiring diagrams and instructions.... <SNIP>
                        Well said and spot on! I'd be super nervous hitting the input of a fairly expensive Focusrite with a speaker level signal myself
                        Electronics engineer, woofer enthusiast, and musician.
                        Wogg Music

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                        • #13
                          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?

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                          • #14
                            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?
                            This is why I prefer ARTA for my acoustic measurements in loop-back mode. When using loop-back mode in ARTA, the soundcard uses one channel to measure the output of the amplifier and one channel to measure the acoustic response. It then does the computation in the background using the MEASURED output of the amplifier instead of assuming that the digital source signal is completely unaltered by any of the analog electronics. Some sources of error can be sound card frequency or phase response, amplifier frequency or phase response, and amplifier output filter/speaker impedance interaction. Using the loop-back mode should virtually eliminate any of those errors in frequency response. The reality is that most systems will only have slight alterations at the ends of the frequency spectrum, and for crossover design, those differences are way outside the bands of interest.

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                            • #15
                              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? https://www.sweetwater.com/store/det...horia-umc202hd
                              John H

                              Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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