| Electronics > Repair |
| Audio… (Amps, THD, channel separation) with analog equipment (scopes, etc.) |
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| TimFox:
--- Quote from: Fried Chicken on December 27, 2024, 03:57:58 pm --- --- Quote from: vk6zgo on December 27, 2024, 11:16:28 am --- Oscilloscopes can be used to measure (for example), distortion , just by looking at the shape of a sinewave, but the distortion has to be quite profound before aberrations become visible. You could compare the sinewave signal at the input with that at the amp output by using the "A" minus "B" function of an analog 'scope, or equivalent on a DSO, but you will run into problems with differing phase. Any "tricks" delivering usable results become increasingly complex to perform, as they require additional circuitry which you will have to design yourself I earned "my daily bread" measuring distortion in Broadcasting for years, & later in testing hearing equipment, & believe me, if there were easy tricks to satisfactorily measure THD using a standalone Oscilloscope,my various employers would have used them! The two "filter" methods I mentioned give repeatable results, so are used, the N & D set in Broadcasting, & the Selective voltmeter in the testing of equipment used in hearing testing. --- End quote --- Well... what other "tricks"? Does the A minus B work off the displayed signal or the fed signal? Can I set the voltages so that the two signals match on screen and then do A minus B? I can alter the phasing by altering the trigger, no? Or do I need a tunable RC circuit to change the phase? --- End quote --- Note that "A minus B" mode of an analog two-channel scope is not a perfect subtraction: the tolerance on the analog combination can be comparable to the THD that you are trying to measure. I do THD measurements (technically, THD plus noise) using an -hp- 339A analyzer that combines a low-distortion generator with a nulling filter and auto-scaling of the remainder to the input level. The nature of the distortion products can be viewed by an analog output of the remainder to an oscilloscope. |
| macboy:
With your equipment you listed, you can do some useful things. You can determine the clipping point of the amplifiers by feeding the amp a sine wave (around 1 kHz, 1 Vp-p), and adjusting the volume up until you can clearly see the flat tops on the output sine wave, observed with the scope. Back off the volume control until the peak is just a little below that, and there you have the maximum unclipped power output. The distribution may already be a little high at this point even if the output isn't technically clipping. The load (e.g. 4 vs 8 Ohms) will alter the clipping point somewhat. Calculate power by P=V2/R . Obviously use a dummy load resistor for this, not speakers. Now without changing the volume control, remove the function generator and short circuit the input jack (I have built shorted phono plugs for this purpose). The input is now zero. Measure the RMS AC voltage of the amplifier output. It will be low but won't be zero. Take the ratio of this compared to the full output power from earlier and you have the signal to noise ratio. Convert to decibels: SNR(dB)=20*log(Vmax / Vnoise). You can repeat the SNR measurement at lower volume levels as well, but the noise will become increasingly difficult to measure accurately as it drops to mV levels. As a rule of thumb, your multimeter's AC voltage reading is only sensible when it is at least 1% of full scale. So at 200 mV scale : 2 mV minimum input, regardless of how many digits it shows. Your fluke DMM should also have decent bandwidth covering the audio range, but you can verify that by feeding it a constant level from your function generator and sweeping through the audio frequency range. The AC RMS reading should remain relatively constant (remember +-10% difference is actually less than 1 dB). If you have a computer with a decent sound card, you can measure distortion and other performance parameters with software such as RightMark Audio Analyser RMAA or others (some free, some are even free open source). With a scope and signal gen, you just can't do it, but you can see gross distortion such as clipping or zero cross distortion. The latter might result from a bias setting much too low. |
| Fried Chicken:
--- Quote from: vk6zgo on December 27, 2024, 05:50:11 pm ---There may be tricks you can do with a DSO, but with an analog 'scope the RC phase change circuit would be the way to go. The difference in shape of the two signals would show up as the result of A minus B, which may require some analysis to work out the relationship between that display & distortion in % (or dB). It will, in fact be a result of THD + Noise, but usually the latter is low enough to be ignored. In TV testing, we expressed both THD + Noise & Noise by itself in dB, making it easier to mathematically remove the latter to give pure THD. --- End quote --- I feel like just looking at and dicking with the signal will start making things obvious to me. Gah; I wish I had a more intuitive understanding of math, the relationship between Distortion and dB. I used to be really good at this, but then I chose to study science instead of EE. --- Quote from: TimFox on December 27, 2024, 06:03:44 pm --- --- Quote from: Fried Chicken on December 27, 2024, 03:19:59 pm ---This classic A/AB amp should reference to ground across everything, including the speaker terminals, right? I can safely probe around bing bing bing bing bing? --- End quote --- Usually, that is true. It is the exceptions that might kill you, for example an analog amplifier with an H-bridge output. --- End quote --- Why would someone put an H-Bridge on an amplifier output? --- Quote from: macboy on December 27, 2024, 06:43:14 pm ---With your equipment you listed, you can do some useful things. You can determine the clipping point of the amplifiers by feeding the amp a sine wave (around 1 kHz, 1 Vp-p), and adjusting the volume up until you can clearly see the flat tops on the output sine wave, observed with the scope. Back off the volume control until the peak is just a little below that, and there you have the maximum unclipped power output. The distribution may already be a little high at this point even if the output isn't technically clipping. The load (e.g. 4 vs 8 Ohms) will alter the clipping point somewhat. Calculate power by P=V2/R . Obviously use a dummy load resistor for this, not speakers. Now without changing the volume control, remove the function generator and short circuit the input jack (I have built shorted phono plugs for this purpose). The input is now zero. Measure the RMS AC voltage of the amplifier output. It will be low but won't be zero. Take the ratio of this compared to the full output power from earlier and you have the signal to noise ratio. Convert to decibels: SNR(dB)=20*log(Vmax / Vnoise). You can repeat the SNR measurement at lower volume levels as well, but the noise will become increasingly difficult to measure accurately as it drops to mV levels. As a rule of thumb, your multimeter's AC voltage reading is only sensible when it is at least 1% of full scale. So at 200 mV scale : 2 mV minimum input, regardless of how many digits it shows. Your fluke DMM should also have decent bandwidth covering the audio range, but you can verify that by feeding it a constant level from your function generator and sweeping through the audio frequency range. The AC RMS reading should remain relatively constant (remember +-10% difference is actually less than 1 dB). If you have a computer with a decent sound card, you can measure distortion and other performance parameters with software such as RightMark Audio Analyser RMAA or others (some free, some are even free open source). With a scope and signal gen, you just can't do it, but you can see gross distortion such as clipping or zero cross distortion. The latter might result from a bias setting much too low. --- End quote --- These are some clever tricks. I know the old analog guys had some wizardry that probably surpasses what the digital people can do. I suspect my function generator doesn't have a linear output on a sweep... at least it didn't when I hooked it up to my scope, I think... low frequencies it was very high p-p, and then it went down as I went up the frequencies... I think. Why measure with the multimeter and not the scope for the sweep? I don't have a soundcard in my computer, but I think I might get one as my USB headphone amplifier thing from Behringer caused bugs in software (I suspect). I look forward to getting some free time to hook up the amplifier. I suspect once I start dicking around something obvious might appear. I guess I should mention the model, it's a Yamaha M-65. I recently did a recap on it, and while I heard a difference, I'm not convinced it was an improvement. I feel like the right speaker is fuller and better, but only across certain frequencies. I should have measured the caps before installing them... possibly I screwed something up, but I don't think so, I worked carefully. If I want to measure a crossover, do I also need a dummy load? |
| TimFox:
"Quote from: TimFox on Yesterday at 01:03:44 pm Quote from: Fried Chicken on Yesterday at 10:19:59 am This classic A/AB amp should reference to ground across everything, including the speaker terminals, right? I can safely probe around bing bing bing bing bing? Usually, that is true. It is the exceptions that might kill you, for example an analog amplifier with an H-bridge output. Why would someone put an H-Bridge on an amplifier output?" To work with a low DC voltage. |
| Fried Chicken:
--- Quote from: TimFox on December 28, 2024, 03:01:37 pm --- "Quote from: TimFox on Yesterday at 01:03:44 pm Quote from: Fried Chicken on Yesterday at 10:19:59 am This classic A/AB amp should reference to ground across everything, including the speaker terminals, right? I can safely probe around bing bing bing bing bing? Usually, that is true. It is the exceptions that might kill you, for example an analog amplifier with an H-bridge output. Why would someone put an H-Bridge on an amplifier output?" To work with a low DC voltage. --- End quote --- How is that different from simply flipping the polarity? (Red --> Black, Black --> Red)? How does an H-Bridge give a low DC voltage? Dumb question: if I'm measuring/testing a crossover, do I need a dummy load? Dummy load on all outputs or just on the output I'm testing? |
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