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| _Wim_:
--- Quote from: ruairi on January 05, 2017, 03:42:27 am ---I'm racing out the door to the studio here but I would advise the O.P. to slow way down and walk before you can run. You are talking about some very high end test equipment but making very basic mistakes in your understanding of audio, electronics and measurements. I totally understand the idea of doing things right and getting the best but understanding the fundamentals and how things are done will help you make better choices when you buy higher end gear later. --- End quote --- I have to agree with this. Good audio measurements are much more about understanding WHAT and HOW you are measuring, then about having very good measurement gear. I would recommend to read a few good books on the subject. I can recommend Floyd Toole "Sound Reproduction", Joseph d'appolito "Testing loudspeakers" (a bit dated, but explains the basics really wel)and Bob Cordell "Audio Amplifiers" (contains a good section about measuring amplifier and electronics). To understand "how" we hear, I can recommend JanSchnupp "auditory neuroscience making sense of sound". This book will give you a better idea what parameter a important, and which need to be only good enough. |
| montemcguire:
--- Quote from: _Wim_ on January 04, 2017, 07:49:14 pm --- --- Quote from: jackenhack on January 04, 2017, 07:43:29 pm ---I'm currently finishing up a headphone amplifier build and I got the QuantAsylum QA401 when I started developing the amp. With todays extremely low noise and distortion figures on op amps, You'll hit the bottom of what the analyser can measure very quickly. I had to build a twin-T notch filter and get hold of a extremely low THD signal generator to be able to measure below -108dB THD. Getting a industry standard Audio Precision is the dream, but they are way to expensive for me... --- End quote --- But the question is, why would we need to see lower than that? It can be fun as a technical exercise, but it will not improve the sound quality we hear any more⦠--- End quote --- The numbers one gets from an analyzer are completely artificial - nobody listens to sine waves. So, stating that such and such distortion levels are audible or not is hard to defend 'a priori'. It's reasonable to expect that, given a complex musical signal, the distortion products are greater than with a simple sine wave. Despite this uncertainty, I have found that these simple sine wave tests can be used to expose the behavior of a circuit very reliably, and that knowledge can help a designer to avoid specific distortion mechanisms, or optimize the circuit to minimize these distortions. Something as simple as changing the frequency compensation of an amplifier might have a small effect or no effect at all, and relying on a listening test to say whether the change improved performance, worsened it, or had no effect might be extremely difficult. This is the beauty of an analyzer: if one's goal is a clean circuit, an analyzer will provide a tireless, repeatable, and honest assessment of a circuit. With an analyzer, some circuit "tweak" that proves to have no effect can be understood as "irrelevant" right away, without the vagaries of a series of tedious listening tests. It is still up to the operator to choose the test and understand the meaning of the results, but given a high resolution distortion measurement, it's my opinion that one can more accurately and rapidly evaluate a circuit's behavior using a high resolution distortion analyzer than is possible with listening tests. |
| FrankBuss:
--- Quote from: montemcguire on January 06, 2017, 10:57:13 am ---The numbers one gets from an analyzer are completely artificial - nobody listens to sine waves. So, stating that such and such distortion levels are audible or not is hard to defend 'a priori'. It's reasonable to expect that, given a complex musical signal, the distortion products are greater than with a simple sine wave. --- End quote --- You are right, because just measuring the THD (or THD+N) doesn't say anything about the interactions between different signals. This is one reason Audio Precision has the multitone test: https://www.ap.com/technical-library/using-multitones-in-audio-test/ Additionally, any amplifier exhibits some filtering. This means that the phase shift of signals at different frequencies can be different (depending of the kind of filter, even if not intentionally). For this it is useful if you can do a bode-plot: https://www.ap.com/technical-library/using-bode-plots-in-apx/ Disclaimer: I'm not associated with Audio Precision, but it is just the best for audio measurement :) |
| montemcguire:
--- Quote from: FrankBuss on January 06, 2017, 11:54:54 am --- --- Quote from: montemcguire on January 06, 2017, 10:57:13 am ---The numbers one gets from an analyzer are completely artificial - nobody listens to sine waves. So, stating that such and such distortion levels are audible or not is hard to defend 'a priori'. It's reasonable to expect that, given a complex musical signal, the distortion products are greater than with a simple sine wave. --- End quote --- You are right, because just measuring the THD (or THD+N) doesn't say anything about the interactions between different signals. This is one reason Audio Precision has the multitone test: https://www.ap.com/technical-library/using-multitones-in-audio-test/ Additionally, any amplifier exhibits some filtering. This means that the phase shift of signals at different frequencies can be different (depending of the kind of filter, even if not intentionally). For this it is useful if you can do a bode-plot: https://www.ap.com/technical-library/using-bode-plots-in-apx/ Disclaimer: I'm not associated with Audio Precision, but it is just the best for audio measurement :) --- End quote --- I'm a big fan of AP gear as well :-) Still, while multitone and IM tests seem useful, I have found that, for my work, the APx-555 high precision analog generator has a lower residual for simple 1-2kHz sine wave tests, and thus seems to more honestly 'sort' and differentiate different devices under test, especially when the devices are so clean that they approach the AP residual. For gear that is, for example, worse than 1 ppm distortion, the multitone and other DAC based stimulus signals can be OK, and can do some useful, quick characterization. But, if you're like me and you're scraping the residual with 32-64 FFT averages, the IM and DAC based signals won't match the low residual of the high resolution generator / analyzer, and they fail to properly 'sort' DUTs according to their real errors. It'd be neat to have an IM test set that could match this performance, but keep in mind that a (basically linear, time invariant) device that fails an IM test also has to fail a simple sine wave test. One thing that an IM test can do is torture the device with high frequencies, but that can also be done with a sine wave test, measured at several frequencies. This can show an indication of the distortion rise with frequency, which can point to the mechanism for the circuit's behavior - e.g. 6dB/octave rise or 12dB/octave can point to different causes etc. Well, that's my world, and the APx-555 helps me out. But still, I could use another 20dB of distortion floor, just to be able to measure silly things accurately. I suspect that this will be possible in a decade or so, and I'll queue up for that box when I can! |
| FrankBuss:
--- Quote from: montemcguire on January 06, 2017, 12:27:51 pm ---Well, that's my world, and the APx-555 helps me out. But still, I could use another 20dB of distortion floor, just to be able to measure silly things accurately. I suspect that this will be possible in a decade or so, and I'll queue up for that box when I can! --- End quote --- Well, personally I think anything better than -120 dB THD+N (like some DACs and ADCs can do, meaning 20 bits effective resolution) doesn't make a difference and it gets really expensive. Be careful with additional 20 dB, because if you move while measure something, the induced current caused by your body-electricity, or if a truck is moving outside and then the piezoelectric effect of the vibrating capacitors might cause measureable distortions :D |
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