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The Audio Measurement Handbook - Audio Precision
The Audio Measurement Handbook - Audio Precision
Hi FriedMule,
I've been an audio service technician for most of my life (with a few day jobs). I have used everything from separate audio generators and THD meters. I still use my HP 339A for some adjustments where it is just plain easier. Like Suzy, I have an HP 3585A (hers is a "B" series). It is still on my bench and used as it covers high frequencies. I also am lucky enough to have an RTX-6001. I use that every day, and it is a wonderful instrument. If I'm going to pay for an AP, I'll go all the way and buy the Keysight U8903B with the 1.5 MHz option. Keysight/Agilent/HP equipment tends to perform much better than their specs would suggest.
The RTX-6001 unit along with Virtins M.I. has integration. That means that it performs like an AP at a fraction of the price. M.I. software can also execute a series of commands to perform a series of tests with one key press.
Too bad ej25awd hasn't used an RTX yet. I would highly recommend the RTX-6001 and Virtins M.I. combination for your test needs. The 6001 will work with almost any sound card software, you aren't married to a single vendor. I also use ARTA with the 6001. I have license for both software packages. With any luck, other software packages will include integration for the RTX-6001. For now, Virtins does and their software is excellent.
-Chris
I've been an audio service technician for most of my life (with a few day jobs). I have used everything from separate audio generators and THD meters. I still use my HP 339A for some adjustments where it is just plain easier. Like Suzy, I have an HP 3585A (hers is a "B" series). It is still on my bench and used as it covers high frequencies. I also am lucky enough to have an RTX-6001. I use that every day, and it is a wonderful instrument. If I'm going to pay for an AP, I'll go all the way and buy the Keysight U8903B with the 1.5 MHz option. Keysight/Agilent/HP equipment tends to perform much better than their specs would suggest.
The RTX-6001 unit along with Virtins M.I. has integration. That means that it performs like an AP at a fraction of the price. M.I. software can also execute a series of commands to perform a series of tests with one key press.
Too bad ej25awd hasn't used an RTX yet. I would highly recommend the RTX-6001 and Virtins M.I. combination for your test needs. The 6001 will work with almost any sound card software, you aren't married to a single vendor. I also use ARTA with the 6001. I have license for both software packages. With any luck, other software packages will include integration for the RTX-6001. For now, Virtins does and their software is excellent.
-Chris
Chris,
I'm full aware of the RTX 6001, I have followed it from birth and watched the GB 2 years ago. I never wanted one because my intended application doesn't fit the RTX 6001 features set.
I need an analog generator with output greater than 96kHz, ability to analyze frequencies above 96kHz, fully isolated outputs, provision for custom analog filters, tightly integrated software, digital I/O, auto ranging, etc. RTX 6001 doesn't support any of these.
Audio Precision software for the older System One and Two is free, no license required.
I have used AP, HP, SRS, Tektronix and R&S audio analyzers when designing commercial products, I know their limits, reliability, stability and ergonomics. They all have pros and cons, HP gear are not bullet-proof, I have fixed some of their spectrum analyzers and audio analyzers.
We all have our bias but I like to keep an open mind.
I'm full aware of the RTX 6001, I have followed it from birth and watched the GB 2 years ago. I never wanted one because my intended application doesn't fit the RTX 6001 features set.
I need an analog generator with output greater than 96kHz, ability to analyze frequencies above 96kHz, fully isolated outputs, provision for custom analog filters, tightly integrated software, digital I/O, auto ranging, etc. RTX 6001 doesn't support any of these.
Audio Precision software for the older System One and Two is free, no license required.
I have used AP, HP, SRS, Tektronix and R&S audio analyzers when designing commercial products, I know their limits, reliability, stability and ergonomics. They all have pros and cons, HP gear are not bullet-proof, I have fixed some of their spectrum analyzers and audio analyzers.
We all have our bias but I like to keep an open mind.
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My goal is to construct and repair amplifiers and media players.
I would rather have a dedicated unit for measuring and testing, instead of using my laptop. Jazid do I think, is making a great list for the amplifier part but if there is more then just ad it in my "hope to get".
ej25awd writes about several tests then the 8903A/B
I would rather have a dedicated unit for measuring and testing, instead of using my laptop. Jazid do I think, is making a great list for the amplifier part but if there is more then just ad it in my "hope to get".
ej25awd writes about several tests then the 8903A/B
A unit with even more / better test is welcome, as long we are not being insane about the price. (What is insane, it's more then what a home DIY would use) yes that's a non telling amount but I hope that it exclude a 10,000$ unit
Hi ej25awd,
Have you ever played with the RTX? This is the crucial question that shapes your impression of the unit.
I was a bit worried about my purchase at first, but that worry evaporated seeing the support we had and the software that supported the RTX. But without ever using one, I couldn't have come to the strong affinity I developed for them.
As for repairing HP gear, I do and have as well. The old 339A isn't perfect, but it is perfect for aligning tuners. Just as the old 3581A/B I have for setting the traps on the output (19 and 38 KHz). Same for tape bias traps.
You must differentiate between the older 8903x analysers and the current U8903x analyser. These run at least $35K optioned out for me, as does the AP. The cheap AP is over $10K and doesn't really offer much, if anything (been two years since I looked) over the RTX and MI software. The RTX sits around $2,500 US I think, and $500 for MI full license. Again, I haven't looked in a long while. It seemed foolish to me to spend that great a difference over two very similar products. Never mind the fact that I couldn't afford the AP at those prices. You do get a calibration certificate with a new RTX btw.
-Chris
Have you ever played with the RTX? This is the crucial question that shapes your impression of the unit.
I was a bit worried about my purchase at first, but that worry evaporated seeing the support we had and the software that supported the RTX. But without ever using one, I couldn't have come to the strong affinity I developed for them.
As for repairing HP gear, I do and have as well. The old 339A isn't perfect, but it is perfect for aligning tuners. Just as the old 3581A/B I have for setting the traps on the output (19 and 38 KHz). Same for tape bias traps.
You must differentiate between the older 8903x analysers and the current U8903x analyser. These run at least $35K optioned out for me, as does the AP. The cheap AP is over $10K and doesn't really offer much, if anything (been two years since I looked) over the RTX and MI software. The RTX sits around $2,500 US I think, and $500 for MI full license. Again, I haven't looked in a long while. It seemed foolish to me to spend that great a difference over two very similar products. Never mind the fact that I couldn't afford the AP at those prices. You do get a calibration certificate with a new RTX btw.
-Chris
Hi FriedMule,
Have a look at the options and prices for the different units. The "HP" Keysight unit is the U8903B currently. It is a very capable unit with a 1.5 MHz option. I consider audio to include 1 MHz where nasty things can happen, like oscillations for example.
The more you look at this, the better the RTX looks.
-Chris
Have a look at the options and prices for the different units. The "HP" Keysight unit is the U8903B currently. It is a very capable unit with a 1.5 MHz option. I consider audio to include 1 MHz where nasty things can happen, like oscillations for example.
The more you look at this, the better the RTX looks.
-Chris
I am sure that it is a great peace of gear but we are way over my budget!
I think it's about 1,500-2,000$ that maybe is my max.
If I want to buy an ATS-1 to a more realistic price, what to do? As stated earlier they is priced to around 2000$ on eBay.
Nope, no future plans. I already told you why I didn't buy an RTX. Please read my complete post.
You are barking up the wrong tree. I've used FFT analyzers for 15 years now including the HP (dynamic) signal analyzers which I forgot to include on my previous post.
I was talking to the OP about the older 8903A/B and AP ATS-1 which are what he was interested in. At that point, it is clear how much his budget was. No point mentioning to him the new Keysight U8903x that I'm also familiar with or anything new from other vendors.
I also gave him an example using my method that a used Audio Precision System One 22A for only $150 is possible. I don't make things up, I back my post with documentation when warranted to save myself from long unnecessary discussion.
I can't justify spending money on something I don't NEED. What is wrong with this belief?
I can fully vouch for ej25awd's point here, he kindly supported my efforts, and put his time and shoulder where his mouth was and I got this (see enclosed).
There were some issues, but with his kind help and some good work, it's fully up and back to factory spec +. I also have a fully calibrated 8903B + HP USB cable and PM's apps and there's no comparison to what the AP can do. And, for an apples to apples point of view, I personally paid (a bit) more for the HP than the AP.
I definitely know now what I'd had done, with the benefit of hindsight. No question in my mind the AP is the better path.
Attachments
Going back to the OP's original question, He is on a journey to learn what to measure and how to measure it, so laying out $00's at the start should not be necessary. I assume he is in it for the enjoyment and not as a profession.
The computer path with a good solid software package such as ARTA, DiAna or REW, coupled with a good quality sound card (24 bit, 192K sampling), are good enough in getting used to the terminology and making measurements.
Adding further bits of kit, like a sound card prescaler/attenuator and a very low distortion oscillator such as Viktor's, is something that can be done as part of the learning that meet the needs of the amateur.
Only when you have got confidence in what they do and mean, and perhaps their limitations, should you think about more expensive test gear - if necessary.
So I would recommend the above: start slowly and enjoy the learning process.
Cheers
Mike
The computer path with a good solid software package such as ARTA, DiAna or REW, coupled with a good quality sound card (24 bit, 192K sampling), are good enough in getting used to the terminology and making measurements.
Adding further bits of kit, like a sound card prescaler/attenuator and a very low distortion oscillator such as Viktor's, is something that can be done as part of the learning that meet the needs of the amateur.
Only when you have got confidence in what they do and mean, and perhaps their limitations, should you think about more expensive test gear - if necessary.
So I would recommend the above: start slowly and enjoy the learning process.
Cheers
Mike
Before spending a lot of money on test gear, invest in Bob Cordell's second edition of "Designing Audio Power Amplifiers" as he has a lot of insight into what you will really need.
If I were to start on a budget, I would get a pair of Viktor's signal generators, 1kHz and 10kHz, discussed here in the forum and sometimes seen on EBay.
If I were to start on a budget, I would get a pair of Viktor's signal generators, 1kHz and 10kHz, discussed here in the forum and sometimes seen on EBay.
I enjoy being a minimalist when it comes to valve amp testing, and typically use a couple of cheap Aneng multimeters for absolute voltage measurements, and a USB soundcard with 10:1 or 100:1 scope probe, a few simple resistive loads with plugs, and REW software. That informs me of noise floor, hum and unwanted resonances, as well as frequency spectrum performance of test signals, frequency response spectrum sweeps, and levels of harmonics and THD. I can do basic feedback performance assessment of gain and phase as long as it is between 2Hz and 96kHz. I can easily look at current signals with small isolated LEM transducers, as well as a few other niche sensors I sometimes use including accelerometers, test mics, and even a mag field sensor.
About the only time I've needed to go beyond that is to use a scope in X-Y and sig gen for below 2Hz, and the same sig gen and HP3400 for greater than 100kHz with vintage HiFi amps.
Sometimes not having the 'best' provides a better experience and awareness about what you are testing and whether any result or spectrum is real or has test setup artifacts, and how test equipment items interact on a workbench.
About the only time I've needed to go beyond that is to use a scope in X-Y and sig gen for below 2Hz, and the same sig gen and HP3400 for greater than 100kHz with vintage HiFi amps.
Sometimes not having the 'best' provides a better experience and awareness about what you are testing and whether any result or spectrum is real or has test setup artifacts, and how test equipment items interact on a workbench.
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Also, don't overlook the value and utility of the QA401. The software is still in active development and new features all the time. Plus you can get results pretty quickly out of the box. This can be very important to a Noob. However you can do some pretty sophisticated tests with them. Still for $449 new its a good value.
The distortion floor of the RTX is lower and the price higher by similar dB ratios. and then the same dB ratio to SOTA APx or tweaked Shibasoku setup.
The AP's are not hard to use but the giant feature set and particular mindset of the UI mean a fair amount of learning ahead.
In practice getting below -100 dB THD+N should mean that you are good enough and look elsewhere for improvement opportunities.
If you need to check frequency response get a good function generator. The Krohn-Hite generators are excellent and very flexible. You can check response with a scope or wide band AC voltmeter.
More important is where this thread started- what to measure and why?
I would do the following on a new design-
1) Gain (can tell if it works and per your plan)
2) Look at the output waveform. Is it right or obviously distorted
3) Frequency response flat or correct EQ.
4) SNR is the noise low enough to meet the goals with the intended input and output?
5) Distortion need to look at the spectrum. The reconstructed or residual waveform can tell you a whole lot about how the circuit is working and some clues as to where to look for problems. DiANA is the best available tool for this: DiAna
6) Power Amplifiers- reaction to load, both resistive and reactive. And, if you are confident in your protection circuit, a dead short which WILL happen.
You can keep fiddling forever with more esoteric tests but maybe you should start using your stuff instead.
(I'm working on a presentation about this for Burning Amp so its all fresh.)
The distortion floor of the RTX is lower and the price higher by similar dB ratios. and then the same dB ratio to SOTA APx or tweaked Shibasoku setup.
The AP's are not hard to use but the giant feature set and particular mindset of the UI mean a fair amount of learning ahead.
In practice getting below -100 dB THD+N should mean that you are good enough and look elsewhere for improvement opportunities.
If you need to check frequency response get a good function generator. The Krohn-Hite generators are excellent and very flexible. You can check response with a scope or wide band AC voltmeter.
More important is where this thread started- what to measure and why?
I would do the following on a new design-
1) Gain (can tell if it works and per your plan)
2) Look at the output waveform. Is it right or obviously distorted
3) Frequency response flat or correct EQ.
4) SNR is the noise low enough to meet the goals with the intended input and output?
5) Distortion need to look at the spectrum. The reconstructed or residual waveform can tell you a whole lot about how the circuit is working and some clues as to where to look for problems. DiANA is the best available tool for this: DiAna
6) Power Amplifiers- reaction to load, both resistive and reactive. And, if you are confident in your protection circuit, a dead short which WILL happen.
You can keep fiddling forever with more esoteric tests but maybe you should start using your stuff instead.
(I'm working on a presentation about this for Burning Amp so its all fresh.)
Here's the simplest and most effective test system ever. Not big $$$, can use REAL MUSIC as a test source:
Sound Impairment Monitor (SIM) - Is This The Answer?
I think this will tell the truth about ANY amplification device better than any sine-wave tester.
Sound Impairment Monitor (SIM) - Is This The Answer?
I think this will tell the truth about ANY amplification device better than any sine-wave tester.
Hi dotneck335,
Sine wave testing is perfectly valid as it has but one frequency and exercises an amplification stage enough to scare out the weaknesses and problems. A two sine wave signal coaxes out other problems. And many classic instruments that use FFT can average the noise floor down to show other characteristics of the performance.
The classic distortion analyser with a meter only gives you a number. While it can be helpful, it's not the whole story. So I hung a spectrum analyser off the residual output of my HP 339A THD meter and learned a great deal about what was going on. Today I use an RTX for most of this work, and sometimes a spectrum analyser for higher frequency show and tell. The RTX takes the signal directly from the output of the device and presents you with an FFT display using various selectable windowing schemes. It is by far the best tool I have for looking at amplifier performance. The old HP 339A? It's about the best instrument for adjusting an FM receiver for minimum distortion and so remains on the bench.
With sound cards it is easy to blow the input section up, so a normal (good) distortion analyser would be the best using your sound card on the now safe residual output of the THD meter. So the THD meter acts as a front end for the sound card.
You can use the method you linked to, but it won't tell you very much about the problems in an amplifier or other device. It might just be better for critical listening gatherings where it can be trotted out to play with. Just don't expect to quantify anything with it, and you're still looking at using real test instruments to get meaningful results.
I wish I didn't have to spend the money I have over my lifetime in order to get meaningful measurements as I am an audio service technician. But it keeps coming back to, "if you wanna play, you gots to pay". It really is almost as simple as that. OF course, it also means that you learn how to use your chosen instrument properly.
-Chris
You wish!
Sine wave testing is perfectly valid as it has but one frequency and exercises an amplification stage enough to scare out the weaknesses and problems. A two sine wave signal coaxes out other problems. And many classic instruments that use FFT can average the noise floor down to show other characteristics of the performance.
The classic distortion analyser with a meter only gives you a number. While it can be helpful, it's not the whole story. So I hung a spectrum analyser off the residual output of my HP 339A THD meter and learned a great deal about what was going on. Today I use an RTX for most of this work, and sometimes a spectrum analyser for higher frequency show and tell. The RTX takes the signal directly from the output of the device and presents you with an FFT display using various selectable windowing schemes. It is by far the best tool I have for looking at amplifier performance. The old HP 339A? It's about the best instrument for adjusting an FM receiver for minimum distortion and so remains on the bench.
With sound cards it is easy to blow the input section up, so a normal (good) distortion analyser would be the best using your sound card on the now safe residual output of the THD meter. So the THD meter acts as a front end for the sound card.
You can use the method you linked to, but it won't tell you very much about the problems in an amplifier or other device. It might just be better for critical listening gatherings where it can be trotted out to play with. Just don't expect to quantify anything with it, and you're still looking at using real test instruments to get meaningful results.
I wish I didn't have to spend the money I have over my lifetime in order to get meaningful measurements as I am an audio service technician. But it keeps coming back to, "if you wanna play, you gots to pay". It really is almost as simple as that. OF course, it also means that you learn how to use your chosen instrument properly.
-Chris
I think you could quantify it---say, by calculating the rms value of the subtractor divided by the rms value of the input. That number could be used to compare it to another amp. Its beauty is that it can be used with sine/square/triangle/double sine waves as well as music.
Now, with your distortion analyzer + spectrum analyzer (a formidable test method, I must say!), do you agree with Gnobuddy's postulation that all amplifiers with less than 0.1% THD sound the same and cannot be differentiated between?
To be clear: this is not my postulation. Rather, it is the result of a lot of hard work, thousands of carefully conducted controlled listening tests, and rigorous statistical analysis of the results, during the era when such tests were actually done, and done by educated people - engineers and scientists and mathematicians - working for rich and well-endowed laboratories like Bell Labs and the research branches of top-notch companies like Phillips.
My unsubstantiated postulations would be worth about as much as yesterday's newspaper (if anyone still remembers what a newspaper is.) This is something else entirely!
-Gnobuddy
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