Audio distortion measurement techniques - Page 1
Forum section: electronics / projects

author: commie
author_rank: Frequent Contributor
timestamp: September 10, 2016, 07:56:23 pm
content: Hi Fella'sI'm playing around with audio at the minute and there is something I can't understand regarding the audio range noise floor.We have all heard of some punters claiming a THD figures of 0.001%?I have a Rigol 815 and a GWinstek 2202E spectrum analyser and scope respectively. So what does 1% distortion actually mean? 1 part in 100, if I have 1Volt r.m.s. @ 1KHz(0dBVrms-ref), then we have all the sum of the squares equating to 10mVr.m.s.with presquare root for 1% distortion.No problem so far?Now, for the above figures to be real you will require a noise floor of at least -45dBVrms, right? Otherwise the harmonics will be lost in the noise.In fact, to support 0.1%, 0.01% and 0.001% distortion figures, the latter requires a noise floor of at least -110dBVrms!I aint a noise specialist but I am aware of thermal and 1/f noises and my instruments give a noise floor of -70dbVrms on a good day so the question I would like to ask is how do you get the audio frequency range noise floor down to around -100dB??CheersCommie

author: ruairi
author_rank: Frequent Contributor
timestamp: September 10, 2016, 08:33:19 pm
content: This is a complex issue but the short answer is that measuring low distortion in audio gear is not done with standard lab gear, to measure the lowest levels of distortion you need a dedicated audio analyzer - Audio Precision, Prism Sound, Rohde & Schwarz etc.How distortion correlates with listening pleasure or the lack thereof is an even deeper subject. I've spent 22 years as a professional audio engineer, the last 15 as a mastering engineer. Listening is my job and I've spent the last 10 years studying electronics to gain a deeper understanding of why things sound the way they do.If you are interested in distortion measurement for audio spend some time looking through the documents available on ap.com, there difference between THD and THD+N etc.

author: dom0
author_rank: Super Contributor
timestamp: September 10, 2016, 09:19:30 pm
content: Yes, general purpose gear like spectrum analysers (or scopes) aren't suitable to this kind of measurement.Distortion can be measured using the residual method (ie. a practically perfect (very, very low noise, distortion and other spurious signals) sine generator and a similarly perfect frequency/phase-locking notch filter followed by a RMS converter), which was fairly common and has been implemented many times.Modern audio analysers usually are "just" the combination of somethingsimilarto a good sound card and a computer, ie. the signal is amplified and digitized, all further processing is digital. Similarly signal generation is often digital as well; although there are still analog generators in production, some of which are still exceeding the performance of even the best digital generators.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 10, 2016, 11:16:28 pm
content: I've also got some interest with various audio measurements, and I'm wondering why THD is such a difficult thing to get without specialized gear. Is it that you're basically doing a high sensitivity harmonic analysis and then doing additional computation over top, and there just isn't a good way of doing it without computational muscle? Is it just that with a low noise floor front end and a very low jitter timebase you end up with an expensive piece of gear just because of the components?Given the low bandwidth of audio and the comparatively high sample rates of various instruments, you'd think you could make a good measurement so long as your front end was designed properly. Could you get a reasonable approximation of a good THD measurement by sending your (known accurate) tone for a longer time and just averaging samples - or is the off-slightly-for-a-period frequency distortions sort of the point of the THD measurement?As for the OP, it seems like DSAs and audio analyzers do require the extremely low noise floor - most of the well regarded ones advertise better than -100dB, though there are a few in the 90s. In fact, most computer sound chips will advertise better than -100dB noise floor, with the premium ones close to -120dB. I really doubt that they actually achieve that inside a PC case and sometimes questionable pcb layouts (you're not going to hear-112dB on your motherboard-integrated sound, regardless of the spec), but it does seem that even decent grade audio parts are rated below 0.01% THD and a -100dB noise floor.

author: Gary.M
author_rank: Regular Contributor
timestamp: September 10, 2016, 11:37:24 pm
content: Google Quantasylum QA-401

author: uncle_bob
author_rank: Unknown
timestamp: September 11, 2016, 12:53:46 am
content: HiOk, here's a quick simple answer:Measure your 1 KHz fundamental signal in a 0.1 Hz bandwidth centered at 1 KHz. Measure each of the 19 relevant harmonics in a 0.1 Hz bandwidth centered at each of the harmonics. You now have 19 x 0.1 Hz = 2 Hz of noise. If your noise spectral density messes that up, you have major issues in your design. A spectral density at -110 dbV is a really noisy circuit.Bob

author: commie
author_rank: Frequent Contributor
timestamp: September 11, 2016, 11:30:48 am
content: Okay Chaps,Thanks for all the replies, next time I see 0.001% distortion figures, I'll view with a bit of scepticism

author: Pjotr
author_rank: Frequent Contributor
timestamp: September 11, 2016, 11:58:09 am
content: Depends on how it is measured. As pointed out by Bob: How deep you can measure depends on thenoise bandwidth.With a good 24 bit sound card, a -150dB noise floor is readily possible with off the shelf low cost software (http://www.artalabs.hr/index.htm). That is < 0.1 ppm. Unfortunately the distortion of the sound card itself is not that low and becomes the limitation. To measure really low you need an ultra low distortion sine source and notch filter (also ultra low distortion) for 1st harmonic suppression.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 11, 2016, 03:46:09 pm
content: The noise level depends on bandwidth, so you don't really need that low noise to measure low THD values. The trick is using longer time averaging and only looking at the harmonics.Usually you are limited by the harmonics from the detecting ADC and the signal source. With quite some math and extra measurements one can get around that limits a little.There also is the classical notch filter to reduce the fundamental and that way allow higher THD for the ADC side. Than even a scope may be OK, though even low cost sound cards are usually better at audio frequencies. Still there should be a high quality source.

author: Pjotr
author_rank: Frequent Contributor
timestamp: September 11, 2016, 04:33:25 pm
content: This is what I archived with my (quite old) M-Audio Audiophile192 on-board PCI card with input and output connected (loop back). Note that 1-bit oversampling cards have lowest distortion around -10dB FS.

author: uncle_bob
author_rank: Unknown
timestamp: September 11, 2016, 06:01:40 pm
content: Quote from: commie on September 11, 2016, 11:30:48 amHiNo reason to be skeptical of the reading it's self. The number can be measured and the measurement its self is quite repeatable. Put another way, when they say 0.001%, that number can be verified.Is a device at 0.001% better than a device that "only" does 0.002%? Yes, indeed the first one is better than the second one. Is this the *only* spec that matters on an audio device? Of course not. I'd like to see some proper IMD and transient data as well if we are talking about linearity.Lots of specs, lots of combinations, how much money is also a spec.Bob

author: commie
author_rank: Frequent Contributor
timestamp: September 11, 2016, 06:28:33 pm
content: Well Hi again,I have just been reading the spec. of Marconi's TF2331/a 1975 distortion factor meter and it seems to make a lot of sense and realism to me.Basically with its most sensitive range being 0.1% fsd, so 0.02% should just about be readable on its 0-100 meter i.e., as 20% of fsd. It's noise floor will then be about 0.015% in its 100KHz bandwidth(switchable).Not sure what the limiting noise floor of HP's 334 is? but I'll bet it's not much better than the Marconi TF2331/a DFM.

author: uncle_bob
author_rank: Unknown
timestamp: September 11, 2016, 06:42:19 pm
content: Quote from: commie on September 11, 2016, 06:28:33 pmHiKeep in mind that both of those meters are ancient. I was selling surplus 334's back in the 1970's ....The advent of digital signal processing has changed a lot of the limits on a whole bunch of instruments. It also has created the tools to go a lot further into optimizing circuits.Bob

author: commie
author_rank: Frequent Contributor
timestamp: September 11, 2016, 07:46:19 pm
content: Quote from: uncle_bob on September 11, 2016, 06:42:19 pmIt's now 2016 and they are still selling as surplus at eye watering prices, check this one out on Ebay.....http://www.ebay.co.uk/itm/Marconi-Instruments-IFR-Aeroflex-TF2331-Audio-Distortion-Meter-/282139564758?hash=item41b0d41ad6:g:j20AAOSwHoFXtDfGQuote from: uncle_bob on September 11, 2016, 06:42:19 pmAs already stated my measuring instruments are both dsp based and both give -70dBVrms noise floor in the audio band.

author: uncle_bob
author_rank: Unknown
timestamp: September 11, 2016, 08:37:31 pm
content: Quote from: commie on September 11, 2016, 07:46:19 pmHiWhat really matters is the noise floor in dbV (or dbm or dbf or dbW) / sqrt(Hz). That lets you get an idea of how quiet they actually are. A number without a bandwidth could indicate a very low floor (if it's a wide bandwidth) or a really crummy floor (if it is a narrow bandwidth). Since it's audio, you could (and probably should) have a bandwidth close to or less than 1 Hz for decent measurements. There are analyzers that date back to the 1970's that will get you into the <-150 dbV / sqrt(Hz) range. These days "something good" would be in the <-180 dbV/sqrt(Hz) range with the input terminated with a 50 ohm resistor.Bob

author: JS
author_rank: Frequent Contributor
timestamp: September 12, 2016, 02:11:54 am
content: Hi, it's quite a topic this one, I invite you to GDIY to talk about this, you will have quite a master class there in this respect.The most critical part is the generator, you won't be able to get any better than you'r generator does because you can't identify what was generator's fault or what was introduced by the DUT. Even so there are quite nice generators, HP classic topology archives some of the best analog generators for low distortion sine waves. You can filter that out then, to get even better numbers, RC filters could be quite linear using NPO caps and decent resistors (avoid tiny SMD and carbon and you are probably good to go)Once you get pass the generator isn't as tricky, you don't need such low noise floors or THD on your ADC. Once the signal is out of the DUT first thing it will see is a filter, again, some cleverness would make an 8bit ADC enough to measure few ppm of distortion, but converters and processing comes cheap now days. Use a nice filter, a HPF is good enough, you don't have any useful information till 1 octave later than your fundamental so it's easy to get it XdB lower. Those XdB you will win in your dynamic range of the ADC. Then the software will make sure it reads the values at the points of interest.In my opinion the importance of a very low THD has to do with eliminating the characterization of the distortion, you may be good all day with a 0.1% THD or not, depending on the content. If you have a heavy weight on higher harmonics it get's much more noticeable very fast. If you have 0.00001% THD it doesn't matter where it is, you won't hear it. Adding the noise to the spec reflects the dynamic range of the device, if your input level goes higher your THD+N will increase, if it goes lower too. So you have 0.001% at +7dBu input, if you put +10dBu the distortion will be higher than optimal, if you put +4dBu your S/N ratio will decrease.Here goes an over simplification of the application of all this. In user grade audio doesn't really matter, the dynamic range of the signal packed in a CD is quite low, not because of the media but culture, the louder the betta, if you listen to Californication (the whole album) converted to 8bit WAV you won't notice a difference, bit more noise between tracks at most, 20dB dynamic range it's all is really used in most records. When working on a production is quite different, the signal in the room may have twice that easily, probably more. A mic preamp would probably round the 90dB dynamic range, but we still want more in later stages, so if we are 10dB higher or lower than the optimal place it won't affect the final result, and we don't need to worry about the level of the signal at each point (which are a lot, really helps). Then the last step is to crunch all into those final 20dB which will take up signal and noise, but we worked which such lower noise and high dynamic range all the way noise doesn't matter at that point. 32bit floating point audio is used for this reason, it doesn't have any more quality than 24bit but it's much easier to work with, you'll never have a peak you won't be able to handle but you are not worried about digital noise floor. Working on 24bits if you are -40dBFS of your peaks your noise may start to suffer on the lower passages once you take the level up again. I'm careful enough to work with 24bits but it's a very good thing for people who isn't.JS

author: uncle_bob
author_rank: Unknown
timestamp: September 12, 2016, 01:31:41 pm
content: HiThere are a lot of other things done at or below 20KHz that have very little to do with consumer audio. They still get tested with the same gear and use a lot of the same specs. Each of those areas has its own quirks and limits. For some things 400 Hz is "high frequency".Bob

author: dom0
author_rank: Super Contributor
timestamp: September 12, 2016, 04:08:03 pm
content: QuoteWith some digital signal processing you actually can.

author: uncle_bob
author_rank: Unknown
timestamp: September 12, 2016, 05:00:48 pm
content: Quote from: dom0 on September 12, 2016, 04:08:03 pmHiYou can also fake yourself out ..... one stupid example is clipping (= both the input waveform and the DUT clip the same way). There are others. The approach works in general, but there are always corner cases where it may have issues.Bob

author: commie
author_rank: Frequent Contributor
timestamp: September 24, 2016, 04:01:34 pm
content: Hi Folks,As I said before, I'm analysing audio distortion.Here are some facts recently discovered...1. hp 331/2/3/4 automatic distortion analyser (average responding and using wien bridge for notch) designed around 1965 giving the most sensitive range of 0.1% fsd currently £200.00 on Ebay.2. hp 339A automatic distortion analyser (true r.m.s. responding and using twin tee as notch, contains its own oscillator) designed around 1977 giving the most sensitive range of 0.01% fsd. Cost from dealer ~£1000.003. There is currently high demand for hollow state audio amp kits or ready built, bringing with it a high demand for distortion analysers and low distortion oscillators.Current contemporary distortion analysers are mostly out of reach due to price.4. Hollow state amps exhibit about 0.7% distortion whilst solid state amps can achieve a factor of 10 or better producing distortion levels of around 0.05%.However, users of hollow state amps claim they sound better than ss amps albeit at higher distortion levels?5. I have found the best frequency notching filter arrangement to be the twin tee

author: uncle_bob
author_rank: Unknown
timestamp: September 24, 2016, 04:10:18 pm
content: HiWhat I have found is that the same distortion measurement gear that you can buy for $1,000 on a "buy it now" basis, sells for about $150 when they come out in bulk and are sold at auction. You pay a *lot* for the convenience of getting it right now compared to spending time shopping. You may indeed wait quite a while for the next batch to come out. You will save a lot of money ....I can buy an 8903B for $1200 right now with no problem at all. Neither one of the two I have cost more than $200 at auction on eBay.Bob

author: commie
author_rank: Frequent Contributor
timestamp: September 24, 2016, 04:22:06 pm
content: Quote from: uncle_bob on September 24, 2016, 04:10:18 pmActually Bob you are right and wrong, I live in the U.K. remember?, try buying a hp339a over here and in fact the hp339a I quoted was from a dealer fully furbished 100% working.Even buying from Ebay through an auction and making the assumption the analyser is based in the USA we, here in the U.K. are clobbered with import duties and vat.

author: ruairi
author_rank: Frequent Contributor
timestamp: September 24, 2016, 04:35:52 pm
content: You can do much better with a computer based solution and an external sound card/interface. I have a Prism dScope on the bench but when I'm on location in studio I'll happily use REW (Room EQ Wizard), a free program with a USB audio interface. It works wonderfully. It'll also do frequency response, phase, impulse response and lots of other good stuff. There are lots of other free or cheap options too - Audio Tester etc.When you dig a little deeper you'll discover that the outright THD+N figure is only a very rough guideline of audio quality, being able to view individual harmonics is much more useful. I've seen those HP analyzers come up for $150 locally and in my neighborhood and had zero interest (and I love dedicated hardware).

author: commie
author_rank: Frequent Contributor
timestamp: September 24, 2016, 04:56:03 pm
content: Quote from: ruairi on September 24, 2016, 04:35:52 pmI'm not sure feeding sonic waves into a computer for analysis is such a good idea. I have a RIGOL dg4102 digital oscillator claiming 0.1% distortion in its audio range, when I measure it I do get 0.1% distorton but the signal after the notch is full of spikes in the time domain.Quote from: ruairi on September 24, 2016, 04:35:52 pmNobody was claiming thd figures to be the holy grail however it can be used as a figure of merit, also when checking the spec. of any commercial amp, thd figures are always given but no reference to individual harmonics.Quote from: ruairi on September 24, 2016, 04:35:52 pmWhat do the individual harmonics tell you?

author: uncle_bob
author_rank: Unknown
timestamp: September 24, 2016, 05:41:51 pm
content: Quote from: commie on September 24, 2016, 04:22:06 pmHiI hear this a lot, "the only good stuff is not where I am". Oddly enough, when I go shopping, that's *exactly* what I find. All the good stuff at low prices is being sold in the UK and it will cost me a fortune to get it..... time to spend some more time shopping. It's not a function of location as much as *time*. If you don't have the time, you will pay somebody else to have it for you right now.Let's go through the numbers:8903B for $150 at auction.VAT at 20% (or whatever) is $30Shipping is $70 to $120 depending on where it's fromCustoms processing is $30Duty at 20% is $30Price delivered is $260 to $310. Still a *long* way away from the $1200 you pay somebody to have it right now.Bob

author: ruairi
author_rank: Frequent Contributor
timestamp: September 24, 2016, 06:41:29 pm
content: Quote from: commie on September 24, 2016, 04:56:03 pmI can't tell if you are joking here? Needless to say that almost every record you've heard in the last 20 years has been created by feeding "sonic waves into a computer". A reasonable audio interface will have low enough distortion to do meaningful work. Yes it will add it's own distortion, and has frequency response limits but it is way more useful than a cheap old hardware unit.QuoteThe specs provided on many audio products are either optimistic, or manipulated in ways to make them look better, for my purposes as a working audio professional for 20+ years they are effectively meaningless. Sure, they can be used for very broad stroke analysis but that's about all.We don't need to know the levels at each harmonic but trends can be very telling - more odd than even, only low order harmonics, lots of higher order harmonics. These can be correlated to actual listening experiences in a way that THD+N often cannot.QuoteSee above.The DIYaudio.com forum is a really useful resource for affordable distortion measurement techniques and information.

author: commie
author_rank: Frequent Contributor
timestamp: September 24, 2016, 08:20:39 pm
content: Quote from: uncle_bob on September 24, 2016, 05:41:51 pmHi Bob,Yep, I agree with what you have said, if I wait something viable should come my way.Cheers and thanks for talking to me.Commie

author: deadlylover
author_rank: Frequent Contributor
timestamp: September 25, 2016, 03:50:34 am
content: I recommend just making do with a cheapish sound card or audio interface until you find a good deal on used test equipment (or building your own).The 8903B is a bit...obsolete, so unless you just want one for fun (who doesn't? =P), I would steer clear unless you find one very cheap. Its residual of ~-95dB THD+N @ 1kHz is in the range of a sound card, so unless that's good enough for your needs then you'll quickly run into its limitations in modern audio.If you're a bit daring and willing to purchase overseas, look at Yahoo Japan auctions for an old Panasonic or ShibaSoku analyser. IMO they are the only 'old dogs' that hold up today in terms of residual distortion and noise performance. You can sometimes find them for around JPY30,000 before shipping, maybe add 20k yen on top for that, they're usually a bit under 20kg or so.My Panasonic VP-7722A was around $500 delivered and its residual is ~0.00023% (-112dB) THD+N @ 1kHz, 30kHz BW at a couple of volts or so.And if that's not quite good enough for you, the 2 decade old ShibaSoku 725B distortion analyser (no built in osc.) can do -116dB THD+N stock and about -120dB with a few easy mods, mainly op amp swaps and a ground rerouting. That's near flagship Audio Precision performance by the way.

author: Pjotr
author_rank: Frequent Contributor
timestamp: September 25, 2016, 10:15:52 am
content: It is not clear OP is up to: Designing and building audio gear/amplifiers, marketing amplifiers or just toying around?For designing amplifiers you actually don't need a distortion meter at all. Although it helps to see if you make progress in areas you are interestedNowadays THD levels between 1 ppm and 10 ppm for audio power amps are not a miracle and a sensitive spectrum analyser set-up is more of help --> AP etc. Although with a good PC sound card you can do also a lot.If OP is in the vacuum tube area where distortion levels between 0.1% and 10% are common place, then an ancient distortion meter from steel, alu and moving coil meters can do a good job. If he is in the solid state league of 1 - 10 ppm THD, better look for something more modern. It is amazing what a modern good soundcard can do then.

author: calexanian
author_rank: Super Contributor
timestamp: September 25, 2016, 11:30:27 pm
content: I got my HP 334A for next to nothing at a ham radio swap meet. Audio fools sell them off cheap because they generally don't know how to use them. The principal is you start with a known distortion free sine wave. The analyzer has a very very good filter in it you tube to remove the direct fundamental sine wave. Whatever signal is left therefore must be distortion and that remainder is measured. Pretty simple stuff, just very hard to implement in a DIY fashion. You can pick them up for very little because they are rather tedious to tune in to work and most people just think they are broken when in reality they are pretty much bullet proof.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 26, 2016, 12:12:05 am
content: First, read the Cordell article on building your own THD meter here-http://www.cordellaudio.com/papers/build_a_thd_analyzer.shtmlI built mine many years ago and it has a mid-band floor of about 0.00015%. Next, get a copy of Doug Self's power amplifier design book. He shows how to routinely build amplifiers at the 0.001% level. You might also like this thread on the 8903-http://www.diyaudio.com/forums/equipment-tools/295895-hp-8903a-audio-analyzer-mods-upgrades.htmlI use one because it's good enough for most things, but especially because it can be run over GPIB, which speeds measurements dramatically. I usually discover problems or areas for improvement just because it's easier to run routine tests you might not otherwise take the time to do.

author: Gary.M
author_rank: Regular Contributor
timestamp: September 26, 2016, 07:54:07 am
content: Quote from: Conrad Hoffman on September 26, 2016, 12:12:05 amSeriously guys, I already gave you the answer. Google QA401. It's a highly competent all in one Analyzer at an affordable price.Sent from my x600 using Tapatalk

author: Pjotr
author_rank: Frequent Contributor
timestamp: September 26, 2016, 09:26:46 am
content: Quote from: Gary.M on September 26, 2016, 07:54:07 amLooks like a hobbyist project USB sound card. But at 2x - 4x the price of a comparable off the shelf card.

author: uncle_bob
author_rank: Unknown
timestamp: September 26, 2016, 02:38:28 pm
content: Quote from: Pjotr on September 26, 2016, 09:26:46 amHiMake no mistake, it's not a hobby card. The converter on the board and it's support electronics (isolation etc) are first rate. My main knock on the product is that they don't really let you get at the data stream from the device directly. Simply put - you can't easily use it like a sound card. That limits it's utility in really nutty applications (say you want the even and odd harmonics summed up independently). I got very close to buying one and ultimately decided that software wasn't going to work for me.Bob

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 26, 2016, 03:39:23 pm
content: Similar results can be had nearly for free with a decent sound card and a copy of Visual Analyser, albeit with compromises, but good enough for the majority of needs. The point of reading the old articles is to get a better understanding of the measurement floors and how these things have been done previously. The Self book shows you how to build amps an order of magnitude better than in the past, as the OP doesn't seem to realize just how good one can do, or that measurements at that level can be done with a great degree of confidence. Seriously!

author: Pjotr
author_rank: Frequent Contributor
timestamp: September 26, 2016, 03:46:03 pm
content: Hi Bob,look at post#9 on the previous page for what I achieved with an off the shelf M-Audio Audiophile192 card (also with AKM DA/AD). I have modded it with better op amps and distortion became 6 db lower than the picture in post#9 (2ppm). It is similar to the QA401. The M-Audio card was cleared last year for under 80 bucks. Such low distortion is only that low a specific signal level. The QA401 will not differ to that.If you are at that distortion levels and want to measure lower you can't do without a really low distortion/noise separate sine generator and accompanying notch filter etc. That is not included with the QA401.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: September 26, 2016, 04:50:27 pm
content: Hi Pjotr,would you care to share details of your Audiophile 192 mod?There are many descriptions of Audiophile 192 mods. But mostly withoutgiving any measured data, like you do. I've been planning to modify the two Audiophile 192sI have been using for measurement purposes for quite a while now.I expect any Audiophile 192 mod to have only an influence on the measurementswith no audible impact at all.Yours Messtechniker

author: Pjotr
author_rank: Frequent Contributor
timestamp: September 26, 2016, 07:07:07 pm
content: Quote from: Messtechniker on September 26, 2016, 04:50:27 pmHi,I only replaced all 5532A op-amps by LM4562 op-amps. Nothing else. IMHO replacing electrolytic coupling caps with film ones doesn't have any use. There is virtually no AC voltage across them and almost no current through, so why should they distort?Your expectation about the sound is right, sonically I couldn't notice any difference. I doubt this mod is worth the effort, only 6 dB measurement improvement. IMO better use this really good op-amps to build an ultra low distortion spot sine generator. With a good notch filter and amplifying the residual distortion components improves measurement floor orders better.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: September 26, 2016, 08:22:41 pm
content: Hi,thank you for your hint. Just had a look at my Audiophile192k boards only tofind a grand total of 11 NE 5532 SOICs on each board. Since I have todate never soldered any SOIC chips, I will for the moment shelve thisproject and dig it out again should the need arise - which I doubt.Here once more the law of diminishing returns applies.Yours Messtechniker

author: echu
author_rank: Newbie
timestamp: September 26, 2016, 10:01:09 pm
content: Quote from: Pjotr on September 26, 2016, 03:46:03 pmYou can plug the QA401 directly into a 100W class D amp output and not worry about it blowing up. At all. It's fully isolated from the PC. No electrical path between the audio section and the PC interface.Sounds cards were a reasonable starting point, but there's much better out there today...http://www.quantasylum.com/content/Home/tabid/40/Post/283/First-Look-Evaluating-Class-D-Power-Amps-with-the-QA401

author: Pjotr
author_rank: Frequent Contributor
timestamp: September 27, 2016, 10:09:09 am
content: Quote from: echu on September 26, 2016, 10:01:09 pmDuhhh.........PC's that blow the rest of your equipment are likely to blow the user as well. Trash them immediately! PC's have to be grounded to a safety ground anyway. Although it is often ignored, even in a lab environment.B.t.w. the M-Audio card at hand has differential inputs and outputs. No grounding issues at all nor mains related noise. With those low noise levels a pitfall can be mains induced noise that comes from power cords coupling inductively into signal cables. Keeping them carefully separate helps a lot.

author: echu
author_rank: Newbie
timestamp: September 28, 2016, 06:31:22 am
content: QuoteM-Audio has differential IO, but the IO isn't isolated from the PC. There's a DC path directly from the audio IO to the PC ground. You really want and need the isolation.And how would the M-Audio do if you connected it to a class D push/pull stage being driven at +/-40V? Very likely, yes, that's right, smoke. And the lack of isolation means the 40V could kill the front end, jump to the ADC, then to the USB data part, and then into your PC. That is why you want isolation. Isolation is a 1000KV barrier between your DUT and your PCAs I said, the world has moved on from soundcards for audio analysis...---"I survived Typhoon MEGI and all I got was this shirt"

author: Gary.M
author_rank: Regular Contributor
timestamp: September 28, 2016, 07:12:37 am
content: Quote from: Pjotr on September 26, 2016, 03:46:03 pmQA401 includes a generator. Google up some loop back tests on it. Pretty good noise floor and very clean spectrum.Sent from my x600 using Tapatalk

author: Gary.M
author_rank: Regular Contributor
timestamp: September 28, 2016, 07:14:56 am
content: Quote from: Pjotr on September 27, 2016, 10:09:09 amYou guys must be very short of a meagre few dollars if you need to cobble together sound card solutions when there is such a good ready made option out there. The QA401 is targeted at factory test on production lines.Sent from my x600 using Tapatalk

author: Messtechniker
author_rank: Frequent Contributor
timestamp: September 28, 2016, 12:37:00 pm
content: You guys must be very short of a meagre few dollars if you need to cobble together sound card solutions ...Not really, because the sound card solution is more than good enough for aligning tape machines and measurements on audio valve amplifier equipment. Am using suitable input :10, x1 and x100 and output x1 and x10 buffer amps to prevent damage to the sound card. As a rule of the thumb the measuring equipment parameters should be better by a factor of 10 compared to the data expected form the unit under test This is true for the above uses cases.Yours Messtechniker.

author: 2N3055
author_rank: Super Contributor
timestamp: September 28, 2016, 12:57:09 pm
content: I think he want to say that when you put together price of good external audio card and handmade atenuator box and tek into account time needed to do it right, you will come close to 400 € which gives you fully tested and specified solution that woks out of the box. If you have time and like doing it yourself then go for it.. If you just need something to work right away 400€ is not bad..

author: JS
author_rank: Frequent Contributor
timestamp: September 29, 2016, 01:27:37 am
content: Quote from: echu on September 28, 2016, 06:31:22 amSorry, how many KVs? I take that as a typo...QA has some good stuff but there are a lot of things that can be done with just a soundcard, a lot! I've used my soundcards to do the task many times, of course you want to be careful with what you connect, the inputs are rated for audio gear, line level at most, not for power amps driving them directly. That being said, you don't want to blow your soundcard or your computer, so you double check before connecting, DC, ground currents/loops, etc. I guess that should be standard procedure anyway, poke with abefore running a full analysis and wandering specs.JS

author: technotronix
author_rank: Regular Contributor
timestamp: September 29, 2016, 07:51:29 am
content: You need a source for a signal that has a low distortion and then see the difference between signal and output. May be it is not 100% accurate.