Measuring the iPOWER - Much Ado about Nothing (part 4 uploaded and complete)

[AMR/iFi audio]

The following is a 4-part paper we shall upload to Computer Audiophile and Head-Fi. Once complete, we will also pdf it for the iFi audio website. We hope you enjoy it and find it mildly enjoyable.

 

Table of Contents

Part 1: Precision in Audio

Part 2: Instrument Ghostbusting

Part 3: GSI-Ground Scene Investigation

Part 4: Let's measure the iPOWER down to 1uV

 

 

1. Precision in Audio

“O, what men dare do! What men may do!

What men daily do, not knowing what they do!”

Much Ado About Nothing (IV, i, 19-21)

 

So there we were, kicking back as England #Euro2016 had just scraped through to the knockout stages (albeit in 2^nd place) and reading the FT, covering the reaction to all this In/Out Referendum malarkey #Brexitandchill.

 

 

Suddenly several private messages/emails disturb our peace and chillout. Seems at Computer Audiophile and other places, readers noticed some threads about some measurements, which read; ‘Dear AMR/iFi, please address the measurements made by this person and look at the measurements made of the iPOWER by that other person.’

 

Frankly, we really don’t see what all the fuss is about as it is only one person taking measurements and another taking another set of measurements. One somewhat tallied with the published iPOWER numbers while the other didn’t. No big deal. Neither looked quite right, but in different ways.

 

 

The Art of (Measuring) Noise

Those who studied ‘Messtechnik’ (lit. the technique of performing measurements) during the ages of needle meters and before digital systems, FFT analysers etc. don’t lose sleep because X measured Y. They know there is a measurement error and it just needs to be tracked down.

 

 

The actual technical term for measurement results that show things are not really there in reality is ‘Instrument Ghost.’

 

No, it does not suggest actual demonic possession of an electronic component where it starts to levitate and spin 360 degrees whilst vomiting profusely.

 

 

No, it actually refers to something far more mundane; what is seen in the actual test results is not real, but a trick of lights; interference in the test system etc.

 

Synopsis:

With measurements of any sort, one needs to know the limits and pitfalls otherwise ‘false positive’ results can deceive.

 

 

AP changed the game or did it?

Back in what some might now view like the Middle Ages barely removed from the times of the Bard (think ca. 1985), making electronic measurements was a very serious craft. Many a test were far from straight forward. Carrying out a comprehensive performance test on an amplifier could and would occupy days on end. If one was unfortunate to find an Instrument Ghost, one could lose weeks of work chasing a non-existent ‘problem.’

 

Circa 1985, enter Audio Precision and their System One, a computer-based audio test system. The System One combined an integrated generator + analyser + connected PC to fully automate test procedures. This setup allowed a radically different visual presentation of results and to do so rapidly. Over the years, Audio Precision[1] has become the de facto standard for Audio Testing.

 

 

---

[1] The System One was soon followed by the System Two and now the System X (APX) series.

 

 

 

Does the fact that it is the industry standard for audio testing mean that an Audio Precision system is truly precise and will never show ‘Instruments Ghosts?’

 

 

Of course not. Under the cover sits the same sort of circuitry we had in the old, hard to use analyser; it is just that the use of the computer to control the test system and easy user interface simply hides the great complexity that is still there. Numerous opportunities for ‘Instrument Ghosts’ still exist.

 

Having used Audio Precision for the better part of over a decade here at AMR/iFi, we respectfully believe we have a fairly good grasp of what the limitations are and crucially, can tell if what we see is a real or perceived problem a.k.a. ‘Instrument Ghost.’

 

Synopsis:

The Audio Precision is just as fallible today as with earlier generation measurement tools.

 

One STILL needs to know, be aware of the limits and pitfalls otherwise the results mean little.

 

 

Next time, Part 2: Instrument Ghostbusting…

 

 

 

2. Instrument Ghostbusting

One look at the contentious test results we have been asked to look into by our readers instantly suggested that indeed what we were seeing was a distinct case of severe instrument ghosts. Holy water won’t work in this instance.

 

We commend those who have a stab at measurements. It is always easy to thrash the keyboard to within an inch of its life on forums about this person or that person but life is too short. Let’s all just chill and instead, share our knowledge/experiences;

 

One set of test results done using an older Audio Precision System was a prime example of one of the AP’s main limitations but unfortunately blame was laid at the door of the iFi product.

 

Somewhat worrying was some of the discussions showed people comparing the results of:

 

 

1. Two very different tests
   
2. Tests done by different people
   
3. Using very different test gear
   
4. …and with different scaling
   
5. …and with very different outcomes.
   

 

The poster then declaring blithely that the second set of test results confirmed the first. Whereas in fact, they clearly repudiated the first set of results and showed them to be instrument ghosts.

 

 

It seems that few today know those arcane disciplines of proper testing and how to spot Instrument Ghosts. They may or may not even know of the existence of Instrument Ghosts. Many try their utmost to make technical measurements but are sadly undone by an error here and an error there.

 

“If there’s something weird and it don’t look good, who you gonna call? Ghostbusters!”

 

Okay, we hear you ask:

“Yes, but what is the problem with the Audio Precision System then?"

 

Answer: Lack of galvanic isolation AND multiple earth connections.

 

Measuring Noise requires full Galvanic Isolation

In order to avoid ground loops when testing audio gear the AP features a galvanically-isolated output from the signal generator (which is fairly easy to do), but the Inputs are ground referenced and ground is linked to Earth. And the AP is linked to the Host PC WITHOUT Isolation. The PC usually has its own earth. This arrangement makes the AP system susceptible to ground/earth/mains-related noise.

 

That is not a problem per se for most tests; it only affects cases where we want to actually measure system noise and fairly low levels of this noise at that! Of course these issues are known and it is often necessary to experiment with ground and removing grounds; using isolation transformers to power gear under test etc.

 

At AMR/iFi we have a fairly complex setup with multiple isolation transformers separating AP2, Host PC and the Audio device being tested. Overall this setup massively reduced the tendency for instrument ghosts but took an age to develop and then extra time and a chunky budget to arrive at a completely safe, reliable package.

 

Any AP2 and PC just plugged into common mains does MUCH worse. We’ve been there, done that. Got the t-shirt and the key ring.

 

What we will do in this system, we shall see what the AP2 can do under those conditions and if we can provoke major noise problems WITHOUT touching the actual audio signal at all; but just by messing with grounds.

 

Synopsis:

Care and attention to setup the optimal environment is needed to rule out spurious noise sources. Even the expensive AP does not have FULL galvanic-isolation and is hampered by multiple earth connections.

 

One must address these before commencing any sort of precise noise measurements.

 

Next time, 3. GSI: Ground Scene Investigation…

 

 

 

 

3. GSI: Ground Scene Investigation

So, could it be that the very expensive Audio Precision system is susceptible to Instrument Ghosts simply by getting connections wrong?

 

Please bear with us as we run through a forensic examination of the ‘crime scene.’

 

 

 

Exhibit 1 – Ground Loop to Major Tom

For our first exhibit, we set the AP2 to output a 0.1mV (0.0001V) signal (also used in following measurements) and used a good quality cable (RG-58 plenum with BNC connectors) to link the AP2 output to the input.

 

As seen in the cyan trace below, this shows a FFT noisefloor at around 10-20nV (that is 0.00000001V to 0.00000002V) with no noise/distortion spikes higher than 50nV (0.00000005V). Compared to a nominal 2V DAC output we are looking at -152dBFS! Sterling performance. This is what one buys for five figures.

 

But wait, what if we take a clip lead, attach a crocodile clip onto the ground of our BNC Plug and connect the other side to the mains earth?

 

This would be like having a laptop feeding a USB DAC where the laptop power brick has a 3-pin mains plug?

 

Note, we have changed nothing whatsoever in the signal path. Our 0.0001V signal still comes from the AP2 generator via 1.5m of high grade coaxial cables. All we have done is simply added an earth wire…

 

 

This calls for an F-Bomb. $%#@! What is that?

As the yellow trace shows, our simple wire between ground and earth has added distortion at multiples of the 50Hz mains frequency with 50Hz being especially awful. It sits at around 7uV (0.000007V) instead of 50nV (0.00000005V), this is over 140 times more noise. The overall noise floor has risen nearly 5 times as well. Even isolating the generator output as shown in the green trace only drops the 50Hz noise slightly to 5uV, still 100 times worse than without that pernicious earth wire!

 

 

Exhibit 2 – Harmonic spray smells fishy

For our second exhibit we replace the wire to earth with a generic SMPS (we used one included with another manufacturer’s USB Repeater product as it was lying around) plugged straight into mains.

 

We just connect the power supply DC plug ground instead of our Earth. This is still totally separate from the signal! We have retained the cyan trace from before for reference. Will we get a major dose of noise?

 

 

As the green trace above shows, we sure do; not just 50Hz but we get an extended spray of harmonics all the way to the frequency limit.

 

The spiky noise / distortion components push the overall background noise up to around 25uV RMS noise, according to the AP2! Yup, that is 500 times more noise than the reference test and we have not touched the signal AT ALL!

 

 

Signs of ‘Para-electrical’ activity

There is one more key 'strange' factor in the green trace that is really noteworthy:

 

i. Peaks at 50Hz (mains frequency) and at 150Hz (3^rd harmonic) and at 250/350/450Hz and so on….

ii. but at 100/200/300Hz and so on, there is very little noise???

 

No EMF meter needed here; if it looks like one, walks like one, we would say this is a sure indicator of ‘para-electrical’ activity by instrument ghosts!

 

 

Applying common ‘electrical’ sense

 

• All power supplies work on rectified mains. If the noise originated with the power-supply – we would expect to see a lot of noise at 100/200/300Hz etc. but very little at 50/150/250/350Hz etc.
  

 

• Given it is the other way around we know reliably we are looking not at power-supply noise, but at ‘instrument ghosts.’ Now that we know this, can we bust these ghosts in any way?
  

 

BTT – Balanced Transformer Technology

In the FFT graph there is also a yellow trace. This shows only a little more noise than the reference test! This is actually exactly the same setup with the SMPS ground to the signal cable ground, but with one crucial difference. The SMPS is no longer plugged into the mains, but instead into a balanced isolation transformer. An inexpensive, off the shelf unit costing £25 was used.

 

The result is impressive. Noise is now around -146dBFS relative to a common DAC output. We can actually do something here people!

 

So, it would seem if we really want to measure noise with an AP, we need to take not some, but a lot of care and a little lovin.

 

 

 

Rehash to those in the know

This not ‘new news’ to those skilled in the electronic arts, but challenging to those who lack the necessary background and experience. There is no substitute for an EE degree AND decades of practice.

 

Further, measuring noise below 10uV (0.00001V) using common gear requires extreme care. We know of many setups that are useless below 1mV (0.001V) because of inherent and unresolved (and oft unrecognised) problems.

 

 

Synopsis:

The limitations of test gear are never covered in the user manual and it takes knowledge + experience to spot ‘false positive’ results. Care, care and more setup care is even more crucial than having expensive equipment to obtain the true, accurate and informed measurements.

 

 

 

Next time, Part 4: Let's measure the iPOWER down to 1uV…

 

 

4. Let's measure the iPOWER down to 1uV

Three outs in one week

It has been a turbulent few days of ‘outs’ for the UK - out of the EU, out of Euro 2016 and now the 3^rd strike is ‘out’: measurements of the iPower.

 

 

We’d be the first to admit the 3^rd is inconsequential. We hope everyone understands the lightheartedness surrounding the ‘measurements’ situation. Let’s all sit back, enjoy the tunes and #brexitandchill.

 

So without further ‘ado’ (we couldn’t resist) let’s out the iPower.

 

The 52%:48% question

The last question we have not yet answered in all this evaluation of how instrument ghosts can FUBAR AP2 measurements:

 

‘By Jove, we don’t want to know all of this – we only want to know how does the iPower REALLY measure?’

 

Had we not shed light on and validated our test setup and how we ensured accurate, measurable, very low-noise levels, we might just have plugged things together and gotten the green trace from the second graph in section 3. That would actually not have been the noise of the iPower, but the noise of poor ground management and of using test gear with a known sensitivity to noise.

 

Measuring the iPOWER

Now for the part you have all been waiting for, let us measure the iPOWER.

 

The lowest ground noise setup was used with a 9V iPower delivering 750mA current (into a 12 Ohm/50W resistor) and the signal on the +/- side of the resistor was fed into the AP2 unbalanced input.

 

 

The FFT noisefloor is around three times that of the AP2 itself at around 30nV (0.00000003V). We can also see that, as it should be for a power-supply, 100Hz is the highest peak at 3uV (0.000003V). And other high peaks are at multiples of 100Hz (not 50Hz…).

 

The rest of the noise is all below the 1uV line and of course, in-line with the stated specification.

 

 

Conclusion

1. If one sees much more noise than shown above 1uV in ones' measurements of an iPOWER

 

and

 

2. In particular, if 50Hz or 60Hz noise is much higher than 100/120Hz instead of being much lower, one's setup is being ‘haunted’ by Instrument Ghosts.

 

 

Thank you for bearing with us. Refill please kind sir.

 

[Richard Dale]

Much Ado About Nothing

 

The following is a 4-part paper we shall upload to Computer Audiophile and Head-Fi. Once complete, we will also pdf it for the iFi audio website. We hope you enjoy it and find it mildly enjoyable.

 

...

 

 

Thanks for posting this - a most entertaining read and I'm looking forward to the next parts!

 

I would also be interested in measurements of the 5 volt output of the iFi USB3.0 Nano - is it cleaner than the 9 volt iPower that powers it?

[sbank]

Thanks for posting this - a most entertaining read and I'm looking forward to the next parts!

+1 Thank you!

[Liam]

[MG32]

Is iFi and AMR suggesting that because inputs on the APrecision 2 are not isolated and the computer connection is also not isolated - this creates noise problems that would not exist with a normal HiFi system!?! can you explain

[AMR/iFi audio]

 

I would also be interested in measurements of the 5 volt output of the iFi USB3.0 Nano - is it cleaner than the 9 volt iPower that powers it?

 

Hi,

 

Yes -it is cleaner than the iPOWER because:

 

1. The iUSB 3.0 nano and micro include double-conversion of the power (which actually makes them quite resilient to what power feeds them) and in actual layout, we implement distinct noise barriers between incoming power and signal section.

 

2. Each output is then fitted with additional passive LC noise filtering and the ANC system to remove audio band noise.

 

The result is very low noise, even lower than the iPOWER itself. We have internally verified this performance previously using the AP2. But the measurements at this level are 'challenging' to say the least and we must delineate for the time being, due to lack of this essential commodity.

 

Cheers.

[Richard Dale]

Hi,

 

Yes -it is cleaner than the iPOWER because:

 

1. The iUSB 3.0 nano and micro include double-conversion of the power (which actually makes them quite resilient to what power feeds them) and in actual layout, we implement distinct noise barriers between incoming power and signal section.

 

2. Each output is then fitted with additional passive LC noise filtering and the ANC system to remove audio band noise.

 

The result is very low noise, even lower than the iPOWER itself. We have internally verified this performance previously using the AP2. But the measurements at this level are 'challenging' to say the least and we must delineate for the time being, due to lack of this essential commodity.

 

Cheers.

 

OK, thanks for the info, that's very interesting. I use iUSB Nanos in three different systems, and in one of them I'm driving a Bel Canto uLink USB to SPDi/F converter with a Nano, but powering it with an AQVOX linear PSU. It sounds like it would be worth listening to the Nano powering the uLink, even with it powering a Raspberry Pi too through the power only USB socket.

[Man in a van]

OK, thanks for the info, that's very interesting. I use iUSB Nanos in three different systems, and in one of them I'm driving a Bel Canto uLink USB to SPDi/F converter with a Nano, but powering it with an AQVOX linear PSU. It sounds like it would be worth listening to the Nano powering the uLink, even with it powering a Raspberry Pi too through the power only USB socket.

 

 

Richard, I have a iUSB Nano powering a Nano Dac.

 

Also used the usb power output to drive an Odroid C1+ running Squeezelite on Max2Play.

 

I have now connected a SBTouch instead of the Odroid and will also power the SBT from the iUSB Nano when I can find the cable.

 

Odroid and SBT sound pretty fine through the Nano combination.

 

atb

 

Ronnie.

[Paul R]

Great post! Love the humor as well as the great tech explanations in there.

[AMR/iFi audio]

OK, thanks for the info, that's very interesting. I use iUSB Nanos in three different systems, and in one of them I'm driving a Bel Canto uLink USB to SPDi/F converter with a Nano, but powering it with an AQVOX linear PSU. It sounds like it would be worth listening to the Nano powering the uLink, even with it powering a Raspberry Pi too through the power only USB socket.

 

 

Hi,

 

It should be worthwhile to draw the power for the uLink directly from the iUSB 3.0 nano. However the rPI might throw a wobbler.

 

Powering everything from the same supply may reduce ground issues, but some noise from the rPI may leak into the USB output, so it is not easy to predict which way this cookie may crumble.

 

As you already have the AQVOX PSU, you might try using it on the rPI and let the iUSB nano power the uLINK.

 

As it is easy to try, please do and report back.

 

Thanks.

[AMR/iFi audio]

Is iFi and AMR suggesting that because inputs on the APrecision 2 are not isolated and the computer connection is also not isolated - this creates noise problems that would not exist with a normal HiFi system!?! can you explain

 

Hello,

 

It is not quite as simple as that. But the short answer, if the hifi system is set up correctly, yes, then it will not be subject to this noise problem (neither would be the Audio Precision BTW, if care was taken in setting up the test).

 

With an Audio Precision + computer we have two devices that are directly electrically connected (via USB or AP's proprietary interface) and usually both have their own earth.

 

This condition fundamentally holds the potential for an earth loop. The provision in the Audio Precision is to have dedicated earth connections on each input and an isolated output.

 

If testing a single device, preferably with its earth lifted and the earth from the AP attached and all signals sources directly from the AP, then there is no problem with earth loops.

 

Where problems arise is, if for example the device being tested has its own earth or any other path (e.g. a EMC regulation required "Y" capacitor) from the mains into the ground.

 

 

The next question is...does this only happen with an Audio Precision and a PC or does it also happen in a HiFi system?

If the HiFi system is centered around a computer and has multiple pieces of equipment in it that all have earth, then yes, the same problems that affect the Audio Precision can affect this system, for the same reason.

 

If on the other hand, due care was taken in the design and setup of the HiFi system, then the problem that affects the Audio Precision setup will not affect the HiFi system.

 

Due care in this context means to ensure the HiFi system has ONE ACTIVE EARTH AND ONLY ONE EARTH AND NO LESS THAN ONE EARTH (this is not meant as trying to shout, we just wanted to be crystal clear!).

 

Any other condition in an audio/hifi system (or a measurement setup) may give rise to erroneous noise, that may be avoided by applying the correct setup.

[firedog]

Again, when do we get to see the rest of your paper and the actual measurements? Without the measurements, the rest of what you are writing is only great theory, and not a real answer to the claims on the other forum.

[AMR/iFi audio]

Again, when do we get to see the rest of your paper and the actual measurements? Without the measurements, the rest of what you are writing is only great theory, and not a real answer to the claims on the other forum.

 

 

Hi,

 

We have already shown measurements that illustrate the sensitivity of the typical Audio Precision setup to any form of earth loop or noise current flowing in the ground line.

 

Our aim in posting this lengthy paper (hence it has been divided into parts) was less so for a specific test, but more for the several out there - with varying and conflicting results we lack the time and resources to comment on all of them (and any future ones too). This is why we took the time to go in-depth and outline the backdrop as to how we go about measuring. And at the end, we wanted to explain how we arrived at our measurements for the iPower as this is our tried and tested methodology.

 

We felt it was important to illustrate the potential pitfalls in doing such tests, as well to provide tools to spot problems in measurements posted and a target result which they should be able to replicate, if tests are implemented and set up similarly.

 

This way those who wish to take measurements and who publish them have in effect a solid base line against which to compare and with which to replicate.

 

Cheers.

[Adyc]

Great post!! Learn a lot stuff. It shows there are a lot of people who think they are expert and in reality they have no idea of what they are measuring.

[SoundAndMotion]

Much Ado About Nothing

[snip]

Next time, Part 4: Let's measure the iPOWER down to 1uV…

 

Again, when do we get to see the rest of your paper and the actual measurements? Without the measurements, the rest of what you are writing is only great theory, and not a real answer to the claims on the other forum.

 

Hi,

 

We have already shown measurements ... [snip]

 

Our aim in posting this lengthy paper (hence it has been divided into parts) ... [snip]

 

Cheers.

 

Thanks for a thorough and useful post! I think what firedog was asking is when to expect Part 4. I'm also looking forward to the last part, for the complete description.

[AMR/iFi audio]

Thanks for a thorough and useful post! I think what firedog was asking is when to expect Part 4. I'm also looking forward to the last part, for the complete description.

 

 

Hi,

 

We have the draft and are working it. So in next day or so. We're more of a tortoise though we are on occasion, a hare!

[Liam]

One has to admire your patience and thoroughness. I am constantly appalled by behaviour on the Internet. Loose and often reckless, if not vindictive behaviour is common. Companies not alone have to navigate all that goes with running a business, but much is in danger of being damaged by some using the great resource of the Internet.

 

Once the megaphone statements are made it is difficult to calculate the residual damage to a Company, even if, as here, every effort of time and resources is made available to answer. There appears to be no appreciation by some of the potential knock on effects of their striving for a moment in the Sun. What ever happened to providing the courtesy of a private communication if there is something to be discussed?

[YashN]

Let’s all sit back, enjoy the tunes and #brexitandchill.

 

Haha!

 

Great write-up! Who's writing, Thorsten? Someone in the Technical Team?

[YashN]

Once the megaphone statements are made it is difficult to calculate the residual damage to a Company, even if, as here, every effort of time and resources is made available to answer.

 

Seeing how some mis-measurements have been propagated with sycophantic over-zealousness on a certain forum making a farce of science recently, I hear you totally!

[Paul R]

One has to admire your patience and thoroughness. I am constantly appalled by behaviour on the Internet. Loose and often reckless, if not vindictive behaviour is common. Companies not alone have to navigate all that goes with running a business, but much is in danger of being damaged by some using the great resource of the Internet.

 

Once the megaphone statements are made it is difficult to calculate the residual damage to a Company, even if, as here, every effort of time and resources is made available to answer. There appears to be no appreciation by some of the potential knock on effects of their striving for a moment in the Sun. What ever happened to providing the courtesy of a private communication if there is something to be discussed?

 

There is the other side of that too - not very many companies are like iFi you know. (i.e. honest, communicative, pricing stuff for a reasonable profit, helpful, engaged with the community, etc.)

 

Many companies put on a nice face, and then will do everything they can to maximize profit, including berating customers with issues, or questions. Right up to telling customers to f&(#-off! when they point out issues or problems. See it here on CA even, which is in general, far more civilized than most other places.

 

The most common thing you find is a sin of omission from companies like that. They will conveniently forget to tell you that they dropped the price in half, or decided they can make more money another way, or that this or that upgrade is like buying the entire product again, or you could get a nice discount by buying from say, Amazon. Or any of a 100 other little tricks like that - tricks which really skirt the edge of ethical behavior, even when they are perfectly legal.

 

The internet - to the great benefit of consumers - makes that kind of behavior very hard to pull off for most companies. They will almost always get caught. And in that case, they deserve whatever negative publicity they get. In the audio world, getting hit with piss poor build quality for "custom" audio gear is a not uncommon thing. But sine the person paid a grand or more for the device, and had to wait six to eight weeks or more to get it, they are disinclined to complain.

 

I do not think they should be.

 

I also think it would be nice if there was some kind of absolutely neutral arbitration available for cases like that. Not going to happen though, so the public stage of the internet is where some one these little drama's play out. It is a small price to pay to gain transparency, honesty, and a general sense of good value.

 

Again, iFi and AMR both exhibit the best possible behaviors, as do many other companies, large and small. You can tell that by, if nothing else, their internet reputation.

 

-Paul

[MG32]

Again, when do we get to see the rest of your paper and the actual measurements? Without the measurements, the rest of what you are writing is only great theory, and not a real answer to the claims on the other forum.

 

 

great to read these posts, at least they explained the background to those without a full knowledgeable background in this field like me. so far, I can at least try to follow. seems we all want things and want it now. this post was done 4 days ago and over weekend also. people today have no patience and demand others serve them now. a please or thank you would be nice so I will say it here - thanks ifi

[mmerrill99]

Thanks for the measurements, AMR

Can you clarify one thing, please? What are the details behind this - "The lowest ground noise setup was used with a 9V iPower"?

Does it mean that an iPower is plugged into a balanced isolation transformer?

[AMR/iFi audio]

Haha!

 

Great write-up! Who's writing, Thorsten? Someone in the Technical Team?

 

 

Hi,

 

Some peeps in the STS.

 

Chairman and Madam

 

They sit at the top of the company org chart and preside over things. Though on occasion, they do get off their furry butts and do some work like answer technical emails.

[AMR/iFi audio]

Thanks for the measurements, AMR

Can you clarify one thing, please? What are the details behind this - "The lowest ground noise setup was used with a 9V iPower"?

Does it mean that an iPower is plugged into a balanced isolation transformer?

 

 

Yes, we used an isolation transformer on the mains side. After all, we want to measure the noise of the iPower not that of the earth loop between AP2 and Host PC (otherwise, the Audio Precision analyser measures noise that is not really there; but the result of the ground-loop).

 

As a reference, we verify against our own audio test system (the iAnalyser) which we developed in-house for our use, not least due to our experience with the AP System and its limitations as well as its high $$$ cost.

 

 

The iAnalyser delivers:

 

- full galvanic isolation (where the AP2 does not)

- a low noisefloor (though limited to around 1uV noisefloor or < -130dB FFT Noisefloor)

- each unit is calibrated to 0.05dB against the AP2

 

It lacks the lacks the built-in gain and automation of the AP2. Hence, cannot measure noise as low as the AP2 can (AP2 reaches down to around 10...20uV FFT noisefloor.

 

The iAnalyer within its limits, measures similar noise levels WITHOUT the iPower being plugged into a balanced isolation transformer as the AP2 does with the balanced isolation transformer, due to having complete galvanic isolation!

 

This re-confirms again that with the AP2, we are dealing with ‘instrument ghosts’ caused by limitations in the AP2's inherent design.

 

As the AP2 can however show lower noise than the iAnalyser, we have illustrated the AP2 results for the iPower reaching 1uV, we just needed to make sure to setup the AP2 correctly to do so.

[Liam]

Just great to know the lengths you go to proving your specifications. Thanks.