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Review and Measurements of QuantAsylum QA401 Audio Analyzer

QAMatt

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#42
Hi @Amir, thanks for the analysis! I'm scratching my head a bit on the measurement with -60 dBr (relative to 2V) because that noise floor seems lower than if input was shorted. But I can report back in 24-48 hours. Is there a chance you could make another measurement if I had a clear request? Thanks Matt
 

amirm

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#43
Hi @Amir, thanks for the analysis! I'm scratching my head a bit on the measurement with -60 dBr (relative to 2V) because that noise floor seems lower than if input was shorted. But I can report back in 24-48 hours. Is there a chance you could make another measurement if I had a clear request? Thanks Matt
Hi Matt. Of course, just ask.

For now, the signal it is analyzing is from my APx555, not its own internal generator.
 

tomchr

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#44
Thanks for the review. -110ish dB THD is about what you'll get from a good external sound card. The Focusrite Scarlett will get you there.

I generally don't mind custom controls ... if they add value or functionality. Sadly, that doesn't seem to be the case here. That's unfortunate.

Tom
 

audio_tony

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#45
Thanks for the review. -110ish dB THD is about what you'll get from a good external sound card. The Focusrite Scarlett will get you there.

I generally don't mind custom controls ... if they add value or functionality. Sadly, that doesn't seem to be the case here. That's unfortunate.

Tom
I have a Scarlett 2i2 (version 2) and I can't get much better than -100dB out of that.
 

tomchr

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#46
You can always attenuate the fundamental with a notch filter (passive twin-T, for example). That'll buy you an additional 60-80 dB of dynamic range.

Tom
 

QAMatt

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#47
>> Hi Matt. Of course, just ask.

Awesome and thanks!

Some notes/observations from various comments in the thread:

> I am not a fan...The rotary controls for example show no values.

Yes, agree it could be better. But it's not without precedent as this is true for modern desktop scopes too: You have a free-spinning hardware knob, and you must look to the display for feedback. The ability to hover over a knob and change it with the mouse wheel or finger slide is pretty is tried and true and borrowed from DAWs (Digital Audio Workstations) where you are looking at a screen (mixing console) with hundreds of knobs and sliders. The point about context menu I agree with as it's a big source of confusion that will get fixed. It'll move to a tap-and-hold so that you can do it from a tablet one-handed. And yes, probably we shoudl add right click for instant activation. That's a very good point.

> Telling the AP to analyze itself resulted in [THD+N of] 116 dB. So we have quite a shortfall there compared to much more expensive AP.

Yes, for sure. Roughly, the economics of this space are about 10 dB improvement costs you about 10 dB in price :) While the QA401 THD+N sweet spot is around -100 dB, you'll more commonly see 90 to 95 dB in most areas of use because it has a single front-end relay per channel with just 2 gain ranges. AverLab will do ~15 dB better on THD+N (I think 9 gain ranges). But it's about $3K versus the QA401's $450, which is 16 dB more expensive. And AP will hit about 25 dB better than the QA401 in THD+N, but it's approaching $30K versus $450, which is 36 dB more expensive. Now, at each step of the way, you get more features included with that higher price (such as 1 MHz bandwidth, digital IO, etc). So these aren't hard and fast numbers. But they hold as a general rule: Performance costs.

The aim of the QA401 was to build a low-cost product with a solid API and performance that was good enough to measure 95 to 99% of consumer and pro-audio equipment out there (and if you look at volume of products sold, the 99% figure most certainly holds).

> Our playback equipment has gotten so good that it requires very high-end measurement gear to handle it.

Yes, so true. We're in an era where a top-end audio analyzer cannot reliably measure the THD+N in a modern op-amp that costs $2. For example, the OPA1612 has typical THD+N of -136 dB, which is quite a bit better than what the best measurement gear today. Even a headphone amp like the OPA1622 has THD+N performance so good that it can't really be reliably measured until it's driving 10's of mW. This doesn't mean devices built from these parts can't be tested, it just means you cannot test them in one shot.

> [Regarding max input level: ] 56Vp-p looks too low even to test [email protected] amps. Is there any mistake about this number, maybe 56VRMS yet?

+26 dBV is the max in. In practice it's not generally an issue because it's so easy to build attenuation into your load. For example, here's the QA401 measuring a class D proaudio amp advertised as 1000W:

https://quantasylum.com/blogs/news/basics-power-amps

> So that would let you use it with REW, Virtins MI or various other software. Sounds like a good addition.

Indeed! The Asio driver Edechamps has created has proven rock solid for me.

> I have read that using QA401 as both generator and adc, measurements are quite better due to synchronization. Does it make sense ?

In some ways, yes: When the ADC and DAC are running from the same clock (as on the QA401), your jitter on both DAC and ADC are the same and thus idea.. Inside the attached pdf are a few slides speculating on the phase noise of the QA401 contributing to the rise of the input noise. But it's an extremely complex topic. But soundcards can do this too.

> Thanks @amirm that confirms my own experience

There are likely 3 dominate mechanisms at work here. I guess you could lump them all under the "noise modulation" banner, but they aren't. The ~21 dB gap can be explained 3 contributors: ADC change in noise floor versus input level (8 dB) (and this could be loosely considered noise modulation), the killer noise floor of the AP generator (5 dB), and perhaps 8 dB of noise floor increase due to phase noise. And the last 2 likely interact in a way that's more complex than has been presented in the pdf.

But walk through the pdf when time permits and let me know what you think. Yes, the effect is the same: A change in noise floor depending on level, but the causes are pretty mundane, present on all analyzers when measuring DUTs that are substantially better than the analyzer, and readily compensated for--they needn't be treated as a blackbox under a single label.

Some notes on the PDF:

Pages 1 and 2 map the Amir-measured data to the factory data on that particular unit. No surprises there, but note the lower generator noise of the AP hints at how the THD+N curve might look if the QA401 had a comparable DAC/generator. It would still be limited by the at the top, but clearly, at lower levels the DAC is limiting today. This is the red line on slide 1. So, as Amir noted, the top-end is limited by the ADC and the lower-end is limited by the DAC

Slide 3 looks at the noise floor gap on the QA401 in loopback and also wiht the input shorted. Note how the gap grows with the input shorted.

Slide 4 looks at Amir's measurements

Slide 5 looks at AKM measurements on QA401 ADC/DAC

Slide 6 summarizes the likely sources of the various gaps

Slide 7/8/9 get more speculative, and delves more into phase noise.

Note that if part of the gap is explained by phase noise, then putting in a better reference oscillator would likely reduce the noise, but it wouldn't reduce the THD+N because the THD+N is dominated by the harmonics. The QA401 has pads for a Crystek low-phase noise oscillator if anyone wants to make the switch. Over the coming months I'll see if I can better quantify what that part could do.

Certainly this isn't definitive, but hopefully some interesting background. If anyone has questions or sees something that doesn't make sense, I'll try to answer/correct.

Thanks again, Amir!
 

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tomchr

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#48
The ability to hover over a knob and change it with the mouse wheel or finger slide is pretty is tried and true and borrowed from DAWs (Digital Audio Workstations) where you are looking at a screen (mixing console) with hundreds of knobs and sliders.
It's still not an intuitive interface, though. A round knob is operated by turning it, not by sliding a finger across it (or rolling a wheel forwards/backwards). It's a clear mismatch between the expected action and required action.

Tom
 

dreite

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#49
I've been turning actual knobs and wheels on test equipment for decades, but you get used to the QA401 software interface very quickly.
But as noted, it helps greatly to use a wheel mouse, IMHO.

Dave.
 

QAMatt

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#50
It's still not an intuitive interface, though. A round knob is operated by turning it, not by sliding a finger across it (or rolling a wheel forwards/backwards). It's a clear mismatch between the expected action and required action.
Yes, true, but for better or worse that ship has sailed long ago. The days of a menu item for each settings have given way into grouped sea of controls, with mouse wheel playing a big role in selection and actuation. Take a look at the video below on how scroll wheels get used on ProTools. This is an app used by people that create music--they are using it for 12 hours a day. It's a lot of hovering over UI elements and using the scroll wheel with modifier keys (shift, control, etc) to perform actions. On the QA401, if you press CTRL while turning the wheel the knob moves 10X faster. If you press ALT while turning the wheel the knob spins 0.1X faster.

Visually the controls might look like knobs, but think of them more as easy-to-hit targets (much easier than standard UI elements like a dropdown list) that change value when the wheel is turned. It's really very comfortable after its explained. But yes, on first use, it is hard to figure out.

 

Rja4000

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#51
Just purchased one.
I measure 104dB SINAD (source is RME Pro fs).
SNR at full scope is 113dB, but HD3 is -109dB.
SINAD for RME loopback at same level is almost 113dB.
Probably my power supply is too weak.
I need a bit more time to really test more...
Stupid question: is there a way to use the DAC spdif as a clock source (through ext clock)?
 
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Dana reed

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#52
So looking at this UI, it looks very much like the national instruments LabView GUI. Does the vendor offer the ability to access this directly through LabView source code, or is it only via automation of their application's UI.
 

QAMatt

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#53
So looking at this UI, it looks very much like the national instruments LabView GUI. Does the vendor offer the ability to access this directly through LabView source code, or is it only via automation of their application's UI.
Hi Dana, with the QA401 app (written in C#), the API interface is DotNet remoting. It's really easy if you are writing in managed C++, C# or VB. But if you are writing in a language not inside the MSFT "managed" languages bubble, then it's not so easy. Now, LabView has the ability to load a managed DLL and make calls into that code, so never say never. But I think you'd need to be very experienced at both to make it work.

There's a headless (console only, no GUI) version of the QA401 app called QA401H, and that interface is REST. The QA401H also runs on Linux/Mac and Windows. The best part of the REST interface is that just about any language out there can make REST queries (including LabView, Matlab, C, etc). Below is a link showing how you can set up and make a THD measurement in 5 lines of Python, pretty much out of the box.

REST is a way of using HTTP POST, GET and PUT to do things.

Longer-term, the QA401 will pull in all the QA401H REST interfaces.

https://quantasylum.com/blogs/news/qa401h-rest-and-python
 
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