Audio measurement gear

[RayDunzl]

Just maybe interested...... if you guys will assist me in learning how to make the proper tests.

The above answers are appropriate options for electrical testing.

For measuring in the air - I use a miniDSP UMIK-1 USB microphone (about $85) and REW software (free).

If concerned about the manufacturer's included calibration file, CrossSpectrum sells the UMIK with their own calibration file.

 

[Blumlein 88]

You can use any cheap usb soundcard if you want to have a play and learn about the measurements and techniques, not that there is that much too it . You will find plenty of people here willing to assist. However making really meaningful acurate measurements is another thing altogether. The major purpose of the above measurements is so that we can compare to Amirs Audio Precision analyser to confirm the QA analyser is really good enough for the job.

Yes a good suggestion for learning. Also, just for learning, the ADCs built into desktop or laptop computers aren't terrible. They typically fall down on noise floors. They have too much noise for some measurements. They are plenty good for learning techniques however.

 

[Wayne]

Yes a good suggestion for learning. Also, just for learning, the ADCs built into desktop or laptop computers aren't terrible. They typically fall down on noise floors. They have too much noise for some measurements. They are plenty good for learning techniques however.

Apparently I will need a good quality ADC to work in conjunction with the QA. How much would a ADC of similar quality -- specs that would allow the QA unit to measure to its maximum accuracy, but not exceed the accuracy of the QA unit -- cost?

 

[March Audio]

Apparently I will need a good quality ADC to work in conjunction with the QA. How much would a ADC of similar quality -- specs that would allow the QA unit to measure to its maximum accuracy, but not exceed the accuracy of the QA unit -- cost?

Hi Wayne, no you dont need an additional ADC, the QA is an ADC and DAC (signal generator) rolled into one.

 

[Blumlein 88]

@BE718
A suggestion on determining how much noise is coming from the QA unit. Measure some resistors. They have a known level of noise from thermal noise, Johnson noise, shot noise etc. So measure some metal films at different values. A mega-ohm is only -94 dbV while a 100 ohms is -134 dbV.

You probably have seen this handy calculator.
http://www.sengpielaudio.com/calculator-noise.htm

That and the noise floor of your DACs at different signal levels should let you parse out what is going on.

 

[amirm]

Hi

The test tracks I used are here:

http://alanmarch.hopto.org/sharing/tr2LJaFZp

That's great. I am downloading now. I will have to use one of my DACs as the source so its signature will be there outside of what Audio Precision does.

 

[March Audio]

@BE718
A suggestion on determining how much noise is coming from the QA unit. Measure some resistors. They have a known level of noise from thermal noise, Johnson noise, shot noise etc. So measure some metal films at different values. A mega-ohm is only -94 dbV while a 100 ohms is -134 dbV.

You probably have seen this handy calculator.
http://www.sengpielaudio.com/calculator-noise.htm

That and the noise floor of your DACs at different signal levels should let you parse out what is going on.

Thats a great idea. However it wont really establish if the rising noise floor with signal level is the adc suffering some kind of noise floor modulation effect. Im suspicious that its the QA and not the dacs, especially as Rob Watts at Chord makes a big thing about the fact that the Mojo suffers no noise floor modulation.

I'm going to have to build a passive notch filter to take out the 1kHz fundamental. This will help the distortion measurement and eliminate any potential noise floor modulation issue.

 

[March Audio]

That's great. I am downloading now. I will have to use one of my DACs as the source so its signature will be there outside of what Audio Precision does.

Cool. Ill PM you later regarding sending my explorer2 over.

 

[Blumlein 88]

Actually JA's test results on the Meridian Explorer 2 and the Chord Mojo look something like yours. He doesn't do a -60 db signal. He does do a -90 db signal. So looking at the noise floor of his near full scale 1 khz tone and his -90 db tone you see the noise floor move up and down dramatically by similar amounts to your results.

https://www.stereophile.com/content/chord-electronics-mojo-da-headphone-amplifier-measurements

https://www.stereophile.com/content/meridian-explorer2-da-headphone-amplifier-measurements

So you may have simply been incredibly lucky to get two DACs that both modulate the noise floor with signal levels and do so just about the same exact amount. You were really, really lucky.

Chord Mojo with -90 db 1 khz tone.

He doesn't show the full scale 1 khz tone, but here is a Jtest. See the noise floor is up 15 db or a bit more. It is similar in level with other high level signals he shows.

Here is the Explorer 2. The lower line is 24 bit and upper line 16 bit.

Here is the Explorer 2 with -6db 1 khz tone. Again the noise floor jumps up.

 

[March Audio]

Thats encouraging

Meridian Eplorer 2 Line out -90.31dB 16 bit and 24 bit

-6dB Headphone out (not loaded with 300 ohms)

 

[amirm]

Thanks for the white background. The black ones were very hard to see.

 

[Blumlein 88]

Thanks for the white background. The black ones were very hard to see.

Seems just the reverse to me. But if the consensus is people like the white background, I can do that in any future graphs I show.

@BE718 your results look almost identical to those of JA's AP measurements. Which is encouraging yes.

 

[amirm]

Seems just the reverse to me. But if the consensus is people like the white background, I can do that in any future graphs I show.

No it is not a matter of always liking one or the other. As you may know, I use black myself:

The key is amount of contrast. I think the measurements you all do are in much higher resolution window making the graph lines very thin. That reduces their contrast a ton. Even on AP, I highlight the one graph by clicking on it which makes it bolder/thicker.

 

[Blumlein 88]

So which is better here. I used a 1090x392 original image. So not such high resolution.

 

[amirm]

For me, white background.

 

[restorer-john]

In relation to the QA-410, I've been about to hit the 'buy' button for a while. Thanks BE718 for the superb photos- however they got me looking...

It appears there is a part they have not included for some reason. It's clearly supplied with 3.3v and looks to run around to the masterclock to the A/D along with another via going where? Ground trace on the backside most likely. The outline and spacing suggest a metal cased SMD crystal oscillator to me. Perhaps the Q101 oscillator is good enough?

I currently use old school gear, including a 1995 AudioLab setup (I need to keep a win95box alive for it- DOS-direct port access), along with a few computers running FFT (VA etc) using M-Audio Audiophile 24/96 PCI cards for aquisition. The QA-401 is going to be on my bench soon enough after passing on a nice R&S unit the other day (it was simply too expensive to justify).

So if a group buy is on the horizon, it could be a good option. My only issue is the 20V RMS ceiling. (I'm always slipping with my x10 switches on Cro probes) ad I do a lot of work on high powered amplifiers, with rails +/-80v+. For testing D/As and line stage levels it looks fantastic.

 

[March Audio]

Yeah I wondered about that myself. I drew the same conclusion that Q101 was adequate and they had possibly experimented with a lower phase noise metal can unit.

You can see the multiple measurements I have made with the unit around this forum. For the price its a no brainer. Although high quality sound cards can be a good option, tjis is better. If they release ASIO drivers, as mentioned on their website, it will become far more flexible.

Unfortunately I doubt you will get enough interest for a group buy, which is why I went ahead and just ordered it.

 

[restorer-john]

I think I will do the same. How well balanced are your two channels BTW?

 

[March Audio]

I think I will do the same. How well balanced are your two channels BTW?

Very close, 0.01dB ish. You can barely see the difference between the 2 overlaid channels.
The absolute levels in the plot below are obviously a combination of the sig gen DAC and the ADC in the 401. Combined error is possibly +-0.1dB

 

[March Audio]

OK, did it properly and dumped the data into excel. Biggest difference between channels is 0.02dB (6Hz to 24kHz)

Maximum deviation from flat (10Hz to 21kHz) +- 0.06dB and (5Hz to 22kHz) 0.26dB

File here
http://alanmarch.hopto.org:5000/sharing/9inuW7g4A