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Audio Interface Suitable for Measurements

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RayDunzl

RayDunzl

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Looking at the links posted above...

Will try with no ground.

Might try across both +/- of the amplifiers (they have differential output)

Will have to think about that, don't like bad surprises.
 

Blumlein 88

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Do you have any kind of microphone so you could use REW via the 202HD? If I'm understanding you rightly you've not done that have you? You've been putting the speaker/amp leads into the 202HD. I think it is partly a connection issue, but the ADC of the 202HD is likely not as good as the DAC.
 
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RayDunzl

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Well...

I guess it doesn't like "unbalanced"...

Here's the balanced out from the preamp:

1558059924401.png


The fat 2nd harmonic is gone.

It raises a question in my little mind, though:

"How is it I can exceed digital full scale?"
 

Blumlein 88

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So do you think you need another interface now?
 

March Audio

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Well...

I guess it doesn't like "unbalanced"...

Here's the balanced out from the preamp:

View attachment 26351

The fat 2nd harmonic is gone.

It raises a question in my little mind, though:

"How is it I can exceed digital full scale?"

You cant :) Mine goes red when I hit 0dBFS

Possibly something to do with setting a "calibrated" level" so FS = a particular voltage (1.414) in your screenshot.

...but then again that doesnt make sense - your ADC 0dBFS is what it is, thats just putting a scaling factor against it..........mmmmmm.......!

1558060930781.png


Thats interesting, thats my Logitech webcam microphone. It clearly is brick wall filtered at 8 kHz
 
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March Audio

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You cant :) Mine goes red when I hit 0dBFS

I think you have set a "calibrated" level" so FS = a particular voltage (1.414)

View attachment 26353

Thats interesting, thats my Logitech webcam microphone. It clearly is brick wall filtered at 8 kHz

Good old Windows mixer had limited it to 16000Hz sample rate. This is better

1558061686506.png
 
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RayDunzl

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RayDunzl

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...but then again that doesnt make sense - your ADC 0dBFS is what it is, thats just putting a scaling factor against it..........mmmmmm.......!

Yeah...

Hmm...

The calibration is in REW, so, take it from there.
 
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RayDunzl

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The calibration is in REW, so, take it from there.

Now I try to calibrate the Behringer ASIO input and it says "Soundcard calibration is not applicabe when using a USB microphone".

May have upgraded REW to a new version along the way... Shows I did, yesterday... v5.20 beta10
 

JohnPM

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Those level controls appear to be just scaling the digital domain data. Maybe lowering the gain knob for the measurement channel would help. Or maybe it's just broken :)
 

JohnPM

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Now I try to calibrate the Behringer ASIO input and it says "Soundcard calibration is not applicabe when using a USB microphone".
That will probably be because the UMIK was previously selected when using the Java drivers. There was a spell when the mic name wasn't appearing correctly in the ASIO4All driver so REW is checking to see if it appears in the Java drivers, that seems fixed nowadays so I'll change it to only check the current input to see if it is a USB mic.

Calibrating doesn't have any effect on the 0 dBFS level though. I wonder if the ASIO driver is returning data outside the -1..1 range? The limit for -1..1 data is 3 dBFS if using "full scale sine is 0 dBFS" or 0 dBFS otherwise. Need a square wave to hit the max.
 
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RayDunzl

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Well... Using a balanced output on the preamp...

Setup:

REW at PC 1khz @ -3dB ->
Generic USB cable ->
Topping D10 Optical ->
Switch ->
miniDSP OpenDRC-DI @-9dB ->
Optical ->
Benchmark DAC2 HGC ->
2ft 18awg magnet wire, unshielded, signal twisted, untwisted ground, XLR ->
Krell KCT preamp 70/151 volume ->
30ft generic XLR ->
UMC202HD with 10dB pad and minimum input knob level ->
25 foot generic USB ->
PC second instance of REW


1558323851928.png


I guess that satisfies my immediate curiosity about the preamp distortion.

Any higher levels at the input to the UMC202HD, the harmonics start rising, I blame the UMC202.

The 70/151 preamp level represents normal listening. Wide open 151/151, have to attenuate more elsewhere, else the harmonics rise at the UMC202.

All that low frequency stuff is UMC202 noise.
 
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RayDunzl

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Ooh...

I beat the new $9500 Pass Labs XP22 preamplifier on distortion with my old Krell at a reported 0.00035% (probably someplace along the green line).

Maybe.

1558327387253.png
 

Interference

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I have come across the same question lately.

The RME ADI-2 Pro would be a nice piece of gear for the purpose, but not properly cheap.

As for cheaper options, despite 15+ years of age it's still hard to beat the E-MU interfaces. The 0404 USB is the most versatile, the 0204 USB should have same performance but less features. Not only they have excellent objective performance but they provide one of the most extensive spec sheets I have seen for an audio interface (that allow you to better assess the intrinsic limits of the ADC). The PCIe 1212M and 1616M are even better, but you need to have a dedicated PC.

There should be no drivers available for Windows 10, although being "mature" products they seem to work flawlessly on Linux (where you can run REW).

I spent quite a lot of times around spec sheets of sub-200 EUR interfaces (Behringer, Focusrite, Steinberg) but spec sheets are underwhelming and often insufficiently detailed. It's surprising that E-MUs are still so hard to beat.

Of course if anybody knows something that can compete I would be happy to look it up ;)
 
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RayDunzl

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I measured an amplifier powering a speaker.

Can't swing past 3.22Vrms/4.55Vpk without overloading the Behringer ADC and making the reported harmonics go nuts.

Maybe that's not too far below this site's 4Vrms/5.656Vpk amplifier standard measure.

Note that that voltage is the differential between the + and - amp outputs, and both channels driven.

The Behringer input is fully attenuated and 10dB pad applied.

0.0036%, -88.9dB on the 3rd harmonic... looking clean as far as anything else I can see with this ADC.

Good enough for now, I suppose.


1558502147002.png


At least it looks cleaner than the BehringerA500

1558502793039.png
 

Blumlein 88

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I measured an amplifier powering a speaker.

Can't swing past 3.22Vrms/4.55Vpk without overloading the Behringer ADC and making the reported harmonics go nuts.

Maybe that's not too far below this site's 4Vrms/5.656Vpk amplifier standard measure.

Note that that voltage is the differential between the + and - amp outputs, and both channels driven.

The Behringer input is fully attenuated and 10dB pad applied.

0.0036%, -88.9dB on the 3rd harmonic... looking clean as far as anything else I can see with this ADC.

Good enough for now, I suppose.


View attachment 26527

At least it looks cleaner than the BehringerA500

View attachment 26529
I would think there is a good chance the ADC is still the limit here. But if around -90 db is good enough for your purposes then you don't need more. Lots of interfaces can take up to 7 or 8 volts at the line inputs. Backing off to a standard 4 v rms would get a clean result with those.
 
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RayDunzl

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I would think there is a good chance the ADC is still the limit here.

Looks that way.

Another tenth of a volt from the amp and the harmonics shoot way up to like -30dB or so.

As with the preamp measures, the speakers verify that it isn't the playback gear.

At least I've finally been able to verify to my satisfaction that I'm not listening to junk and just convincing myself it's good.

So the interface is real noisy below 1kHz, has an odd slope up above 1kHz. Not ready for prime time, but sufficient to fool around with. Let it be another "cheaper isn't always better" reminder.

I'll live with it some more, and watch for a good deal on a better device.

Thank you everyone for the suggestions above.
 

6speed

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How about taking 2 measurements.
  1. Loopback
  2. With DUT inserted
Then just subtract the two to get the distortion of the DUT alone.
 
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