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Howto - Distortion Measurements with REW
Howto - Distortion Measurements with REW
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Old 13th October 2021, 08:59 PM   #1031
phofman is offline phofman  Czech Republic
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Behringer UM2 is a USB fullspeed (12Mbps) soundcard. It sends/receives data every 1ms. IMO that is the 1kHz signal you see in your spectrum, very typical for fullspeed soundcards. The measurement signal with this card should differ a bit from 1kHz so that the results do not overlap with the 1kHz USB interference.
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Old 14th October 2021, 12:03 AM   #1032
trobbins is offline trobbins  Australia
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xrk971, that shows how well many cheap soundcard interfaces can do the job as a basic audio tool (ie. allow practical assessment of distortion products and any other signals down at a comparatively low noise level) without having to be aesthetically pleasing to the eye with a subterranean and flat noise floor with no extraneous signals poking out. I'd even expect that loopback using its own DAC output would be similarly as good with REW, especially for distortion testing as harmonic cancellation can be deployed and a test frequency away from 1kHz can be used.
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Old 14th October 2021, 04:44 AM   #1033
xrk971 is online now xrk971  United States
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Howto - Distortion Measurements with REW
I even tried using it as a DAC and the music coming out of it isn’t half bad. For $30, it’s got a lot going for it. There’s really no excuse saying measurement gear costs too much anymore.
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Old 19th October 2021, 06:16 PM   #1034
drinkingcube is offline drinkingcube  United States
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Finally got this working using a Focusrite Scarlet 2i2.


I think it was you, xrk, which suggested that you need to use T and R, not T and S, on a TRS for the input to the interface. After many attempts and finally reading the documentation *VERY* carefully, it seems the Scarlet 2i2 automatically configures itself to Line Input when you use a TRS (not TS) jack.



I was not able to get good distortion measurements using the direct out of the scarlet, decided to use my iPhone's DAC, seemed to work fine.



I did not use an attenuator, so max power I was able to send into this thing I believe is:


P = ( 9 * 9 ) / 8 = 10 W


It was fun playing around with the different gain stages: 2i2 preamp input, phone DAC, amplifier preamp.


Anyway, I am pleased with the results. Thanks for all the info in this thread!
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File Type: png distortion_1W.png (51.4 KB, 305 views)
File Type: png distortion_10W.png (51.6 KB, 303 views)
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Old 26th October 2021, 05:06 AM   #1035
kozard is online now kozard
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Quote:
Originally Posted by wtnh View Post
It is amazing how fast they seem to get snapped up on eBay. I can't find any either.
If someone needs one for measurements there is one E-MU 0404 USB presently on eBay with box and packing materials:

creative e mu 0404 | eBay

I got mine a little cheaper but they didn't have the box or anything like that.
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Old 2nd November 2021, 07:37 PM   #1036
Peppe is offline Peppe  Europe
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What an amazing thread this is. I have played around myself with recycled parts using my wife's old MacBook (still has the CD-ROM!), an old Echo Audiofire 4 (firewire sound card) that has collected dust for 10 years or so and a case I had around where I placed four 8 Ohm / 100 W resistors (unknown chinese stuff), few switches and the Aktika-style attenuator. BNC goes to the oscilloscope, TRS to the soundcard.

I know that the resistors are not ideal and will be replaced at some point. However, I have been able to chase and learn more about hum and buzz for instance. Quite revealing has been the measurements of the Aleph P1.7 where the BAL connection has almost eliminated the 50Hz peak.
Attached Images
File Type: jpg 2021-05-23 22.34.56.jpg (732.0 KB, 163 views)
File Type: png 2021-09-25 - Aleph P 1.7 BAL FFT.png (95.1 KB, 163 views)
File Type: png 2021-09-25 - Aleph P 1.7 SE FFT.png (99.1 KB, 160 views)
File Type: jpg 2021-11-02 19.43.48.jpg (634.1 KB, 89 views)

Last edited by Peppe; 2nd November 2021 at 07:47 PM.
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Old 2nd November 2021, 09:02 PM   #1037
danny_66 is offline danny_66  Belgium
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I've been using the Tascam US-1x2HR for measurements and it's doing its job very good
Here's the selftest
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File Type: gif Tascam US-1x2HR 192k.gif (162.1 KB, 97 views)
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Old 3rd November 2021, 06:05 PM   #1038
Alexium is offline Alexium  Ukraine
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Trying my EMU 0404 USB as a measurement device for the first time, just plain loopback for now with a single stereo cable. Having some issues.
1. Got it working with RMAA, yesterday the best noise floor I could get was -86 dB. Tried the exact same thing today and it's instantly down to -112 dB, multiple measurements with different settings - not a single bad one, only minor variation. Any idea what the problem could be? Bad cable / connection, perhaps?
2. While RMAA works fine, I don't understand what REW is doing and can't get it to work. Why no signal in one channel?

Untitled.png
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Old 3rd November 2021, 06:11 PM   #1039
audio_tony is offline audio_tony  United Kingdom
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Quote:
Originally Posted by Alexium View Post

Why no signal in one channel?
You only have the right channel selected for output. Change it to "L+R"
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Old 3rd November 2021, 06:13 PM   #1040
audio_tony is offline audio_tony  United Kingdom
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Quote:
Originally Posted by Peppe View Post

I know that the resistors are not ideal and will be replaced at some point.
I use those resistors and they work well for me. But you should consider attaching them to heatsinks.

Also - to keep noise low, you potential divider should be relatively low impedance.
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