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Howto - Distortion Measurements with REW
Howto - Distortion Measurements with REW
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Old 15th June 2019, 12:01 AM   #91
enochRoot is offline enochRoot  United States
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Any idea why the signal level on this is showing 70db or so? I can worry about the noise later (probably a fan plugged in on same circuit). Just wanted to figure out how to have my main test signal around 0.

EDIT: Why is it scaling my image down?

Click the image to open in full size.

Last edited by enochRoot; 15th June 2019 at 12:04 AM.
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Old 15th June 2019, 12:12 AM   #92
6L6 is offline 6L6  United States
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Howto - Distortion Measurements with REW
What are you measuring? How is it hooked up into the input?
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Old 15th June 2019, 12:21 AM   #93
enochRoot is offline enochRoot  United States
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Just a loopback to test the device (Behringer UM2). Output connected to input.
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Old 15th June 2019, 01:50 AM   #94
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
Quote:
Originally Posted by enochRoot View Post
Any idea why the signal level on this is showing 70db or so? I can worry about the noise later (probably a fan plugged in on same circuit). Just wanted to figure out how to have my main test signal around 0.

EDIT: Why is it scaling my image down?

Click the image to open in full size.
Change your vertical scale to dBFS. Then it will be relative to max of 0dB.
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Old 15th June 2019, 11:21 AM   #95
enochRoot is offline enochRoot  United States
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Thanks!! I wondered if that was the problem. Took forever to find where to do it though.

Now, if I wanted to measure a phono stage (which hopefully will show the limitations of my setup)...would I just crank the output way down on my interface box so as not to overload it? Any idea what dbFS level would approximate the sub mV range? Or is this not a good idea?

And on a side note...does anyone know why the system would be sizing down my images?


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Originally Posted by xrk971 View Post
Change your vertical scale to dBFS. Then it will be relative to max of 0dB.
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Old 15th June 2019, 10:45 PM   #96
Mark Tillotson is offline Mark Tillotson
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For a phone-stage I would use an external passive attenuator with an output impedance matching typical cartridges for meaningful noise results.
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Old 20th June 2019, 07:57 AM   #97
Dave S is offline Dave S  United Kingdom
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Quote:
Originally Posted by xrk971 View Post
That might be your problem. You have a mono 6.35mm TS plug. What you want is a stereo TRS plug ...
I am now getting very similar THD measurements to yours.


Thanks!!
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Old 21st June 2019, 04:28 AM   #98
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
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Originally Posted by Dave S View Post
I am now getting very similar THD measurements to yours.


Thanks!!
Good to hear that the differential input TRS or XLR is important in getting the best measurement. Thanks for letting me know that it worked!
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Old 21st June 2019, 04:32 AM   #99
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
The rather tall "grass" we see at the noise floor is actual noise of the noise. When I used a 4uVrms noise TPS7Axxx voltage regulator as part of my headphone amp, the grass got a lot shorter. Interesting, how everthing matters. This was a super low noise, complicated PSU consisting of a Class 2 12v 1000mA wall wart, which then went to a CLC, then a DC-DC converter, a 4uVrms noise LDO linear regulator, a cap Mx, then a CRCRC, then to the amplifier.

Click the image to open in full size.

Click the image to open in full size.
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Last edited by xrk971; 21st June 2019 at 04:36 AM.
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Old 22nd June 2019, 10:43 AM   #100
poseidonsvoice is offline poseidonsvoice  United States
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Howto - Distortion Measurements with REW
X,

Interesting. So are you saying the DUT should have a low noise supply or the Focusrite unit or both?

Best,
Anand.
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