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Howto - Distortion Measurements with REW
Howto - Distortion Measurements with REW
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Old 12th June 2019, 09:38 PM   #71
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
Quote:
Originally Posted by elwood625 View Post
Like this?
Yes! Click on the 4 arrow cross to adjust the span.

Your PSU looks like what mine used to look like when I reported the “Forest of Noise” issue. Couple things may be going on here: you have power trafo EMI interence being coupled into the Edcor and amplified, your power trafo may be saturating due to dirty mains having DC component, and/or there are ground loops. Having 120Hz mains component only -55dB below your signal is quite audible.

But THD and profile looks consistent with what I recall.

Click the image to open in full size.
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Last edited by xrk971; 12th June 2019 at 09:41 PM.
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Old 12th June 2019, 10:49 PM   #72
elwood625 is offline elwood625  United States
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Here is my latest loopback. I can dial down the noise by adjusting the Akitika to 1.5 vac output and the Behringer UM2 with the Line Gain adjuster turned 90'.
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File Type: jpg Loopback 1.JPG (174.4 KB, 177 views)
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Old 12th June 2019, 10:54 PM   #73
elwood625 is offline elwood625  United States
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I'm using 4 ohm resistors for loading both outputs, with my Fluke DMM on AC voltage, I was measuring 2 vac to the resistors. I adjusted the Akitika to get the 2vac output from the amp. Is this a correct adjustment to measure 1 watt at 4 ohms?
If the DMM measures RMS, then I'm off if peak to peak is the correct measurement.
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Old 13th June 2019, 12:06 AM   #74
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
A DMM in Volts AC measures RMS assuming sine wave.

P = V^2/R so 2Vrms ^2 /4ohms = 1w

The M2X was probably not meant to drive 4ohm loads. You might try 8ohm resistor and adjust for 2.83v rms. Which is 1w rms into 8ohms.

Your loopback looks a lot better.
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Old 13th June 2019, 09:04 AM   #75
suresh is offline suresh  India
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XRK how about using Siglent SDS1202X-E 200Mhz oscilloscope for measuring the FFT plot. any advice much appreciated.

here is the link for the oscilloscope Siglent SDS1202X-E 200Mhz
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Last edited by suresh; 13th June 2019 at 09:06 AM.
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Old 13th June 2019, 10:21 AM   #76
BamboszeK is offline BamboszeK  Poland
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Scopes are good just for basic FFT. I doubt you will get better performance than -60dB THD
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Old 13th June 2019, 11:40 AM   #77
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
Hi Suresh,
O-Scopes are different specialty tool. They are crucial for visualizing time varying or transient events. They generally have extreme speed but lower resolution. Typically 8bit or maybe 10bit ADC’s. So their resolution in terms of noise floor vs signal is not going to be very deep. With averaging turned on, maybe 50dB to 60dB range.

RF Spectrum Analyzers May work better if they have a higher resolution ADC. An audio spectrum analyzer like the one from SRS like SR1 is perhaps even better than what PC sound interfaces can do.
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Old 13th June 2019, 02:42 PM   #78
Dave S is offline Dave S  United Kingdom
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Quote:
Originally Posted by xrk971 View Post

Update June 3, 2019: Here is a loopback using Focusrite Solo gen2 and Akitika 2ppm 1kHz oscillator at 1.0Vpp into 10k:
Click the image to open in full size.
How is it possible to get 0.00033% THD when the Solo is spec'ed at 0.003%?

Using a Victors oscillator I am getting 0.0033% THD using a 1st gen 2i2 which has a THD spec of -91dB (about the same as 0.003%) at minimum gain

I've followed the instructions and settings carefully but wonder if I am doing something wrong?
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Old 13th June 2019, 03:58 PM   #79
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
The amplitude that the oscillator has to drive and the output impedance of the oscillator has an impact on the value. Make sure you are not asking an opamp of to drive a low impedance load at too high of an amplitude. 1Vpp is 0.353Vrms.

How I got a number better than factory spec? Lucky unit?
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Old 13th June 2019, 04:47 PM   #80
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
Maybe it’s not just all luck? I measured 0.00061%THD with my 2i4 and this was a headphone amp driving 47ohms - not a loopback.

Click the image to open in full size.

Careful attention to good quality cabling, clean connections, a quiet laptop (mine has a SSD). Mostly, make sure you are not driving any more amplitude than needed from either the oscillator or the Focusrite front end preamp.
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