Focusrite Scarlett 2i2 Gen 3 Performance Measurements

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mediatechnology
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Focusrite Scarlett 2i2 Gen 3 Input Common Mode Rejection

Post by mediatechnology » Wed Dec 23, 2020 3:09 pm

I've been using the Focusrite Scarlet 2i2 as my test and measurement front-end and found a quirk that users should be aware of.

The left channel of my 2i2 has very poor common mode rejection:

Image

The left channel CMRR measures 13.5 dB; the right channel is almost 48 dB.

With the input cables shorted the difference in hum signature is pretty large:

Image

The input controls are at the 10 o'clock position which is unity gain.

The poor common mode rejection of the 2i2's Left input makes a THAT1646 balanced output appear noisier than it is.
Both channels are connected to the same 1646 output:

Image

The left channel measures about 13 dB higher because the common mode noise in the 1646 output is being converted to a differential signal by the 2i2's "bad" input. (FWIW a DRV134 or SSM2142 would show similar characteristics due to all three parts having high CM gain.)
The right channel is near the noise floor of the 2i2 - the 1646 noise is a few dB lower.

When the 1646 OutSmarts circuitry is defeated the common mode output noise drops and the noise measurements are close to one another:

Image

I don't know if I have defective 2i2 or not.
It seems hard to believe that component tolerance could be bad enough to make CMRR that low.

I've got a second 2i2 on order and am anxious to test it.

I would recommend everyone check the CMRR of their 2i2 inputs because it introduces measurement errors with active balanced outputs and has greatly reduced hum rejection.
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mediatechnology
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Re: Focusrite Scarlett 2i2 Gen 3 Performance Measurements

Post by mediatechnology » Thu Dec 24, 2020 1:20 pm

I received a second Focusrite Scarlett 2i2 and it has good common mode rejection:

Image

Left channel CMRR 68 dB; Right channel 58 dB.

I'll use the first unit in an unbalanced environment and use the "good" 2i2 on the work bench.
https://ka-electronics.com

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Directing Orson Welles: "What Is It That You Want?": https://waynekirkwood.com/content/Orson ... u_Want.mp3
Americas Frontline Doctors Lawsuit Motion For Preliminary Injunction Of Covid-19 Vaccine Emergency Use Authorization: https://waynekirkwood.com/images/pdf/Am ... zation.pdf

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