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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
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Old 22nd January 2017, 10:43 PM   #11
cwtim01 is offline cwtim01  Hong Kong
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Agree.

On the other hand, at -3dBFS level, a THD of 0.0005% (-106dB) seems not so bad, even for a good sound card. The output with the OPA1622 has a THD of 0.0004% (-108dB), so better but not by far, and THD+N are about the same.
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Old 24th January 2017, 06:23 PM   #12
OlegSh is offline OlegSh
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
I agree that looking at the numbers the difference is minor but looking at the difference in terms of present/absent harmonics above the noise floor is encouraging. I'll try to further amplify the output to reach higher absolute levels and see if it brings further improvements.

Since it is now clear that the problem is with the output level, the next question is which parts are really responsible for the THD rise: are these the output op-amps or the D/A chip itself? I guess there is no easy way to test it without swapping the op-amps. I am fine to experiment with my unit but I have no idea how to open it. The 1st generation interface has screws at the back and TRS sockets have fixing nuts but the new unit has no visible screws or nuts to start with. Does anybody have an idea where to start opening?
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Old 24th January 2017, 11:00 PM   #13
cwtim01 is offline cwtim01  Hong Kong
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Have fun searching the sweat spot with the right opamp.

In my case with my own sound card, the sweat spot requires a specific output level and input level, as both the generator and the ADC are contributing to the THD, so there is an optimal compromise.
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File Type: png Loopback THD.png (49.3 KB, 420 views)
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Old 24th January 2017, 11:05 PM   #14
peufeu is offline peufeu  France
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Subscribing, because I'm interested in a photo of the board inside this soundcard.
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Old 25th January 2017, 06:22 AM   #15
OlegSh is offline OlegSh
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Quote:
Originally Posted by cwtim01 View Post
Have fun searching the sweat spot with the right opamp.

In my case with my own sound card, the sweat spot requires a specific output level and input level, as both the generator and the ADC are contributing to the THD, so there is an optimal compromise.
Your result is spectacular. What sound card are you using if I may ask?
My Focusrite 2i2 showed its best when generator out is set to -16dBFS (around 0.185 Vrms) and input gain is increased to -16dBFS(it's 0dBFS is at around 14Vrms). Even then the THD I get is around 0.00029 %.

I also researched a little more about the AD/DA chip CS4272 (which people claim is inside) and indeed it might be the DAC which is responsible for the THD limit.

Anyways, I would be interested to look inside of my unit but as I wrote earlier, I have no idea how to open it

Last edited by OlegSh; 25th January 2017 at 06:24 AM.
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Old 25th January 2017, 06:35 AM   #16
wintermute is offline wintermute  Australia
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Hi Oleg, I found with the Focusrite 2i2 first gen that it is important to output to a balanced input to get good distortion performance, for input from SE source do not tie ring to sleeve just connect tip and ring.

I made up a circuit on breadboard using a couple of LM4562's to covert the balanced output to SE and that helped a lot.

The way to tell if it is necessary is to do a loopback with full balanced TRS cables and then do another loopback with cables that are either TS to TS. ie single ended output to single ended input. If the performance drops substantially then you ideally need both balanced to SE for output and SE to balanced for input, though as I said doing TS on the input with the balanced to SE on the output worked quite well for me.

Tony.

edit: attached the circuit I did for the balanced to unbalanced 0.1% resistors advised. This certainly helped with the gen1 2i2 it may not be necessary with the gen2...
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File Type: png bal_to_unbal.png (17.1 KB, 408 views)
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Last edited by wintermute; 25th January 2017 at 06:47 AM. Reason: add pic of balanced to unbalanced circuit
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Old 25th January 2017, 06:44 AM   #17
OlegSh is offline OlegSh
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Thanks a lot Tony! I'll resolder the connectors since I recently made a couple of balanced to SE cables to use with the 2i2 interface and, indeed, tied ring to sleeve... I also noticed significant amount of mains harmonics if using my bal-SE cables, so I'll definitely give your suggestion a try. Going all balanced is another possibility.
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Old 25th January 2017, 06:52 AM   #18
wintermute is offline wintermute  Australia
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
It seems like the correct thing to do but certainly on the gen1 it drops the performance substantially!

I used 12V SLA batteries for the powersupply on the balanced to unbalanced converter, note that the other advantage is it doubles the output magnitude, so effectively 6db more signal

Tony.
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Old 25th January 2017, 07:00 AM   #19
OlegSh is offline OlegSh
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
I think gen 2 is not much different from gen 1, except that there is no obvious way to open the unit anymore

6dB extra is always a good thing I have an assembled low noise PSU, a bunch of quality op-amps and suitable small PCBs (see my signature), so I'm nearly ready to try bal-SE and SE-bal route. I've also started putting together an attenuator for the 2i2 input, so all these bal-SE/SE-bal can go in there.
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Old 25th January 2017, 08:01 AM   #20
wintermute is offline wintermute  Australia
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Note I did put 100nf decoupling caps on the opamps in the real world implementation

One difference looking at your graphs is the noise floor. definitely lower than my gen1. Below are the first measurements I took (not optimized) comparing balanced loopback with SE loopback. As you can see the distortion goes through the roof with the SE.

Also attached is measurement made with the balanced to SE circuit with with se to TR cable (third pic) you can see the improvement! Note that the third one was done with different levels probably after I had more idea what I was doing Unfortunately I can't find a straight balanced loopback done at the same time as the third pic, but the fourth one is probably more representative of the real performance...

Tony.
Attached Images
File Type: png balanced.png (22.4 KB, 403 views)
File Type: png unbalanced.png (22.4 KB, 319 views)
File Type: png 2i2_distortion_test_bal_to_se.png (22.6 KB, 316 views)
File Type: png 2i2 loopback_distortion.png (18.7 KB, 128 views)
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