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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 7th November 2017, 09:15 PM   #51
OlegSh is offline OlegSh
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
An afterthought: there may be a start-up/shutdown transient which requires AC coupling. I should probably test it to be sure.
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Old 8th November 2017, 01:01 AM   #52
wintermute is offline wintermute  Australia
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Hi Oleg, I did a sim, and it certainly looks like you can add the gain you want direct to the converter without needing a separate gain stage.

See the attached. It's a great idea, and I will try it myself on my breadboard when I get a chance. 0.2V in 2V out

Real world of course won't be as good as the sim, but the sim indicates that distortion should be minimal.

It sounds like they've created a virtual ground, there must be a dc-dc circuit to as usb only supplies 5V . Will be interested to hear the results of any mods you make!

edit: note also that the 5.6p was to stop some peaking in the sim around 20Mhz (with the original circuit). The version with gain does not have the peaking so I think it could be eliminated.

Tony.
Attached Images
File Type: png bal2se.png (41.3 KB, 276 views)
File Type: png bal2se_ac.png (19.6 KB, 275 views)
File Type: png bal2se_fft.png (21.0 KB, 272 views)
File Type: png bal2se_log.png (24.1 KB, 266 views)
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Last edited by wintermute; 8th November 2017 at 01:19 AM. Reason: add comment on 0.2v in 2V out.
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Old 8th November 2017, 06:09 AM   #53
OlegSh is offline OlegSh
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Great info, Tony!

The only issue with the added gain is the increase in the noise floor, but since the harmonics level relative to noise should stay the same I guess it won't be too bad. I'm mostly interested in THD rather than THD+N since real noise is impossible to measure with such type of interface.

Anyways, I'll play with this idea and see how far I get with it. I will also try to short the output caps to DC couple the outputs and see if it improves anything. Also, since the line input impedance is 52k I can probably replace the electrolytics at the inputs with 1u~2.2u polyester film caps which fit the lead spacing if AC coupling is a must on this interface. The value of the AC coupling capacitors at the outputs are probably sized such that even if someone connects 600 Ohm loads the low end roll off does not become too severe. But I guess the distortion at low end will skyrocket in this case.
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Old 8th November 2017, 06:47 AM   #54
OlegSh is offline OlegSh
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
I checked my parts bin and found a couple of LME49990, LME49720, OPA1612 and OPA1642. I guess I will not waste LM49990 on this but am thinking about LME49720 or OPA1612. The datasheet of the OPA1612 suggests that the THD+N at frequencies above 3kHz goes down significantly with added gain (see figs. 7 and 9). I will also try to keep the input impedance low by using 5 times lower resistor values than you used in your simulation, just enough to arrive at 2k load per leg.

Regards,
Oleg
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Old 8th November 2017, 07:56 AM   #55
wintermute is offline wintermute  Australia
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
I'm actually using LM4562's The 49860 in the sim is basically the same. Just having a look at the LME49720 and it looks pretty much identical (from the brief specs on the first page) as the LM4562, so it should work fine

I didn't think about the noise floor. Once I breadboard I will have an idea of what effect there is, as I don't know if the noise floor is limited by the output or the input of the 2i2.

the opa1612 also looks good, and yes it seems it performs better with 2K (or higher I guess) load and gain of 10+

Tony.
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Last edited by wintermute; 8th November 2017 at 08:03 AM. Reason: add comment on opa1612
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Old 11th November 2017, 06:51 PM   #56
OlegSh is offline OlegSh
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
After a bit of testing I can say that electrolytic capacitors at the inputs and outputs are not a problem at all. I tried shorting them (DC coupling) and haven't noticed any change in measured THD and noise floor. I also found a new sweet spot (see attached). The front panel controls are as depicted, the output of the generator is set to -8dBFS in REW, 96kHz sampling rate, balanced loopback connection. This results in measured THD of only 0.00016% which I find very good.
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File Type: jpg FronPanelControls.jpg (561.7 KB, 230 views)
File Type: png Focusrite.2i2.Loopback.BAL.png (141.6 KB, 121 views)
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Old 11th November 2017, 06:55 PM   #57
OlegSh is offline OlegSh
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
For clean measurements at higher frequencies the input level should be no more than -20dBFS.
Attached Images
File Type: png Loopback.6kHz.BAL.png (132.1 KB, 119 views)
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Old 11th November 2017, 11:10 PM   #58
wintermute is offline wintermute  Australia
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Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
looks good Oleg
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Old 22nd February 2018, 05:29 PM   #59
flyingfishtw is offline flyingfishtw  Taiwan
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Just bought gen 1, How's the test going?

I just measured -8dbFS, it's about 235mV. so that means the maximum input for 2i2 is 560mVrms?
The front panel I just tried, I think it's gain control for pre-amp, turning it up will only add more noise to measurement.

it's blinking red around 560mV, but arta predict around 800mV rms will be peak input.
Dose that mean when I measure noise floor, it will be referring to 800mV?
So -100dBFS will be 8uV?

I did some test with SE, it was miserable. lucky that I saw this thread

Last edited by flyingfishtw; 22nd February 2018 at 05:43 PM.
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Old 22nd February 2018, 07:17 PM   #60
flyingfishtw is offline flyingfishtw  Taiwan
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Silly me, I measured hot pin, so -8dbFS and full swing should times 2.
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