Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Equipment & Tools From test equipment to hand tools

Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 24th December 2016, 01:24 PM   #1
OlegSh is offline OlegSh
diyAudio Member
 
Join Date: Dec 2010
Location: Germany
Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Default Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?

I've just got Focusrite Scarlett 2i2 (2nd gen) USB audio interface. The intended use is measurements of audio circuits. So far I've done basic loopback tests (posted here). Since it is the first time I measure audio signals I need some guidance where to start to make my results meaningful.

The first question is what kind of probes do I need? I have a couple of 1/4" TRS terminated cables to start with. Since the inputs/outputs of the 2i2 are balanced it probably makes sense to use balanced probes instead of single ended. Is there a recommended probe construction to start with? So far I plan to measure line level circuits and headphone amps. I may also measure power amps and speakers later which means I will need switchable probes with different ratios (1:1, 1:10 etc.). Also should I worry about proper probe's impedance termination?

Hope to have some pointers!

Merry Christmas!
Oleg
  Reply With Quote
Old 24th December 2016, 01:39 PM   #2
sangram is offline sangram  India
diyAudio Member
 
Join Date: Sep 2002
Location: India
Hi

It is probably a good idea to have some protection for your investment. This would involve using back to back zeners across the card's input and to common, to ensure your maximum levels are never exceeded.

Capacitor coupling for both input and output are a good idea to prevent amps with DC offset at either end sending it down the card and taking out the circuitry. A large capacitor (I use 80uF bipolars here) will ensure there is no sonic impact. The card can work with a variety of source and load impedance, no further corrections should be needed in your probes.

Finally, at the card's inputs you would be best advised to have some sort of rotary attenuation or selection to tailor in input to a variety of possible input levels - an amp at clipping will be putting out a lot more voltage than a preamp. To achieve all of this you can create a small jig with the required circuitry and have the card permanently connected to it. Then it can have binding posts and other terminals to connect various equipment. To begin testing attenuation must be maximum and then you can bring up the trimmer to the level you need for a good measurement.

Balanced inputs will always make more sense for rejection of 50Hz hum. However I'm not sure the Scarlett range uses properly balanced inputs, maybe pseudo-balanced. This works well for common-mode noise but does not reject the ground noise. In which case you might end up with harmonics of mains frequency in your results.

I looked at your other thread, and note that THD will rise with sample rate, this is a fact with all DACs. For most meaningful measurements you should be testing at 24/48.
  Reply With Quote
Old 24th December 2016, 01:48 PM   #3
OlegSh is offline OlegSh
diyAudio Member
 
Join Date: Dec 2010
Location: Germany
Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Thanks a lot sangram! I'll sketch a simple protection/attenuation circuit for the interface. I am also monitoring the autoranger thread by Jan Didden. Depending on the price it may be a solution for me too.

Regards,
Oleg
  Reply With Quote
Old 30th December 2016, 11:05 AM   #4
OlegSh is offline OlegSh
diyAudio Member
 
Join Date: Dec 2010
Location: Germany
Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
I just realized that I have already almost all I need for the input protection attenuator. I have a bunch of R-2R attenuator PCBs as well as input selector PCBs and other staff like arduino interface, encoders etc. I think I'll also build in a variety of loads for line stages and headphone amps measurements and control all of it by the arduino.

One question remains, what input impedance for the attenuator should I use? The 2i2 interface has rather high input impedance thus not to add more noise to the measurements I guess I should not go higher than 10k attenuator input impedance (which results in output impedance in the range of 0 to 4k), or should I select constant output impedance of let say 5k for the attenuator to keep the noise low? Or should I increase the input impedance up to 1M and use an op-amp buffer after the attenuator?
  Reply With Quote
Old 22nd January 2017, 03:50 PM   #5
OlegSh is offline OlegSh
diyAudio Member
 
Join Date: Dec 2010
Location: Germany
Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
I noticed that the sweet spot for the generator output of only -16 dBFS is severe limitation in measurements. If the output level is increased even a bit the THD rises so quickly that there is no way to test the amps at full swing. Therefore I have a question if anybody tried changing the line out op-amps to lower noise ones with higher max voltage swing before THD starts to degrade? As discussed here this interface probably uses NJM4565 at its line outputs and line inputs. May be OPA1612 would be a better option here with its lower noise and higher undistorted voltage swing?

Last edited by OlegSh; 22nd January 2017 at 04:13 PM.
  Reply With Quote
Old 22nd January 2017, 05:10 PM   #6
voltwide is offline voltwide  Ireland
diyAudio Member
 
Join Date: Jan 2010
Quote:
Originally Posted by OlegSh View Post
I noticed that the sweet spot for the generator output of only -16 dBFS is severe limitation in measurements. If the output level is increased even a bit the THD rises so quickly that there is no way to test the amps at full swing. Therefore I have a question if anybody tried changing the line out op-amps to lower noise ones with higher max voltage swing before THD starts to degrade? As discussed here this interface probably uses NJM4565 at its line outputs and line inputs. May be OPA1612 would be a better option here with its lower noise and higher undistorted voltage swing?
Are you sure distortion originates from line out and not from overdriven input amp?
  Reply With Quote
Old 22nd January 2017, 05:30 PM   #7
OlegSh is offline OlegSh
diyAudio Member
 
Join Date: Dec 2010
Location: Germany
Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
The output is around 0.2 Vrms at -16dBFS generator setting. At the same time interface calibration states that line input sensitivity is around 14Vrms (0dBFS). So it is unlikely to be input overloading, IMO.
  Reply With Quote
Old 22nd January 2017, 05:51 PM   #8
cwtim01 is offline cwtim01  Hong Kong
diyAudio Member
 
cwtim01's Avatar
 
Join Date: Aug 2012
The THD rise could be from the generator, the non-linearity of the ADC when dealing with large signals (doesn't need to be overdriven), or both.

I would try a 1Vrms signal from the generator, use a resistor voltage divider to drop signal to 0.2Vrms and measure. If the THD is better than measuring 1Vrms directly, then the ADC is contributing to the increased THD.
  Reply With Quote
Old 22nd January 2017, 08:11 PM   #9
OlegSh is offline OlegSh
diyAudio Member
 
Join Date: Dec 2010
Location: Germany
Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
Thanks! Good idea I'll try it.
  Reply With Quote
Old 22nd January 2017, 09:04 PM   #10
OlegSh is offline OlegSh
diyAudio Member
 
Join Date: Dec 2010
Location: Germany
Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?
I just measured the behaviour of the 2i2 generator output. I used my OPA1622 headamp with the gain of 2 to attenuate/amplify the output to the same level (seen by the 2i2 input). The resulting spectra are attached. If I set the generator to -16dBFS and amplify the output to -22.7 dBFS (seen by the 2i2 input) I get a small third harmonic indication but not more. If I set the generator output to -3dBFS and attenuate the level to -22.6dBFS (seen by the 2i2 input), I get relatively high second and third harmonics. I interpret it such that the generator is responsible for the rise of the THD at higher output levels.
Attached Images
File Type: png 3dBFSAttenuated.png (32.6 KB, 620 views)
File Type: png 16dBFSamplified.png (33.5 KB, 618 views)
  Reply With Quote

Reply


Got Focusrite Scarlett 2i2 (2nd gen) for measurements. What's next?Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
2nd gen 225hcca missing parts JBigDog Car Audio 5 15th September 2014 09:30 PM
Alpair 10M Gen. 3 on/off axis measurements spekr Full Range 18 4th August 2014 03:26 AM
CHR-70 Gen 2 vs Gen 3 mt490 Full Range 21 4th April 2014 12:21 PM
[IDEA] Focusrite Scarlett 2i2 hack forart.eu Digital Source 1 25th June 2013 04:22 PM
PUNCH 45 2ND GEN NON HD seankane Car Audio 4 22nd November 2011 12:57 AM


New To Site? Need Help?

All times are GMT. The time now is 04:36 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki