I'm in no hurry. Can work with a good guide for dummies, too.

Been re-reading on the 2i2:

http://d3se566zfvnmhf.cloudfront.net/sites/default/files/focusrite/downloads/7317/scarlett-2i2-user-guide-v2.pdf

The Line-In option offers much better specs for measurement than the Instruments option (which at -72dB THD seems quite horrible for low distortion DAC measurements), proly related to being lower gain. According to specs the best the Line-In interface can do is about -91 dB of THD+N which is about 0.0028%, which I think is not too bad if correct. But need to check how to tweak things to achieve that and if it's practical to do so.

I'm currently checking results with the ODAC and the interface seems to like 0.319 Vrms. I will see if I can relate that to the gain setting in the 2i2 spec for line-in, but it could be that this corresponds to 0 dB of gain (i.e. min gain) and if so, will run a 2i2 loopback test at this setting to verify claims.

I will also try to see separately what the 2i2 can do when configured for the tests performed here: https://www.quantasylum.com/content/Products/QA400/QA400Specs.aspx

It seems to me that most interfaces have a sweet spot for measuring. But I'm still getting up to speed.

For measuring things like a Clip+, I think the 2i2 can do pretty good, but not sure if it's upto snuff for certain low distortion equipment... yet.

If feasable, hopefully out of this a tutorial or whatever can be eventually put together to use the 2i2 for measurements, again, if feasable or at least to get the most out of it. Hopefully too, some of the things found can be applied to other interfaces.

Recently I posted some ODAC measurements using the 2i2 here:
http://www.changstar.com/index.php/topic,442.msg78440.html#msg78440

I do not believe these measurements are accurate, specially the 2.0 (0 dBFS) ones. I believe the problem comes down to the 2i2 (and other entry level pro interfaces) unbalanced line-in performance.

Consider the following loopback measurements done at 0 dBFS (1.14Vrms for the 2i2):

Unbalanced

THD 1kHz



SMPTE



Balanced

THD 1kHz



SMPTE



Clearly balanced shows superior performance. Stuff like this has been discussed elsewhere in the context of the EMU0404:
http://www.diyaudio.com/forums/equipment-tools/276823-usb-vs-pcie-sound-card-audio-analysis-14.html#post4415566

However, in my setup, it seems the different connections make a sizable difference, as was discussed here:
http://www.changstar.com/index.php/topic,1163.msg31210.html#msg31210

In the Vali thread the unbalanced measurement was at -7 dBFS, while here it is at 0 dBFS. Unbalanced performance improves as the input levels hit somewhere around 0.3 Vrms, but are worse as voltage levels increase. For the ODAC measurement, 0 dBFS is about 2.0 Vrms and measurements would be completely off IMO. To get the most of this interface w/o modifications I would recommend to measure using levels close to 0.3 Vrms which is of course a severe limitation (and even then performance will not be all that great).

So I do not currently recommend the 2i2 for accurate measurement of low distortion unbalanced DACs and Amps. Can help get an idea though.

A QA400 is likely to yield better results. Getting an AP, SR or D-Scope is proly even better. But we are going from a $150 do-it-all interface, to a $300 dedicated setup, to kilobuck gear. And my audio piggy bank is currently drained. Will have to wait.

Derp, I forgot to post my measurements last night. I have measurements for the ODAC revB both on the Scarlett and ZxR. The latter looks much better. All of my measurements on the ZxR have kinda low dynamic range compared to what they claim, so I think I'm doing something a bit wrong there.

I'm also planning on bringing my gear home from work to verify my measurements.

What software are you using for your measurements? I've just been using REW (Room EQ Wizard) for earphones and RMAA (RightMark Audio Analyzer) for amps/dacs.

Derp, I forgot to post my measurements last night. I have measurements for the ODAC revB both on the Scarlett and ZxR. The latter looks much better. All of my measurements on the ZxR have kinda low dynamic range compared to what they claim, so I think I'm doing something a bit wrong there.

I'm also planning on bringing my gear home from work to verify my measurements.

What software are you using for your measurements? I've just been using REW (Room EQ Wizard) for earphones and RMAA (RightMark Audio Analyzer) for amps/dacs.

I acutally use REW for Amps/DACs also. One could use ARTA as well, but it was easier for me to get things going with REW. I basically have two crap laptops (one over 8 years old). One runs REW as a source and the other one runs REW as a line-in receive end. The latest REW has distortion numbrs on their Real Time Analyzer, but you have to set the source and the receiving end generators the same, even if the receiving end is not being used as a source. I'm not certain if the distortion numbers work with non-standard exitation (SMPTE and so on), but you can certainly get a plot and do a dB to % conversion by hand.

Like Marv said, be very careful about RMAA. NwAvGuy was right to question it. You can get it to work but it's a bit of a pain and there is little flexibility in the freeware version. Not sure if there is a paid version that is more flexible. But I can tell you that I had a terrible time getting it to work with the 2i2 using ASIO at high rates. REW should run ASIO fine at all rates on the 2i2 and likely the 2i4. Furthermore, you want to use ASIO all around. It makes a difference in your results. For the ODAC, you can use ASIO4All or something like that. It took a little work, but I got that to work in the end. ASIO makes a difference in the ODAC performance.

You should be able to do all of the measurements that Marv linked. The agreement was to use 0.55 Vrms for Amps, but for DACs you may want to do 0 dBFS (full-scale) or maybe -1 dBFS (if DAC is clipping badly for x or y reasons, or even less if your interface has issues at those levels like my 2i2). Usually 0 dBFS this is about 2.0 Vrms or so. But some other DACs may go lower or higher. For example, the 2i2 is about 1.14 Vrms I think at 0 dBFS. The reasons for using 0 dBFS is that in many cases the DAC does not have volume control and are indeed run full scale (digital volume control all the way up) while the amp in the chain takes care of gain (pretty, shinny, sexy volume control knob).

Furthermore, don't use loads with DACs. A DAC will usually see a high impedance anyway. That loading stuff applies to Amps which have to deal with headphone/speaker crazyness.