Looking for a better way to measure my line-level and amplifier projects, I decided to investigate some USB prosumer interfaces. Rather than options like the QA401 with its required software suite, the Keithley 2015 with uninspiring THD specs, or HP 8903 with a footprint and compatibility penalty, I wanted something small, flexible, and with performance good enough for tube audio. A USB audio interface will require voltage dividers for many measurements (whereas the lab equipment usually allows a higher Vrms input), but recording interfaces are inexpensive and flexible with software. Also, I don’t have the play money for an AP or dScope rig.
These were all measured back to back on the same laptop with latest drivers and the same unbalanced cables. The same -1db level was used for all interfaces to get a relative distortion/noise baseline. RMAA doesn’t necessarily give an absolute and repeatable spec, but it is good enough for relative comparisons. All interfaces were measured several times; the displayed specs capture the “average” performance (calculated by eye).
- The MBox 3rd Gen is an obvious winner in just about every regard. It was also a much more expensive interface when it was new. MBoxes are no longer produced, but used interfaces aren’t difficult to find. This would be a decent basis for THD measurements of amplifiers (my intended use).
- The M-Track 2×2 did rather well (as much as I hate to admit it) but doesn’t have two identical channels for this kind of thing (one TRS and one combo jack with mic pre). Wouldn’t recommend it for measurements for that reason.
- The old Fast Track Duo (Avid branded bu made by M-Audio) blew chunks. Can’t rule out that my unit has some kind of issue.
- The AudioBox USB looks good but there is a cross-talk issue. Possibly grounding with the unbalanced cables. The knobs are also too fiddly for fine adjustment in my opinion.
- I really wanted the iConnectivity to perform the best here. In my opinion it’s the nicest piece of hardware. Unfortunately, the relative measurements don’t make it the best choice. It can be run from a 9V supply rather than the USB bus though and I may try that to see if there’s any improvement. No supply handy for this test.
- The AudioBox 44VSL does pretty well (this is what I had been using for measurements). It also requires a 12V external supply and is a larger 4 mic pre interface, making it a little less convenient for a bench-top test setup. The 22VSL is smaller and may measure just as well (don’t have one to play with).
Here’s after some fine tuning the MBox levels in REW (sampling rate set to 96khz):
This is close to the -110db THD Avid spec’d. All in all, I think I can live with the MBox for a while for my testing. Although all the caveats of RMAA and testing conditions/methodology apply, performance is on par with some specs I’ve seen on the cheaper audio analyzers and definitely a cut above the other USB interfaces here.
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