Read the asterisk on the datasheets. They disclose on at least one of them that after 6kHZ or so, they substitute in their IMD measurements. Review by separate persons have all measured at least .1% to .2% at 20kHz, which is not SOTA by a mile. The nc400 measurements you posted above that Amir did show the same thing, albeit not as drastic. Look at the NAD M22 for a representative IMD measurements. Posted third party measurements of Hypex products have never (to date) measured to the datasheets. They are still good, but I find it odd that a product that is little more than an amp-on-a-board has that issue, repeatedly (including in the designers own Mola Mola amps):
https://www.soundstage.com/index.ph...la-kaluga-mono-amplifiers&catid=97&Itemid=154. I'd love it if someone would spend some serious time measuring one of these things to figure out why. Are there repeated implementation errors, measurement errors, or are the datasheets just a bit optimistic?
Ok, wasn't aware of this.
I've found this in the NC400 datasheet:
I presume that's what you're talking about?
My interpretation of this is that the two-tone IMD measurement is an actual measurement of the device (albeit using atypical frequencies of 18.5 & 19.5KHz), but that the THD vs frequency measurement is not. This would be consistent with the Soundstage measurements, which show rising THD above 3-5KHz as power increases beyond 1W, but which also show an SMPTE vs power measurement that fairly closely tracks the THD vs power measurement (which I presume was taken at 1KHz).
Personally I wouldn't be worried about harmonics outside the audio band in any case, but it is a little strange that they don't use standard measurements in their datasheets.
It's also strange that this note appears only in the consumer-level NC400 datasheet, but not that of any of the OEM NCore products (eg NC252, NC500). Does this mean it doesn't apply to those amps?
Also puzzling me atm is that Soundstage states they are using an AUX-0025 to do their measurements, which is spec'd as having a 20-20,000Hz passband only. Yet looking at the
AP website, I'm struggling to understand what this filter's cutoff is. There are two separate and very different frequency response measurements shown. If the latter is the filters actual response, I can't see how higher order harmonics could even make it into the measurement.
Would be interested to know if others know more...