Bruno Putzeys wrote on Thu, 22 May 2008 09:14 |
Stereophile always seems to be grappling with measuring class D amps. I've also had problems getting an AP system 1 to return reliable results when measuring audio devices with out-of-band content (noise shaping DACs, class D etc). This is a result of the AP1 having its band-limiting filter downstream of the notch filter. This causes the autorange circuitry to range the detector output based on the carrier residual instead of the audio signal. At low signal levels you don't get any meaningful distortion or noise reading. Starting from the system 2, the AP test sets no longer have this problem, especially the ones fitted with an AES17 filter. Audio Precision's passive external filter is not necessary here (I've never gotten different results with or without) but this filter does allow correct measurements with the AP1.
Anyhow, Stereophile has tried designing their own measurement filter to get round the problem and I am sorry to report that the distortion shown on their plots is entirely due to that filter. I have re-measured the amplifier and got the exact same results as those we publish on our web site.
They were correct in identifying an issue with long-term power output though. This has led to us using slightly larger output inductors in the UcD180 modules for the last two years or so.
As a result of these 2 items, the stereophile review is the only one showing substandard measurements. Subsequent reviews by other magazines have been fully in line with our published specs (bar one where a manufacturer bypassed our input buffer and put in one that distorted more than the amp, not naming names).
|
Bruno.
Thanks for pointing that out.
Stereophile really have to get their act together WRT
measurements, they are not doing any favours for manufacturers.
In the absence of AP2, would not something like a Prism Dscope
be at least a major improvement over the AP1.
I checked the graphs from Hypex site of UCD700 and 400 HG series.
They certainly are better than the magazines results. Having said
this, they are still a way off a really good linear amp.
Can you comment on sonic differences between something like the
ExtremeA and UCD modules? I know you built extremeA as a
yardstick, and I am thinking it is something along the lines of a
low powered Halcro.
T