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RE: Pass labs...

Not sure about the newer X250.5 but here are the measurements from the X1000 monos.
http://www.stereophile.com/content/pass-labs-x1000-monoblock-power-amplifier-measurements

IMO, they fail in a few ways. 1) there is a large rise in distortion at higher frequencies. 2)Distortion vs. power first goes down with increasing power, which indicates a moderate amount of feedback. 3)The distortion measured at 50Hz still shows quite a few harmonic components (mostly odd harmonics) and that doesn't account for the HF distortion rise. 4)The IM distortion has quite a few harmonics but this might not matter much

The Pass Aleph 1.2 has a similar problem with distortion vs. frequency but I don't think the FFT in this one is fair becuase it was done at 201W and this is about clipping for this amp. I don't think TJN really knows what to measure to be honest.

http://www.stereophile.com/content/pass-aleph-12-monoblock-power-amplifier-measurements

Bizzarely, the XA30.5 has a very bad distortion vs. frequency plot.

http://www.stereophile.com/content/pass-labs-xa305-power-amplifier-measurements

I haven't heard this amp but I would bet that it has not the best sounding highs despite being class A.

The 1Khz FFT shows a number of peaks and a lot of power supply related IM products with the signal (all that fuzz in the "noise" floor). The poor IMD results are largely due to the distortion vs. frequency problem.

Now, I have heard the original X250 and thought that is was a quite good amp...for SS with some negative feedback and not class A but that it can be bettered by a number of other competitors. I remember a review in Bound for Sound where the X350, which sounded worse to me than the X250, was not so favorably compared to a pair of Monarchy Audio SE250 monos, which sound very good to me and fit most of my criteria.

So, in answer, I think the X250 is an ok amplifier but not really special and I see that its brothers (none of which i have been terribly impressed with) fail a few criteria but that doesn't necessarily mean a fail sonically. Just means its less likely to be stellar.


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