Pass XA200.8 Review - Fun measurements

My passdiy amp sounds really nice, no idea about measurements but it sounds more tubey than my 300B tube amp. Warm and musical.

The measurements, as John Atkinson suggested, indicate a relatively low level and a very restricted open-loop bandwidth, which appears to be about 500 hertz.  This is unusual as a very restricted open-loop bandwidth is normally used to ensure stability where a relatively high degree of corrective feedback is used.  The loss of feedback at high frequencies results in comparatively poor high frequency distortion readings.  That said, the sound was good.  As well, the amp provides true true class A operation at its rated power (200 watts) into an eight ohm load.  The manufacturer specifies the current draw at idle at a very high 6.1 amperes, which if one applies the formula that output power is the square of the current (I) x resistance (R), means that the class A power into eight ohms is 6.1x6.1x8 or 297.8 Watts.  The manufacturer claims that it leaves class A at 430 peak watts (no impedance specified) whatever those are. Into a four ohm load, ithe amp derates (leaves class A below the rated power into that impedance of 400 Watts) at 148.8 watts, but this is still impresssive in comparative terms.  More to the point, the reviewer found it had some unique, desirable qualities in the midrange, even though the Class AB Momentum monoblocks beat it in bass slam and tightness, capture of recording venue ambience, and lack of a sonic fingerprint.  

My previous post should have said in the first line, “relatively low level of corrective feedback”.  That said, the very low measured output impedance, including cable, of 0.03 ohms suggests a very healthy degree of corrective feedback, although there may be other explanations for this.  Would explain the very limited open-loop bandwidth and consequent increase in high frequency THD to 1% at 20K into an eight ohm load.  Still, the proof of the pudding is in the eating, as the saying goes.

@erik_squires 

I think it depends on set up and taste as to what works best. My point is you don’t need to spend a fortune on power amplification to get a warm tubey sound. SS power amps that try to emulate tubes like McIntosh are going to colour everything coming through them. The beauty of a tube preamp is you can play your DAC directly or through the tube preamp - just a couple of interconnect connections....

I think that the Pass X series is their best all around amp. I have the new Pass XP-22 preamp and the Pass X350.8 amp. They produce a very detailed and somewhat tube like sound with excellent sound stage. 
While this system is expensive, you are paying for excellent sound, reliability and product support that is second to none. In fact I had Mark at Reno Hi Fi steer me away from their class A amps because of their lower power and high heat output. I run this system with an 
Allnic H 7000 phono amp (tubes) and achieved an excellent result. 
The trick with a Pas system is to have a tube phono amp like a Herron,
Audio Research or Allnic.

Finally, to me, tube power amps are a real hassle to maintain. Their reliability is far below that of a Pass which is built to last and maintain its sound level. Tubes (especially power tubes) deteriorate over time.
And they have reliability issues because of poor QC in the factories that produce them.