- Written by John E. Johnson, Jr.
- Published on 04 April 2011
- Pass Labs XA-100.5 Pure Class A Monoblock Power Amplifier
- Page 2: Design of the Pass Labs XA-100.5 Power Amplifier
- Page 3: The Pass Labs XA-100.5 Power Amplifier In Use
- Page 4: The Pass Labs XA-100.5 Power Amplifier On the Bench
- Page 5: Conclusions About the Pass Labs XA-100.5 Power Amplifier
- All Pages
It became very obvious that the XA-100.5 has huge overhead when I played the Telarc version of Copland's Fanfare for the Common Man. I could get the needle on the meter to move a little to the right, meaning that it was going into Class AB operation, but I didn't hear any clipping. The thunderous bass drum beats probably were drawing several hundred watts from the Pass amp during those short transients. But just think about it for a moment. The amp weighs 100 pounds and has 40 output transistors. That's 2-1/2 watts per transistor for the 100 watt output specification. If there is a short transient high demand, the Pass XA-100.5 can handle it. The bench test THD+N vs. Output is for continuous output, not peak demands. I suspect that this amplifier can easily deliver 400 watts for a short transient.
Stringed instruments work well for detecting IM distortion, which can make them sound very congested. This Telarc SACD of Grieg, Dvorak, and Elgar has plenty of violins, and through the XA-100.5, they were crystal clear. Easily distinguishable in their complex harmony. No harshness at all. That's a characteristic of Class A sound.
Mozart's Requiem in D Minor was unfinished (later completed by Franz Sussmayr), but the composition that Mozart did complete is still a masterpiece. It has a choir and soloists, and multiple human voices are another good test for congestion caused by harmonic distortion, because the voice is located in the lower midrange frequencies, so there can be several orders of audible harmonics. Nonetheless, the XA-100.5 withstood the onslaught of so many people singing. Soloists stood out from the choir's soundstage rather than being buried in mush.
Enough of this Requiem stuff (a Mass for the dead). Here is something for the living, and The Manhattan Transfer know how to live, or in this case, swing. Along with John Pizzarelli and Janis Siegel, The Cincinnati Pops have their hands full, but the XA-100.5's power meter remained rock steady in spite of a full orchestra and jazz musicians doin' their thing. This amplifier delivers tremendous dynamics and is lightning fast.
Over the course of listening to the XA-100.5, it became obvious to me that having plenty of Class A instead of just a few watts, really makes a difference when the music needs to be cranked up. There are many small Class A amplifiers, usually tubes, often using 300B triodes, and the sound is breathtaking, but you can't party without more power.