Power Amplifiers

Pass Labs XA-100.5 Pure Class A Monoblock Power Amplifier



Most amplifier manufacturers produce power amplifiers that are biased as Class AB, meaning that they run a few watts in Class A and the rest in AB. This is usually a push-pull configuration, and the AB moniker refers to the slight overlap in the + portion of the waveform that is delivered by one section of the amplifier with the - portion handled by the other amplifier section. This keeps some current flowing during the transition from the + to - and - to + portions without there being a point where both halves are completely off, which would produce a lot of distortion. During the Class A activity, both the + and - sections of the amplifier are producing the entire waveform.

The problem is that Class A operation produces a lot of heat dissipation when it is idle, and it is only about 30% efficient, so a 100 watt Class A amplifier is dissipating about 300 watts of heat. Because of this, the 100 watt Class A amplifier requires a massive power supply, and that transfers to EXPENSIVE. In a world where most consumers want maximum power for the least amount of money, Pass Labs bucks the trend by building Pure Class A power amplifiers. They are heavy, they are inefficient, they get hot, and they are mucho dollars, but there are customers who are willing to pay a premium for such a product. Pass builds a number of models, including the XA-100.5 monoblock reviewed here. Does it deliver a sound worth its price for just 100 watts? Read on . . . .


  • Design: Pure Class A Solid State Monoblock Power Amplifier
  • Power: 100 Watts RMS Pure Class A; 175 Watts Class AB (into 8 Ohms)
  • MFR: 1.5 Hz - 100 kHz
  • THD+N: 1% at Full Output
  • Input Impedance: 30 kOhms
  • Inputs: XLR and RCA
  • Dimensions: 9" H x 19" W x 21.5" D
  • Weight: 100 Pounds
  • MSRP: $16,500/pair USA
  • Pass Labs