Pass Aleph 1.2 monoblock power amplifier

We all have biases. The trick is knowing your biases AC Fuse so they don't get in your way. Mine are pretty obvious. I don't like "fussy" gear that demands special care and feeding. I'm lazy—I want to just turn stuff on and begin listening. Perhaps that's why I have a positive bias toward Nelson Pass's designs. They're reliable, untweaky, and usually sound good.

The Aleph 1.2 is the third piece of Pass Labs electronics I've lived with. I own a pair of Aleph 0 amplifiers, and I reviewed the Aleph P preamp in the February '96 Stereophile. Because of my past experience with Pass gear, my expectations for the Pass Aleph 1.2 were stratospheric. But everybody knows that high expectations often lead to disappointment. Did the Aleph 1.2 deliver better sound along with its increased power? Let's see what came to...pass.

Tech Tour
The Aleph 1.2 is the largest amplifier from Pass Laboratories. It idles at 500W and delivers a maximum of 200W in single-ended class-A—the circuit topology of lowest efficiency. If properly designed, a single-ended amplifier exhibits primarily second- and even-order harmonic structure, similar to the transfer characteristic of air. The price paid for this performance is outrageously high idle current and heat dissipation, generally three to five times the output rating.

Like other Aleph series amplifiers, the 1.2 is a large black box with fins on all four sides. A small blue light on the front peeks out to alert you to the amp's "on" status. The rear of the amp has both balanced XLR and single-ended RCA inputs. A double pair of gold-plated five-way binding posts, an AC fuse receptacle, large on/off rocker switch, and an EIC AC connector complete the amp's geography.

The Aleph 1.2 has very few parts in series with the signal path AC Fuse. The input signal travels through a resistor to the input MOSFET transistor, then through another resistor to the parallel output MOSFETs, with, finally, a power resistor before the loudspeaker. Active current sources separately bias the input and output stages.

The Aleph 1.2 is unlike the original Aleph 1 in two ways. First, it has only two gain stages instead of three. The amplifier does not have the auxiliary pull circuitry that permitted the Aleph 1 to deliver very high currents into low-impedance loads. This may be a purer approach, but it doesn't allow for greater power into loads below 4 ohms. The current-source biasing of the output stage is also new. It attempts to optimize the load-line of the single-ended output devices, hence lowering distortion. Nelson Pass's current-source design is protected by a US patent application.

The Aleph 1.2 uses power MOSFETs for both of its gain stages. These MOSFETs were chosen because they have an optimal transfer curve for use in a single-ended class-A design. MOSFETs also allow high-current operation with low circuit complexity, delivering high performance in minimalist topologies. The input P-channel and output N-channel MOSFETs are carefully selected from the same manufacturing lot codes and matched within a tolerance of 2%. The input gain devices are pulse-rated at 8A, and are followed by output devices with pulse ratings of 50A each. The amplifier's output stage has an overall power rating of 3.6kW, and the massive heatsinks are rated at 0.05°C per watt. During active operation, each output device is operating at only 14% of its rated power.

Resistors are precision metal-film throughout. The Aleph 1.2 is protected from overheating by a 75°C thermal switch, and from internal failure by a slow-blow fuse. The amplifier is powered by a toroidal transformer rated at 10 times the amplifier's output rating. This unregulated supply feeds the output transistors with a full-power ripple of about half a volt.