Pass Labs Aleph 3 power amplifier Page 2

The Aleph 3's is of modest weight compared to other high-end amps. Also, its relatively compact dimensions facilitate placement within the room. It's necessary to give it plenty of space for ventilation, though. Pass recommends 6" of space on all sides and top. The amp's constant 250W heat output feels like a lot in the summer, but it isn't enough to require extra air conditioning. (For comparison, that's only about a third of the heat output of an old Krell KSA-250.)

All external connectors are located on a cramped panel at the rear, between two heatsink fins. This maximizes the heatsink area of the amp, but it also makes the power switch a pain to flip, and the speaker binding posts difficult to work with. Ah, the price we pay for compactness.

Also on this panel is the fuse receptacle. The only other protection devices are a thermal shutoff device mounted to the heatsink, and a power-line voltage-spike absorber for each channel. All of these devices are electronically far removed from the signal path, so they can't hinder the musical performance of the amp.

Inside the black box
When I removed the Aleph 3's top plate, the view inside reminded me of a Navajo rug. There was symmetry in all four directions, with different-colored square and circular shapes arranged within each other. Many lines radiated out to connect some of the significant shapes.

It was obvious that Pass's respect for simplicity applies to structural design as much as it does to electronic design. For example, the same four aluminum bars that the output MOSFETs are attached to as primary heat conduits also constitute the amplifier's main chassis. All the heatsinks, and the top and bottom panels, are attached directly to these bars with 3/16" bolts, creating a very strong box. The pressure from these bolts also ensures good heat transfer to all external surfaces. The four small bolts that hold two power transistors to each bar also hold each satellite circuit board in place, eliminating the need for any additional hardware here. Even the big bolt that holds the 450W toroidal transformer to the bottom panel doesn't mind lending half an inch of its end to serve as a connecting platform for the central circuit board. Servicing this amp would be a breeze; one can gain access to any part of it quickly.

The circuit itself is direct-coupled, all-discrete, power-MOSFET, low-feedback, single-ended class-A. The signal enters the amp on a Teflon-insulated gold-plated RCA jack, then passes through a Dale RN55D ¼W metal-film resistor. The signal is then amplified by an International Rectifier IRFD9110 in a 4-pin DIP package, one half of a differential pair. The signal moves through another Dale resistor, and then is amplified by a matched pair of TO-3-cased IRF244s operating in parallel. There are four paralleled Matsushita metal-oxide power resistors in series with the speaker binding posts, and that's it! Simple, no?

The International Rectifier MOSFETs were chosen for their sound quality and their gain characteristics, but they have other advantages. IR reportedly uses the strictest quality control in the power semiconductor industry, with seven separate checks performed during manufacture. Their MOSFETs are reputed to be very reliable. However, only 10% of the IRFD9110 input devices that Pass Labs receives meet their low noise standards. The IRFD9110s are matched to within 0.2%, and the output devices to within 2%.

The driver and output transistors are fed by constant-current sources. Pass feels the current sources are well worth having, since they improve both power efficiency and power-line noise rejection. From the owner's manual: "Previous methods of loading the output stage have used networks consisting of resistors, coils, transformers, and active current sources, all of which offer an optimal load line based on a resistive load. The Aleph 3 has a unique current-source topology which optimizes performance for a wide range of impedance and reactance in the load, improving all aspects of performance into real loudspeakers."

Pass has a patent pending on the design of his special output-stage constant-current source. Its "ghost" loading effect increases the actual power efficiency of the amp to 33%, somewhat above the expected theoretical ceiling of 25% for a single-ended amp with a conventional constant-current supply. The design also provides some extra current beyond the linear region, postponing clipping, albeit at higher distortion levels.

Other high-quality parts making appearances include the excellent Panasonic HFS low-ESR electrolytic caps for all the driver power supplies, and four Panasonic TSU 22,000µF caps per channel for the output stage supplies. There is almost no wire at all inside; short solid-core wire jumpers connect the five circuit boards to each other. These jumpers are soldered on both ends, so there are no press-on, unsoldered connections anywhere in the music circuits. The power cord is 14 gauge with a standard female IEC connector.

Company Info
Pass Labs
21555 Limestone Way
Forest Hill, CA 95631
(530) 367-3690
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