Threshold CAS-2 Power Amp

Description

The Threshold CAS-2 is a particularly interesting early power amplifier from the company founded by Nelson Pass, and it occupies an important place in the development of Threshold’s legendary solid-state designs. Produced from approximately 1979 to 1981, the CAS-2 was developed from Pass’s work on cascode circuitry, an approach intended to reduce nonlinearities in the output devices while achieving some of the sonic advantages associated with Class A operation without its enormous heat and power requirements. In that respect, the CAS-2 can be viewed as an important precursor to Threshold’s later Stasis amplifiers.

The amplifier was rated at 125 watts per channel, making it considerably more powerful than the smaller CAS-1. Its design was unusually ambitious for its time. The CAS-2 uses a full dual-mono architecture, with separate power supplies and amplifier circuitry for each channel, and employs twelve high-speed output devices per channel. Another source examining the amplifier’s construction identifies 24 output transistors in parallel, giving the CAS-2 considerable current capability and the ability to handle difficult loudspeaker loads.

The cascode circuit was central to Nelson Pass’s thinking. The objective was to minimize signal compression caused by voltage variations in the power transistors, thereby producing very low distortion and extremely wide bandwidth. Contemporary Threshold literature positioned the CAS-2 as a less expensive alternative to the company’s highly regarded Class A amplifiers. A vintage equipment listing from High Fidelity placed its original price at $895, compared with $1,395 for the Threshold 400A.

Listening impressions of a properly restored CAS-2 are particularly impressive. In a retrospective review for TONE Audio, Jerold O’Brien initially heard some treble haze and bass roughness from an unrestored example, but after replacing aging capacitors and output devices, he found that the character changed dramatically. He described the improved amplifier as having a “pristine even-handed quality”, with improved bass detail and grip.

O’Brien ultimately concluded that “this amplifier is something special.” He praised its large soundstage, transparency and ability to remain composed during demanding orchestral and high-energy recordings. Even when compared with substantially more modern and expensive amplifiers, he wrote that he never felt compelled to replace the Threshold, instead appreciating what it brought to the system.

Modern owner experiences are similarly enthusiastic. One CAS-2 owner described it as “lively and engaging”, with excellent pace, rhythm and timing. He also reported good bass extension and definition, increasingly solid imaging and strong dynamics. Importantly, he found the amplifier’s presentation reminiscent of the highly regarded Naim NAP 250, while noting that the CAS-2 is a moderately high-bias Class A/B design rather than a true Class A amplifier.

The CAS-2 offers a fascinating combination of Nelson Pass engineering, high current capability, dual-mono construction and a distinctive musical presentation. For vintage-audio enthusiasts, it is one of the more intriguing early Threshold amplifiers and an important link between Pass’s early Class A work and the company’s later Stasis designs.

Product Specifications

• Type: Stereo solid-state power amplifier
• Manufacturer: Threshold Corporation
• Designer: Nelson Pass
• Production period: Approximately 1979–1981
• Circuit topology: Cascode / high-bias Class A/B
• Power output: 125 watts per channel
• Channels: 2
• Architecture: Full dual-mono
• Output devices: 12 high-speed output devices per channel; 24 total
• Rated distortion: 0.03%
• Power supply: Separate supply sections for each channel
• Design objective: High current capability and reduced transistor nonlinearities
• Inputs: Line-level RCA
• Speaker outputs: Heavy-duty speaker terminals
• Front-panel display: Dual rows of LED power/output indicators
• Weight: Approximately 28 lbs.
• Design significance: Early Nelson Pass cascode amplifier and precursor to later Threshold Stasis designs