Used Good
The Threshold 400A is a legendary high‑end solid‑state stereo power amplifier introduced in the late 1970s by Threshold Corporation, the company founded by renowned amplifier designer Nelson Pass. It is widely considered one of the defining amplifiers of its era—not just for its power and engineering, but for the way it helped establish the reputation of Class‑A solid‑state amplification among audiophiles.
Overview
At a time when most solid‑state amplifiers were class‑AB designs with modest power and limited sonic refinement, the Threshold 400A broke new ground by delivering 100 watts per channel of continuous power into 8 ohms while operating predominantly in Class‑A mode. Its innovative circuitry allowed it to maintain Class‑A operation much further into the power curve than conventional designs, contributing to exceptionally low distortion and a very high level of musical coherence.
Internally, the 400A featured a massive heatsink structure and a large output stage using multiple output transistors per channel, all thermally coupled to dissipate heat efficiently and maintain signal integrity under demanding conditions. Its design was purpose‑built to achieve a wide bandwidth, ultra‑low distortion, and robust current delivery—attributes that allowed it to control even challenging loudspeaker loads with authority.
Physically, the 400A carries the substantial footprint and weight typical of high‑end amplifiers from its era. With a sturdy front panel featuring distinctive LED meters that display both peak and average output levels, it presents a visually engaging and functional user interface. Homes with high‑end systems in the late 1970s and early 1980s often showcased Threshold amps alongside premium preamplifiers and speakers.
Sound and Review Impressions
Among audiophiles, the Threshold 400A has earned a near‑mythical reputation. On Audiogon’s discussion forums, one long‑time listener described it as “the amp that started high end as we now know it”, noting that its control, musicality, and *“sonics… were to die for”—*from jazz to rock and classical.
Users consistently describe the 400A as having a solid, authoritative bass foundation, a rich and engaging midrange, and a smooth treble that never becomes abrasive, even at high listening levels. Its Class‑A biasing and oversize power supply contribute to a tube‑like warmth and linearity that can make it difficult to compare to contemporaries from the same period.
Some owners note that the amp’s performance is system‑dependent: while it can drive a wide range of speakers, pairing it with efficient and revealing designs tends to showcase its strengths best. Others emphasize its robustness and longevity, with many 400A units still running decades after their manufacture.
Key Features
Class‑A biased solid‑state amplification with conservative continuous power ratings
Designed by Nelson Pass, a recognized pioneer in high‑end amplifier design
Large heatsinks and robust output stage for stable thermal operation
Dual LED meters for peak and average output indication
Wide bandwidth and low distortion topology
Substantial power supply and output transformer design
Long service life and upgradable components
Product Specifications
Power Output: ~100 watts per channel into 8 Ω (continuous, stereo)
Frequency Response: 20 Hz–20 kHz
Total Harmonic Distortion (THD): ~0.1 % at rated power
Damping Factor: ~100
Gain: ~23 dB
Bandwidth (small signal): 1 Hz–250 kHz (+0,‑3 dB)
Input Impedance: ~75 kΩ
Slew Rate: ~50 V/µs
Output Current: Max ~20 A/channel (pulse)
Dimensions: ~19″ × 7″ × 11″
Weight: ~51.5 lb (23 kg)
Summary:
The Threshold 400A stands as an iconic piece of high‑end audio history. Its combination of Class‑A performance, solid‑state reliability, and Nelson Pass’s innovative design helped redefine expectations for power amplifiers in the late 20th century. Even decades later, it remains a revered and collectible amplifier—capable of delivering musical performance that still surprises listeners today.