Dynaco Stereo 400 (1971–1978)

Name: Dynaco Stereo 400
Brand: Dynaco

The Dynaco Stereo 400 is a serious, no-frills solid-state power amplifier. It is a brute-force design of the kind that defined high-power home audio in the early 1970s. David Hafler designed it, and it was one of the most successful amplifiers in Dynaco's history. It also marks the company's move from its famous tube designs into the solid-state era.

The Stereo 400 is rated at 200 watts RMS per channel continuous at 8 ohms. Frequency response is 20Hz to 20kHz ±0.25dB, and total harmonic distortion is less than 0.25% at full output. Signal-to-noise ratio is 100dB, input sensitivity is 1.5V for full output, and the damping factor is greater than 100 at 8 ohms, 1kHz. It was sold as a kit ($249) and fully assembled ($349). The chassis measures 19"W × 6.5"H × 11.5"D and weighs 42 lbs, though some sources list 12.5 lb, which is likely a misprint.

The design is distinctive. It is a quad solid-state amplifier with dual-differential output stages and a Class A input stage. The power supply uses a large toroidal transformer, and the transformer sits in a separate enclosure connected by cable, so it stays physically isolated from the amplifier chassis. Heat sinks on the rear panel handle cooling, and a relay speaker protection circuit guards the speakers and the amp. Inputs are a single pair from a preamp. There are no tone controls and no switching.

Audio (September 1974, p.19) ran Dynaco's own advertising for the Stereo 400, which claimed "your ultimate amplifier" — 400 watts at 8 ohms in the ad's language (the spec sheet more conservatively rates 200W RMS per channel continuous), with "unsurpassed stability with any speaker" and "unmatched protection for both amplifier and speakers." The ad campaign continued into 1975, with High Fidelity (August 1975, p.87) running the same message alongside the introduction of the smaller Stereo 150, which was positioned as delivering "the sonics of the 400 with more modest power."

The known weaknesses are few. The bias setting tends to drift with age, and the speaker relay contacts may need cleaning over time. The damping factor is relatively low compared to more modern designs. Replacement parts are generally available from third-party sources, and upgrading the power supply capacitors is a common improvement among owners. Asking prices today typically run in the $400 - $600 range.

Surviving documentation is limited. No manual or schematic surfaced in the research for this page. The amplifier's reputation is well established among collectors, though, who know it for a clean, detailed sound, legendary reliability, and a heavy, solid build. A bias check and relay cleaning are the sensible first jobs on any example.

Power output: 200W RMS/ch continuous at 8 ohms
Frequency response: 20Hz–20kHz ±0.25dB
THD: <0.25% at full output
S/N ratio: 100dB
Damping factor: >100 at 8 ohms, 1kHz
Input sensitivity: 1.5V for full output
Dimensions: 19"W × 6.5"H × 11.5"D
Weight: 42 lbs (some sources list 12.5 lb, likely a misprint)
Price: $349 assembled / $249 kit