Denon AU-320 (1973)

Name: Denon AU-320
Brand: Denon
Availability: InStock

At just 0.8kg, this unassuming box hides a triple-shielded transformer built to elevate moving-coil cartridges with near-silent precision

Overview

The Denon AU-320 isn't an amplifier, preamp, or receiver—it’s a dedicated MC step-up transformer (SUT), engineered for one purpose: to boost the tiny signal from a moving-coil cartridge to a level suitable for a standard MM phono stage. Introduced around 1973 with an original price of ¥16,000, the AU-320 emerged during a period when Denon was solidifying its reputation in high-fidelity transducers, particularly with its now-legendary DL-103 cartridge. Crafted in Japan, the AU-320 was designed for purists who valued signal integrity and minimal coloration, offering passive gain through precision-wound transformers rather than active electronics. Its compact size—97 mm wide, 65 mm high, and 155 mm deep—belies the serious engineering inside, including a 3-fold magnetic shield and gold-plated terminals, all aimed at preserving the delicate dynamics of analog playback.

Key Features

1:10 and 1:36 Step-Up Ratios with Dual Input Impedance

The AU-320 provides two selectable winding ratios—1:10 and 1:36—allowing compatibility with a range of moving-coil cartridges of varying output levels. This dual-ratio design, combined with switchable primary impedances of 40 Ω and 3 Ω, enables users to fine-tune the load for optimal cartridge performance. The secondary impedance is fixed at 4k Ω, matching well with most MM inputs.

Triple Magnetic Shielding in Iron Enclosure

Housed in a rigid iron case, the transformer benefits from a 3-fold magnetic shield designed to minimize the influence of external induction noise—a critical consideration for MC systems, where even minor interference can degrade the signal-to-noise ratio. This shielding contributes to the unit’s ability to maintain a quiet background, The iron case helps reduce the influence of SN ratio due to external induction noise.

Gold-Plated Input/Output Terminals with Low-Capacitance Wiring

Signal integrity is further preserved through the use of gold-plated terminals for both input and output, ensuring low resistance and resistance to oxidation over time. Internally, low-capacitance wiring——is employed to minimize high-frequency roll-off and phase shift. The approximately 1-meter cord length provides sufficient reach for typical setups without encouraging cable clutter or inductive loops.

Historical Context

The AU-320 was part of Denon’s early commitment to the moving-coil cartridge ecosystem, a technology the company had been refining since the 1950s. HiFi-Stereo (September 1978, p.45) noted that Denon’s moving-coil designs had become a “reference standard for the Japanese broadcasting industry,” underscoring the company’s technical authority in this niche. While the AU-320 lacks the fame of Denon’s DL-103 cartridge, it served as a critical companion piece, enabling audiophiles to extract maximum performance from their MC setups. It shared design philosophy with later models like the AU-300LC—both featuring a 1:10 step-up ratio—and was succeeded in the used market by units like the AU-340, which commanded higher prices, suggesting a tiered product strategy.

Collectibility & Value

Working examples of the Denon AU-320 typically sell in the $228 to $350 range, with HifiShark reporting a 2026 median price based on 22 trusted listings. Its value is tied closely to condition and noise performance—background hum is a known issue, often prompting users to seek modifications for optimal grounding. A dedicated thread on AudioKarma titled “The mighty Denon AU-320 S.U.T. modification” includes internal photos and wiring tips, indicating an active community of users willing to tweak the unit for modern systems.