Denon AU-340 (1978)

Name: Denon AU-340
Brand: Denon

A compact, no-frills box that became a favorite among MC cartridge fans

Overview

The Denon AU-340 weighs 2.1 kg and measures 155 mm wide. It doesn't look like much next to a flagship receiver. But for vinyl people running low-output moving coil (MC) cartridges, this plain metal case holds one of the most respected step-up transformers of the late 1970s. Some circles market it with preamplifier-like features, but the AU-340 is an MC step-up transformer - a passive device that boosts the tiny signal from an MC cartridge before it reaches the phono stage. Denon released it in 1978, while it was refining its analog front-end technology. Owners report a clean, transparent gain that keeps detail and adds no coloration. It has two input switching functions and a 3-stage impedance selector - 3 Ω, 40 Ω, and PASS - so you can match it to your cartridge's output and internal resistance. The input/output terminals are gold-plated, and the factory 1-meter output cable has low capacitance (55 pF/m) and minimal core resistance (0.055 Ω/m).

Key Features

1:10 and 1:33 Step-Up Ratios with Impedance Matching

The Denon AU-340 has two step-up ratios: 1:10 with a 40 Ω load and 1:33 with a 3 Ω load. That lets it pair with a wide range of MC cartridges. Combined with the 3-position switch (3 Ω, 40 Ω, PASS), it allows impedance matching, which is critical for both frequency response and cartridge performance. The PASS setting bypasses the transformer entirely and turns the unit into a direct line-through for non-MC sources. Owners say it is rarely used. HiFi-Stereo (1984, p.70) noted that such transformers were increasingly essential as high-end turntables adopted lower-output cartridges, stating, “the demand for quiet, wide-band step-up units has never been higher.”

Tamura TKS-83 Transformers with Sandwich-Wound Coils

Each channel has a Tamura TKS-83 transformer, chosen for its linearity and low distortion. Denon's engineers used a sandwich-wound structure, balancing the primary and secondary coils to minimize leakage inductance and distributed capacitance. That improves high-frequency extension and gives a flatter response across the audible band. The core is Super Permalloy 0.2 mm laminations, heat-treated for low-level signal applications, which helps keep it magnetically stable with microvolt-level inputs. A double cushion material floats the transformer inside its shielded case, protecting against mechanical interference and preserving magnetic properties under physical stress.

Permalloy and Silicon Steel Shielding

Electromagnetic interference is a constant threat in gain-sensitive analog circuits, and the AU-340 deals with it through heavy shielding. Each transformer sits in a permalloy multiple shield, and additional silicon steel sheets shield the internal boards and switches. This layering reduces noise and crosstalk, and the unit has a reputation for a quiet background and precise imaging. The internal wiring is low-loss and the terminals are gold-plated, which keeps the signal path clean for a device whose only job is amplification by transformer.

Frequency Response from 10 Hz to 120 kHz

The rated response is 10 Hz to 120 kHz (+0.5 / - 1.0 dB), well beyond the range of human hearing. The wide bandwidth supports transient accuracy and phase coherence. It was reportedly engineered to handle the extended high-frequency output of contemporary cutting lathes and high-compliance MC cartridges. The large inductance in the transformer also helps low-frequency performance and reduces bass distortion, a common weakness in lesser step-up transformers. Channel balance holds within 0.5 dB or less, so stereo imaging stays precise even at low signal levels.

Distortion Factor Below 0.05%

Under test conditions of 1 mV input, 100 pF load capacitance, and 50 kΩ load resistance, the AU-340 reaches a distortion factor of 0.05% across 20 Hz to 20 kHz on the 4 Ω terminal. That was exceptional for a passive transformer-based device in 1978. Collectors note that surviving units often measure close to original spec when cleaned and properly maintained. High-grade materials run through the whole signal path, from the core alloy to the output cabling, and that contributes to the low distortion. It is a favorite among those who prioritize transparency over warmth.

Historical Context

The Denon AU-340 appeared in 1978, when high-end audio was shifting toward separates systems and improved phono-stage design. MC cartridges were gaining popularity among audiophiles for their tracking and detail retrieval, and the need for quiet, high-gain pre-amplification grew. The AU-340 filled that niche with a focused design. It was part of a small family of Denon step-up transformers, though exact model hierarchies are not documented. Active preamps of the era could introduce noise and coloration. The AU-340's passive transformer design offered a neutral alternative, especially with mid-sensitivity MC cartridges like the Denon DL-103.

Collectibility & Value

Denon AU-340 units in working condition typically sell for $200 to $400 USD, though some listings have reached $580 depending on condition and region. Prices briefly dipped below $400 on eBay as recently as early 2022, according to user reports on Steve Hoffman TV. A 2020 purchase on Yahoo Buyee closed at $277, with total cost including fees and shipping around $350. Owners consistently praise the build quality and sound, and one user called it “my favorite SUT” alongside the Ortofon T. Maintenance is minimal. There are no tubes, transistors, or internal power supply to fail. The common issues are surface-level: oxidation on the gold-plated RCA jacks, which can be cleaned with isopropyl alcohol, and occasional contact noise from the push-button switches, which respond well to contact cleaner. Since the AU-340 has no active circuitry, aging capacitors in associated gear (such as the phono preamp) are a greater concern than the unit itself.