Denon AU-340 (1978)

Name: Denon AU-340
Brand: Denon
Availability: InStock

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

Overview

At just 2.1 kg and measuring 155 mm wide, the Denon AU-340 doesn’t announce itself like a flagship receiver or a towering amplifier. But for vinyl enthusiasts running low-output moving coil (MC) cartridges, this unassuming metal case houses one of the most respected step-up transformers of the late 1970s. Despite being marketed with preamplifier-like features in some circles, the AU-340 is fundamentally an MC step-up transformer—a passive device designed to boost the tiny signal from an MC cartridge before it reaches the phono stage. Released in 1978, it was built during a period when Denon was refining its analog front-end technology, and owners report it delivers a clean, transparent gain that preserves detail without adding coloration. The unit features two input switching functions and a 3-stage impedance selector—3 Ω, 40 Ω, and PASS—allowing users to match it precisely to their cartridge’s output and internal resistance. With gold-plated input/output terminals and a factory-included 1-meter output cable featuring low capacitance (55 pF/m) and minimal core resistance (0.055 Ω/m), the AU-340 was clearly engineered for signal integrity from end to end.

Key Features

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

The Denon AU-340 offers two step-up ratios—1:10 when used with a 40 Ω load and 1:33 with a 3 Ω load—giving users flexibility in pairing with a wide range of MC cartridges. This dual-ratio design, combined with the 3-position switch (3 Ω, 40 Ω, PASS), allows precise impedance matching, which is critical for optimizing both frequency response and cartridge performance. The PASS setting bypasses the transformer entirely, effectively turning the unit into a direct line-through for non-MC sources, though owners note this function is rarely used in practice. 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

Inside each channel resides a Tamura TKS-83 transformer, a component chosen for its exceptional linearity and low distortion. Denon’s engineers implemented a sandwich-wound structure, balancing the primary and secondary coils to minimize leakage inductance and distributed capacitance. This design improves high-frequency extension and ensures a flatter response across the audible band. The core itself is constructed from Super Permalloy 0.2 mm laminations, specially heat-treated for low-level signal applications, which helps maintain magnetic stability even with microvolt-level inputs. To protect against mechanical interference, a double cushion material floats the transformer within its shielded case, preserving magnetic properties under physical stress.

Permalloy and Silicon Steel Shielding

Electromagnetic interference is a constant threat in gain-sensitive analog circuits, but the AU-340 combats it with serious shielding. Each transformer is enclosed in a permalloy multiple shield, while additional silicon steel sheets shield internal boards and switches. This layered approach reduces noise and crosstalk, contributing to the unit’s reputation for a quiet background and precise imaging. Combined with the low-loss internal wiring and gold-plated terminals, the shielding ensures that the signal path remains as pure as possible—critical for a device whose sole job is amplification-by-transformer.

Frequency Response from 10 Hz to 120 kHz

Rated at 10 Hz to 120 kHz (+0.5 / –1.0 dB), the AU-340’s frequency characteristic far exceeds the human hearing range, a design choice that supports transient accuracy and phase coherence. This wide bandwidth was reportedly engineered to accommodate the extended high-frequency output of contemporary cutting lathes and high-compliance MC cartridges. The large inductance in the transformer design also enhances low-frequency performance, reducing distortion in the bass region—a common weakness in lesser step-up transformers. Channel balance is maintained within 0.5 dB or less, ensuring precise stereo imaging even at low signal levels.

Distortion Factor Below 0.05%

Under test conditions—1 mV input, 100 pF load capacitance, and 50 kΩ load resistance—the AU-340 achieves a distortion factor of 0.05% across 20 Hz to 20 kHz when used with the 4 Ω terminal. This level of performance was exceptional for a passive transformer-based device in 1978, and collectors note that surviving units often measure close to original spec when cleaned and properly maintained. The use of high-grade materials throughout the signal path, from the core alloy to the output cabling, contributes to this low distortion, making the AU-340 a favorite among those who prioritize transparency over warmth.

Historical Context

The Denon AU-340 emerged in 1978, a time when high-end audio was shifting toward separates systems and improved phono-stage design. As moving coil cartridges gained popularity among audiophiles for their superior tracking and detail retrieval, the need for quiet, high-gain pre-amplification grew. The AU-340 filled this niche with a focused, no-compromise design. It was part of a small family of Denon step-up transformers, though exact model hierarchies are not documented. Unlike active preamps of the era, which could introduce noise and coloration, the AU-340’s passive transformer design offered a neutral alternative—especially valuable when paired 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 unit’s build quality and sonic performance, with one user calling it “my favorite SUT” alongside the Ortofon T. Maintenance is minimal—there are no tubes, transistors, or internal power supply to fail. The most 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. Because the AU-340 contains no active circuitry, aging capacitors in associated gear (such as the phono preamp) are a greater concern than the unit itself.