Accuphase DG-28 (1997)

Name: Accuphase DG-28
Brand: Accuphase

Digital voicing equalizer with 64-band correction and automatic room calibration

Overview

The Accuphase DG-28 is a stereo digital equalizer released in May 1997 as a successor to the DG-18. Built by Accuphase Laboratory, Inc., it operates entirely in the digital domain using dual DSPs per channel for 48-bit internal processing. Designed for audiophiles seeking precise acoustic correction, it includes a calibrated AM-28 measurement microphone and automated room analysis using a warble tone sweep. The unit corrects speaker and room response anomalies and stores up to four user presets (A–D) in non-volatile memory. It functions as a stand-alone processor and integrates with compatible Accuphase gear like the DC-300 preamplifier and DP-75V CD player via optional digital I/O boards.

Specifications

Format	Digital 1/6 octave 64-band (switchable to 1/3 octave 32-band)
Frequency Range	16 Hz to 22.4 kHz (ISO standard, ±0 dB at 48 kHz sampling)
DSP Configuration	Two DSPs per channel (four total), 48-bit internal processing
Dimensions & Weight	475 x 150 x 393 mm, 13 kg
Power	100 VAC, 50/60 Hz, 12W consumption
Memory	Four user profiles (A–D), retains settings when powered off
Inputs/Outputs	Optical & coaxial digital I/O (EIAJ CP-1201), 16–24-bit, 32/44.1/48 kHz auto-detect
Remote Control	RC-21 infrared, powered by two AA batteries
Included Accessories	AM-28 microphone, 5m mic cable, stand, two 75Ω coaxial cables, power cord

Design

The DG-28 uses Accuphase’s proprietary filter algorithm with parallel-connected filter engines per channel, minimizing calculation errors and noise. Each channel employs 64 filter cells in a "filter engine," with unused cells deactivated to prevent signal degradation. Clock stability is ensured by an ultra jitter-free PLL circuit using a crystal oscillator and preamble detection. The unit features a 320x240 dot LCD under a screw-mounted front cover that protects the microphone jack and less-used controls. Digital tone generation and analysis are DSP-based, enabling automatic correction in about 5.5 minutes for both channels. Optional analog I/O boards (up to four) can be added via rear slots.

Context

Positioned as a high-end solution for room acoustics, the DG-28 competed with units like the TacT RCS 2.0 and Behringer Ultracurve Pro DSP 8024. It was marketed as a tool to correct listening room anomalies unaddressed by passive methods. When paired with the DC-300 digital preamplifier, the signal remains fully digital from source to output. The DG-28 was introduced during a period when digital equalization was gaining credibility among audiophiles for improving spectral balance, imaging, and soundstage stability without adding audible artifacts.

Market

Discontinued and available only on the used market, typical DG-28 listings range from €2,200 to PLN 7,400 (~$2,000–$3,500), with rare outliers as low as €10. Common issues include front panel display failures and loss or damage to the proprietary AM-28 microphone. Service is challenging due to the complexity of the DSP circuitry and difficulty sourcing legacy components. Optional accessories include the AO-B1 analog output board and the AO2-U1 D/A converter module.