Sansui AU-alpha507NRA
The AU-alpha507NRA is an integrated amplifier from Sansui's late-period Alpha-NRA generation. It was sold on the Japanese domestic market in 1998 at ¥93,000. It is the third-plus-plus-plus iteration of Sansui's X-Balanced circuitry - period sources give the family designation as "X-Balanced 3rd+++ gen.," covering 1998-2001. This was a late-career Sansui design, arriving well after the brand's 1970s separates, and it carries the Alpha badge that marked their top-tier amplifier engineering of the era.
The NRA series is built around the "Ultimate α-X balance" circuit. Sansui's own documentation describes a fully balanced architecture. The driver stage uses a "NEW diamond differential circuit" that can take positive and negative output without phase inversion. The output stage has four dedicated amplifiers arranged on the + and − sides of each channel, driving the speakers from both sides under identical conditions. The power supply is balanced too - a closed-loop configuration independent of ground, with separate windings on the transformer's secondary side. Sansui also claims the driver stage runs in class A with double the conventional current margin, and that a new elliptical-insulator radiator disperses vibration effectively.
Rated output is 80W + 80W into 6Ω and 60W + 60W into 8Ω, with total harmonic distortion of 0.003% or less at rated output into 8Ω. Damping factor is rated at 150. Frequency response runs from DC to 300 kHz (+0/−3dB) at 1W, and signal-to-noise is 120dB or better (A-weighted). Dynamic power is substantial: 280W into 2Ω, 180W into 4Ω, and 130W into 6Ω. The phono stage handles both MM (2.5mV/47kΩ) and MC (300µV/100Ω) cartridges, with line-level sensitivity of 150mV/20kΩ. Slew rate is a fast 180V/µs. The unit weighs 15.5kg and draws a rated 160W.
Very little independent documentation survives for this model. One Russian-language source says no schematics are available and that, on visual inspection of the boards, little appeared changed from prior models. That discussion centers on the related AU-Alpha907NRA, though, so it should not be read as specific to the 507NRA. Period reviews in English-language publications have not surfaced in the Archive's search. The qualitative descriptions that exist come from marketplace listings, which praise the "noble, thoroughbred sound" of the NRA series and note the unit's 100V Japanese-market rating. A sold listing in Vietnam fetched 6.7 million đồng, and a Russian listing was offered at 113,590₽ before a price reduction. Those are informal figures, but they suggest modest collector interest.
The record on the AU-alpha507NRA is thin beyond its factory specifications. What is clear is that this was a fully balanced integrated amplifier from Sansui's final high-end era. For an owner outside Japan, the 100V rating means a step-down transformer is required. No service manual availability has been confirmed, and owners should expect the usual vintage-electrolytic concerns of a late-1990s unit.
Known Specifications
- Output: 80W + 80W (6Ω), 60W + 60W (8Ω), 20Hz–20kHz, both channels driven
- THD: 0.003% or less (8Ω, at rated output)
- Damping factor: 150 (8Ω)
- Frequency response: DC–300kHz (+0/−3dB, 1W)
- S/N: 120dB or better (A-weighted)
- Dynamic power: 280W (2Ω), 180W (4Ω), 130W (6Ω)
- Slew rate: 180V/µs
- Phono: MM 2.5mV/47kΩ; MC 300µV/100Ω
- Line inputs: 150mV/20kΩ
- Dimensions: 432 × 165 × 450mm (W×H×D)
- Weight: 15.5kg
- Voltage: 100V (Japanese market)
- Price at launch: ¥93,000 (1998)
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