Luxman SQ 507X (1971)
A solid-state integrated amplifier released in August 1971 with 50W + 50W output and advanced transistor design for its era.
Overview
The Luxman SQ 507X is a solid-state integrated amplifier introduced in August 1971 as the higher-power counterpart to the SQ 505X. It delivers 50 watts per channel into 8Ω with exceptionally low distortion and noise, making it a standout in early Japanese high-fidelity audio design. The amplifier features a fully symmetric power supply, direct-coupled amplification stages, and a sophisticated preamplifier section with switchable input impedance and flexible tone controls. Known for its clean, powerful sound and precise engineering, it represents one of the first Japanese transistor amplifiers of a new audio era.
Specifications
| Continuous output (both channels, 8Ω) | 50W + 50W |
| Continuous output (single channel, 8Ω) | 60W |
| Total harmonic distortion factor (8 Ω, 50W) | 0.04% or Less |
| Cross modulation distortion factor (8 Ω, 50W, 70 : 7 khz = 4 : 1) | 0.04% or Less |
| Output bandwidth | 5 Hz to 50 kHz -3dB (0.04%) |
| Frequency characteristic | 10 Hz to 60 kHz (within -1dB) |
| Input sensitivity (for 50W into 8Ω) | 580 mV |
| Input sensitivity (Phono MM) | 2mV |
| Input sensitivity (Line) | 80mV |
| Input sensitivity Phono1, 2 | 2 mV |
| Input sensitivity Aux1 | 90 mV or more (adjustable) |
| Input sensitivity Aux2, 3 | 80 mV |
| Input impedance | 50k Ω |
| Input impedance Phono1 | 30k Ω, 50k Ω, 100k Ω (switchable) |
| Input impedance Phono2 | 50k Ω |
| Input impedance Aux1 | 50k Ω |
| Input impedance Aux2, 3 | 200k Ω |
| Damping factor (8 Ω) | 40 |
| Damping factor (16 Ω) | 80 |
| Residual noise | 1 mV or less |
| Preamplifier Section Frequency characteristic (Aux1) | 10 Hz to 50 kHz -3dB |
| Preamplifier Section Total harmonic distortion factor (1 kHz, 1 V, Aux1) | 0.05% or Less |
| Signal-to-noise ratio Phono1, 2 | 63 dB or more |
| Signal-to-noise ratio Aux1, 2, 3 | 80 dB or more |
| Maximum Allowable Input (1 kHz) Phono | 300mV |
| Crosstalk (1 kHz) Phono | -60dB or less |
| Crosstalk (1 kHz) Aux | -65dB or less |
| Tone control | LUX System NF Type with Bending Point Switching |
| Low Bend Point | Defeat, 150 Hz, 300 Hz, 600 Hz |
| High Frequency Curve Point | Defeat, 6 kHz, 3 kHz, 1.5 kHz |
| Filter Low-cut | 70 Hz (-6dB/oct.) |
| Filter High Cut | 6 kHz (-6dB/oct.) |
| Low booster | 100Hz(6dB/oct) |
| Attachment Attenuator | -18dB |
| Speaker switch | 2 systems |
| Premain isolation switch | Yes |
| Tape monitor | 2 systems |
| Recording output terminal | 2 systems |
| Semiconductor used | Transistor : 33, Diode : 2 pcs, Varistor : 4 pcs |
| Power consumption (8 Ω simultaneous load at maximum output) | 200W |
| Power consumption (no-signal) | 15VA |
| Supply voltage | AC100V - 240V |
| External dimensions | Width 450 x Height 160 x Depth 268 mm |
| Weight | 11kg |
Design
The main amplifier section uses a ±2 power supply with all stages directly coupled, eliminating capacitors in the signal path. The complementary output stage employs pure complementary symmetry with PNP and NPN power transistors to prevent crossover distortion. The preamplifier’s equalizer stage uses a three-stage direct-coupled PNP-NPN-PNP transistor circuit, while ultra-low-noise transistors are used throughout the preamp for minimal noise. Tantalum and Mylar coupling capacitors are selected for reliability, and driver- and power transistors are directly soldered to the PCB. The design includes switchable input impedance for Phono1, level adjustment on Aux1, and filters for low-cut, high-cut, and low-boost. No speaker protection circuitry is included.
Context
Released in 1971, the SQ 507X was one of the first Japanese transistor amplifiers marking the beginning of a new audio era. It was positioned as the higher-power model in the SQ 505X line and part of a series that includes the SQ 503X, SQ 505X, and SQ 700X.
Market
Known for potential DC leakage in the preamp due to aging tantalum capacitors, a fixable issue by replacing them with polypropylene types. The low-cut filter can be modified into a subsonic filter, and the power supply can be upgraded with larger capacitors and split rectification for improved performance.
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