Denon TU-520 (c. 1980)
The Denon TU-520 is a "Servo Locked AM/FM Stereo Tuner" from around 1980. Denon was putting serious engineering into analog FM reception at the time. It was priced at ¥33,000 in Japan and appears in Denon's July 1980 "System 80" component stereo catalog alongside amplifiers like the PMA-530 and PMA-510. That puts it in the middle of the company's system lineup.
The main feature is the servo lock, a frequency control system meant to keep the local oscillator stable when the temperature changes. Tuning took two steps. You found the station using the three-LED tuning display (red LEDs showed which way the frequency was off, a green LED showed just tuning), then engaged the servo lock to hold it. A five-LED signal strength meter showed antenna input level for both FM and AM. The front end used an FET in the first stage with a 3-row varicon for better sensitivity. The IF stage had three linear-phase ceramic filters, for low distortion and high selectivity. A newly developed PLL-MPX IC handled the multiplex section. It had a line amplifier built in, and the coupling capacitor between the MPX section and the audio amplifier was removed, so the path is DC-coupled. The AM section used a coil combined with two stages of high-selectivity ceramic filters and a double-series varicon to eliminate 9 kHz beat interference.
The TU-520 also has a built-in 440 Hz oscillator for checking recording levels, electronic switching for FM/AM/Rec-Cal functions, and a muting circuit to suppress noise between stations. The detachable low-impedance loop antenna was designed to resist electric-field noise. A Japanese-market unit covers 76-90 MHz FM. Some units were made for the European market with the 88-108 MHz band.
Known Specifications
- Type: FM/AM tuner, servo lock mechanism
- FM sensitivity: 1.7 µV / 9.8 dBf practical; S/N 50 dB at 3.3 µV mono, 30 µV stereo
- FM S/N: 84 dB mono, 80 dB stereo
- FM THD (1 kHz): 0.05% mono (100% mod.), 0.06% stereo (90% mod.)
- FM selectivity: 65 dB (±400 kHz); capture ratio 1.0 dB
- Stereo separation: 55 dB (1 kHz)
- Frequency response: 20 Hz–15 kHz, +0.2 / −1.5 dB
- AM sensitivity: 18 µV (S/N 20 dB); selectivity 36 dB (±9 kHz)
- Output level: 0.6 V FM (100% mod.), 0.2 V AM (30% mod.)
- Power consumption: 7.5 W, 100 VAC 50/60 Hz
- Dimensions: 434 × 93 × 300 mm (W×H×D)
- Weight: 4.5 kg
Surviving documentation is thin - most of the technical record comes from a single Japanese database entry. A 2025 Dutch marketplace listing called the unit "classic" and "perfect to complete your hi-fi set," and asked €35. No production years are explicitly documented, though the catalog evidence points to 1980-1981.
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