Marantz Model 115B (1974)
An FM/AM tuner built for precision reception with advanced interference suppression and quadradial output for 4-channel broadcasting.
Overview
The Marantz Model 115B is an FM/AM stereo tuner from 1974. It covers the Japanese FM band (76-90 MHz) and standard AM broadcast frequencies (535-1605 kHz). Tuning is the traditional gyro-touch system. It has special circuitry to keep the signal clean when reception is weak or noisy. A high-blend switch suppresses negative phase noise on weak FM signals, and an attenuator switch cuts the input by approximately 20 dB when engaged.
Specifications
| Tuning frequency range | 76 MHz to 90 MHz (Japanese FM band) |
| Practical sensitivity (IHF) | 2.3 μV |
| 50 μV S/N | 70 dB |
| IF interference ratio | 96dB |
| Image interference ratio | 70dB |
| Spurious interference ratio | 96dB |
| AM suppression ratio | 60dB |
| Selectivity (IHF) | 60dB |
| Capture ratio | 1.6dB |
| Signal-to-noise ratio | 70dB |
| Stereo separation | 42dB (1kHz) |
| Harmonic distortion factor | mono: 0.15%, stereo: 0.3% |
| Frequency response (50 μsec, de-emphasis) | ±1 dB (50 Hz to 15 kHz) |
| AM Tuning frequency characteristic | 535 kHz to 1605 kHz |
| AM Practical sensitivity | 20 μV |
| AM Selectivity | 26dB |
| AM Image interference ratio | 70dB (1400kHz) |
| AM Signal-to-noise ratio | 46dB |
| Power | 100 VAC, 50Hz/60Hz |
| Power consumption | 25W |
| External dimensions | Width 390 x Height 146 x Depth 300 mm |
| Weight | 8 kg (Body), 13.6 kg (Packing Weight) |
Design
The Model 115B uses a double-tune FM front end with a four-row varicon and a two-stage FET configuration. The IF amplifier has six transistors and a six-element phase-linear filter. The limiter is a multi-stage symmetrical circuit with a hot-carrier diode and a transistor. The MPX section uses a PLL-type IC. The muting circuit takes two transistors for noise amplification, plus two transistors and one FET for switching. On the AM side, the RF amplifier, local oscillator mixer, IF amplifier, detector and signal strength indicator all sit in a single IC, with one more transistor for audio amplification. The AM RF amplifier uses two tuning circuits with a 3-row varicon. The front panel has a monaural switch, a high-blend switch and an attenuator switch. The transformer, FM front end, AM tuner and power supplies are shielded in black to prevent unwanted radiation. There is also a QUADRADIAL OUTPUT connector for 4-channel broadcasting.
Context
A 1974 review noted: "Nice. Nicer still is seeing the wife all smiles listening to Stravinsky's Firebird, up loud..."
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