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 produced in 1974. Designed for high-fidelity reception, it supports the Japanese FM band (76–90 MHz) and standard AM broadcast frequencies (535–1605 kHz). It features a traditional gyro-touch tuning system and includes specialized circuitry for improved signal clarity under weak or noisy conditions. Notably, it includes a high-blend switch to suppress negative phase noise on weak FM signals and an attenuator switch that reduces 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 system with a four-row varicon and a two-stage FET configuration. Its IF amplifier includes six transistors and a six-element phase-linear filter, while the limiter uses a multi-stage symmetrical circuit with a hot-carrier diode and transistor. A PLL-type IC is used in the MPX section, and the muting circuit employs two transistors for noise amplification and two transistors plus one FET for switching. The AM tuner integrates an RF amplifier, local oscillator mixer, IF amplifier, detector, and signal strength indicator in a single IC, with an additional transistor for audio amplification. The RF amplifier uses two tuning circuits with a 3-row varicon. The tuner includes a monaural switch, high-blend switch, and attenuator switch. The transformer, FM front end, AM tuner, and power supplies are shielded in black to prevent unwanted radiation. It also features a QUADRADIAL OUTPUT connector for 4-channel broadcasting.