What is stereo?

There are now two system of high fidelity, monophonic (monaural) and stereophonic. Monophonic is a system that starts from one microphone and is fed through a single high fidelity set. Stereophonic is a double system. Two separate microphones are placed at different sides of the orchestra and two different systems are used to keep the two signals or channels separated. Two separate speakers are used, placed on different sides of you room. Stereo is much like 3-D photography, two slightly different sound reach your ears giving you a new dimension in sound.

H.H. Scott '59

Marantz Model 125

 The Marantz Model 125 is an all solid state AM/FM streophonic tuner incorporating the advanced innovative design and unparalleled technology that have made marantz famous in the audio component industry. The Model 125 features a highly selective AM tuner, a superb Dual Gate MOSFET FM front end, a phase locked loop stereo multiplex demodulator, variable muting and output level controls, linear phase IF LC filters, provisions for the connection of a four channel adaptor, and a front panel selectable de-emphasis network for Dolbyized FM broadcasts.

The Model 125 is equipped with two meters, a Signal Strength Meter and an FM Tuning Meter. The Signal Strength Meter indicates the signal strength of any AM or FM broadcast. The FM Tuning Meter operates only when FM is selected and indicates correct station tuning.
The FM tuner section of the Model 125 is equipped with electronically triggered circuits which automaticaally mute interstation noise and automatically switch to the proper mode of operation for stereo and monophonic FM broadcasts. In addition, the Stereo indicator light automatically indicates a stereo broadcast when the Mono pushswitch on the front panel is in its normal out position.
 Output Level Control
This control attenuates the level of both stereo output channels to match the line level of other components in the system.
Muting Level Control
This switch controls the muting circuit to eliminate interstation noise. Three muting threshold levels can be selected, each activating muting at a different signal strength. Low (5 µV), Med. 920 µV) or High (100 µV). If desired, the muting circuit may be defeated by placing the switch in the Off position. This prevents the tuner from muting very weak stations along with the noise.
Hi Blend Function
The Hi Blend functionis useful for quieting weak stereo FM signals with excessive noise and/or interference. When activated, the Hi Blend slightly reduces separation at high audio frequencies. This offers moderate separation along with moderate suppression of out-of-phase noise and distortion components. Unlike most noise filters, the Hi Blend does not sacrifice full frequency response.
 Technical Description
Front End
FM antenna signals are applied through a balun transformer and an Antenna Attenuator switch to the antenna coil which drives a Dual Gate MOSFET RF amplifier.
Antenna Attenuator "Out" - the attenuator circuit is cut off and the FM signals are directly fed to the FM antenna coil.
Antenna Attenuator "In" - the signals are attenuated approx. 20 dB by the attenuator network, then fed to the antenna coil.
A five section tuning capacitor tunes antenna, interstage and oscillator circuits which provide exceptional selectivity and spurious signal rejection. The signal from the RF amplifier are fed through the triple tuned Butterworth type RF tank circuit to the Dual Gate MOSFET Mixer stage, which is also fed by the signal generated by the local oscillator. This mixer converts the carrier frequency to the 10,7 MHz intermediate frequency. Careful attention to the thermal and electrical characteristics of the local oscillator has minimized drift, thus eliminating the necessity for AFC. The 10,7 MHz converted signal is then fed to four linear phase IF LC filters which include and are followed by five stages of limiting. It is then, in turn, processed through the FM discriminator. The output of the FM discriminator is fed to a buffer amplifier which then drives the stereo multiplex demodulator.

IF Stages
The IF sectionconsist of five transistors with four stages of IF LC filters (one 6-pole filter and three 4-pole filters. The characteristics of these filters are ideal in that the 200 kHz passband is phase linear, with sharp cut-off slopes. Its exceptional phase linearity assures the elimination of a major source of high frequency distortion and a loss of stereo separation. The sharp cut-off slope provides outstanding selectivity, permitting reception of closely spaced channels.

The Model 125 utilizes five multi-staged IC limiter amplifiers, eliminating the need for an IF AGC circuit which introduces low frequency distortion. Undesirable Amplitude Modulation (AM signals, AM nise, AM distortion) are removed from the IF signal by the limiters.
 FM Stereo  Demodulator
The stereo composite signal obtained from the buffer amplifier is fed to the phase locked loop stereo demodulator IC circuit and decoded into both left and right channel signals. Each left and right channel signal, is then applified to the 19 kHz and 38 kHz filters and de-emphasis networks, to remove the undesired switching carrier signal in the audio signals. Next, each audio signal is applied to the IC, which includes such functions as audio signal amplifier and muting circuit and is amplified to the required signal level of about 1 volt RMS. Finally, each amplified signal is fed to the FM switch. The phase locked loop iC is equipped with a separate automatic Stereo/Monaural switching circuit. The circuit examines the input signal intensity and activates the stereo demodulator circuit and the stereo indicator lamp automatically when the input signal is strong enough to provide quality stereo reception.  When the input signal strength is below the threshold level, the FM stereo broadcast is processed as a monarual signal. Improved signal-to-noise ratio is obtained in the Mono mode during low signal levels.

Muting Circuit
In the absence of an FM carrier, all FM receivers inherently produce interstation noise. The muting circuit eliminates this noise, providing noise-free tuning from station to station. A muting circuit consisting of a two transistor noise amplifier and an IC has been incorporated in the Model 125. the muting circuit perfectly mutes out all the interstation noise and also completely mutes out the side slope spurious response of the unit. The circuit has been designed to minimize annoying switching noise as the tuning band is scanned.

AM Tuner
The AM tuner section is composed of one IC  and three transistors.  One of them  is a signal strength indication amplifier, while the other two are used for amplifying detected audio signals. A three section variable capacitor is used to insert two tuned circuits into the RF stage for high selectivity and improved spurious signal rejection performance. The ceramic filters utilized in the AM IF amplifier are designed for higher selectivity and wider bandwidth for interference-free high quality AM reception.following the AM IF amplifier, the AM detector recovers the audio modulation and presents this signal to the mode selection switch. The AM tuner and IF amplifier are subjected to the action of an effective automatic gain control circuit which maintains a constant signal level for all stations in the AM band.
FM Section
Quieting Slope:  1,7 µV for 30 dB, 5 µV for 57 dB, 10 µV for 62 dB, 50 µV for 70 dB
Ultimate Quieting:  50 µV for 70 dB
Selectivity:  alternate channel, better than 85 dB
Capture Ratio:  better than 1,5 dB
Stereo Separation:  40 dB at 1 kHz, 27 dB at 15 kHz
Total Harmonic Distortion: 
Stereo :  less than 0,3%
Mono :  less than 0,2%
Frequency Response:  30 Hz to 15 kHz  ±1 dB
Total Spurious Rejection:  better than 100 dB
Image rejection:  better than 100 dB
AM Suppression:  better than 60 dB
IF rejection:  better than 100 dB
Antenna Impedance:  300 Ω (balanced), 75 Ω (unbalanced)
Quadradial Output:  300 mV, 15 K Ω for ±75 kHz Dev.
AM Section
Am Sensitivity:  better than 40 µV
Selectivity:  Adjacent channel, 1000 kHz, better than 30 dB
AM Bandwidth:  7 kHz (-6 dB)
Image Rejection:  better than 70 dB (14000 kHz)
Power Requirements:  220 V, 50/60 Hz
Power Consumption:  25 watts
Dimensions ( W x H x D):  15-3/8" x 5-3/4" x 11-7/8"

Weight [net/shipping]:  17,2 lbs/ 23,8 lbs

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