Model S Jr. Amplifier

Model S Jr. Description

The Model S Jr. is a smaller version of the Model S. It is a very low power Class A Stereo Amplifier for only the very highest efficiency loudspeakers. It features a headphone jack and will provide excellent sound as a headphone ampilifer.

Specifications:
Power Output w/ 4 Ohm Load: .75 Watt / w/8 Ohm Load: 1 Watt / w/16 Ohm Load: .75 Watt
Sensitivity 4 Ohm: •3 VRMS / 8 Ohm: .4 VRMS / 16 Ohm: .5 VRMS
Damping Factor: 4 Ohm: 4.5 / 8 Ohm: 9 / 16 Ohm: 16
Output Impedance: 1 Ohm
Negative Feedback: None
Input Impedance: 120 K
Power Requirement: 110 - 120 V / 220 - 240 V 50 / 60 Hz.
Power Consumption: 105 Watts
Dimensions: 8" W x 12" D x 6 1/2" H
Weight: 5Lbs.

Circuit Details: The signal is applied to the grid of the voltage amplifier via the volume control. The amplified signal is direct coupled to the grid of a cathode follower stage. This output is taken from a voltage divider in the cathode circuit.
The DC voltage developed across the 27 K resistor provides a 24 volt bias to put the MOSFET into Class A operation. This bias, along with the audio signal is applied to the gate of the MOSFET. The Drain terminal of the MOSFET is connected to the Plate of the 25CG3 which is a low impedance vacuum tube diode. The cathode of the 25CG3 is connected to chassis ground. The audio signal is developed across this diode and is coupled to the speaker load via a 6600 mfd. capacitor. The 50 volt power supply is applied to the Drain of the MOSFET and the two stage, dual triode is powered from a low current 300 VDC power supply. The 25CG3 heaters are also supplied, in series, from the 50 volt supply. The dual triode stage heaters are supplied with 24 VDC, also in series, from a small DC - DC converter. A cooling fan, mounted on the chassis bottom plate, keeps the two MOSFET heatsinks at a temperature of about 60 degrees C. A .1 Ohm resistor in the 50 volt power supply for each channel is provided for measuring MOSFET current of approximately 700 - 800 mA. 10 mv across these resistors= 100 ma.

cdm 8-7-2026

 

The Circuit and Associated Parts

(Click to enlarge)