Model S (Special) Amplifier

The Model S (Special) is a custom build avaibale only upon request.

Model S (Special) Description

Model S Class A, Single Ended, Hybrid OTL Stereo Amplifier (Special)
Build Quantity: Upon Request Only

The Model S (Special) consists of an amplifier chassis and a power supply chassis, each is connected via a 3 Ft. cable.
The amplifier is designed for use with very efficient speaker systems such as the Klipsch Heritage Series.
Maximum Power Output is 6 Watts per channel wl 4 - 8 Ω nominal load and 3.6 Watts w/ 16 Ω load, both channels driven.
Input Sensitivity for maximum power output: 6 V 42/.8 V 82/1 V
16 Ω
Input Impedance: 390 K
Output Impedance: .12 Ω
Damping Factor: 35(4 Ω) 70(8 Ω) 95(16 Ω)

Circuit Description: The input signal is amplified with a conventional, two stage voltage amplifier circuit using a 5814 / 6189 / 12AU7 vacuum tube (V1). The amplified signal is then capacitively coupled to the gate of an N channel MOSFET (Exicon ECF10N20) The gate is also supplied with a positive bias via a 100K resistor (R 10) The bias is adjustable via a 10K Potentiometer (P1) to Obtain approximately 2.3 Amps measured across a g Resistor Resistor (R 13) V V= 1 Amp of Drain current (This current is adjusted to supply the amplifiers rated 6 Watts @ 1 kHz with an 8Ω load connected).

The Source terminal of the MOSFET is connected to the plates of three 6CM3 vacuum tube diodes (V2, 3, 4) the cathodes of the three diodes are connected to common (ground) This results in a unique current load for the audio signal which is then coupled to the speaker load via a 15,000 Mfd capacitor. The feedback signal is taken from the + speaker terminal and fed via a 680 Ω resistor and a 510 pf capacitor (R9 and C2) to the cathode of the input voltage amplifier.

The MOSFET drain is supplied + 70 Volts from two 36 Volt switching power supplies in series (Mean Well UHP350-36) Note: Serial Number 2507S uses a linear power supply for the 70 Volt drain supply.The vacuum tube heaters are supplied from a single 100 VA switching power supply (Mean Well 100N2-12).

The heater power supply should be adjusted to supply 12.6 VDC as measured at one of the 5814 tubes after a sufficient warm up time.
Each of the 36 volt supplies should be adjusted for 35 volts each as measured at the + input to the amp after warm up time. (the linear 70 volt supply used with Serial No. 2507S will settle out at approximately 70 volts +/- 2 volts or so.)

The higher "B" voltage required for the two stage voltage amplifier is supplied via a conventional, low current full wave rectifier power supply of approximately 170 VDC.

cdm
05-13-2025

Important Note: This is a Class A type amplifier that produces large amounts of heat. It must have plenty of ventilation and must not be located near any flammable items or where children or pets could come in contact with it.

 

The Circuit and Associated Parts

(Click to enlarge)

(Click to enlarge)