Here is the pre-amplifier diagram circuit (including the low gain bass loudness). R3-R9 is a 20K ohms pot for louness gain adjustment:
Here is below the schematics of the audio preamp circuit. This is a current-feedback design. The circuit uses jfet in cascode configuration for input, bjt in enhanced VAS cascode (refer to Design Details of Power Amp - VAS stage section) and medium power mosfet for output.
The headphones amplifier uses the same circuit as the above line pre-amplifier and is in series with it. The residual T.H.D. of both line pre-amp and headphone amplifier is below 0.005% (THD equipment resolution is 0.005%).
Here is below the schematics of the power supply discrete regulator circuit. This circuit has a LM317-337 pre-regulator circuit not shown sourcing the +/- 28 Vdc to have an output of +/- 24 Vdc.
The pre-amp includes a motorized volume with remote control function pcb (below) ordered from this site: Remote control
Interior of pre-amplifer chassis. Note the transparent plexiglass on top:
HeadPhones Amplifier circuit:
Line Amplifier circuit (including discrete voltage regulator circuit as scematics above):
Loudness circuit with 20K ohms potentiometer:
Input source selector circuit: