Zeus Preamplifier - Mosfet

Zeus Stereo Mosfet Preamplifier & Power Supply - Concept CAD Sketch for new Prototype.

This concept CAD sketch is for my new version preamp prototype. It is designed to incorporate my 24 way reed switch attenuator and newly designed radial reed switch XLR input selector. The latter stereo pair are mounted side by side and connected by toothed timing belts, the overall assembly being accommodated in the rear section of the preamp.

24 way reed switch attenuator PCB (with operating magnet in centre).

The 24 way reed attenuator switch is built up in sections and allows for either discreet wiring to a TVC or on the reverse has surface mount resistor sites for conventional or ladder attenuators. Each section has a radial arm mounted on the central shaft which contains a magnet that operates the reed elements.

Radial reed switch XLR input selector - PCB drawing.

An 8 way reed input selector switch plus tape loop using the same switching principle as the attenuator.

All three pins on the XLR connectors are switched, with an individual option to ground or leave open circuit for Pin 1. This arrangement reduces to an absolute minimum the internal signal paths of any source that is not specifically selected.

The individual XLR reed groups are arranged to ensure that the XLR Pin 1 ground is always made before the audio path is made and is always held until after the audio path is broken.

In addition as the magnet arm is moved from channel to channel it passes over a mute reed.

Zeus Stereo Mosfet Preamplifier Prototype.

The prototype front panel layout is going to be reorganised with the volume control in the centre and the input selector to one side. A hard mute/standby switch will be on the other side to balance the visual appearance.


  1. Each channel has it's own voltage regulator (I used LM317HVs set to 27 volts) and has a diode on the input to guard against reverse polarity protection.

  2. (Not shown in photograph) As the preamp is single ended I use a reed relay in the output circuit of each channel to open circuit the transformer secondary to prevent power cycling transient to the power amps.

Zeus Stereo Mosfet Preamplifier - Original Concept CAD Images

(Prototype shown using commercially available hardware and 3U front panel.)

Preamplifier Highlights

  • Class A operation.

  • Zero feedback.

  • Balanced and isolated XLR inputs

  • Reed switch channel selection and volume control.

  • TX-102 transformer variable attenuator.

  • Single Ended mosfet Class A operation.

  • Balanced and isolated transformer output.

  • Output configurable to drive 600, 150 or 32 ohm impedance load.

  • Configurable AC/DC coupling and 0 or +6dB per channel.

  • Auto standby-mute when volume set to Zero (red LED at 9 o'clock).

  • External 24 volt power supply.

  • Good EMI/RFI performance.

  • "Direct" signal connection from input to output.

  • Large 300 x 40 x 100 mm heatsinks for each channel.

General Preamplifier Schematic

Zeus SE MOSFET Line Driver - Further information & FFTs

Test bench development information, will be updated.

Zeus System - Pre + Power

Full end to end configuration with TX-102 TVC input.

MOSFET SE Pre-amp & PP MOSFET Power Amp Tests.

Test bench development information, will be updated.

Design by: Susan Parker, MIET.

The information contained here may be used to construct one audio system specifically for personal NON commercial use only.

N.B. Personal liability disclaimer applies.

Email: susan@audiophonics.com

All design and other information, drawings and images on this website are
Copyright 1992-2010 Susan Parker MIET (unless otherwise credited).

These designs and other information may be used to construct systems specifically for personal NON commercial use only.

N.B. Personal liability disclaimer applies - see T&C.