Zeus Power Amplifier

75 Watt Output Transformer

Sowter type number 9840

Dual chamber bobbin version

Dual Chamber Bobbin.


  • EI 1-1/2" M6 lamination - 2" stack ( dimension drawing ).

  • 120 x 2" Dual chamber bobbin.

  • Quad filar wind each side.

  • 1.0 mm double or triple insulated magnet wire.

  • c. 60 turns hand, 70 turns machine wound - per group of four wires.

  • Insulate with polyester tape.

Both sides to have identical number of turns and length of wire i.e. mirror image twins.

All ends brought out and insulated with coloured sleeves i.e. there are eight separate windings.

This allows maximum configuration flexibility and the transformer can be used as:

  • a centre taped inductor with the speaker across the ends.

  • a centre taped inductor with two mid taps for speaker connection.

  • a transformer with four equal output windings for the speaker which can be configured as:
    4 series, 3+1, 2+2 or 4 parallel.

Although seemingly more complicated the use of a split dual chamber bobbin allows greater flexibility of configurations and ensures that all windings are exactly matched. The winding operation does not have to be stopped and restarted half way through as would otherwise be the case for a single chamber bobbin.

These transformers are now available direct from Sowter to order online, see Parts for DIY page on my Audiophonics website.

Provisional wire colours:

        Pri          Sec
        _____ ||| ______
BROWN  1A    )|||(   3A   RED
MAUVE   _____)|||(______  BLUE
        _____ ||| ______
BROWN  2A    )|||(   4A   RED
MAUVE   _____)|||(______  BLUE
        ===== ||| ======
        _____ ||| ______
YELLOW 1B    )|||(   3B   ORANGE
GREY    _____)|||(______  GREEN
        _____ ||| ______
YELLOW 2B    )|||(   4B   ORANGE
GREY    _____)|||(______  GREEN
N.B. To be confirmed.

Dimensions and Weights:

As per Sowter N type transformer.

See Sowter dimensions info for the package style required.


Shroud: CadmiumPlated
Laminations: Painted

Package Type "S" Fully Shrouded Example

Note: the S type is 3" deep lamination stack, but will otherwise look like this.

Construction Information

Single Chamber 2:1 configuration

75W output transformer fully shrouded for chassis mounting...

... and the bits that go inside (single chamber bobbin version).


Inductance of each winding = 99 mH @ 1 kHz, 468 mH @ 120 Hz.

Impedance of each winding = 3.9 K @ 1 kHz, 1.25 K @ 120 Hz.

Inductance between input and output windings (wired for use) = 2.68 H @ 1 kHz, 181 H @ 120 Hz.

Inter-winding capacitance between input and output windings (wired for use) = 9.34 nF

Windings to laminations/frame capacitance (wired for use) = 176 pF

Output impedance in PP NMOS circuit when powered = 0.94 @ 1 kHz, 0.90 @ 120 Hz.


EI 1-1/2" Lamination Dimensions

  • Material = M6 (M4 or M2)
  • Size = EI-1.1/2"    (Standard dimensions = 19.1, 19.1, 31.1, 57.2, 5.6, 114.3, 9.5 mm )
  • Thickness = 0.35 mm
  • Frame Mounting (has bolt holes in the corners).

Single chamber bobbin, EI 120 x 2"
Quad Filar wound, 1.0mm insulated copper wire.
No insulation between layers.
Terminations: none, wire out with sufficient length to connect as appropriate.

Overall size: 110 x 95 x 115 mm; 4-3/8" x 3-3/4" x 4-5/8".

Weight: 4.7 kg; 10 lb 4 oz.


This transformer version is for standard 8 ohm impedance (or greater) speakers.

2 x Primary:
2 x Secondary:

Starting from same bobbin end, wind all four simultaneously with enough layers to fill bobbin.

Approximately 120 x 4 turns.

N.B. The 4 simultaneous windings should be even without crossings over the width of the winding. Cross-overs allowed at ends when changing direction.


If possible, vacuum impregnated varnish.

Back to Zeus Main Page

Design by: Susan Parker, MIEE.

The information contained here may be used to construct one set of power amplifiers specifically for personal NON commercial use only.

N.B. Personal liability disclaimer applies.

This page last modified on: 11th November 2006

All information, drawings and images Copyright 1995 - 2006 Susan Parker unless otherwise credited.