Zeus Power Amplifier

Toroid Transformer Demonstrators


This page has various configurations for demonstrating the mode of operation of the Zeus amplifier using mains toroid transformers instead of custom wide bandwidth audio transformers.


Output Transformer Version

Output transformer using toriod's primaries AND secondaries: approx 2:1

  • 500 VA mains toroid 120 + 120 Vac primaries, 45 + 45 Vac secondaries.

  • Interstage 15 VA toroid as previous tests.

  • Input 1:4 Gardener 600 ohm line transformer, Rterm = 10K.

  • IRFP140 mosfets, low 100 mA bias (max) per stage.

  • Amplifier Bandwidth -3dB = 20 kHz.

  • Output Impedance = 1.5 ohms.

  • THD+N at 1 watt into 8 ohms = 0.167%

Amplifier:1 kHz (-9dB) FFT at 1 watt into 8 ohms: THD+N = 0.167%

Note: See next graph for test setup measurments.

EMU1212: 1 kHz (-20dB) FFT test balanced line out to line in: THD+N = 0.006%

Note: In the EMU 1212 test above there is the same disturbance pattern to the harmonics as seen in the amplifier test.

Both tests 192 kHz, 1 million point transforms.


Output Inductor Version

Toroid Transformer Test Amplifier


Toroid Transformer Swinging Inductor Schematic

Updated schematic:

  1. Added R3 and R4 plus specified 5W zeners to maintain protection when used with larger VA rating input transformers.

  2. Added R5 0R1 25W wire wound resistor in power supply for measuring bias current.

  3. Changed value of R1 and R2 to 390R

  4. Changed mosfets shown to STW34NB20.

Notes:

  1. Set Vbias for 750 mA per mosfet (needs large heatsink), tracking bias circuit recommended.

  2. Input transformer may be larger - 50 to 120 VA. Depending on manufacturer and source driving impedance this may give wider bandwidth.

  3. Chose Rterm to give flat top end frequency response (may be quite a different value to that shown).

  4. Output transformer VA rating to be four times the power in watts expected in use. E.g:
    20 Vac across 8 ohms = 50 Watts. Use a 200 VA transformer (larger is okay).
    30 Vac across 4 ohms = 225 Watts. Use a 1000 VA transformer.

  5. Output transformer winding voltage in AC volts to be approximately the same as the power supply DC voltage. E.g. for a supply of 34 to 32 volts (under load) use a 2 x 30 Vac transformer.


15VA Input Transformer Update - 27th Oct 04

Low level bandwidth is 10Hz to 25.5 kHz (-3 dB), 31.5 kHz (-6dB).

I used the centre tap on the 115 + 115 to Vbias.


Initial Test Amplifier - 25th Oct 04

Low level bandwidth is 10Hz to 24 kHz (-3 dB), 28 kHz (-6dB).

Input Transformer 30 VA, Pri single 240 : Sec 15 + 15 volts at 1 amp
Output Transformer 217 VA, Pri 120 + 120 : Sec 31 + 31 volts at 3.5 amp
MOSFETs 2 off IRFP140s
Computer CPU heatsink (with fan - suggest 7808 regulator).
Vreg for Vbias + twiddle pot.

Rterm is 3.7K - 2 off 1K8 with a 100R trip pot in the middle to set balance as the input transformer is not center taped.

2 off 0.47 caps for smoothing of Vsupply and Vbias.

Bias is set low at circa 15 to 20 mV on the mosfet source as the heatsink is small (distortion is a bit higher because of this).

I would recommend a bigger heatsink and higher bias level - but this was built from "bits to hand" rather than specially made/wound transformers.

The amp is driven from my computer's CD-ROM headphone output, using the two 15 volt secondaries one for each channel.

Power is from a bench PSU using a 30 volt 500 mA power supply (which has current limiting and a meter).

Using toroid transformers one can make a test amp to try out and whilst not as good as using custom transformers should still perform more than adequately and prove the concept.

And if you don't want to keep the amp in this configuration you can always use these toroids in power supplies for your other projects.


Input Transformer - 30VA

Bandwidth: 10Hz to 24 kHz (-3 dB), 28 kHz (-6dB)

Spec: 30 VA, Pri single 240 : Sec 15 + 15 volts at 1 amp

Inductance of  240 V winding = 9.19 H @ 1 kHz, 26.3 H @ 120 Hz.

Impedance of 240 V winding = 339 K @ 1 kHz, 112 K @ 120 Hz.

Inductance of each 15 V winding = 31.5 mH @ 1 kHz, 273 mH @ 120 Hz.

Impedance of each 15 V winding = 1.2 K @ 1 kHz, 620 @ 120 Hz.

Rterm: 1.8K - 100R trim to Vbias - 1.8K (total 3.7K).

Driving impedance = 80 ohms Series, 20 ohms Parallel.

Input Transformer - 15VA

Bandwidth: 10Hz to 25.5 kHz (-3 dB), 31.5 kHz (-6dB)

Spec: 15 VA, Pri 115 + 115 volts : Sec 12 + 12 volts at 650 mA

Inductance of  115 V winding = 4.6 H @ 1 kHz, 20.9 H @ 120 Hz.

Inductance of each 12 V winding = 96.2 mH @ 1 kHz, 586 mH @ 120 Hz.

Impedance of each 12 V winding = 1.3 K @ 1 kHz, 903 @ 120 Hz.

Rterm: 3.6K - 115 + 115 centre tap to Vbias.

Driving impedance = 59 ohms Series, 15 ohms Parallel.

Output Transformer

Spec: 217 VA, Pri 120 + 120 (not used) : Sec 31 + 31 volts at 3.5 amp

Inductance of each 31 V winding = 95 mH @ 1 kHz, 541 mH @ 120 Hz.

Impedance of each 31 V winding = 1.77 K @ 1 kHz, 922 @ 120 Hz.

Output impedance in circuit when powered = 4.3 @ 1 kHz.

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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: 3rd March 2005

All information, drawings and images Copyright 1995 - 2005 Susan Parker unless otherwise credited.