Zeus SE Power Amplifier

 

Fig 1. General Single Ended Amplifier Schematic.


Introduction

A single ended test configuration of the amplifier with a single mosfet output stage. This uses the same output transformer as in the push-pull configuration, with 1:1 interleaved EI laminations (no gap).

Single Ended Amplifier with one STW34NB20


Distortion Measurements

  • Signal generation: HP 8903B > TPA6120 Line Driver.

  • Amplifier: 1:10 Sowter > STW34NB20 > 75W Tx SE

  • Bias current as per graph.

  • Measurement: HP 8903B.

75W Tx SE (2:1 Output Configuration) - 33.5 Vdc Supply - 8 ohms

Distortion at 1.0 and 1.25 Amp Bias Levels

THD versus Vac output at 1 kHz, Load 8 ohms.

10 Vac - 12.5 watts - maximum swing with 33.5 Vdc power rail.

  • Vsupply = 33.5 volts, 252 mV ac ripple.
  • Core cross section with EI 1-1/2" laminations is 3.0" square.
  • Bias current when 1.0 amp - Vbias level 6.52 volts.
  • 1.63 volts across transformer windings.
  • Output impedance = 0.856 ohms at 1 kHz, 0.768 ohms at 120 Hz.

75W Tx SE (2:1 Output Configuration) - 46.5 Vdc Supply - 8 ohms

Distortion at 1.5 Amp Bias Level - 47 Vdc Supply

THD versus Vac output at 1 kHz, Load 8 ohms.

14 Vac - 24.5 watts - maximum swing with 46.5 Vdc power rail.

  • Comparison between STW34NB20, APT20M20 and 2SK1929 mosfets.
  • Vsupply = 46.5 volts.
  • Bias current 1.5 amp.
  • Output impedance = 0.856 ohms at 1 kHz, 0.768 ohms at 120 Hz.
  • Note: Vbias needs a separate supply of less than 40 volts.

75W Tx SE (4:1 Output Configuration) - 46.5 Vdc Supply - 4 ohms

Distortion at 0.75 and 1.5 Amp Bias Levels - 47 Vdc Supply

THD versus Vac output at 1 kHz, Load 4 ohms.

7.5 Vac - 14 watts - maximum swing before clipping to the 46.5 Vdc power rail.

  • Vsupply = 46.5 volts.
  • Bias current as shown.
  • Output impedance = 0.212 ohms at 1 kHz, 0.201 ohms at 120 Hz.
  • Below 1 Vac output power supply harmonics are adding to amplifier distortion - see FFTs.
  • Note: Vbias needs a separate supply of less than 40 volts.

75W Tx SE (2:1 Output Configuration) - 46.5 Vdc Supply - 8 ohms

Distortion at 2 watts

THD and Vac output at 2 Watts from 20 Hz to 40 kHz, Load 8 ohms.

  • Vsupply = 46.5 volts.
  • Bias current 1.5 amp.
  • Note: Vbias needs a separate supply of less than 40 volts.


FFTs - TX 2:1 Output Configuration

  • Signal generation: E-mu 1212M > TPA6120

  • Amplifier: 1:10 Sowter > STW34NB20 > 75W Tx SE

  • Bias current 1.0 amp.

  • Load: 8 ohm.

  • Measurment: E-mu 1212M, balanced XLR cable to 1/4" balanced input.

  • FFT Analysis by WINaudioMLS software

1 kHz at -1dB, 7.32 Vac into 8 ohms, 0.0935% THD -60.8dB

Note the magnetic flux induction at 50 Hz and harmonics from power supply transformer.

The intermodulation between the 1 kHz fundament and it's harmonics can be clearly seen at the base of each signal. See later section for more details.

1kHz at -4dB, 5.19 Vac into 8 ohms, 0.0585% THD -64.7dB

 

1 kHz at -7dB, 3.67 Vac into 8 ohms, 0.0409% THD -67.7dB

 

1kHz at -10dB, 2.60 Vac into 8 ohms, 0.0326% THD -69.7dB

1 kHz at -13dB, 1.84 Vac into 8 ohms, 0.0316% THD -70.0dB

19 and 20 kHz IMD at -16dB, 1.95 Vac into 8 ohm

Amplifier on, generator mute, 8 ohm load

Amplifier linear power supply mains cycle frequency of 50 Hz and harmonics can be seen up to 2 kHz, and the Computer and Monitor switching power supplies at 38 kHz and above.

Amplifier noise without these components is below -140 dB.


FFTs - TX 4:1 Output Configuration

  • Signal generation: E-mu 1212M > TPA6120

  • Amplifier: 1:10 Sowter > STW34NB20 > 75W Tx SE

  • Bias current 750 mA.

  • Load: 4 ohm.

  • Measurment: E-mu 1212M, balanced XLR cable to 1/4" balanced input.

  • FFT Analysis by WINaudioMLS software

1 kHz at -6.2dB, 4.0 Vac into 4 ohms, 0.0461% THD -66.7dB

1 kHz at -12.2dB, 2.0 Vac into 4 ohms, 0.0216% THD -73.3dB

1 kHz at -18.2 dB, 1.0 Vac into 4ohms, 0.0251% THD -72.0dB


Intermodulation Between Harmonics and Fundament

The low noise floor of the Zeus amplifiers allow details to be seen that is normally not shown on amplifier measurements.

Of particular interest is the intermodulation sidebands which are now apparent with a -150 dB measurement capability in conjunction with 64 bit precision 1 million point Fast Fourier Transforms.

1 kHz Fundament plus harmonics to 10 kHz at 4.0 Vac into 4 ohms, 0.0460% THD -66.7db

1 kHz Fundament plus harmonics to 10 kHz at 1.0 Vac into 4 ohms, 0.0086% THD -81.2db

The Single Ended Zeus shows 20 dB higher intermodulation sidebands than the Push-Pull version. Is this the elusive "Classic" Single Ended sound signature which is normally buried in analog noise which test instruments don't show but which people can nonetheless hear?


E-Mu 1212M Soundcard + TPA6120 Loop-Back Tests

For comparison with the above tests to give an idea of the base line measurements.

Soundcard + TPA loop-back - 1 kHz FFT - 0.007% THD+N.

Soundcard + TPA loop-back - 19 + 20 kHz IMD.

Soundcard + TPA loop-back - Mute.


Amplifier Direct Connectivity

Amplifier feedthrough when not powered.

This scope trace shows that the output transformer is passing a signal, although small and distorted, even when the power to the amplifier is off.

This is a unique feature of the Zeus amplifier and demonstrates the direct connection between input and output.


FFT Analysis by WINaudioMLS software

PRO EX Version + 64-bit ultra high precision FFT; 192kHz/24 Bit ASIO support.


This page last modified on: 15th March 2005

All information, drawings and images Copyright 1994 - 2005 Susan Parker unless otherwise credited.