Zeus Power Amplifier

Toshiba 2SK1529 Audio MOSFETs

FFT Tests - 8 ohm Load


Note:

The Toshiba 2SK1529 Audio FETs are vertical not lateral MOSFETs, although VGS is much lower than with other types (e.g. IR and ST) I have been using which has caused some confusion.

Gate−Source cutoff voltage VGS (OFF) at VDS = 10 V, ID = 0.1 A:

 2SK1529 VGS min-max of 0.8 2.8 V

compared to:

IRFP150N VGS min-max of 2.0 4.0 V, and

STW34NB20 VGS min-max of 3.0 5.0 V

A true lateral MOSFET such as the Exicon ECW20N20 has a VGS min-max of  0.1 1.5 V

The distinction is "Gate threshold voltage is less than 1 volt for lateral parts and over 2 volts for vertical parts."

(I now have some Exicon 20N16s to test, and will put the results up on a separate page.)

My thanks to those who have brought this matter to my attention.

P.S. Subsequent testing with the Exicon 20N16s were very disappointing. They were very prone to parasitic oscillation and even when "tamed" did not give as good results as the STW34NB20s!


4:1 Output Transformer Tests


Test setup

  • Signal generation: E-mu 1212M > TPA6120

  • Amplifier: 1+1:10 Sowter > 2SK1529 > 4:1 75W Tx

  • Load: 8 ohm

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

  • Amplifier Output Impedance = 0.25 ohms @ 1 kHz


Distortion Test at 1kHz into 8 ohms

V ac % THD+N Watts
1.52 0.0141 0.3
1.92 0.0115 0.46
2.00 0.0098 0.5
2.28 0.0089 0.65
2.56 0.0082 0.8
2.71 0.0073 0.9
2.87 0.0070 1.0
3.04 0.0067 1.2
3.41 0.0062 1.5
3.61 0.0058 1.6
3.83 0.0056 1.8
4.05 0.0058 2.0
4.29 0.0058 2.3
4.55 0.0059 2.6
4.82 0.0058 2.9
5.01 0.0059 3.1
5.41 0.0061 3.7
5.72 0.0063 4.1
6.06 0.0066 4.5

N.B. 6 V ac is this test setup maximum limit.


Tests: 43 Hz to 96 kHz FFT

1 kHz sine (see TPA graph for comparison of harmonic structures).

THD+N at 2 watts into 8 ohms = 0.0058%

19 kHz + 20 kHz Intermodulation.

Residual noise for above tests (signal = mute).

TPA6120 Line Driver Comparison Tests

1 kHz sine (see amplifier graph for comparison of harmonic structures).

TPA6120 THD+N  = 0.005%

19 kHz + 20 kHz Intermodulation.

TPA6120 Residual noise for above tests (signal = mute).


Notes:

These tests were performed "open" on the bench.

It is interesting to compare the harmonic structure of the TPA6120 Line Driver loop-back only tests with that of the amplifier.

In these and the other tests on this page and elsewhere is is apparent that the signal source has an appreciable effect on the test results, for example with the relative levels of the second and third harmonics.

A valve based source should therefore maintain the valve quality of the sound, and a solid state source should also retain it's own signature.

The FFTs results are all produced on scales which are +10 to -180 dB. The signal floor is discernable below 140 dB and whilst "noisy" still shows effects and structure. For example in the three amplifier test graphs the wavy undulation is a function of the FFT sampling aliasing with 50 Hz pickup in the transformers.


2:1 Op Tx: Intermodulation - 19 kHz + 20 kHz


80 Hz to 96 kHz FFT

  • Signal generation: E-mu 1212M > TPA6120

  • Amplifier: 1+1:10 Sowter > 2SK1529 > 2:1 75W Tx

  • Load: 8 ohm

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


First Tests (2:1 Op Tx)

Test setup

  • Signal generation: HP 8903B > SSM2015 > Attenuator > OP275 > TPA6120

  • Amplifier: 1+1:10 Sowter > 2SK1529 > 2:1 75W Tx

  • Load: 8 ohm

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

Test Conditions

  • 1.45A Quiescent i.e. 750 mA per 2SK1529 FET
  • 3.116 Volts Bias
  • Q1 = 832 mV
  • Q2 = 836 mV
  • Gate resistor = 330R
  • -3 dB Bandwidth = 60 kHz
  • Load = 8 ohms
  • Output impedance = 0.99 ohms @ 1 kHz, 0.95 ohms @ 120 Hz.
  • 2 Watt THD+N @ 1 kHz = 0.027%, -71.3 dB

First FFT results.

1 kHz sine wave, 800 Hz to 96 kHz, 4.11 Vac, 2 Watts into 8 ohms.

  • 1kHz peak is at -6db on the graph = 4.11 Vac RMS .

  • 1 kHz signal, 4.11 Vac RMS into 8 ohm load = 2 Watts.

  • 75 W transformer, 2:1.

  • 8 ohm load @ 1 kHz =  8.024 ohms, 3.6 uH on 12 foot cable.


A series of tests to confirm basic operation of the test setup and put the above results into context.

E-mu 1212M residual input noise with XLR cable - open circuit.


Amplifier ON, Input Open Circuit, 8 ohm load.

I guess the stuff up above 30 kHz is the switching power supplies in the computer and monitor.

Apart for the interference pickup the residual noise is about the same as the soundcard.


Amplifier OFF, Input Open Circuit, 8 ohm load.

As previous test but with the amplifier power off for comparison. That pickup above 30 kHz still there.


TPA6120 Line driver output.

This is the output from the TPA6120 headphone amp chip (with OP275 and SSM2015), source HP 8903B 1 kHz sine wave (peak is at -6 dB on the graph = 4.11 Vac), direct to the 1212M input.


Amplifier ON, input from TPA6120 nominally zero signal, 8 ohm load..

This is the output from the TPA6120 headphone amp chip, source HP 8903B 1 kHz sine wave, with attenuator set to minimum. Theoretically no signal to the amplifier.


FFT Analysis by WINaudioMLS software

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


Email: susan@audiophonics.com

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Copyright 1992-2010 Susan Parker MIET (unless otherwise credited).

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