Amplifier Schematic (a reminder of the circuit complexity).

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Monoblock Amplifier Front View

Top Inside View

Rear View - Note 2:1/4:1 Tx switch (RH side).

General Internal Layout

Audio Path Transformers

Heatsink Mounting on Rear Panel

Notes:

• The heatsink needs to be a lot bigger for high bias implementations e.g. 300 x 250 x 40 mm or 300 x 200 x 84 mm.

• The heatsink is mounted away from the rear panel with spacers to reduce thermal heating of the inside of the enclosure (M4 washers are sufficient).

• I use the amps in 4:1 mode for most speakers. 2:1 mode is only suitable for LS3/5As or other "high" impedance speakers.

• Yes, these amps are still working (without any modifications, updates or adjustments). The only time the top has come off is to take these pictures.

Input Transformer Specification

35W Output Transformer Specification

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Testing

Test setup Signal generation: HP 8903B > TPA6120 Amplifier: 1+1:10 Sowter > IRFP150 > 4:1 and 2:1 35W Tx Load: 8 ohm Soundcard: E-mu 1212M, balanced XLR cable to 1/4" balanced input. Distortion v. Power = Audio Analyser: HP 8903B

Test Conditions Load = 8 ohms Output impedance 4:1 = 0.25 ohms @ 1 kHz, 0.29 ohms @ 120 Hz.

THD+N % Distortion v. Power - 8 ohms

Vac RMS	2:1 Tx - THD+N %	4:1 Tx - THD+N %
1.00	0.123	0.094
1.25	0.109	0.070
1.50	0.096	0.057
1.75	0.089	0.049
2.00	0.088	0.040
2.50	0.099	0.032
2.84	0.118	0.031
3.00	0.128	0.030
4.00	0.224	0.029
5.00	0.369	0.033
6.00	0.512	0.043
7.00	0.598	0.060
8.00	0.646	0.086
9.00	0.665	0.116
10.0	0.669	0.161
11.0	0.664	0.542
12.0	0.652
13.0	0.637
14.0	0.620
15.0	0.605
16.0	0.588
17.0	0.620
18.0	0.744
19.0	0.944	PSU Supply Limit

Zeus35 2:1 and 4:1 distortion figures

Note the much lower figures for the 4:1 setting - the trade off is the lower power output.

N.B. These results are similar to but not quite the same as the FFT derived figures.

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FFT Tests (2008)

4:1 Op Tx - 1 kHz sine wave, 8 ohm load.

4 Watt THD @ 1 kHz = 0.0353%

2 Watt THD @ 1 kHz = 0.0254%

1 Watt THD @ 1 kHz = 0.0148%

0.5 Watt THD @ 1 kHz = 0.0154%

2 Watt THD @  50 Hz = 0.0532%

2 Watt THD @ 100 Hz = 0.0295%

2 Watt THD @ 200 Hz = 0.0183%

2 Watt THD @ 500 Hz = 0.0149%

2 Watt THD @ 1 kHz = 0.0185%

2 Watt THD @ 2 kHz = 0.0284%

2 Watt THD @ 5 kHz = 0.0642%

2 Watt THD @ 10 kHz = 0.1358%

2 Watt THD @ 20 kHz = 0.2460%

2 Watt THD @ 50 kHz = 1.735% (HP8903B measurement)