4:1 Output, 1 Watt THD+N @ 1 kHz = 0.020% 2:1 Output, 1 Watt THD+N @ 1 kHz = 0.114% 2:1 Output, 2 Watt THD+N @ 1 kHz = 0.231%

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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 will need to be at least twice this size for high bias implementations
  i.e. 300 x 150 x 40 mm.

• The heatsink is mounted away from the rear panel with spacers to reduce thermal heating of the inside of the enclosure.

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Testing

Test setup Signal generation: HP 8903B > SSM2015 > Attenuator > OP275 > 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 = x ohms @ 1 kHz, x ohms @ 120 Hz.

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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

 

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 results

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4:1 Tx - 1 kHz sine wave, 800 Hz to 96 kHz, 1 Watt into 8 ohms.

4:1 Output, 1 Watt THD+N @ 1 kHz = 0.020%

1kHz peak is at -9.2db on the graph = 2.84 Vac RMS .

N.B. Compare with TPA6120 Line driver graph.

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2:1 Tx - 1 kHz sine wave, 800 Hz to 96 kHz, 1 Watt into 8 ohms.

2:1 Output, 1 Watt THD+N @ 1 kHz = 0.114%

1kHz peak is at -9.2db on the graph = 2.84 Vac RMS .

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2:1 Tx - 1 kHz sine wave, 800 Hz to 96 kHz, 2 Watts into 8 ohms.

2:1 Output, 2 Watt THD+N @ 1 kHz = 0.231%

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

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