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A4 Power Amplifier

A4 Power Amplifier

This amplifier uses four LM4780s working in inverted parallel mode. Each of these offers around 50 watts into 8 ohms, but are capable of delivering significant peak currents into lower impedances. A large custom made torroidal transformer forms the heart of the amplifier, designed with a low flux density for reduced mechanical noise, and incorporating an electrostatic screen to attenuate interference.
The sides of the amplifier are formed by a pair of heat sinks. Each heat sink assembly has two power amplifiers and a mode select switch. The following options are as follows

In normal use, the signals are applied to the inverting inputs of the differential amplifiers. These cancel the inversion of the power amplifier, which saves having to reverse the polarity of the loudspeaker terminals. Also, the input op-amps work in shunt feedback mode, thus avoiding the common-mode distortion that some FET-input op-amps such as the OPA2134 suffer from when the inputs are fed from non-equal source impedances.

The resistors surrounding the LM4780 give a voltage gain of 22.6, or 27dB. I chose 0.5% resistors here as well because in parallel-mode operation it’s vitally important to match the gain of the two amplifiers to ensure equal current sharing. As before, the resistors were hand-selected precisely. Incidentally, I paid a premium to buy 0.5% RC55 resistors, but I could have spent even more to have 0.1% devices. Happily, all of the 0.5% devices were comfortably within 0.1%, so I was pleased I didn’t spend more

The OPA2134 is powered from split 15 volt supplies. The unregulated supplies to the amplifier modules are around ±36V, which is on the limit for 7×15 regulators. So I chose a simple shunt power supply using two 12mA current sources and a pair of Zener diodes. The noise generated by the Zeners is filtered by a low-pass filter formed by a 15 ohm resistor and 47uF capacitor, turning over at around 200Hz. This frequency is comfortably low enough to ensure that the op-amps PSRR is sufficiently high to provide good ripple rejection. Finally, a low-ESR 1uF capacitor is connected between the power pins of the op-amp.

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