Overview

As the stunningly original (!) name suggests, the A4 contains 4 separate power amplifiers. This compact unit offers great flexibility - the following modes of operation are available:

  • Four-channel 50 watts per channel operation for surround-sound or multi-room operation.
  • Two-channel bi-amped mode, for suitable bi-wireable loudspeakers.
  • Two channel Bridged mode, offering around 150 watts per channel.

Combinations of the above are possible. For example, in a home theatre application, rear speakers could be driven separately, while a centre speaker could be bi-amped or bridged.

This amplifier uses four LM4780's working in inverted parallel mode. Each of these offers around 50 watts into 8 ohms, but are capable of delivering significant greater powers 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.

Operating mode selection

Mode select switch (28K)

Each side of the amplifier has two power amplifiers and a mode switch which selects from the following options:

  • DUAL

    Each amplifier is completely separate.

  • BI-AMP

    The signal applied to the top input socket is sent to both amplifiers.

  • BRIDGED

    Again, the signal applied to the top input socket is fed to both amplifiers, but the lower amplifier inverts the signal. The negative terminal of the loudspeaker is connected to the lower positive output terminal, resulting in bridged operation. The output power is increased to around 150 watts.

  • SINGLE

    When a channel is not required, selecting this option will mute the lower amplifier. This option helps ensures that the heat dissipation is shared across both heatsinks in a stereo 50 watt per channel application.

Control circuits

The microprocessor-based control circuits respond to the front panel switch, the 12 volt trigger input, temperature sensors, a DC detection circuits and mains supply failure. A soft-start circuit is incorporated to prevent large inrush currents.

Front panel switch

Power switch - click to see more! The stainless steel front panel switch incorporates the blue power LED. Brief presses will toggle between "on" and "standby" modes, and holding the switch for more than half a second will mute the amplifier; this is indicated by the power LED gently flashing. A brief press will cancel this.

During Standby, the brightness of the red LED gently rises and falls, indicating that the amplifier is happily "sleeping". While an interesting programming exercise, the concept was shamelessly copied from Apple!

12 Volt trigger inputs

This amplifier has a 9 way D-type connector which is compatible with my remote-controlled preamp. A rear-mounted toggle switch determines whether the amplifier should respond to the main or surround trigger signal (the preamp generates the "main" signal when brought out of standby, and will additionally generate the "surround" trigger when the multichannel source has been selected). A rear-panel LED lights to indicate the presence of a 12 volt trigger signal, which confirms the toggle switch is in the right position. And while a 12 volt signal is present, the front panel switch still works as normal. While this might seem like an obvious thing to say, it should be realised that this is not always the case with similar products!

When mains power is applied to the amplifier, the status of the 12V input is checked, and the amplifier is turned on if 12V is present. Otherwise, the amplifier defaults to standby mode.

Supervisory and protection circuits

Should the amplifier overheat, the outputs are muted, and the front panel power LED is flashed rapidly to indicate why the output has disappeared. This flashing is quite distinct from the gentle rise and fall of the LED during the user-selectable mute mode.

If the overtemperature condition persists for more than around 5 minutes, the amplifier is shut down, and the red standby LED flashes an error code. To recover from this state, a "hard reset" is required (switching the mains off and on again) once the amplifier has cooled sufficiently.

The presence of a DC potential on any output will cause the amplifier to be shut down immediately. As above, an error code will be displayed on the standby LED, and the amplifier must be reset before operation can be attempted.

The control circuits manage the power supply, and ensure that large inrush currents are prevented at power-on (a common problem with audio power amplifiers). Anti-cycle routines ensure that the amplifier isn't damaged by being rapidly turned on and off. A rapid-action mains-failure detection circuit is employed to ensure the control circuits are properly reset, ensuring noise-free operation and freedom from "lock-ups" (always a potential problem with microprocessor-based control circuits).