AVR400 Power Amplifier Circuit Description

Apart from the power transformer, the power amplifier electronics is fully contained on the large double sided PTH PCB and heatsink located at the bottom of the AVR400. This is called the Main Board on the schematic diagrams (pages 11 and 12). The Main Board also contains the mains input circuitry, in- cluding the safety fuses and standby power transformer, so great care should be exercised when probing this area of the board.

Note that some small surface mount components are soldered to the underside of this PCB.

The 7 power amplifiers are identical in terms of circuitry, although necessary compromises in the physi- cal layout may give rise to slight differences in measured noise, crosstalk and distortion performance. The two channels at the extreme ends of the heatsink have less radiating area available to them and will run hotter under load – this is not normally an issue as these are assigned to the SBL and SBR channels.

Looking from the rear of the AVR, facing the flat face of the heatsink, the channel order is SBR, SR, FR, C, FL, SL and SBL, the same order as the loudspeaker terminals. The SBL and SBR channels can also be assigned to zone 2 or as duplicates of the FL and FR channels for when passively biamping the main stereo loudspeakers. In this latter condition we recommend assigning the SBL and SBR outputs to the tweeters of the FL and FR speakers, in order to minimize the power amplifiers’ heat dissipation.

Note that the 8 pre-amplifier outputs are also on this PCB – apart from SUB their phono sockets are effectively in parallel with the power amplifier inputs, which are fed from the Input Board via a ribbon cable and CON103. This connector also carries 5 power supply and power amplifier control signals to and from the system microprocessor (μP) IC151 situated on the Input Board above the Main Board.

The amplifiers’ power supply is provided from a centre-tapped secondary winding on the toroidal power transformer, via the connector BN508, to the bridge rectifier D5830. This is mounted on a small PCB near the top of the heatsink. The rectified AC is then sent to the main PCB via connectors BN581/582. To avoid induced hum and distortion it is important to keep these cables twisted tightly together and well away from the actual power amplifier circuitry. The main 15,000μF 80V reservoir capacitors, C835 and C836, are positioned on the main PCB well away from the power amplifier input traces and close to the system star ground. The smoothed DC is fed to the power amplifiers’ Vcc and Vee lines near the centre of the heatsink via a twisted-pair cable, again to minimize induction into the power amplifiers. Vcc and Vee are typically +/- 52V at 234VAC with no signal. Q5845 sends a fraction of Vcc to the muting control on the Input Board.

The FL (front left) channel will now be described in detail.

The input stage is a long tailed pair Q5101 and Q5102, with local degeneration provided by R5105 and R5107. The tail is fed from the negative rail via an approx 3mA ring-of-two constant current source, Q5109 and Q5110. R5101 and C5101 at the input provide high frequency rolloff and help keep residual DAC ultrasonic noise above 100kHz out of the power amplifier. DC blocking is provided by C5102 at the input and C5107 in the feedback loop so that the whole power amplifier has unity gain at DC. The midband AC gain is 22000/680 = 32.35 after allowing for the attenuation provided by R5101 and R5102. Thus 875mV at the input produces 100W into 8 ohms at the output.

The long tailed pair’s collectors are loaded by a current mirror, Q5103 and Q5104. The resistors R5103/4 and R5105/6 are 1% tolerance to minimize even order distortion. The collector of Q5101 also feeds the Darlington class A voltage amplifier stage (VAS) made up of Q5115 and Q5116. Q5113 is loaded by the output stage and the 8mA constant current source made up by Q5125 and Q5126. The amplifier’s main frequency compensation network (for stability) comprises C5115 plus the combination of C5116 and R 5115. This adds gain inside the loop (two pole compensation) at high audio frequencies so that the

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Arcam service manual AVR400 Power Amplifier Circuit Description
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AVR400 specifications

The Arcam AVR400 is a high-performance AV receiver designed to provide an immersive home theater experience. As part of Arcam’s esteemed lineup, the AVR400 combines advanced technology with exceptional sound quality, catering to audiophiles and home cinema enthusiasts alike.

At the heart of the AVR400 is its impressive power amplification, providing 100 watts per channel across 7 channels, ensuring a robust performance capable of driving a wide range of speakers. This power output is complemented by a high-current design, which means the receiver can handle dynamic audio passages without distortion, offering clarity in both subtle and explosive soundtracks.

One of the standout features of the AVR400 is its support for various surround sound formats, including Dolby TrueHD and DTS-HD Master Audio. This allows for an unparalleled audio experience, transporting users right into the heart of the action. Additionally, the AVR400 is equipped with the latest HDMI inputs, ensuring compatibility with a multitude of high-definition sources, including Blu-ray players, gaming consoles, and streaming devices.

The AVR400 also boasts advanced video processing capabilities, supporting 4K pass-through as well as upscaling of lower-resolution content to 4K. This makes it an ideal choice for users looking to future-proof their home theater setup, as it can seamlessly integrate with modern 4K televisions while enhancing the quality of older content.

User-friendliness is another essential characteristic of the Arcam AVR400. It comes equipped with an intuitive on-screen menu system that simplifies the setup process. The inclusion of an automatic calibration system helps optimize speaker settings in accordance with the room acoustics, ensuring users achieve the best sound possible from their setup.

Furthermore, the network features of the AVR400 allow for streaming audio from various online services and wireless devices. Built-in support for DLNA enables users to access their media library wirelessly, while the USB input allows for direct playback from flash drives or external hard drives.

In terms of build quality, the Arcam AVR400 embodies the brand’s commitment to excellence. With a sleek design, high-quality materials, and an engineered chassis that minimizes vibration and interference, the AVR400 is not only visually appealing but also built to deliver performance over time.

In summary, the Arcam AVR400 is a feature-rich AV receiver offering a perfect blend of power, precision, and versatility. Whether for movie nights or music listening, it stands out as a reliable choice for those seeking high-quality audio and video reproduction in their home entertainment systems.
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