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Inside an AV receiver 2 - VHF tuner and power amps

Article Index
Inside an AV receiver 2
Inputs and outputs
Video connections
Signal processing
Loudspeaker tuning
VHF tuner and power amps
All Pages


RADIO TUNER

Most receivers have a VHF/FM and AM Medium Wave tuner. Some come with Internet radio,  DAB (Digital Audio Broadcasting) and Sirius satellite radio (USA). These are conventional tuner modules and work in the same way as any tuner, giving stereo output. It can be processed to surround-sound if wished, using Dolby Pro Logic or DTS Neo 6. DAB and Sirius compress the audio signal to reduce data rate. DAB offers more variety than VHF/FM in the UK, but the latter offers better quality if an aerial is used to avoid hiss. Internet radio offers mediocre quality from (mostly) MP3 compressed files at low data rates, but internet connected stations from around the world (circa 14000) become available.

 

DAB remains unavailable in many UK regions and even in London may need a dedicated DAB aerial, because obstruction by local buildings and hills substantially reduces signal strength, due to the high carrier frequency used. Basements are DAB black spots too.

 

Surround-sound radio and television broadcasts are available from Freesat in the UK, but a quality Freesat receiver is needed (see Humax). Connection to a receiver is via S/PDIF. Programmes are coded in Dolby Digital (AC3) or Dolby Digital Plus. Only a limited number of programmes, like music concerts and nature programmes, use surround-sound because it does not suit situations where forward facing directional microphones are used to minimise unwanted background noise.

 

 

 

rear-panel-onkyo

 

THE POWER AMPLIFIERS AND POWER OUTPUT

Modern receivers are fitted with seven on-board amplifiers, usually each of 100 Watts into 8 Ohms and more – around 140 Watts – into 4 Ohms, one channel driven, our measurements show. The bigger receivers manage 120 Watts or so, but have better regulation from bigger mains transformers and typically will produce 160 Watts or so into 4 Ohms. Rarely will all channels be required to deliver full power in use, so the issue of power output  under these conditions, when the power supply would be severely loaded, is academic. With reasonably sensitive hi-fi loudspeakers (87dB SPL from 1W) the 700 Watts available produces very high volume and, unlike a stereo system, the sound fills the room evenly. A perceptible benefit is that with this power distributed among five to seven loudspeakers, each is less stressed, giving surround-sound a sense of easy dynamism.

 

Manufacturers like Onkyo and Marantz suppress low level non-linearity by biasing their output stages substantially into Class A in the usual Class A/B configuration and, over seven channels, this makes the receiver run hot. Cooler running receivers may seem preferable but sound quality suffers, our listening tests show. Whilst this would seem to be an ideal role for ‘digital’ switching amplifiers, they are now not used, a major U.S. manufacturer telling us failure rates were unacceptably high.

 

The distortion characteristics of high quality AV receivers are nowadays (2010) similar to those of hi-fi amplifiers (e.g. <0.1% thd at 10kHz, 1W), although not up with the best. Good quality AV receivers (e.g. Marantz) sound very good, on par with stereo hi-fi amplifiers. Onkyo are in the race too. Unfortunately, most other brands seem unable to meet hi-fi standards and their amplifiers sound coarse, dimensionally flat, papery and somewhat crude against better hi-fi components. Much of this is  attributable to poor component quality; every part must be ultra-cheap and this is no way to produce a good sounding amplifier.

 

AV receivers are built to a tight budget and their loudspeaker terminals are usually wobbly, close together and difficult to use (see above). In this environment 4mm plugs make a lot of sense, and it is best to put ID tags onto the many leads from the loudspeakers. Onkyo use a neat colour coding system that other manufacturers seem unwilling to adopt. They supply colour stickers that attach to the speaker cables and match colour coded terminals, a great help in the spaghetti junction that is the back of an AV receiver. But Onkyo also fit safety inserts that are difficult to remove and they don’t explain in their user handbooks how to remove these plugs; a self tapping screw must be used to haul them out. The safety plugs in other receivers are either non-removable or can be flipped out with a sharp knife or screwdriver, so 4mm plugs can be used.

 

Our block diagram shows that the 7.1 inputs run directly through the volume control and into the analogue output stages, missing the DSP, so loudspeaker distance and level compensation and all other processing is unavailable. If a multi-channel player is connected this way, perhaps an old SACD player, then ideally it needs loudspeaker compensation on-board.

 

Although complex and difficult to operate AV receivers can provide excellent results from a wide variety of sources, including the internet and even LP. The technology is daunting but the results – in the best products – are outstanding.




 
Comments (1)
AV receiver technology guide
1Thursday, 21 July 2011 09:06
Ian Walker
Very easy to understand & well written.

Look forward to similar articles on other subjects.

Ian Walker

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