Power to the People – the Benefits of the Power Amplifier
27/11/2001
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by Gavin Womersley
The vast majority of home theatre systems contain at their centre the ubiquitous surround receiver. Costing anything from around $500 to more than $6000 for the top models, they are by far the most popular choice for anchoring surround systems. On the whole they offer amazing value with good quality chips handling advanced processing of the digital formats we enjoy today, plenty of connection options and a level of general sophistication that would have been unheard of just a few years ago. They are not perfect however, and their biggest weakness lies in their most basic task – amplifying speakers.
Why this is so will be examined below. While the traditional alternative – the surround pre-amp power-amp combination is still the best choice for those who can afford it, our focus here will be making good use of the popular surround receiver as a processor/pre-amp and adding high quality muscle for a high value as well as high performance combination.
Surround Receivers - Limitations
Surround receivers are sold largely on the basis of price and specifications. Manufacturers elbow their way through the competitive market with models that have Dolby Digital, DTS, component video switching etc, etc at ever-lower price points. These are features that help stake out their products as great value. It is unfortunate that some important parts of the amplifier do not get the attention they deserve, yet they are vital to the amplifiers core function of powering speakers. I am talking about the amplifiers power supply, the size of its transformer and heat sinks. Things like this will tell us how well the amplifier is able to generate the current required to drive a loudspeaker.
In a typical surround receiver you have crammed into a small space five amplifiers, the main power transformer, heat sinks, preamplifier circuitry, front display circuitry, video switching and so forth. In short there is a dam lot of stuff in that box - too much in fact. There simply is not enough space both in the manufacturers budget and in the amplifiers chassis to accommodate items such as a decent size power supply and heat sinks. Shane Buettner Equipment Review Editor from Widescreen Review compares dedicated power amps to surround receivers when he states:
“As an example, the dedicated power amplifier that I use as a reference has a power transformer rated at 2.2 kVa for a five-channel amplifier. The Denon AVR-5800, one of the best sounding A/V receivers I’ve heard, has a transformer rated at 1.1 kVa. Under no circumstances will the AVR-5800 receiver be able to deliver as much current to the output stages of its amplifier section as my dedicated amplifier. The number and quality of the output devices on each channel will be limited in the A/V receiver as well, due to physical size restraints and to the limited amount of real estate in the A/V receiver along with cost considerations." (Widescreen Review; 46; p. 92)
He goes on to say that higher current designs generate more heat and thus require more heat sinks, which again are not easy to do in a surround receiver. In short the manufacturers of dedicated power amps do not have the constraints that surround receiver manufacturers have upon them.
All this would only be of marginal interest if it did not have an effect on sound quality. However these things affect the sound quality of music and movies to a massive degree. Any speaker regardless of price will sound better when supplied with all the current it requires. Whether large or small, whether you listen loudly or softly or whether it is music in stereo or a film soundtrack, the differences are immediately apparent. Again, Buettner wraps it up succinctly:
“Now the common misconception here is that all of the above only translates into higher volume levels from the dedicated amplifier. Strain, congestion, distortion and dynamic compression are all terms that can be used to describe what an amplifier sounds like when it can’t deliver the current necessary to drive a loudspeaker during transient peaks (which occur during music playback as well as movie soundtracks).” (Widescreen Review; 46; p.92)
It is the transients in music and music that the differences between dedicated power amps and surround receivers are most strikingly apparent. However at all times the sound is superior, not just during peaks. The amplifier simply has a much better hold of the speaker allowing for a cleaner more precise and yet relaxed and smoother sound.
The Sound Alternative
As mentioned in the introduction the traditional alternative to the one box surround receiver is the pre-power combo. This is still the best way to go but is often very expensive. Surround pre-amps are not thick on the ground and with the exception of two models from Rotel ($2500 & $3700), virtually all cost upwards of $4000, with many giving little change from double this figure. Add the price of a power amp and things start to get expensive.
The alternative to this however lets you enjoy the potent dynamics of a movie soundtrack and the subtle nuances of the quietest passages of your favourite music without breaking the bank. This alternative lies with a cluster of sockets on the back of your surround receiver called pre-outs.
Line level outputs for all channels from the back of your surround receiver allow the connection of a multi-channel power amp. The power amp would then power all the speakers. Surround receivers down to just under $1000 have been seen sporting these outputs, so it is an option open to many. With the relatively high chip quality and processing power of many of these receivers they can make surprisingly good surround pre-amps. Adding a surround power-amp will take the performance to a whole new level as will be seen below.
Shane Lord, a reviewer for Australia’s most respected DVD website michaeldvd.com.au uses his accurately calibrated surround system as a tool for critically reviewing DVD soundtracks. With a small room and small speakers one would not have thought he was obviously in need of a powerful surround amp as he had just bought a advanced new surround receiver, one with great chips and supposedly plenty of power. This is what he found when he took the Elektra Theatre multi-channel power amp home for a test drive:
“The sound difference was stunning. Even on smaller speakers such as my own the power-amp enabled sound had greater depth and clarity, and far less "muddiness". I could hear more information than I had before. Whether it was music or movies, the power amp proved it could outclass the far less capable amp in the receiver. It was as big a difference as that between mono and stereo. Simply put, I was stunned. Such a major difference and it didn't require replacing the receiver. Even in my relatively small room, with my mid-priced speakers, the quality difference is unquestionable. Friends and family now prefer to come here to see a movie than go to the cinema. The sound is so much better now that I am thoroughly enjoying going back through my DVD collection & CD's just to listen to them again.”
Needless to say, Shane now owns the Elektra Theatre.
Normally we keep power amps in a separate area to surround receivers and it was not possible to do an A/B comparison. Now however we have a multi-channel power-amp in our home theatre comparison room at all times so you can hear the incredible difference the power-amp makes. The difference is absolutely huge.
Of course we have a number of different surround power-amps. Brands include top end gear like Krell and Musical Fidelity, mid-range gear like Perreaux and cheaper models from companies such as Rotel. We have five channel power-amps starting from the Rotel RMB-1075 120w x 5 at $1900 although the standout model in terms of quality and value for your dollar is the Australian designed and built Elektra Theatre ($3000). Weighing in at 27kgs and with 150w x 6 it sounds superb for music and movies and has a degree of slam and attack that has to be heard to be believed. Unlike all surround receivers under about $3500, power-amps, with their huge power supplies, can run all their amplifiers at their full power simultaneously and most will come close to doubling their power output when impedance halves. This means that they can run almost any speaker you care to name, even in big rooms.
Conclusion
I would urge anyone in the market for surround receivers over $3000 to first hear what an inexpensive surround receiver (such as Marrantz’s brilliant new SR4200 or SR5200) sound like running with a power-amp via the receivers pre-outs before doling out their hard earned on an all singing and dancing surround receiver. If you do the comparison you will not ever choose even a $5000 or more receiver on sonic grounds over a cheaper one with a power-amp. For those of you already with a flashy surround receiver - do yourself a favour and check out its back panel. If it has pre-outs you have the easiest upgrade choice you will ever have to make, just be sure to strap yourself in when the surround power-amp arrives!
