Q&A # 254 - September 30, 2001

Staff

Q You guys haven't been doing many updates lately, and that usually means you're up to something big. Am I right? Care to let us in on the secret?

A It is a combination of being bummed out from the disaster in New York City, having been traveling, I am the one (JJ) who has to write most of the answers (it takes research time), getting ready for the individual DVD player reviews that we tested at Benchmark 2, and writing a large review on the new Theta Casablanca II 24/384 DACs that no one else has reviewed before.

Q After reading your primer on amplifiers, I am confused about the definition of class AB mode of operation. In this primer class AB operation is explained as a combination class A for low current and class B for high current. This is in contradiction with the explanation given in another article by Rod Elliott (http://www.sound.au.com/class-a.htm). In this article class AB operation is explained as a mode of operation partway between class A and class B: the output devices do not conduct 360� of the sinewave (like class A) or just 180� (like class B) but rather between 180� and 360� so that two output devices of a push pull amplifier conduct partially overlapping in time.

A Think of it as being that during Class A operation, there is current already flowing in the output circuit even with no music (the CD is in pause mode), but when the music comes through, that current is diverted to the speakers. During the no-music period, that current is dissipated as heat. In Class B operation, there is no current flowing in the no-music period, and when music comes through, the current has to be switched on. A Class AB amplifier is biased so that part of the current that is necessary for full power output is flowing during no-music, but not all the current that is necessary for full power output. Let's say it is a 100 watt amplifier. If enough current is flowing at no-music to supply 20 watts, then the amplifier is said to be biased 20 watts into Class A. The remaining 80 watts are biased for Class B. Up to and including 20 watts operate in Class A and watts 21 through 100 operate in Class B.

Q I just purchased the Cambridge Soundworks T500. It comes with an active subwoofer and a passive one. How does this all work? And is this type of setup better than having two active subwoofers?

A An active subwoofer has its own built-in amplifier, while a passive subwoofer is just the driver in an enclosure. You have to supply the power amplifier. However, there are also subwoofers that have an active driver connected to the power amplifier, and a passive radiator (often just the cone with a weight on the back) in the same enclosure. The active driver compresses and decompresses the air, which causes the passive radiator to move too.

Q I have stereo speakers (Bose AM5) and had a stereo amp. Now I bought a DD and DTS amp (Marantz SR-5000). I read many reviews that said that Bose aren't good especially for home cinema. I'm confused. What should I do?

A Bose is designed for people who like the compact modular look and feel. The are very spouse friendly and will fit just about anywhere. In my opinion, they are pretty high priced for the product, and that is because they spend a lot of money advertising in high profile places like Time Magazine. They sell more speaker systems than any other company because of superb marketing. Besides the price, their drawback is that the satellite speakers are so small, the crossover to the bass module has to be high, and you can localize the sound to the bass module which handles all the channels. Whether they are good or not is a matter of your own taste in sound. If you like the sound, get more Bose. If you don't like the sound, then get something that has larger drivers for the satellite speakers.

Q My friend is about to assemble a home theater on a budget, and so far, here's the setup: Polk RT55i, RT35i, and CS400i (JBL PB-10 sub) leaving about $400 for a receiver. We're looking for a suitable AVR to compliment the Polk speakers. I read your review of the RT800 towers, and I've concluded that it takes good, clean amps to make Polk RT speakers shine. I suggested the Harman Kardon AVR-210 rated at 45 watts per 5 channels and the new Denon AVR-1802 (80wpc). The Onkyo 595 is third. HK claims to have large capacitors and high current design leaving me to believe that dips in speaker impedance are easily compensated. Moreover, HK seems to have a reputation of producing deep, warm sound despite being less powerful than its  competitors. Given the specs, can HK receivers handle dynamic Dolby Digital, SACD, and DVD-A sound tracks or do we need the extra few decibels headroom from a receiver twice as powerful? What's the real story with high current design?

A High current is a relative thing. A 10 watt amplifier (at 8 Ohms) that doubles to 20 watts at 4 Ohms and 40 watts at 2 Ohms could be called a high current design. A 100 watt amplifier (at 8 Ohms) that goes to 140 watts at 4 Ohms and is not rated into 2 Ohms, would not be a high current design. In my opinion, 45 watts per channel into 8 Ohms with 8 Ohm speakers will not cut it with DD, DTS, DVD-A, and SACD unless you play your discs at very low volume. The dynamics of digital surround are just too demanding. However, keep in mind that there are 50 watt amplifiers out there that will outperform 100 watt amplifiers because of the non standard way in which various amplifiers are rated. Marketing departments like to stretch the specs as much as possible. Most power amplifiers are Class AB, so I just look at the capacitors and power transformers to get an idea of their muscle.

Q I have a big laserdisc collection and need to buy a combination LD/DVD player. The one I looked at seems to imply its digital output jack only works for the DVD and not for LD. Do you have any idea on this?

A As far as I know, the digital output jack will work for all the discs you play in the combination players. However, you will need the AC-3 RF output for DD LDs. DTS digital out for LDs and DVDs should go through the digital output jack and not the AC-3 RF output jack. DD digital out for CDs will go through the digital output jack as well. If readers know of some exceptions, please let us know so we can post them.

Q I can't find B&K processors on the web.

A Their website is http://www.bkcomp.com.

Q Does anyone make a simple multi-channel line stage, analog preamp? I would need inputs for six-channel sources and a couple of two-channel sources, switching between them, volume control (overall assuming the balance between channels is handled by the player - or individually for each channel) and output to amplifiers.

A John Kotches is testing one now, but it is very expensive. I assume you are asking because you want to bypass the processor and just go from the player to the preamp and to the power amplifiers, as we did before surround sound. Processors coming out now seem to have pretty good preamplifier sections, and you just use the 5.1 pre-in jacks, which bypasses everything but the volume control. Another possibility though would be to purchase three stereo preamplifiers and just wire them so that each preamp receives two channels of the analog output from the player. I would wire the front left/right to one preamp, center and LFE to the second preamp, and the rear left/right to the third. You could use three Pure Class A Single Ended Triode preamplifiers this way, and the sound would be spectacular.

� Copyright 2001 Secrets of Home Theater & High Fidelity
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