Q&A # 273 - January 25, 2002

Staff

Q I have a passive subwoofer driver (15") that I want to drive with my stereo amplifier. How do I bridge the amplifier to get more power?

A Bridging two amplifiers requires that you send the signal to one channel as is, but inverted to the other channel. Then, you connect the speaker to the two + speaker terminals, one from each of the two amplifier channels. Some amplifiers have a bridging switch that does this (inverts on one channel) for you, but if yours does not have it, you would need a phase inverter. Inverters can be found on the web, for example, at http://www.avahifi.com/bridge.htm. They can be pretty expensive, so you might try building one yourself, if you are handy at such things. Here is a link for DIY http://sound.westhost.com/project14.htm. You get crossover distortion with bridging, but it is not so important at subwoofer frequencies.

Bridging a stereo amplifier to operate as a monaural amplifier adds opposite voltage values from each output, so that the voltage swing capability will double compared to that of a single amplifier. This also doubles the gain 6 dB over a single channel, so that bridged amps will usually sound more powerful just because the volume is louder at a given volume setting. However, it doesn't increase the current delivery capability over a single channel, as each channel must produce X amps (each) to output the same amount of current, as the outputs are running in series with each other. The result is that you will get twice as much power into the given load as you would with a single channel driving half that impedance. For instance, if a stereo amp could deliver 60 watts per channel into 8 ohms, and 100 watts/channel into 4 ohms, you could then deliver roughly 200 watts into 8 ohms. However, since each channel of a bridged pair is "seeing" half the impedance, i.e., 8 ohms looks like 4 ohms, and the current drive capability of the bridged pair is no more than a single channel, the bridged configuration compromises low impedance drive capability, so that distortion levels may be higher. When driving a relatively low impedance, say 4 ohms or so, and both channels of the stereo amp share a single power supply, it may make more sense to simply use just one of the amplifier channels, so that it can monopolize the power supply, and deliver up to twice as much current.

Q What do you think about the Rotel RMB-1095 power amplifier? I'm wanting to add a power amp to my system without going over board. If you have any other suggestions give me some ideas. I'm currently running my system with a Sony # DA59ES and B&W speakers all the way around with a Paradigm #PS1000 subwoofer. Don't laugh. It's what I can afford.

A The Rotel RMB-1095 is an excellent five-channel power amplifier. One of the best buys in home theater. In fact, we are using it right now in one of our labs.

Q I have a Sony KWP-65HD1 and its display format states that 480p input will be displayed as 480p, 720p will be displayed 1080i and 1080p as 1080i. Is 480p better than 1080i display?

A This all has to do with the TV's maximum scanning frequency capability. 480i uses 15.75 kHz, and 480p uses 31.5 kHz. 1080i uses 33.75 kHz, which is just a little more than what is needed at 480p. 720p requires 45 kHz, and it is more expensive to make a TV with high scan rate capability. Your TV is engineered to scan at no more than 33.75 kHz, so that is why 720p and 1080p are downconverted to 1080i. 480p and 1080i are fine as is, so they do not get changed. Native 1080i will look better than 480p.

Q Are there some generic hints for eliminating hum from an A/V receiver? I have a Philips all-in-one home theater system which emits a hum through the center and surround speakers beginning about 2-3 seconds after it is powered on. The only solution I have tried so far is to remove all other components plugged into the same power strip.

A Most of the time, hum is produced by a ground loop, and this kind of problem increases in proportion to the number of different components (TV, Cable box, DVD player, receiver, power amplifiers) that are all connected together. There are lots of things to check when trying to get rid of the problem, and here are some links that can help: http://www.smr-home-theatre.org/Ground-Loops/        http://pages.prodigy.net/jforgione/FT_HUM.html         http://www.laaudio.co.uk/laaudio_jan01/pdfs/DIXDAT.pdf. One of the most common causes is the cable TV connection, because it uses a different ground than the one for your home's electric breaker box. Try disconnecting that first to see if it solves the problem. If so, then get a ground isolation transformer, such as sold by Mondial. It goes between the cable coming out of your wall, and the TV.

Q I currently have four Bose 301 speakers, front L&R and rear L&R for surround. I am about to start upgrading my home system for 5.1. Are these speakers adequate for a 5.1 systems or should I consider a non-reflecting type speaker?

A They should work fine. Their Direct-Reflecting technology is similar to other companies who make speakers with drivers on more than one surface of the enclosure. That is just their marketing term for it. You will need a center channel speaker, which could be a similar Bose model, but also a subwoofer. Get a sub with at least a 12" driver, and preferably 15" or 18".

Q I have a Sony DVP-S7000 and a library of around 200 DVD movies I have acquired over the past three or four years. All of the discs are commercial releases (no DVD-Rs). Two or three times during that period I would buy a DVD that would not play on the S7000. I would exchange it and the replacement would play fine. In the last year I have seen the percentage of discs that won't play increase significantly. The player will make a few clicking sounds and either the player display goes blank and the TV on-screen message says "Insert Disk" or the player will occasionally lock up. Recently I bought two discs that would play the first time, but fail to play on any subsequent attempt. A replacement would not play at all. My first thought was that my S7000 had gone out of alignment, but I have yet to find any disc in my library that will not still play properly. Obviously something has changed. If my player is the problem, why does it still play all of my existing library? Has there been a change in the media?

A The problem is that disc manufacturers have not all adhered to the specifications. DVDs are software. They have menus and execution files, and all the things that a computer program on your PC has. If the software does not match the spec for the operating system, it crashes, whether it is being run on your player or on your PC. The S7000 is an older player, and its operating system version is not as up to date as the newer players which don't seem to have as many disc playing problems. So, don't fret over it, because there is nothing we can do about it. Most discs work fine. To heck with the ones that don't.

Q Are Onkyo and Integra the same company? I read your review on the Integra 9.1 and realized I was looking at the backend of my Onkyo 989.

A Onkyo and Integra are two separate companies, but they do use the same product templates. Integra is a high-performance line that they wish to separate from their Onkyo mass market oriented line. Integra has larger power supplies. To me, it is like buying a car. You can choose the basic model with a V-6 and cloth seats, or the version with a V-8, leather bucket seats, and an 8 speaker audio system. There is a third company called Integra Research. They have the real high performance products, sort of like opting for the custom T-Roof and super charger in the new car. All three companies are separate in sales and accounting. Frankly, I think they should have changed the face template for all three lines, because it has caused some confusion, and you are not the only reader to ask us about them.

Q Does anybody make a flat wire for my rear speakers? I don't want to tear up the ceiling!

A Several companies make flat wiring for speakers. Nordost is perhaps the most well known, because they were the first to really develop the flat technology into an entire line of cables. I have used their 2-Flat wiring for rear speakers, with good results. It's inexpensive and goes under the rug.

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