Q&A # 156 - January 12, 2000

Staff

Q Do balanced RCA type connectors work the same way as balanced 3 prong type connectors? Which is better and why - with reference to subwoofer use? I just purchased a Velodyne HGS 15. Should balanced (3 prong type) be used, or will the RCA balanced type work just as well (the sub is about 8 feet away from the preamp)? What kind of improvement in sound should be expected to be heard using such balanced cables over non balanced type RCA?

A There is no such thing as a balanced RCA connector, since the RCA connector (the most common type of connector used in audio/video) is a two conductor system, and balanced requires three. Here is a photo of the two types of connectors. Notice that the balanced XLR connector (right side of photo) itself is much larger than the RCA (left side of photo). The RCA has two conductors, the pin in the center and the surrounding shell. The XLR connector has three conductors, defined by the three pins. One of the pins is the ground and the other two carry the hot signal, each of which is inverted with respect to the other. For use with your subwoofer, just check to see if you have any hum problems with a standard RCA connector. Even if you do, you might be able to reduce it by moving the cable around. If that does not reduce the hum to acceptable levels, then you can get a balanced cable, but they are pretty expensive.

Q My local cable company broadcasts the big three (ABC, NBC, CBS) on the same channels as the air signal. Therefore, ghosting is a big problem on these channels. I have upgraded all my internal and external cabling . I installed a Radio Shack line booster that seemed to do nothing at low gains, and as I increased the gain, it added interference. At this point I'm not ready for a satellite dish. Are there other options?

A The only other option is a roof antenna. I get better reception with an antenna than with cable. However, satellite (DSS) has just begun transmitting local network channel affiliates for the major cities, so you can get regional programming now. You might also experiment with some of the high tech antennas that are sold in several markets, for getting your local stations. Some of them don't need to go on the roof.

Q I can't tell you how much I enjoy your site. Now to embarrass myself, can I ask you the difference (and preference) of amps that are MOSFET vs. bipolar and the difference between Class "A" and Class "B"?

A There is no need to feel embarrassed. Secrets is the source for all the info you need, basic and otherwise. MOSFET transistors are more "musical" than bipolars (hard to say what that means exactly, but that is the reputation anyway), but a designer needs more of them in the circuit because they are not a high current transistor. Bipolars, on the other hand, have more of a snap to their sound, and they produce high current, so designers can use less of them in the circuit. MOSFETs sound more like tubes, so as you can imagine, there are heated arguments about which is "better", MOSFETs or bipolars. Most companies use bipolars, but not necessarily because less transistors are needed. As to specific companies, Adcom uses MOSFETs in some of their amplifiers, as do Balanced Audio Technology and LLano.

Class A means that the transistors are biased so that current is flowing all the time regardless of whether or not there is a music signal going through. This allows the transistor to send the music to the speakers faster when a signal comes in. Class B means that there is not any current in the circuit when no music is playing, and when the signal does come through, the flow of current has to be switched on. This makes it a little slower to respond. Most hifi amplifiers are Class A/B, meaning that the transistors are biased such that a little current is flowing all the time, so up to a certain percentage of the rated output power, it is Class A. The rest of the output power is in Class B. For a mass market product, it might be the first couple of watts out of, say, 70 watts, that are in Class A. For a high performance amplifier, it might be the first 20 watts. A pure Class A amplifier would have all 70 watts in Class A, meaning that enough current is flowing at idle to supply 70 watts to the speakers when needed. Class A requires huge power supplies, and they are very inefficient. A 70 watt pure Class A amplifier would burn about 250 watts in heat when it is not playing music.

Q I would like to know what kind of Sound Blaster PC card I need to install a home theater system on my PC. Have you a suggestion about a complete "home theater system"? My actual configuration is Sound Blaster AWE32, Pentium II - 400, and DVD SCSI.

A If you want a digital output to feed a Dolby Digital decoder, the Sound Blaster Live is a great card.

Q  I have a surround system, but I purchased a new house and instead of trying to hide rear speaker wires, I am looking for some wireless rear speakers. Can you help me with this?

A That depends how picky you are. Recoton makes some wireless speakers that work. You still have to run an AC power cord, so they're not exactly wireless. I wouldn't go so far as to call them really good, but they'll fill the rear portion of the room. It's been awhile since I've seen them, but I think they sell for $200-$400 per pair. As an audiophile, I wouldn't recommend wireless speakers because they're usually not very satisfactory. You're paying for a transmitter, receiver, and power amplifier which leaves very little for the speakers themselves. As someone with a background in home installation, I'd lean towards running wire under the floor, in the ceiling, and up/down the appropriate wall, perhaps resulting in a little sheet rock work. You'll then have NO wire except for the six inches or so directly behind the speaker (maybe mounted on a wall bracket), better sound, and maybe even a little pride about a job well done. As for placing the surround speakers, even though you didn't ask, try to get them slightly behind the main listening area, in relation to the front speakers, but more to the sides, and perhaps six feet or so up if it's not too much of an eyesore.

Q I want to add an external D/A converter to my DVD & LD front end. I was using the D/A converter of my receiver up to now. If I used the analog line from my front end, the D/A conversion in the players would also not be very good. In any case, this analog signal from the front end to the receiver will be converted to A/D for DD & DTS processing, and later reconverted (D/A) for the power amplifiers. I am not very happy with the quality of my receiver's D/A conversion. If I use an external D/A converter after the front end, my signal will still go through the receiver's D/A,A/D conversion process. Please tell me what to do, in order to reap the benefits of better D/A conversion.

A Many receiver these days have a bypass mode, so you can just use the volume control of the receiver rather than having the signal go through any further A/D and D/A. So, you should look for a good D/A for your transports, making sure you connect the digital outputs of your LD and DVD player using 75 Ohm cables, then route the analog outputs of the D/A converter through the bypass circuit of the receiver.

Q I have a Pioneer DVD player (DVL-9) connected to a Yamaha RX-V992 receiver. I play CDs about 70% of the time, using the D/A converter in the DVD player. That means the amp set to "effect off", and the input (from player) selected for analog (I have a parallel connection for the optical, digital, and analog input). I heard good comments on pairing of ICL Model_2 tube D/A converter with the Pioneer DVD player. You may refer to the web site <http://www.icl.co.jp/audio/english/index.htm> for further info. Do you have any comment on such matching?

A There are several tube DACs out there, but, in general, they just use tubes in the output stage. This tends to smooth the sound and can be useful when a system has a harsh edgy tone to it. However, my suggestion is just to get a better player for use with CDs. As I have mentioned before, DVD players are not very good for playing conventional CDs, with the exception being just a handful of DVD players that have dual lasers.

Q I have a VSX-D308 receiver. When I turn up the volume, it overloads and cuts out. I can barely hear it . Could it be the speaker wires or is it too much power for the speakers?

A You are overloading the receiver, not the speakers. You don't have too much power, you have too little. For receivers, you should use 8 Ohm speakers. Using 4 Ohm speakers can easily overdrive the amplifiers, and they will shut down temporarily. The best alternative, however, is to get a receiver with pre-out jacks, and have an outboard power amplifier (at least 100 watts per channel, preferably more) for those window blasting sessions.

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