Q&A # 78 - August 25, 1998

Staff

Q If I want to use more than one amplifier for my home theater setup, should I just buy a DVD player with digital output and a separate decoder that has 6 discrete lines out so I can run them directly to the amps that will power each speaker? For example, I have 4 speakers that are bi-amped, so there are 8 amps that need to be signaled. And if there is such a beast, how do I keep my CD player and tuner in the setup as well as other audio components?

A Whether the power amplifiers are all in one chassis or in separate chassis does not really make any difference. However, using separate amplifiers will increase the chance of ground loop hum. You can reduce this problem by using the same AC outlet for all the power amplifiers. To use a 5.1 decoder connected directly to the power amplifiers, you will need a volume control somewhere in the signal path. There are several models of power amplifiers with volume controls, such as the Carver AV-705. Keep in mind that a preamplifier's functions include switching in various sources, such as CD, VCR, DVD, TV, etc., producing sufficient voltage to drive the power amplifier, varying the output voltage to suit the required volume, and maintaining a constant output impedance. By using the decoder to drive the power amplifiers directly, you will not be able to switch in a different source, and unless the decoder has a volume control, you would have to use the power amplifiers' volume control which changes the impedance as you change the volume. The best plan is to get a surround sound preamplifier that includes the decoder or has a set of 5.1 inputs for a decoder. Receivers that have 5.1 inputs or built-in decoder, and a set of 5.1 preamplifier outputs ("pre-outs") can also serve as a preamplifier.

Q If one has a surround sound audio system and a stereo television, does one also need a HiFi VCR to enjoy movies in Surround Sound? Or would one accomplish this by hooking the stereo TV into the sound system, as opposed to the VCR?

A As long as your stereo TV has a set of stereo analog output jacks, you can watch broadcast movies using your surround sound system without having to turn on the VCR. In this case, connect the stereo analog out from the TV to an auxiliary or TV input on your surround sound system. If your TV does not have the set of stereo analog output jacks, then you would need to use the VCR's TV tuner set to the station that is broadcasting the movie in stereo, and connect the stereo output jacks of the VCR to your surround sound system. In either case, you need to turn the volume of the TV such that the sound is not coming out of the TV's own speakers. Depending on how your TV is wired, this may involve setting the volume to mute or just turning the TV's volume control down.

Q My question is with regard to "bass management" for Dolby Digital. I have a Yamaha RX-V2092 A/V Receiver, Definitive Tech BP2002 mains, C1 center, BP6 towers, a PF12 sub for rear/surround channels, and a pair of old Radio Shack Minimus 7s for the Yamaha's "front effects". I have the BP2002s tri-wired to high and mid speaker-level inputs and full range line-level (from Yamaha preamp outs) to the BP2002's sub inputs. I also have the Yamaha line-level sub outputs going to the separate LFE inputs on the BP2002s (technically they are quad-wired!!). I have the Yamaha set up as follows: Front speakers - Large, Rear speakers - Large, Center - Normal, Bass Out - Both (choices are main/sub/both).

If I add a sub to the center channel and change from "normal" to "wide", will there be any signal left going to the LFE channel? That is if all 5 (left/center/right/left rear/right rear) speaker channels are set to "Large" is there still a .1 channel in existence? Also, when you select small/large settings does that affect only the speaker-level, or both speaker- and line-level outputs? For instance, could I run the BP6s from the rear speaker outputs as small and still use the PF12 from the rear channel line-level outputs? I'm currently using the speaker-level outputs to the PF12 sub then to the BP6s. This gives a nice addition to the "rear" (my room is over 30' long, but the couch is only 18' from the front) that I couldn't get using the Yamaha sub outputs to the PF12, because with that setup the sub had to stay up front.

A Bass management is getting to be very complicated, but also very flexible. There is always some bass coming from the subwoofer output on the pre/processor regardless of how you set ("Large", "Small", None") your other speakers. Even if you set the subwoofer to "None", the sub-out jack will still have subwoofer frequencies (depending on the brand and model), but they are also sent to your main channel speakers in that case. The "None" setting for subwoofers just assumes you don't have a subwoofer connected in the system. With modern bass management, when you select "Small" for a set of speakers, you can direct the bass that would have otherwise been in those channels (or channel in the case of the center) to other speakers, such as the front mains, or to the subwoofer. However, with DD and DTS, even if you set all the speakers to "Large", there is still a .1 channel (LFE) that contains information. It may very well just be a duplication of low frequency information that is in the other channels, but it is a separate channel. One of the problems in using bass management to direct bass from one channel to another is that the cutoff frequency is too high (in my opinion), usually around 90 Hz, which means some of the redirected signal is directional (you can detect its location) from a speaker that it was not supposed to be coming from. I am hopeful that future bass management will include several cutoff frequencies to choose from, such as 50 Hz, 70 Hz, and 90 Hz, with different cutoff frequencies being selectable for the various channels simultaneously, rather than just one cutoff frequency at any one time.

Q I would like to know if you prefer bipolar speakers or direct radiating speakers for Dolby Digital and DTS.

A It all depends on the room size. With a small room, or where the speakers have to be rather close to the walls, direct radiating work best. If the room is large and you can put the speakers several feet out from the wall, bipolar and dipolar speakers can deliver spectacular sound. However, bipoles and dipoles are more diffuse in their sound compared to direct radiating, and some people prefer a more localizable sound rather than diffuse, regardless of the room size.

Q I have 2 VCRs (1 stereo & 1 mono), and I want to dub a mono tape using these 2 VCRs. How should I do this? Should I playback the tape on the stereo VCR using only an output from the left or right jack into the mono VCR or do I need a Y-Adaptor so I don't lose any of the sound. Is there a difference in the quality of the audio recording or playing back mono tapes in stereo?

A I assume that your stereo VCR is a "Hi-Fi" stereo VCR, meaning that it has the stereo audio recording/playback head on the same rotating head that contains the video recording head. These rotating audio heads do a much better job than the older stationary audio recording heads. Your mono VCR probably uses one of these stationary audio heads, so I would suggest using a Y adapter from the audio out jack from your mono VCR and connect it to both (right and left) audio inputs of your stereo VCR (so you would be copying the tape from the mono VCR to the stereo VCR).

Q I am planning to buy a CRT projector in the next few weeks. However, the cost of the screen is a worry to me. What is the advantage of spending a thousand dollars on a screen, when perhaps you could see just as well with a screen used for viewing slides or a smooth white wall, costing much less.

A The slide projector screen should be fine, but they are usually much smaller than screens designed for TV projectors. Screens are used rather than walls for several reasons: color, diffusion, and reflection. Screens are color balanced so they are white, instead of "off white" that most wall paints are. Secondly, if the projection surface diffuses the light too much, the image will not be as bright or as sharp as it should be. Third, screens are designed to reflect as much as possible, so the image is nice and bright (tied in with diffusion). A flat white wall paint will diffuse the image too much. A gloss white will have hot spots where you can see a reflection of the lens and light bulb behind it. Finding a wall area that is big enough, without any picture frame holes, or faded sections where objects used to be, is not easy. Painting a wall white may not fit in with the decor plans. If you have a slide or movie projector screen, use that until you can afford to buy a ceiling mounted projector screen. Otherwise, you would be throwing away potential performance of an expensive projector.

Q I'm in the middle of rough wiring my future home theater room, and have a question about the rear speaker (dipole) placement. My listening area will be off-center across the width of the room (15 feet in width) because of the location of supporting columns, and approximately 14 feet down a 40 foot room. I have several choices as to where to place the speakers: 1) mounted on a column and wall which would center the speakers, but place them 6 feet behind the seating area, and halfway down the length of the 40 foot room; 2) on the walls in line with the listening area, but then the distance from the speakers would not be equal; 3) on the ceiling (7 foot height), possibly pointing downward; and finally 4) on stands on either side of the seating area (but then I will have a speaker cable run over the carpet). Suggestions?

A I would choose either the column, six feet behind the seats, or on stands next to the seats. If you mount them on the walls at unequal distances from the seats, the difference in volume could be adjusted, but not the difference in sound arrival time. Placing them on stands will give you a lot of control over the effect you prefer, since you can move them slightly and change the effect. For this choice, put them on stands with the cables laying across the rug, until you get them in the position that gives you the nicest sound. Then cut a very small slit in the rug next to the stand and pull the cable through the slit. If you change your mind later, the rug can be brushed and the slit won't show.

Q We recently received a pair of Linn TUKAN speakers and a Sunfire True Subwoofer Mark II as a gift. Unfortunately, my stereo receiver, a 6? year old Denon DRA-435R (55 watts per channel RMS @ 8 ohms; it also has pre-outs), doesn't have the power to drive the speakers (minimum amplifier power is 30W/8 Ohms; efficiency at 87dB for 1W at 1m - 1kHz). So, we're trying to decide what upgrade path to take. Can we get away with simply adding a power amplifier (we've looked at/heard Marantz MA500 monoblocks 125W - $250 each, Bryston 3B - $1250 for the floor model, Dynaco 125W - $700, Rotel and Acurus - 200x2 for $999), or do we need to upgrade the receiver/tuner/preamp? We're not looking to upgrade to home theater (stereo is fine for us). Any thoughts you have would be most helpful (i.e., any comments about the amps/prices). The system is for a 20'x20' living room not specifically designed for optimal listening conditions.

A I would suggest getting a power amplifier at first, and then later on, getting a dedicated preamplifier. From the units you mentioned, the Bryston would probably be most suited to the Linn speakers. It has a very big power supply and a 20 year warranty. As long as you are sure you just want two channel stereo for this system, then you might think about a nice tube preamplifier down the road.

� Copyright 1998 Secrets of Home Theater & High Fidelity
Return to Q&A Index.