Q&A # 112 - April 20, 1999

Staff

Q I've inherited, been bequeathed, had handed to me, 80 meters of 4 pair shielded [foil] cable. Each wire is 20 gauge and looks to be silver over copper. I was thinking of making some DYI speaker cable out of it and have a question or two. There is a ground/drain wire. Do you recommend terminating that drain wire at the receiver or not at all? For bi-wiring, is using a pair twisted together sufficient on a cable run that is around 8 meters? Or should I make 2 runs?

A If you're going to use a shield, terminate it at the amplifier side (connect the drain wire coming out of the cable at the amplifier end to the amplifier chassis as a ground). I wouldn't think that it'd make much of a difference, since it's difficult to pick up noise on speaker cables because the load impedance is relatively low. It can happen, but only under extreme circumstances, such as several hundred feet bundled up close to 120 volt AC. Still, it won't hurt, and may assuage any paranoia which otherwise could develop. You could also use the shield as a conductor itself. If the wire is four pairs of 20 gauge, twisted together it would equate to about 14 gauge. I'd suggest double runs for an eight meter length. The way to achieve the lowest series inductance (which will benefit high frequency response slightly) is to group the conductors so that each pair is split on the positive and negative terminals. The contrasting method would be to have a group of pairs connected to the positive terminal, and the other group connected to the negative. As for bi-wiring, if you choose to do so, I recommend running four lengths, as doubling the cable does not reduce the series inductance between the tweeter and the amplifier, nor the series resistance between the woofer and the amplifier. It does double the capacitance that the cable loads onto the amplifier, which could change the sound with marginally stable amps or amps with a very high output impedance at high frequencies (that might even be desirable, depending on the listener). Bi-wiring may boost output near the crossover frequency, but the benefits of that are just as dubious. You might try bi-wiring while keeping the shorting straps IN. You will not only double your contact area to the benefit of BOTH the upper and lower frequency drivers, but halve the resistance between the aforementioned, and the total series inductance. Have fun with it. A tip, if you like, for cosmetic features. You can get nylon wire mesh that expands to fit your custom cable, and then use shrink tubing to hold it on and tidy up your banana/spade terminations. It's surprising how expensive it can make cables look.

Q I was wondering why many Pioneer Elite Receivers have a speaker impedance rating of 6 Ohms. I have heard two of them, and they sounded in my opinion better than an 8 Ohm receiver with the same THD rating of .09%. How is it that a receiver with a lower impedance and the same THD rating sound better? The salesman told me that 6 Ohms is the best impedance for amplifiers, like it is the magic number. Is there any truth to this, or was he just trying to sell me on the idea? Also who produces good 6 Ohm speakers? I like the Bose 901s, but they are supposed to be used with an amp that is rated at least 8 Ohms. Are there any other speakers that sound as good as the 901s but are 6 Ohms?

A This is an example of relying too much on the specification sheets. Mass market receivers work best with 8 Ohm speakers. They will do OK with 6 Ohm speakers, and some of them have switches on the back to limit the rail voltage for use with 4 Ohm speakers. However, if you are buying a mass market receiver, just stick to 8 Ohm speakers. The 6 Ohm rating you referred to means that the minimum speaker load the amplifier can handle is 6 Ohms. But that does not mean that you must use 6 Ohm speakers. 8 Ohm speakers will still be better for use with the receiver. If you have some 4 Ohm speakers that you simply must keep and use, then it would be best to purchase an outboard power amplifier that is rated into 4 Ohm loads. Manufacturers may specify the output at 6 Ohms because it is a higher number (more watts output) than at 8 Ohms, and their product may not be rated into 4 Ohms. If amplifier "A" is rated at 50 watts rms per channel into 8 Ohms, but is not rated into 4 Ohms, and amplifier "B" is also rated at 50 watts rms per channel into 8 Ohms, but is rated into 4 Ohms, it usually means that amplifier "B" has a larger power supply. In general, all high-performance amplifiers are rated into 8 Ohms and 4 Ohms. Many are rated into 2 Ohms, and some into 1 Ohm.

The THD rating is a different matter. It depends on a number of things, and having a very low THD does not imply great sound. Some of the finest amplifiers in the world have higher THD than mass market products.

Q I have 2 Definitive Technology Celcius bookshelf speakers that I want to use side by side as a center channel under my monitor. My Sony TA- E9000ES has 2 center channel outputs, but my Carver 705x amp only has five channels, so I am one channel short. I was wondering how I could wire the 2 speakers together so they would be comparable to, say, a Definitive CLR-2002 center channel speaker (I think the Celcius bookshelf has the same drivers as the CLR-2002). The Celcius is rated at 8 Ohms impedance, so if I wired them in parallel, this would present my amp with a 4 Ohm load, which I don't want to do. However, I have read that wiring speakers in series is incompatible with solid state amps. There has to be a way to wire these two speakers together to equal a center channel speaker, because, after all, they are the same drivers in two enclosures instead of one.

A The two center channel pre-outs on your unit represent the same signal. You can accomplish the same thing by using a Y connector to split a single pre-out into two pre-outs for use with bi-amping. The presence of two pre-outs for a single channel does not mean that you must use them. If you wish to have two center channel speakers - one above the TV and one below - then wire them in series with one of the amplifier channel outputs. Solid state amplifiers don't have a problem with that. Otherwise, you don't need to worry about one of the center pre-outs not being used.

Q I recently purchased the Yamaha RX-V595 which has built-in Dolby Digital and six channel input. I also have the Technics DD/DTS decoder which I have hooked up to the Yamaha. I tested the Dolby Digital from both the Yamaha and the Technics with a movie, and found that it sounds better to me when I use the Yamaha's own DD decoding than if I listen to it using the Technic's DD decoder. The Dolby Digital from the decoder sounds brighter. Is the reason for this due to my interconnects or the different decoding chips used by the manufacturers? I'm using the cheapest monster cable interconnects.

A There are differences in decoders, and this may be why one sounds brighter than the other. However, it can also be due to the cable that you use between the DVD player or LD player and the decoders. Make sure that they are 75 Ohm cables rather than audio coax cables. Toslink cables will work OK too.

Q I have just upgraded my entire audio system. What do I do about cables, how much should I spend, and what do I look for as far as pros and cons are concerned? I have about $8,000 Canadian total invested at this point. Our tastes in music are varied, and our room size is 12' x 20'. Is there a set percentage of total investment that I should spend for cables? I have a Bedini 150 mk2 power amp, Rotel RC850 preamplifier, ATC10 SCM speakers, Alchemist Nexus 32A CD Player, and Magnum Dynalab FT-11 Analog Tuner.

A I have seen a "rule of thumb" that states you should spend about 10% of your budget on cables. However, cable design is as much an art as it is a science. No one really understands everything that happens with signal conduction. As a result, cables of widely variable construction and price are available. There are plenty of very good models out there that sound great and are relatively inexpensive. Two of my favorites are the Kimber PBJ Interconnect (about $60 per meter pair) and Nordost 2-Flat Speaker Cable (about $3 per foot). If you bought enough of these to cable your system, it would come out to be about 5% of your budget, which is just fine.

Q I have two Velodyne subwoofers that have both left and right line level inputs. On the back of my Dolby Digital receiver there is a single subwoofer pre-out. I wanted to utilize both subwoofers so I split the single subwoofer pre-out and fed them into the left line level input of each subwoofer. There is nothing going into the right line level input. Is this connection proper? I have heard conflicting reports that if you have a subwoofer with dual line level inputs and you have a receiver that has a single sub-out, you should split the signal with a Y connector and feed it into both line level inputs to get maximum subwoofer performance.

A You have the connections arranged correctly. However, this will deliver mono subwoofer sound. If you want to add a little more three dimension to the low frequencies, you can connect the subs in a stereo fashion. This can be done several ways, even if your receiver only has a single sub-out. You can connect the front right/left pre-outs to the two subwoofers (the right pre-out to one sub and the left pre-out to the other sub). You could also connect the front right/left speaker outputs to the two subs in the same fashion, either with using the subs' speaker level outputs back to your front left/right speakers, or just connecting the speakers and the subs in parallel, with one set of speaker cables going to each (4 sets of speaker cables are necessary). You could also use the tape loop line level outputs from the right and left channels of the receiver to drive the two subs in stereo (you would have to use the volume controls on the subwooers to adjust volume, since the tape loops don't change in volume when you adjust the volume control on the receiver). In any case, it is not necessary to use a Y connector to split the sub-out and then go to both line level inputs on a single subwoofer.

Q I've noticed that several new home-theater processors use 32-bit DSP chips. Up until now, most manufacturers have used 24-bit and 20-bit DSPs. What are the advantages of these newer 32-bit processors over the older chips, specifically for Dolby Digital and DTS reproduction? Do they really offer significant improvements over 24-bit processors?

A DD and DTS consist of 20 bit, 48 kHz sampling. What you are referring to is a situation where signals are processed for DSP modes using 24 bits or 32 bits, but it is extra processing outside of the DD or DTS decoding. Theoretically, the larger the word length (more bits per word), the greater the precision in handling the digital data. Whether or not we can hear the difference between 24 bit and 32 bit data handling is another question that must wait for a while to be answered. My guess is that beyond 24 bits, differences in sound quality will be harder and harder to detect with our ears, even if we can see them on an oscilloscope. But, I think that the difference between 20 bit and 24 bit handling may be substantial in terms of audible improvements.

Q My cable Company (Time Warner) has HBO in High Definition, so I bought a Panasonic HDTV decoder to use with my Data Grade projector (NEC XG85) and my Faroudja VP250 line doubler. Now, how do I hook them together? Do I need a switcher of some kind?

A If your Panasonic decoder has RGB + Sync out, then you simply run that output straight to the VGA pass through on the Faroudja VP250. If your Panasonic only has YUV (Component) outputs, then you cannot route through the Faroudja. Some of the new HDTV decoders only offer YUV outputs, while others give you both RGB and YUV.

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