Q&A # 101 - February 3, 1999

Staff

Q Over two years ago my roommate placed an unshielded speaker on top of my TV that caused a greenish discoloration in the corner of the TV. It's highly visible on black & white programs. How do I get rid of it? Can I just do what's done on tape deck heads...place a magnet close to it then slowly pull away?

A You need to "degauss" the picture tube. Newer TVs automatically do this. There are commercial degaussers available, which consist of a large coil of wire, but they are alternating current electromagnets. A permanent magnet won't do it, and in fact could make it worse. If you can't find one (Radio Shack used to sell them), you could probably make one from a coil and a 12 volt transformer. The coil should be about 10" in diameter with enough turns to produce a substantial electromagnetic field, and also to produce enough impedance that you don't burn out the transformer. The transformer primary is connected to the house AC and the secondary to the coil. You place the coil close to the picture tube and then move it slowly away.

Q I have a Sunfire Cinema Grand and was wondering if you could answer a question regarding bi-wiring. The owner's manual says that for a bi-wired system, use the current source for mid and treble, and use the voltage source for the woofers. It's my understanding that I can run one set of wires from each output to the appropriate input on my bi-wireable speakers. Do you know if this is correct and what this does in terms of power going to each input since the 200 watt output is now being split?

A The Cinema Grand has the voltage and current source outputs for two channels (front left/right). However, you can bi-wire all of your speakers if you wish, just by connecting two sets of cables to the single set of speaker binding posts on the amplifier for the other channels. In the case of the front left/right, there is a high wattage resistor in series with each of the two current source outputs. In order to perform bi-wiring, it is necessary to disconnect the metal bus strips that connect the woofer to the mid/tweeter at the speaker binding posts on the rear of the speaker. Then, run one set of speaker cables from the current source outputs to the mid/tweeter binding posts, and another speaker cable from the voltage source outputs to the woofer binding posts. The 200 watt output is not being split since the voltage and current source connectors are actually in parallel with the amplifier's output circuit.

Q I just heard about the Denon AVR-5700. It sounds like it has the latest and greatest. They have already said it is the DSP-A1 replacement as the best of the best, on some newsgroups and forums. What I would like to know is if it would make a big difference in my system to upgrade to the Denon AVR-5700. I have a Denon AVR-3200, Paradigm speakers, Monitor 9, CC-350, ADP-170, and a Cambridge Subwoofer 150 watts 12" driver.

The system gets rave reviews from friends, but I feel like it is still lacking. The audio store where I bought the Paradigm surrounds gave me a demo of B&K separates, and it sounded a lot cleaner and clearer than my system at loud volume. He then demoed the best Marantz receiver using the Monitor 9s at about the same volume, and it was annoying to say the least. I had to tell him to turn it down. He also mentioned that getting a PW200 sub would help my system a lot. Do you think so?

A friend can get me a Denon at about 30% off and I'm interested. Am I fooling myself to think that getting the Denon will improve my system and also offer me DTS with some more power?

A We have not had the chance to listen to the Denon 5700 yet, but I am sure it is very nice. Denon tends to put a little more into the amplifier section, while Yamaha puts more into the processor (more features). Either one would be fine, but you should decide which one gives you the features and ease of use that you like. I would suggest getting one of the two along with a better subwoofer, such as the Velodyne HGS-15 or HGS-18. A good sub makes a huge difference, perhaps more than any other component. If you get the Denon or the Yamaha, you probably won't need an outboard power amplifier, unless you like to listen to movies at high volume. From your comments about the demo, you probably don't like loud movies. I don't either, and I routinely use the DSP-A1's built-in amplifiers along with lots of big subwoofer power. I keep the main volume at modest levels, but turn the deep bass up a notch or two. The low frequencies don't bother my ears at higher volume, but high volume mids and high frequencies do. This way, I get my movie thrills without leaving me with ringing in the ears after the film is over.

Q I have a two part question. At what frequency should I crossover my subs for home theater? I will run one on the rear speakers and one at the sub-out on the receiver. I also seem to be having subwoofer placement difficulties. I know that bass signals are omnidirectional, but I find a sweet spot when I move around the room. Can you help?

A The crossover frequency will depend on how big your other speakers are. If they are bookshelf size, then set your subwoofer to about 70 Hz. If they are floorstanding speakers, then try 50 Hz. Bass frequencies are omnidirectional below 50 Hz, but above that frequency, we can begin to detect where they are coming from. However, your placement problems are not due to the directionality issue. In order to place the subs, put one in the listening position, and turn on a low frequency test signal, such as those found in many receivers. You can also use a test CD for this. Then move around the room and locate the position that gives you the low frequency effect you want. Put the subwoofer in that location. Then do the same thing with the other subwoofer.

Q First, let me say that when it comes to no-nonsense info, your site is the best, whether it be for beginners or experts. My question is, assuming a constant volume/dB output, how does a speaker cone move at 40 Hz in relation to itself at 80 Hz with regard to speed and distance traveled? Do they travel the same distance but the lower octave at a slower speed (resulting in less cycles) or do they travel at the same speed but the lower octave a farther distance (again less cycles). I have read somewhere that as you go down one octave your speakers need to move four times more air. Is this true? If so, then a lower octave does mean that a speaker cone travels farther (but at the same speed?) Does this mean that a mini-monitor's woofer that tries to go as low as 50 Hz does so by making the cone travel farther than its bigger, three way, floorstanding brother's woofer to displace the same amount of air? But isn't speaker movement a function of the electrical input from the amp? If so, how is cone travel distance in passive speakers controlled for more bass?

A Lots of questions in one. First, a given speaker operating at 40 Hz vs. 80 Hz must travel farther at the same output level. As I recall, four times the air displacement is correct. The signal from the amplifier represents the electrical force that will be transferred to the cone as mechanical force, not the position of the cone, assuming that the driver is operating above its low frequency cutoff point, which is an entirely different scenario. The four times displacement is somewhat inversely akin to a car's braking distance. Raise the speed up an octave (double) and assuming linear brakes (no heat fade), the stopping distance quadruples. Also, you are right in the case that the smaller woofer must move farther to displace the same amount of air as a woofer with more surface area. That's why, even though large woofers, used as such, may have problems in the lower midrange/mid-bass due to beaming dispersion patterns and diaphragm resonance, the lower required excursion at any given output allows the voice coils to stay in their most linear operating range, making them ideal for true subwoofer applications (lower than 50 Hz.) The speaker's movement, ideally, is a function of the input of the amp; however it is not a direct relationship of position, but force and therefore acceleration, from which position can be derived, assuming an initial resting position of the center of the gap, and an initial velocity of zero. Of course, voice coils aren't
perfectly linear motors, and suspensions introduce restoring forces which affect the net force on the cone, more so the lower the frequency, the extent of which depends on the resonant frequency of the driver/enclosure combination. So, even with a perfectly stiff cone, the only way to "ideally" control the acceleration of the driver is with a servo feedback loop which utilizes an accelerometer, which effectively increases the linearity of the motor and allows control over the driver's frequency response. On the other hand, most major nonlinearities in terms of frequency response and phase response aren't any different than a crossover network, either passive or active. Whether or not the cost of implementing a servo feedback design justifies the effort at a given selling price, compared to other techniques in speaker design, can be argued both ways. Excellent products can come in a variety of flavors.

Q The last question in Q&A # 98 deals with wire length. My question to you is why do you keep all your speaker cables the same length? Surely it's not propagation time differences......is it!?

A I try to keep the front left/center/right speaker cables the same length and the rear left/right speakers the same length for impedance reasons. The cable impedance for the front channels is the same, and for the rear channels, it's also the same. My feeling is that by having the same impedance, the time domain will be constant for the front channels and for the rear channels. Since cables have impedance, they act like filters, and therefore, phase shift occurs. In general, you should keep the speaker cables as short as possible and not wind any extra length into a coil, since a coil serves as an inductor.

Q I am considering purchasing the Yamaha RX-V2095 as the centerpiece of a home entertainment system that I will slowly be putting together. If I wish to use the Dolby Digital and DTS decoding features that are part of this receiver, will I also need to purchase a DVD player that has built-in Dolby Digital and DTS capabilities, or is it enough that the receiver can handle this?

A You just need to make sure that whatever DVD player you get has DTS digital out. This means that it will recognize DTS signals on DVDs and output the signal accordingly. Most of the new DVD players will do this. All DVD players will output DD signals. The 2095 will decode DD and DTS automatically. To connect the DVD player to the receiver, use the digital output jack on the DVD player and connect it to the DVD/LD digital input jack on the 2095.

Q What size screen, in diagonal measure, is appropriate for a given viewing distance? I am in the market for another TV, but I am not sure what size I should get. Can you provide a rule of thumb for screen sizes?

A The rule of thumb is to sit back about 3 - 5 times the width of the TV. What I have done in the past is to go to the TV store, and see which TV is the size I like, then I buy the next size up. The reason is that when I take the TV home and become used to it, I am always glad I got the bigger one. Also, widescreen movies shown on 4:3 TVs look better if I sit closer. With "Ben Hur" at about 2.7:1 aspect ratio, I usually sit so close, I can put my feet on the base of the TV. So, the bottom line is to buy what is comfortable. Use the rule of thumb just as a starting place.

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