Q&A # 185 - June 28, 2000

Staff

Q I would like to know if there is any real difference between subwoofer cones for home and those for cars. The reason why I ask is because I would like to build my own subwoofer and I found very inexpensive cones (15 inch for $20) and inexpensive amps (1000 watts for $70) at an on-line bidding site. However the parts listed are for mobile systems, not specifically for home theaters. Will this really be a problem?

A The main difference is that car speakers tend to be low impedance, i.e., 4 Ohms. The car amps that drive them tend to be rated into 4 Ohms or even 2 Ohms because they use low voltages, and consumers often connect the speakers in parallel pairs to get more power output. You should be very careful about purchasing something on a bidding website. You can get some good deals, but you can also get taken to the cleaners. The amplifier you mentioned is probably the rating at 1 Ohm, and this kind of output is something you could expect for only a tiny fraction of a second, especially at that price. I think you would be better off getting a name brand through one of the on-line stores that specialize in auto hifi parts. Also, you will probably have better results using a couple of 10" drivers in the car, rather than a single 15". They will be easier to keep under control (less distortion) and will be much easier to find a place they will fit. Then, get yourself a two channel amplifier to drive them, rated at about 100 watts rms per channel into 4 Ohms. Drive each speaker with one channel of the amplifier. Lastly, be extremely cautious about cranking your system, even when you are impressing your friends. You can get tremendous SPL in an enclosed space like a car, and this is very bad for your ears. Permanent damage can result, with tinnitus (ringing in the ears) bothering you for the rest of your life.

Q For my home theater setup, I just bought a multi-channel EAD PM500 amp. The AC plug of my amp has 3 pins and the plugs on the rest of system have only 2 pins (Toshiba 3109 DVD, Nakamichi AV2 receiver, Sub MK). When I connect the whole system , I get a loud hum from all 5 speakers and sub. I use the receiver like a preamp, with 5 pre-outs from the Nak to the amp, and the amp to 5 speakers. The subwoofer is connected directly from the pre-out of the Nak. I have tried different solutions: With the amp powered off but plugged into the wall socket, and the rest of system powered on, I get hum in the subwoofer; When I unplug the amp and the rest of system is on, the problem disappears on the subwoofer; If I plug the amp into AC by using an adapter to convert 3 pins into 2 pins (not using the ground pin), there is no hum. My question is where is the problem (Nak or EAD), and how do I solve it without using the adapter (what is the potential risk for the amp if I don't use a ground pin)?

A From your information, this is very likely a ground loop problem. They are very common with home theater systems, because we have so many components connected together. It is also something you tend to have when some components in the system have grounded AC plugs, and others don't. First, make sure all the components are plugged into the same outlet, through a power strip. Secondly, you can try connecting a ground wire to the chassis of all the components together. If none of these solutions work, you can isolate each component from the rest with high quality audio isolation transformers, such as those made by Jensen. Start by putting one between the pre-out of the Nak and the input of the subwoofer. The ground prong on AC plugs is usually for safety against shock, but sometimes it is required for proper functioning of the component.

Q I am looking for help in setting up my Home Theater, like which test disc for audio and video, the use of a Decibel meter, burn in CD etc. When using a burn in CD what should the volume level be set at?

A Although there are many anecdotal accounts of burn in CDs quickly improving the performance of a system, there's very little scientific evidence that the effect is anything more than psychological. Speakers may exhibit a very quick change in performance with the first few stretches of the suspension, if the drivers had never been exercised before, but this type of thing would not require a specialized burn in CD. Rather, you could play some music at a relatively high level (90 dB at 1 meter) with lots of low frequency content. Some equipment may perform better after reaching operating temperature, but again, this does not require a special signal, if any at all. Some manufacturers and dealers suggest allowing a burn in period, although none that I am aware of have provided substantial proof of real performance changes. However, this does not mean that the equipment will not possibly sound "better" over time, as the listener will likely become more accustomed to the new sonic character, and find it more natural. If you really want a burn in CD, AudioQuest, Bedinin, XLO, and others have them (XLO CD available at Audio Advisor http://www.audioadvisor.com). Of course, anything with white noise (equal energy in equal frequency ranges, for example 1-2 kHz and 5-6 kHz) or pink noise (equal energy in equal octaves, for example 1-2 kHz and 5-10 kHz,) should work just as well. Then again, you could just do it the old fashioned way and enjoy some music, then gradually form an opinion over time. Finding noise somewhat irritating for periods longer than required to calibrate a system, I'd go with the latter myself. For decibel meters (SPL meters), get one from Radio Shack. They are perfectly suited for consumer use in home audio.

Q Could it be said that amplifiers with high damping factors use a lot of negative feedback, and therefore, would be expected to have excessive sibilance?

A We can say that a high-damping factor, by definition, means a low output impedance. The benefits of negative feedback are lower distortion, wider bandwidth, and lower output impedance, at the cost of reduced gain. That, in itself, would not increase sibilance. However, in some situations where the "designer," usually not qualified for the term engineer, will twiddle with the negative feedback circuit in order to either try to cheat the gain/bandwidth tradeoff by using the negative feedback circuit to apply what turns out to be positive feedback on the upper end, creating a slightly rising frequency response in the audible range and a peak in the ultrasonic portion of the amplifier's bandwidth, which then suffers from very poor phase response and poor stability because of it. In such a case, it would be very possible that the rising response, or more likely, distortion elicited by driving a reactive load which made the amplifier unstable, would add a harshness or edge to the upper mid-range and treble, making it appear perhaps fast or revealing, or simply, sibilant. It may also be that the "designer" simply used negative feedback to get good numbers for a spec sheet out of low quality parts and a poor design, and that the amplifier, if measured intelligently, would test very poorly, exhibiting a significant ratio of its distortion spectrum in the higher harmonics. Lots of negative feedback can lower the overall amount of THD, but add some harmonics up around the fifth or seventh, which are very noticeable and very irritating. That is one big reason why class A single ended triode amplifiers, using no negative feedback at all, can have THD of 1% and the listener is in heaven, while a mass market receiver with 0.001% THD and significant amounts of negative feedback, sounds bad.

So while it may be accurate (or may not) to say that some amplifiers with very "good" damping factors may be sibilant because of the particular and individual use of negative feedback in that model, I don't think one could say that an amplifier that uses a good deal of negative feedback and has, as a result, a "good" damping factor, would be necessarily sibilant. The sibilant amplifier would be sibilant because of poor engineering, or rather a poor engineer. I think that it's interesting to note that after testing a bunch of DVD players in Redmond, Washington a few weeks ago, although the specs provided by the manufacturers in the manuals are probably quite similar in nature, NONE of them measured the same. This not only raises questions that measurements aren't sensitive enough to characterize sonic differences, and challenges the usefulness of manufacturer's "specs" as well. Personally, I think they're pretty useless (manufacturer's specs, that is). Lastly, there are lots of very fine amplifiers that use a little negative feedback. It is the overuse of negative feedback that gives it a bad name.

Q I recently purchased two 100wpc Rotel amps to power my two main and two surround speakers. These amps run off the pre-outs of a NAD DD receiver. Due to space constraints, I must place the amps in a well-ventilated but difficult to reach spot on the floor. Would running both amps from the switched outlets of the NAD offer acceptable performance, or would current constraints hamper performance? Reaching around all the time is a pain, and I cannot find a manufacturer of a 120v triggered surge protector or conditioner that I could use to turn these amps on/off. Any suggestions?

A Do not plug in the amplifiers to the receiver's switched outlet, as the outlet is probably not rated for the power draw of the amplifiers. Plug a 12 volt DC power supply into the switched outlet, and use that to trigger a remote voltage triggered outlet, such as a Niles AC-3 or a Xantech AC-1. Either will work fine. The Niles provides two switched outlets and one unswitched outlet. The Xantech has only one switched outlet, although it may be split by a multi-outlet surge protector, which you may want since neither provides surge protection. You could also get something like a Panamax MAX2000 (previously reviewed,) and plug the receiver into the current-sensing outlet, and then program the outlets to the amplifiers to come on after a delay, but that's substantially more money. Rotel markets the RLC-900 for $200 (Secrets reviewed this in 1995), and it has a sensing AC cord that you can plug into the NAD's switched outlet. When you turn on the NAD, the Rotel 900 will then turn on and supply power to your Rotel amplifiers. As to Power Conditioners limiting current flow, those that do not use a transformer (the Rotel does not) shouldn't limit the flow. The only limitation will be the 20 amp fuse in the conditioner and circuit breaker in your wall.

Q In your Speaker primer, you say, "Keep in mind that digital surround will be best with full range speakers even in the rear surround channels (which are full range stereo), so, if you purchase small limited range speakers for the rear channel when using Pro Logic, they will not reproduce the low frequencies present in DD or DTS sound tracks." After many auditions at various stores, my ears are convinced of this. My question relates to the center channel: Would it make sense to use an identical speaker for the center channel (laying on its side)? Salesmen were showing me some great cabinet speakers, then would point out the 'matching' center channel. I noticed similarities in some of the drivers, but the enclosure was not even made of the same material. I am skeptical of some of these center channel speakers handling lower frequencies. If poor axis-response is an issue, what are your thoughts on adding a second, identical speaker to the center channel (wired in series to lighten the load on the amp)? Thanks for being the best HT resource ever.

A You are quite correct in being concerned about small center channel speakers that are supposed to go with big front left/right speakers in a system. The center has most of the audio action, and it should be the same size as the others, or even bigger for that matter. However, if you have some floorstanders as the front left/right, you can't really expect to put one on top of the TV or underneath, even if you turn it on its side. The remedy I have is to use the largest center channel speaker that I can put on top of the TV, and it is very big. You could, as you mentioned, add a second center channel speaker and wire it in series with the other. A third option is to add a subwoofer dedicated to the center channel and place it behind the TV, crossed over at 50 Hz, to go with the large center channel speaker. Anything you can do to beef up the center performance will make your system more enjoyable. If you have some nice bookshelf speakers that you are using for the front left/right, it is quite possible to use a third one (identical) and turn it on its side. Poor off axis response is a possibility, but not necessarily the outcome. However, be sure to check for magnetic shielding.

Q Is there a best type of speaker wire to use when concealing the wire run in the walls? Stranded or solid? 14 awg or larger or smaller? Since the house will be built only once, I want to do it right. Any suggestions?

A I would suggest using Nordost 2-Flat for speaker cables ($2/ft), or as a second choice, 13 gauge zip cord ($0.30/ft) that you can buy in bulk at electronic supply stores. Just to be safe, put some extra runs in the wall for upcoming new formats, such as DTS Discrete 6.1.

Q Recently I added a tube integrated amplifier to my system. After hearing the musicality of the tube integrated, I now prefer to play music through it always, and just use my Onkyo receiver for playing DVDs or VCDs. I have to move my front speakers (with pin-type terminals) to either the Onkyo receiver or the tube amp. My question is, I've been warned that tube amps may be damaged if I heat it up with NO speakers being connected to it? Is this really true, or not? Does this rule apply to the solid state Onkyo receiver? My kid watched his Barney VCDs using the center channel only because I forgot to return the front speakers.

A Welcome to tube sound. It's great isn't it? Tube amps can get hot for several reasons. One is that they actually use heat to begin with (hot filament). Secondly, Class A tube amps use a high bias so that current is flowing all the time. When there is no music playing, the tube has to dissipate all that current as heat rather than delivering it to the speakers. When the music starts, the amp actually runs cooler because the speakers are getting all that current. Your integrated tube amplifier is probably not Class A, and probably not high bias, so it won't run very hot whether there is music or not. As to not having the speaker connected, I have not experienced any problem with heat from them when the speaker was not connected. I think this used to be a problem, but modern designs have gone away from it. This was especially true of solid state amplifiers, but not any more. Regardless, however, just be sure to place your tube amp in a highly ventilated space because they do produce a lot more heat than a solid state component. And, don't tempt fate by running the amplifier on purpose without speakers connected just to see if it does get extra hot.

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