Q&A # 118 - June 1, 1999

Staff

Q Given the power inputs are the same for both 4 Ohm and 8 Ohm speakers, are there any differences between them in terms of sound quality, loudness? If my receiver is capable of handling 4 Ohm speakers, would it be wise for me to go for 4 Ohm speakers instead of 8 Ohm? What should be the reasonable low end frequency response for a subwoofer? Is 40 Hz good enough? Technics has a model of subwoofer that utilizes two 5 1/2 inch drivers facing downward. Theoretically, would that be worse or bettter than one 8 inch driver? I have a 6 x 100W receiver with bass volume and frequency crossover control right on the unit. Would it be wise for me to go with the passive subwoofer just to save a few bucks? What would be the general preferred crossover frequency for a sub? My receiver is capable of varying from 50 to 200 Hz.

A At the same output level (amplifier voltage), a 4 Ohm speaker will draw twice as much power as an 8 Ohm speaker. Which is louder will depend on the voltage sensitivity. This is usually rated as decibels/2.83volts (1 watt@8 Ohms)/meter. The impedance itself has no direct correlation to sound quality from the speaker itself. However, there are a couple of catches to that statement. The first is that it's difficult to build a really good full-range speaker that's also efficient. It can be done, but usually isn't. Since speaker voltage sensitivity determines the perceived efficiency (relative loudness), many manufacturers will make a speaker with an average 4 Ohm load, as opposed to 8, in order to double voltage sensitivity, and make the speaker appear more efficient than an equally efficient 8 Ohm speaker that would measure 3 dB less sensitive. The second part is that even though most receivers are rated at 4 Ohms, they're rarely actually good at driving a loudspeaker that averages a 4 Ohm load, since an average of 4 Ohms indicates an impedance that drops below 4 Ohms, often to 3 Ohms, and occasionally to 2 Ohms. If a receiver is going to be the main power source, it may work with 4 Ohm speakers, but I'd really look for those that average 8 Ohms or so, as they'll probably still dip to 5 or 6.

A 40 Hz limit on a subwoofer? It's not a subwoofer, it's a woofer that got lost. 40 Hz is just below poodle flatulence. If you want to make the air move, look for something that gets below 30 Hz. Theoretically, the superiority of two 5" drivers over a single 8" driver would depend on the total displacement, and the alignment of the enclosure. Knowing nothing about the specifics, I'd go with an 8" woofer. It sounds like you're on a restrictive budget for the subwoofer. Powered subwoofers start at $199 or so. Good ones begin at about $600 retail. Under that, I'd just put the money into larger left and right speakers. A good passive subwoofer, such as an NHT SW-3 12" woofer, will run about the same. If you're willing to fork over a good chunk of money for a good sub, you'll never regret it. Go for something in the 15" range, with several hundred watts of amplifier power if you can afford it. If you're looking for a subwoofer in the $200 range, it's not worth the effort of picking it up, or signing for delivery. It will almost be worse than nothing. Likely, within about 6 months, you would throw it away once you realized it doesn't give you much more than a whimper. If you have a couple of weekends, and want real performance for about $200, I suggest building your own passive subwoofer since you've already have an amplifier channel and the crossover handy. Can't recommend a crossover frequency without knowing what your main speakers are and their performance characteristics. Keep in mind that low frequencies demand lots of power, and using a channel of your receiver to drive a subwoofer will take power from the power supply that your other channels will be looking for. Check out www.madisound.com. The easiest, and probably best bang for your buck would most likely be the NHT 1259 woofer driver, which sells for $150 for DIYs. Throw it into a solid cabinet of about three cubic feet. What you'd end up with, essentially, would be the passive half of the NHT SW-3p subwoofer amplifier combination, that can slam with some of the best in many regards. We did an article on both the DIY version, and the NHT product some time ago, and the DIY article was complete with plans. Madisound could also recommend some less expensive solutions I'm sure. Perhaps a bass-reflex design with a 10" driver, though tuning ports can be tricky if you really want accurate response. I think you'll be much happier either spending the extra money or throwing in some labor. I know of no 5" drivers available that could be called subwoofer material. They may add more bass, but so does the bass tone control on your receiver. A 5" driver is for the midrange, not for woofing. Wouldn't you rather have a theater that can scare you when you need it most? A 5" driver won't even scare a parakeet.

Q I placed my Marantz CD63SE CD player on top of my Onkyo receiver, and these two are housed in an entertainment cabinet. I noticed that at prolongued use, the CD player is being "fried" by the receiver. The CD player's underside gets hot because of its steel chassis and hence conducts heat quite easily (internal and the one radiating from the receiver). 1. Will there be any sound degradation if I glue in a thick fabric mat on the underside of the CD player to insulate it from the hot receiver? 2. Is it true that even if I glue on the mat, there is still the risk of damaging both stacked components due to the receiver's EMI noise and CD player's RFI noise which are allegedy detrimental to each component? If no, then I will just "open" the cabinet back board to allow ventilation (my carpentry tools will come handy on some weekend).

A Your best recourse is to do two things. One is to separate the CD player with more distance from the receiver. You can do this by placing spacers under the CD player feet. You need a good couple of inches between the two components for air circulation. Secondly, you should open the cabinet back. Whenever I use any type of amplifier, such as in a receiver, that is in a cabinet, I leave the cabinet door open while it is in use. Heat is an enemy of electronics. More so than any EMI or RFI that might be there. Your components are insulated from both, but the heat sinks cannot do their job if you keep them in an enclosed box. You might even put a small computer fan in the back of the cabinet, using a wall wart or low voltage transformer power supply to drive it.

Q I have a Yamaha RX-V990 receiver, and I have just purchased a Sony DVP-S550D DVD player with a built-in DDl decoder. I was told to use the 5.1 outputs on the DVD player and connect them to the 5.1 inputs on the receiver. I can only find 5 inputs on the RX-V990. The one that I can't find is the subwoofer. What do I need to do in order for my subwoofer to work properly? And am I getting the full digital effect?

A Connect the 0.1 LFE output from your DVD player directly to one of the subwoofer's line level inputs (either left or right). Connect the subwoofer output from the receiver to the other line-level input on your subwoofer. This way, when you are playing a DD movie, the subwoofer will get the 0.1 output from your DVD player, and also when you are using the receiver for Pro Logic, or just two channel stereo CDs, you will still get subwoofer signal, but it will come from the receiver instead of the DVD player. In the case of DD from the DVD player, you will have to adjust the volume of the subwoofer by using the volume control on the back of the sub.

Q Recall these facts in sound pressure levels:

Quiet Room 30 dB
Office Conversation 70 dB
Opera Climax 105 dB
Rock Concert 120 dB (near the speakers)
Jet takeoff 130 dB

Why is it that in the volume/SPL readings of my receiver, I see negative (-) figures, such as -16 dB, or -20 dB. Also in the manual for my subwoofer, the SPL rating reads -3 dB at 14 Hz. Kindly shed some light on these disparities.

A When one is measuring SPL of a sound, using a meter, the numbers are always greater than zero, because the reference is zero, or absolute silence. In your receiver, the reference is also zero, but that zero represents full output of the amplifier, in watts rather than SPL. When you use your receiver, most of the time you don't have it running at full output. Let's say your amplifier is rated at 100 watts output. This is defined as 0 dB. You probably would be outputting about 10 watts on average, and this would be - 10 dB. An output of 1 watt would be - 20 dB. For every factor of 10 reduction, the output would go down by 10 dB, but that is in watts, not SPL. The readout on your volume control would stay at - 10 dB, but that only represents the average, and may not really be accurate. Your volume control probably has a setting that reads out above 0 dB, and this is supposed to represent driving the amplifier above its rated output. For your subwoofer, the - 3 dB is in reference to the average frequency response, and the - 3 dB measurement is the standard for specification sheets as a limit to the useful output of the subwoofer. In practical terms, it means something like your sub is flat to about 20 Hz and then drops its output level by 3 dB when the frequency reaches 14 Hz (sounds like you have a very good subwoofer).

Q In Q&A #104, you wrote that all current CD players would become obsolete within 2 years due to Pioneer's 24/192 and Sony/Philips' SACD. But then you also wrote that these discs would play on regular CD players since they have 16/44.1 PCM tracks. You also wrote that current DVD players would not play 24/192 and SACD discs. I'm confused. I have a DVD player that plays both DVD and CD, and if a regular CD player can play the two new formats (at least the 16/44.1 PCM tracks), why can't current DVD players do the same?

A Part of the confusion stems from the fact that the new formats may include a second layer that has the conventional 16/44.1 tracks for those consumers who want to buy the disc, but don't have 24/192 or SACD decoding capability. Current DVD players would play these discs, but they would be using that additional layer, rather than the 24/192 or SACD layer. These new formats will only be on DVDs rather than on CDs because CDs don't have enough room, and CDs have only one layer. The DACs in current DVD players only can go to 96 kHz sampling with PCM, and SACD requires a different type of DAC, because it is not PCM.

Q After months of deliberation (and some vacillation), I think I've settled on a stereo/home theater setup. I'm thinking of getting a B&K DD/DTS receiver-preamp and running it into some pro-audio power amps that I have, partly to save money and not have to buy power amps, and partly because the amps are of the very-high-quality recording-studio variety.

My question is whether there is any problem with this kind of "mixed" setup (home audio vs. pro audio), including any potential impedance mismatches? Also, would I, in any way, be able to take advantage of the balanced input connections on the amps, in order to avoid hum and signal loss? If such info would help, my amp "battery" consists of Carvin, Crown, and Soundcraftsman models, the first two types being fan-cooled, and the last being convection-cooled (with some mighty hefty heat sinks!)

A Typical pro amplifiers are built to drive low impedance loads, so they have huge power supplies. The are primarily designed for delivering high volume rather than a finesse of sound quality. They usually have phone plug jacks on the front and/or rear, rather than just RCA phono jacks. Other than that, they should be fine in your home theater. You might experience a little more hiss than with a consumer amplifier, but perhaps not all that much. As to the balanced inputs, these are only useful if the interconnects are really long. If your amplifiers were fully balanced, then I would say yes, use them. But it is more likely your amplifiers are quasi-balanced, meaning the balanced input is a module that interfaces between the balanced interconnect and the amplifier circuits themselves. Using these modules actually puts more circuit in the signal path, and could, with some products, reduce the quality of the sound. In pro-audio situations, such as a rock concert, where the interconnects are long, using the balanced module would reduce the hum, which is more noticeable than a loss of finesse. For your home theater, I would say use the unbalanced inputs, and you should be OK.

Q How many people do you have answering these questions?? If there is only one of you you should be enshrined as a national treasure!!

Now my question. I am from Australia and want to purchase an entry level system. I have $3,000 (Aus) to spend, tops. I was going to base it around the Yamaha RX-V595 which would cost about 1/3 of my budget.

1.Can you suggest a set of mains and sub for me to use as a reference point that would also be adequate for music? (I was going to make do with my 301s for the rears for a while.) 2. Is it more appropriate to simply audition a centre in my home or purchase a matched item?? 3. Should I wait till my finances are more adequate before I do anything? (I plan to use my ears not just the spec sheet to decide.)

A Most of our writers contribute to the Q&A section. It is an enormous amount of work, but we have a large archive of valuable information now that will be useful for many years. Thanks for your kind words.

I would suggest going to the 795, which will give you DTS decoding as well as DD decoding. That way, you won't have to worry about adding an outboard decoder later. You will be able to add an outboard power amp later, since the 795 has pre-outs (the 595 does not have pre-outs or DTS decoding). Secondly, just get the front left/center/right speakers for now (I am attempting here to help you make your budget go far, without discarding anything later when you upgrade). Pick some speakers that you would also be happy with as rear surrounds, and then, later, get a second pair to use in the rear. Something like the B&W DM-602s would be great as front left/right, and their CC6 as the center. Later, then, another pair of 602s for the rear. I suggest these speakers because they are pretty hefty, although not floorstanders, and they will give you reasonable bass until you can afford a good subwoofer, such as a Velodyne 15" or so. This will give you a waste factor of 0, as opposed to getting a complete package of everything now, and having to put some things in the closet when you upgrade. You would have to configure your receiver in three-channel mode (unless you use your current 301s) for awhile, but all good things are worth waiting for.

Q I have a question on a processor's bass management, in particular the Technics DD/DTS processor SHAC500D. According to the manufacturer, when all speakers are set to SMALL, the bass is cut off below 120 Hz and routed to the 0.1 channel. When set to LARGE, they are full range 20 Hz - 20 kHz. My speakers are the Mission 731pro for main L/R, 73C for center, and 73 for surround. Main L/R/C speakers can go down to 70 Hz, and the surround speakers can go down to 100 Hz (all at -3dB point). I have a powered sub (REL100) that can do 25 Hz - 150 Hz, but I prefer to keep as much bass on the mains as possible and let the sub fill in the real low end. What is your recommendation of routing the bass?

A I will be glad when the manufacturers build in digital variable low pass/high pass for all the channels in receivers, because, in my opinion, even 80 Hz is too high for many situations, let alone 120 Hz. I would suggest setting your front left/center/right to "Large" and the rear surrounds to "Small". Set your crossover low pass to about 70 Hz. This may lose some bass in the 90 Hz region for the rears, but you won't get boominess, which is much more noticeable and irritating.

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