Q&A # 49 - February 3, 1998

Staff

Q My system is comprised of PSB Century 400is front and back with a PSB Century 200Ci for the center speaker. We use the system mostly for home theatre, but clarity, low distortion, and deep extension with music is a prime concern in purchasing a subwoofer. I am looking for a subwoofer (or subwoofer pair) that would integrate seamlessly with my system. I have read great reviews of the Mirage BPS-400; however, I wonder if this is "too much subwoofer" for my system. Other options would include a pair of PSB Century Sub IIis or a pair of Hsu TN1220HOs. Also, what would be optimal room size and subwoofer placement with these options?

A There is no such thing as too much subwoofer, only not enough subwoofer. Just as with a big power amplifier, you don't have to turn it up to a high level. In fact, having a big, powerful subwoofer is extremely important to your prime concern of clarity, low distortion, and deep extension. In general, a powerful sub does not have to work hard at modest SPL, so its distortion level will be less than a smaller sub that does have to work hard at that SPL. From your remarks, I would suggest getting one good servo-feedback subwoofer. They are very clean. Your need for deep extension brings to mind the Velodyne F-1800R, which is now the FSR-18. Both of these are superb, and if you can still find an F-1800R, it will probably be at a discount, in preparation for the new model. Otherwise, you will be extremely happy with the FSR-18 too, but it is about $2,500. For home theater, I like a room about 20' x 20', but this is not always available. Put your subwoofer in one of the front corners, about 2 feet out from the front wall (the wall the TV is facing out from) and 3 feet from the side wall. If you can't put it that far out, just be sure that the distance from the front wall and the side wall are not the same.

Q I currently have a NAD 701 receiver (which I will eventually upgrade), and am planning on buying new speakers. I am considering the PSB Century 1000i tower speakers or going for the PSB Century 400i bookshelf speakers with a PSB Century Subsonic 2i subwoofer. My question is: since either choice is within $100, which will be most versatile? Taking one step back, generally speaking, which is better for the same price range - a tower or bookshelf/sub configuration?

A Digital surround sound has changed things quite a bit, since it has full range sound in all channels. Put the fact that home entertainment technology is evolving very fast, together with the fact that most of us are never completely satisfied with what we have, and you can see that the situation is complex. The Century 1000 speakers are 3 dB more sensitive than the 400i, which means your amplifier will have a much easier time driving the 1000s. Also, the 1000s have two 6 1/2" drivers, one for the lows and one for the mids, which will give you a cleaner mid-range, since that particular driver won't have to throw mid frequencies at the same time it would otherwise be handling larger excursion low frequencies. The 1000s are down 3 dB at 40 Hz, which should be fine for most music. Later on, get a subwoofer to handle that last octave (20 Hz  - 40 Hz).

Q I am trying to put a good home theater and music system together for $3000. I get a lot conflicting answers from different audio shops. I have looked at the Denon, Marantz, Sony and Yamaha Dolby Digital receivers. I need to have small speakers that can be mounted on the walls. Can you suggest what I might get in a system?

A The mass market receivers are all pretty good these days. Pick one out based on the features that you like. Each brand is unique in this regard. Make sure that the functions important to you are easily accessible from the remote control. I would suggest getting one of the mid-priced models that has pre-in jacks and pre-out jacks. This way, you can add whatever digital surround decoders you like. Also, you should get an outboard power amplifier. So, the receiver would be $500, DD decoder $500, outboard power amplifier $1,200 (e.g., Carver AV-705). That's $2,200. A set of five PSB Alphas would be about $600. (From what you said, I am assuming your center channel speaker has to go on the wall too.) With all the interconnects and speaker cables, that should bring you up to your budget limit.

Q You have referred to negative feedback in some of your equipment reviews. I thought it was supposed to be good for an amplifier's performance, but I have seen different opinions on this. So, what is it? Good or bad?

A All amplifiers are "non-linear" to a certain degree in performing their job. This means that they produce distortion, i.e., the output waveform is shaped differently than the input waveform (other than the amplitude). If a pure sine wave of frequency "X" is input, the output will have not only the original sine wave, but multiples ("harmonics") of that sine wave. The second harmonic (twice the frequency of the original sine wave) is the largest, followed by the third, fourth, fifth, sixth, etc. Negative feedback is the process of comparing the output with the input and subtracting the difference so that the output is closer to the input in shape. It is extremely difficult and expensive to design an amplifier that is so linear in performance, it can be used without any negative feedback. Most amplifiers have some, and it is stated in terms of dB of negative feedback in whatever stage it is inserted. The problem is that, when negative feedback is used, it decreases the amount of second order harmonic distortion, but increases the amount of third, fourth, fifth, sixth, and so on. Therefore, it might decrease the second order distortion from 5% down to 0.1%, but the fifth order distortion might increase from essentially nothing to 0.01% or so. Even-order distortion, such as the second harmonic, is not irritating to our hearing, but odd-order distortion, such as the fifth harmonic, is extremely irritating. So, the specs may look good on paper, but the truth is that negative feedback has reduced the second harmonic and introduced the fifth. Intermodulation distortion (IM) is also increased because the output distortion products are sent back to the input, and they interact with frequencies from the input signal. These interactions cause abnormal frequencies that our ears find very obnoxious. Good design philosophy is to build an amplifier that is pretty linear to begin with and then use a small amount of negative feedback.

Q I'm in the process of building my first high-end home theater and I have a few questions.

My system will be built around Martin Logan SLs in the front and Aerius in the back. I'm not going to choose the amp on reviews, but I'll take recommendations and then let my ears decide. I'm leaning towards a Sunfire CinemaGrand or Classe. My biggest concern is the preamp. It has to be able to handle all the inputs I add to the system (DVD, LD, DSS, HDTV) with a great switching ability. I'm not looking for bells and whistles, but I am concerned about the ability to upgrade in the near future when DVD audio is out in the stores and with DTS changing their code. Currently I'm leaning toward the Theta Casablanca though I am intrigued with the Casanova and am considering the Proceed AVP, the
Acurus AC-3, the Classe, and the new Sunfire.

I'm lost in this mess. I want to get this over with so I can spend everything on CDs and DVDs. Can you point me in a general direction?

The local stores are great in helping me with the products they have, but no one sells everything.

A The Theta Casablanca uses internal replaceable cards for options and upgrades, so it would be an excellent choice for the A/V preamplifier. It's expensive, but it is also superb quality. I listened to it at the CES, and I would not hesitate to get one. However, be sure to check with the manufacturer when making your purchase, about future upgrade cards that are in the works. Meridian is another manufacturer that uses cards for hardware options and upgrades, as well as Internet downloadable software upgrades.

Q I have a Sony DVP S7000 and Marantz AV-550. I can play CDs through the analog output of the DVD player (so using the DAC of the Sony) or through the digital output (so using the DAC of the Marantz). Which would be the best?

A This is a matter of just trying them both out. But, you should have a 75 Ohm cable if using a coax connection for the digital signal. The 50 Ohm audio coax cables usually don't sound very good when using them for digital signals, and it would be a shame to mistake that problem with one caused by a lesser quality DAC. If you use a Toslink optical cable for digital signals, impedance is not an issue.

Q I have a couple of questions concerning ground loops. I have a Yamaha RX-V2092, Sony DVP-7000S, Boston Acoustic speakers, a Sunfire MKII subwoofer as my main sub, and a Velodyne VA-810 for my CR6 rear surrounds. My first question concerns the Sunfire. I had a loud hum from my main speakers when I plugged the subwoofer into the subwoofer jack of the Yamaha. The Sunfire required the use of a grounded plug adapter from Radio Shack and a little experimentation with the plug orientation before the hum decreased to an acceptable level. What is the purpose of a grounded plug when all it seems to do is add ground loops? The Sunfire itself still has a small hum which doesn't seem to affect sound quality. Could this be caused by the plug adapter, and is it safe to use this device? My second question concerns the DVD player. If I use only the optical cable and no other electrical connection with the Yamaha, I can hear the the Yamaha tuner if I increase the volume level to the nine o'clock position even though I'm in DVD mode. This is distracting and bothersome since it is adding noise to an otherwise excellent player. When I use the optical cable along with an RCA cable from the analog out jacks to the Yamaha, the problem goes away but I suspect I'm adding another ground loop. Am I still chasing ground loops or is there a problem with my hookup? Thanks in advance and congratulations on a super site!

A Grounded plugs are for the purpose of safety. They allow a path to ground in the event of an electrical short. It is better for the short to conduct electricity to the direct ground connection than having it conduct through your body. Some components work fine after removing the ground connection, and some do not. I had a laser printer that kept blowing circuit boards, and finally realized the ground wire in my three-pronged wall socket was not connected to ground. You are getting the ground loop hum when you connect the Sunfire because it is the only component in your system that has a grounded plug. Audio amplifiers are probably not so sensitive as computer equipment, but even so, it is safer to keep the ground connection. But, I do understand the frustration with ground loops. We have to deal with them all the time. The biggest culprit, in our experience, is the cable TV connection. Disconnect it and see if your hum is reduced. A wide band RF transformer will eliminate this problem. Also, make sure all of your components are plugged into the same wall outlet. Obviously, you would need an AC line conditioner with multiple outlets. The Rotel RLC-900 is a good one, for $200. I have heard the tuner problem before. The tuner circuitry is still powered, but its functioning is muted. What you are hearing is crosstalk from the tuner circuit into the amplifier section. Crosstalk is something we just have to live with when we have the tuner, preamplifier, and power amplifier all in one chassis. It should only be audible, though, as faint background when you turn up the volume and there is a quiet section of the movie. If the tuner is loud, then your receiver may have a defect in need of repair. 

Q Now that HDTV is about to be sold to the public, what is going to happen to DVD?

A Since DVD is a digital format, it could be adapted to any of the DTV formats, including HDTV. However, space on the disc would be a problem with the highest resolution HDTV format, 1080 scanning lines x 1920 vertical lines. The current resolution of DVD is 525 scanning lines x 480 vertical lines. So, 1080 x 1920 represents about 8 times the data as 525 x 480. Assuming modest compression, a current DVD will hold a two hour movie on one side and one layer of the disc. With 1080 x 1920, one hour would take up both sides and both layers, and two discs would be needed for each movie, assuming that it was only presented in widescreen format (no need for pan and scan, since HDTVs have a widescreen shape anyway). Videophiles probably would not mind changing discs, but mass market consumers probably would, and it is the mass market that drives sales. The other HDTV format, 720 x 1280 Progressive (720P) has about 6 times the data as 525 x 480, so that is still too much for one movie on one disc. DTV at 480 x 704 Progressive (480P) would be a good choice for mass market DVDs, since the image would be better than the DVDs we have now, and most movies could still fit on one disc. Future players will probably play both sides of the disc, and perhaps carousel DVD players would alleviate the multiple disc problem. Maybe we could have special edition DVDs of classic films at 1080 x 1920 for those of us willing to pay the higher price and willing to change the disc every 45 minutes. 

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