Q I have a home theater with Magnepan speakers all around with the front L&R being the 3.5s. My amp for the L/R speakers is a Rotel 2
channel 100W/ch amp and a bridged 6-ch amp for the center and rears giving 90W/ch. I have a mirage BPS 150 sub. To get more bass, I purchased another one and it has provided me with more oomph but if I turn them up, they sound too boomy. I've set the x-over for the subs at 80 Hz from my Lexicon DC-1 processor. I'd like a tighter bass response with still more ooomph but short of spending $5000 on the Proceed amp, are there any other suggestions you may have, i.e., bi-wiring the Maggies or getting the Adcom GFA 7500, etc.

A Boomy bass from subwoofers is a very common problem. There are several causes. One is that when you turn up the subwoofer volume, the amount of harmonic distortion increases dramatically. Action movies have lots of bass in the 40 Hz region, and the second order harmonic is at 80 Hz, where the boominess is. A second reason is that subwoofers generally do not have any damping material in the enclosure, and this tends to emphasize the sound at frequencies around 80 Hz - 120 Hz. To correct your problem, you need to set the crossover frequency lower, if possible, say at 50 Hz - 60 Hz, and not turn the volume up so loud.

Q I have a home cinema system that is mainly audio-based. Initially I had a Yamaha DSP-A590 Pro Logic amplifier, with Mission 73C for a centre speaker, Mission 733 for the main speakers, and Mission 73S for the paired rear surround speakers. I then exchanged the 73C for the 75C speaker as I was not happy with the 73C performance. Since then, I have bought the Yamaha DSP-A1, and a REL-Q50 subwoofer.

My questions are:

1) The 75C speaker has a nominal impedance of 6 Ohms, and the Yamaha-DSP-A1 is set for speakers of 8 Ohms. I think the other speakers are 8 Ohms in nominal impedance.

What, if anything, should I do to my DSP-A1 settings to overcome the difference in the speaker impedances?

2) My Mission 73S rear speakers are now the weak point in my system. Are there any speakers you would recommend to replace these ?

3) My Television is currently a Sony 25 inch NICAM TV (I live in London). It is a little small, and I find a Lechtronic (or something) screen magnifier that expands the screen to about 29 inches is much better. Unfortunately, High-definition TV is not yet even discussed in the UK, although digital TV is coming. Would you recommend a CRT projector that can produce massive screen images, or a big widescreen TV that can support digital TV, and DVD aspect ratios of 16:9 and component inputs for RGB?

A The DSP-A1 has an impedance selector switch on the back for 8 Ohms or 4 Ohms. Try using the 4 Ohm selection, since your center channel speaker is 6 Ohms. That selection means that all speakers have to be at least 4 Ohms, which they are in your system. You may have to turn the volume of the center channel down a bit in relation to the other channels, if they are connected to 8 Ohm speakers. The 4 Ohm selection reduces the amount of voltage available, and therefore, reduces the current that can flow so that the amplifier will not go into hard clipping. However, at any one voltage, twice as much current will flow into a 4 Ohm load as into an 8 Ohm load, so there will still be plenty of watts at 4 Ohms.

If you like the Mission 733s in the front, then get another pair for the rear. Use the 73Ss for the front effects channels.

Digital TV has lots of different formats, and HDTV represents only a few of them, so if you get a digital television, it should be able to display HDTV signals when they begin broadcasting in the UK. I would suggest getting the digital TV (all of which are 16:9), whether it is in the form of a direct view or projector, although the first ones are all rear projection sets.

Q I have read that placing just a capacitor (of the correct calculated value) in the line from a preamp to a power amp can produce an
effective 6 dB/octave high pass filter. Has SHT tried this and how well does it work? I'm also wondering if an inductor could be put in another line to another power amp, thus forming an effective low pass filter. Of course the inductor would have to be kept away from any metal.

A Yes, a capacitor in between the preamp and power amp does work, and we have tried it (the cap is put in the path of the + conductor). The formula for calculating the proper value for the capacitor is C = 1.59 / R f, where C is in Farads, R is in Ohms (input impedance of the power amplifier), and f is the crossover frequency, in Hertz, that you wish. So, if the input impedance of your power amp is 100,000 Ohms and you wanted the crossover frequency (high pass) to be 50 Hz, it would be C = 1.59/100,000 x 50, and this calculates to be C = 0.000000318 Farads, or 0.318 µF. The rolloff would be 6 dB per octave below 50 Hz. For the low pass, the formula is L = R/6.28 f, where L is the inductance in Henries. You can see from the formula that the necessary inductor would have a very high value, and is not practical. For about $30 a pair, you can get Filter Modules ("F-Mods") from Harrison Labs (http://www.hlabs.com). They are small tubular modules with an RCA jack on one end and an RCA plug on the other. These items are used quite a bit in the automobile sound industry, but they can also be used in home theater. You put them in between the preamp and power amp, and they can be plugged into the power amp input jack, with the interconnect from the preamp plugged into the other end. The F-Mods are 12 dB/octave slope. The modules have buffers so that the impedance of the amplifiers is not a factor. These modules might be very handy in limiting frequencies sent to small speakers. Remember that you will have phase shift at the rate of 90⁰ per order at the crossover frequency, decreasing in amount as you move away from the crossover frequency, so a first order filter (6 dB/octave) results in 90⁰ shift, and a second order filter (12 dB/octave) will result in 180⁰ shift. Also, you need to remember that if you get the low pass modules and high pass modules at exactly the same frequency, the fact that there is a rolloff rather than a straight down drop, will create overlap, and therefore, a hump. So, you should plan on the low pass at one frequency, and the high pass to be somewhat above that frequency.

Q What is the best way to mount rear speakers? I have B&W 601s for rears and I am wonder if mounting them on the wall will have negative effects on the sound. I have pre-wiring in the back walls about 8 feet high already.

Also I have a Velodyne FSR-12 subwoofer. I heard it is better to have subwoofers in the front. Will having them in the back of the room be terrible?

A The 601 is designed for mounting on speaker stands, or on shelves, but you can purchase wall brackets from other manufacturers to mount them if you wish. Since they are front ported, you probably would not have any problems. They could be mounted behind the listening position and above it, angled down towards the listening position. Having a subwoofer in the rear would not sound terrible, but you will get deeper bass if it is in the front, farther from the listening postion. Place it near one corner, about 1 foot from one wall and 2 feet from the other, to start with.

Q A DVD player is my next purchase. I read "things" about the audio laser vs. the video laser. What should I look for Two separate lasers, one for music CDs and one for DVDs or is there any such "animal" as a combined laser? When DVD Audio becomes a fact will the DVD laser be adequate for DVD-Audio? I am confused! I want "good" music so should I keep my Onkyo DX-C606 for music or will the next generation DVD player take care of movies and music? Do you have a DVD player in mind? Please, please, please suggest one.

A Take a look at this week's review of the Yamaha DVD-S700. As you will see, it does not perform very well with audio on CDs, but no mass market DVD player does. The reason is the laser is designed to read the narrow track of DVDs and not the wider track of CDs. Since the transport and laser are there, the manufacturers put CD capability in. I would suggest keeping your CD player for CDs and using a DVD player for movies and Digital Audio Discs (DADs), which have music in the narrow DVD tracks (so the laser reads them just fine). The Yamaha DVD-S700 is an excellent player and would be a good choice. It performs beautifully with video and 5.1 audio.

Q I have a Proceed PAV/DSD surround processor, and the amplifier is a Parasound HCA1206. I like to have sheer dynamic impact of the cinema in my home. So I'm interested in the Bose PA system. They're Bose 802 series II, and the sound is absolutely as I want, but the Bose dealer recommends that I buy PA amps for their speakers, which I'm not very sure is really necessary since the Parasound can be bridged to have 300Watt x 2. The Proceed dealer is against using PA speakers with their product. They said I'm already on the audiophile side, so I should buy audiophile speakers. I believe that no home speakers could ever produce the sound the same way as PA speakers do. What should I do ? My room is 7.5 meters x 10.5 meters, and no matter what the main speakers are, the subwoofer will be the Velodyne FSR-18.

A As long as you have listened to the products you intend to buy, you should go with what you like.

Q Like many other readers of your e-magazine, I am intrigued by the Yamaha DSP-A1. I'm just starting to piece together a home theater that will also be used extensively for good old audio CD listening, and thought I would definitely go with separates, until I read your DSP A-1 rave. I'm starting with Magnepan 10.1s, which are not huge but still 4 Ohm, and I like their power, plus a PSB Subsonic2i subwoofer.

Will the DSP-A1 be able to cleanly drive the maggies? I'm considering the maggie surround and center channel speakers as well, and a Panasonic 310 DVD player. Eventually, I want to add a separate front amp for the maggies, but not for a while ($$$$). Will I be OK for the time being, and what are your thoughts on these pieces as a system?

A It looks like you are on your way to a fine system. The DSP-A1 will be able to drive the Magnepans, but not very loud. They are not only 4 Ohms, but low sensitivity too (85 dB/w/m). You should set the impedance switch on the back of the DSP-A1 to 4 Ohms, and use the volume control judiciously. Later on, definitely go for a good outboard five-channel power amplifier that will handle 4 Ohm loads.

Q I saw a question in #72 about tubes vs. transistors, and as always, you gave good advice about letting our ears being the ultimate judge. Tubes are definitely better sounding to my tastes but are incredibly expensive. Before I plunk down $4,000-$6,000 on a 100 watt per side amp, and another $2,000 - $4,000 on a preamp, can you give me some specifics on upkeep, care, and handling? I know that in general tubes last 2,000 - 3,000 hours, but how expensive is it to replace dying tubes? I have auditioned Sonic Frontiers, Audio Research, and Cary, and like the sound from the ARs best, but the SF amp is very close. What key factors should influence which brand I go with (they are similarly priced through the dealers I have talked to, and have similar prices on the used market)? Is it really important to use a tube preamp with a tube amp? Most importantly why does a tube amp which is rated at 25 watts per side drive speakers sounding like a 100 watt per side transistor amp?

A Probably the main thing with tubes is heat dissipation. You have to be very careful to place the amplifiers in a spot that is well ventilated. Although the power amplifier will produce the most heat, the preamp gets pretty hot too. I carefully dust the tubes every couple of weeks, since dust can get in the way of efficient heat transfer. Tubes can actully last a lot longer than 3,000 hours, and except for the output tubes, are relatively inexpensive. Write down the tube types the next time you are in for an audition, and then check on the prices from websites such as http://www.vacuumtubes.com/. With tube amplifiers, there is probably more variation in sound quality than with solid state because tubes come as triodes, tetrodes, and pentodes, with triodes and pentodes the most common. Also, they come as single-ended designs, push-pull, and also OTL (output transformer-less). Single-ended triodes (they are always run in Class A) are generally regarded as the most pleasant sounding, but they don't have a lot of power (they work fine as preamps though). Push-pull pentode designs are very popular. The reason 25 watt tube amps sound so much more powerful is that the power supply for tube amplifiers has to be massive. Also, tubes tend to distort with even-ordered harmonics, so you can turn up the volume well into clipping and it doesn't bother your ears. The output impedance of tube amplifiers tends to be high, so the characteristics of the speaker will greatly affect the sound. Be sure to listen to the exact combination of tube amplifier and speakers you want to buy, before you actually make the purchase. Pairing a tube preamplifier and solid state power amplifier can be wonderful. You should not have any problem mixing the two.

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