Reader Q&A;

Send your questions to: htletters@primedia.com.

Amp Impedance Investigation
I'm trying to match a new pre/pro to my multichannel amp and have finally taken notice of all the different input impedance values supplied for a particular amp. I understand damping factor, signal-to-noise ratio, THD percentage, power bandwidth, etc., but I can't recall ever having read anything that explained how input impedance should be considered when looking at amps. Is that important to know when I'm looking at a particular processor's voltage output and comparing it to the amp's input sensitivity? I've noticed that some amps print that they may need, for example, anywhere from 1 to 1.65 volts to get their reference power output to the speakers. I've also learned that most receivers don't have nearly the voltage (RMS) output level that dedicated pre/pros do. For example: A Pioneer Elite receiver delivers 1 volt peak into 75 ohms (how does that relate to an amp that has an input impedance of 28, 50, or even 100 ohms?), while an Adcom processor may put out 4.6 volts (RMS, into how many ohms, it didn't say). So, my main aspiration is to bring cohesion to these specifications in order to maximize my amplifier's capabilities. Is there a site online or even a book I can read that explains this concisely?

Matt (Via E-mail)

Gain scaling and impedance matching is a big topic to cover in depth, but here are some pertinent points to consider. This is one of the many details that become academic when using all THX-certified gear.

The rule of thumb for impedance matching is that a lower pre/pro output impedance and a higher amplifier input impedance minimizes the effects of the interconnect cable. The ratio of output impedance to input impedance should be greater than 1:10, often much greater. For example, a pre/pro with a 100-ohm output impedance will be happy with amplifiers having at least a 1,000-ohm input impedance. Most modern solid-state amps and pre/pros will work just fine together in terms of impedance matching. If you're dealing with the few goofy solid-state pieces of gear that don't comply with the concept of low-output impedance/high-input impedance, then you're on your own.

The preamp outputs in some receivers don't have as much output capability as dedicated pre/pros, but they aren't intended to be connected to power amps with unusually small amounts of gain. If the power amp has gain controls, this generally becomes irrelevant, though. Also, amplifiers with a gain of substantially more or less than about +29 decibels will be more difficult to match with a pre/pro. Once again, most modern solid-state pre/pros and amplifiers will work together, but there are exceptions.

Don't hesitate to contact the manufacturers if you aren't sure about compatibility. This was a much bigger issue in the olden days than it is today; but, when you mix solid-state and vacuum-tube gear, then you do have to pay more attention to these details.—MJP

Room Calibration: Which Way Is Up?
I own a $50,000 home theater system, and I want to calibrate it. But my only option here in Lebanon is to use the RadioShack digital SPL meter. My home theater measures 29.5 feet by 16.4 feet. I have a 7.2-channel system with powered speakers. Which of the following approaches should I take?

1. Direct the microphone of the SPL meter to the front of the room toward the center of the screen and calibrate all of the speakers while keeping the microphone pointed toward the front of the theater?

2. Direct the microphone toward each speaker as I calibrate it?

3. Direct the microphone toward the ceiling and calibrate all of the speakers and subs while maintaining the microphone in a vertical position?

Nasri (Via E-mail)

I'd go with option 3. Kneel behind the listening position and hold the SPL meter at arm's length away from your body, with the microphone pointing up toward the ceiling at the height of a seated listener's ear.—MJP

Gain Settings: Sounds like Trouble
I have been a subscriber for just over one year and thoroughly enjoy reading every issue that has been sent to my house. I recently purchased a new receiver, which I had been researching on my own and that you then reviewed. You were right, this is a great product. There is one thing I do not understand—when viewing the satellite receiver, the VHS, or playing my Xbox, the sound output from the speakers is relatively the same. When I switch over to the DVD player, the sound is much lower, and I am forced to turn the volume very high (almost maxing it out). Is there a reason for this discrepancy? Do I have a setting wrong somewhere? I noticed this issue on my previous receivers and just assumed it was somehow tied to them being of mediocre quality. Do I have a problem with my DVD player?

Russ Leadnetter, Scottsdale, AZ

Most likely there isn't anything broken; this is an age-old annoyance. Some receivers have a menu for setting the relative gain of each input, which can help. Even once the level of each input are balanced, though, some discs are mastered with a much higher average program level than others, so volume discrepancies are almost impossible to alleviate.—MJP

Product Quality: Made Where?
I just recently purchased a new amp instead of a soon-to-be-phased-out older model. To my disgruntlement, I found that the new model was not manufactured in the United States and that all the specs (except for the claimed power) were virtually the same as the older model. I'm suspicious that the company stamped a new model number on an existing design, sent the design overseas for production, then unveiled it as new and improved. (Higher power ratings are claimed through EIA standards instead of the typical FTC standards used before—LAME.) My confidence in my purchase is now shot. I'm worried that my new amp isn't on the level of build quality that I expected it to be, and the company wouldn't even respond to me when I asked them why they changed the manufacturing location.

Matt Andersen, Omaha, NE

The electronics industry (among many others) has come to be so international that the "Made In" designation generally refers only to the country of final assembly. Putting aside nationalistic pride for a moment, it's important to realize that the myriad of transistors, integrated circuits, transformers, switches, circuit boards, etc., are going to come from many different countries, even on a unit emblazoned with a "Made in USA" sticker. There may be many valid reasons to want American workers twisting the screwdrivers during the final assembly of your shiny new amplifier, but reliability and sound quality are unlikely to be among them anymore. As is always the case, some manufacturers have better engineering and quality control than others, regardless of where their product is manufactured. Offshoring is a complicated and contentious subject, but, in the final analysis, I'd like for my friends (and myself) to still have jobs here in the USA.—MJP

Subwoofers and Speaker Placement: Bass Desires
First, I am curious about the apparent fascination with bass in home theater applications. I frequently read advertisements for subwoofers that crack foundations and shake neighbors' houses. Apparently, this is what many people crave, as evidenced by discussions about wanting "bass that you can feel." As a proponent of the ideology that "what is good for music is good for home theater," I can't understand anyone wanting overwhelming bass. As a matter of fact, I have experimented with earth-shattering bass in my own system. I cranked up my three subwoofers and found that house rattling came at a price. The low frequency drowned out the detail of the soundtrack. Am I missing something? Second, I love to look at pictures of my fellow enthusiasts' systems. I have noticed that the speakers in two-channel setups often appear to be fairly close together. My understanding is that, as a general rule, 6 to 12 feet is an acceptable distance—the object of the distance, of course, being to maximize the soundstage, imaging, and so forth. My speakers are 10 feet apart. The listening position is 15 feet back from the speakers, thus giving me the "perfect triangle." Are my speakers too far apart?

BKM (Via E-mail)

You're correct that bass should not be overwhelming. Depeche Mode said it best: "Get the balance right." The point is to have improved bass extension and quality, not increased bass level relative to the other channels. Deep, low-distortion, powerful bass at the correct level can literally be breathtaking. If you can hear the bass, it isn't especially low in frequency. If you can feel the bass pushing against your diaphragm and making it difficult to breathe, it's probably the genuinely low bass that all the fuss is about.

As to the placement of main speakers, there are rules of thumb that can provide a good starting point, but personal preference and listening-room conditions should always take precedence. Experiment until the sound makes you grin, then sit back and enjoy the results.—MJP

Aspect Ratios: 16:9 + 4:3 = Confusion
Where can I find information on deciding which screen shape (16:9 or 4:3) to use? I have a 16:9 projector, and my home theater room will accommodate up to a 96-inch wall-mounted screen with the projector back at about 19 feet. I want to maximize the screen width and image while always keeping any image format on the screen. Local TV stations will be a large part of the viewing agenda, so does that drive me to a 4:3 screen?

Ken Paul (Via E-mail)

The short answer is, to maximize the resolution of your 16:9 projector, you should get a 16:9 screen. But, first, your last point. You say that most of your viewing will be local network stations. In most areas, the local networks broadcast in HD. So, for prime-time viewing, you'll be watching 16:9 material, anyway. Plus, your projector is 16:9. So, if you get a 4:3 screen and set the projector to fill it, when you're viewing 16:9 material (HDTV and DVD), the image will have to be shrunk to fit the 16:9 portion of the 4:3 screen. I'm not positive, but I don't think you can even do this with some projectors. Even if you were able to, you'd lose a significant amount of resolution (you'd only be using a fraction of the chip to produce a 16:9 image instead of the whole chip). If you thought of a 4:3 screen because you're worried about blank areas of the screen when you watch an aspect ratio other than that of the screen, you should look into a screen with masking. This will shape the screen to whatever aspect ratio you need. But, it tends to be an expensive solution to something as, to me, inconsequential as black bars.—GM