Q&A # 52 - February 24, 1998

Staff

Q What happens when you have two crossovers working together? Does it really matter? I ask this because the Yamaha 2092 has a crossover at 90 Hz, and my Polk subwoofer has a crossover that cannot be switched off.

A This is the situation most of us are in, and under normal circumstances, it won't present a major problem. There will be more phase shift, and a little more attenuation of the sound, because extra parts are in the signal path. THX processors control the low pass to the subwoofer, but you need to be able to bypass the crossover in the sub to use this feature properly. If you use the processor to divert 90 Hz and below info from the main channels into the subwoofer, just be sure not to set the low pass crossover on the subwoofer to less than 90 Hz. Otherwise, if you set the subwoofer to 60 Hz, there would be a hole in the sound between 60 Hz and 90 Hz. What I do occasionally is use Y-connectors from the front left/right pre-outs of the processor/receiver, with one leg going to the power amplifiers and the other to the left/right inputs on the subwoofer (or one each to 2 subwoofers). This way, I bypass the subwoofer filtering circuitry in the processor/receiver. There is less phase shift that way, and sometimes more signal too, but because human hearing is not so sensitive at low frequencies, I can't tell much difference. It's a "purist" thing.

Q I enjoy your site and I just read Q & A  #51. One of the questions referred to horizontal resolution. According to Toshiba, satellite broadcast is at 360 lines of resolution? That number seems low. Also, I own a Toshiba 48" rptv and DSS. Is there a way to calibrate this set without owning a LD or DVD player and using the Video Essentials discs? DVD is in the future for me, but currently no. I am referring to percent reference like: contrast-50%, sharpness-10%, etc. Thanks for a great site.

A DSS claims to be around 425 lines of resolution or approximately 5.2 to 5.4 MHz.

Without an LD player or DVD player, you would need a signal generator that has the appropriate test patterns for color, tint, and so on.

If you were to pay for an ISF tech to come over and calibrate the gray scale on your set, the techs are usually equipped with the signal generator and would do the adjustments for you. Otherwise I recommend just turning down the contrast to protect your set and hold on until you get your DVD player. If your Toshiba has a theater mode setting, start from there and pull the contrast down even more.

Q I would like to know if center speakers should always be the same brand and come from the same series/model of the mains in order to have perfect tonal balance. What do you mean by voice matched and timbre matched? Are the two things the same? Is it also possible to mix different brands together like for example, one brand of center speaker with a different brand of main speakers or just a different model/series?

A If your main speakers are large, then it is very difficult to match the center channel speaker for the mid-bass and bass, but it would still be important to get a good center speaker regardless of the main front left/right. I particularly look for neutrality in the center channel because it has so much of the dialogue, which is noticeably unpleasant if the sound is boomy, chesty, or nasal (like the person has a cold). This implies peaks in the 80 Hz, 120 Hz, and 800 Hz region, respectively. These sorts of peaks are not so noticeable if they occur in the main speakers, where the environmental sounds occur (cars passing, jets whooshing, explosions, guns, etc.) Whether you call it voicing, timbre, tonality, or whatever, having the sound quality as neutral as possible is important for all the speakers, and when I test them, I always use vocals as one of the major points of evaluation. Listening for that neutrality will help you get a nice speaker match, even if they are different brands or models. If they are not neutral, the best you can hope for is an equal unpleasantness in all the speakers. Even if you have exactly the same speakers for front left/center/right, the center will sound different because it is on its side, and has bass loading from the TV screen. So, listen for a natural, comfortable sound quality, and it won't matter so much as to matching brands or models.

Q Can any preamp go with any power amp? For example, can I use a Yamaha RX-V793 to drive a Marantz MA-500 monoblock amp? Someone I know is saying that Marantz only goes with Marantz. Is this really an issue?

A The Yamaha will drive the Marantz just fine. Pretty much all preamps and power amplifiers these days have proper input and output impedances for using them together when mixing brands.  Sometimes a brand will use proprietary triggering sockets that are designed to be used with other components of the same brand, but even that is becoming more standardized. Also, most remote controls have learning features so you don't always need to use two remotes to control two components. A problem could occur if you used a single pair of output jacks on a preamp to drive, say, three power amplfiers per channel (for tri-amping), with Y-connectors. In that case, it is best if the power amplifiers have high input impedances. Otherwise, you could end up with less volume and a loss of high frequencies.

Q In trying to get the most out of the inputs that I have available (like any home theater enthusiast, I'm always wishing I had more), I am using some of the switcher boxes commonly found at the venerable Radio Shack. While their convenience is unquestionable, I am a little concerned about their effects on signal quality. "Amplified" models are also offered, and they seem to elicit an even greater concern. Are my worries unfounded?

A Your concerns are definitely valid. Anytime additional circuitry is placed in the signal path, deterioration of the signal occurs. This is true whether it is just a few more inches of wire, capacitors, inductors, resistors, sockets, or switches, no matter how good the quality of the parts. But, as you mentioned, it comes down to convenience, and how much signal deterioration you are willing to put up with for the sake of being able to accomplish something more easily. There is an old saying that the best amplifier would be a single wire with gain. We could go one further and suggest no wire with gain (like the vacuum between the cathode and anode in a tube). Everything is downhill after you connect the first wire into the circuit. By using switchers, you will probably notice more noise, both audio and video. But, those switchers are necessary sometimes. We use them too, but not everywhere. In one of our home theater labs, we disconnect the cable TV from the video monitor when watching DVD or laserdisc. This gets rid of hum and video noise coming in from the TV cable. Then, when watching a broadcast signal from the cable, we just reconnect it to the monitor.

Q How much is enough?

In four little words I have opened what is quite possibly volumes worth of discussions, opinions, experiences and measurements. Nonetheless, I would be interested in your take on the subject.

I was prompted to bring up the question after skimming last weeks Q&A where you recommended that a reader not strive for 105 dB but keep things at a respectable and safe 80ish. All in all, I could not agree more. And
according to a Radio Shack SPL meter, I can achieve just about that with my Paradigm Mini-Monitors and CC-350 being driven by a modest Yamaha rated for 70 watts per.

That is all well and good. The problem that seems to plague people like myself is the God awful word: Clipping.

I'll start by saying, rather confessing, that I am not even positive if I have ever heard it. Yet it constantly fills my mind, is used prolifically in magazines (even your Q&A column), and has attained a reputation of mythical
proportions, the likes of which they sing songs about in the hills.

But I ramble on. I was convinced once that my system was clipping at a moderate volume. I could hear a very scratchy sound during loud passages in a film. As it turns out, this was merely a bad sound track on a VHS movie (this seems to be the case far too many times).

Clipping . . . . My brother called me up long distance one night and said, "Brian, I think I just drove my system to Clipping. Have I ruined my speakers?"   "What happened", I asked. He answered, "I turned off the power amp while the rest of the system was still playing the CD, and as the sound faded, it got all scratchy. Sounds fine but . . . have I ruined the speakers?"

This poor guy, and myself at times, stay awake at night pondering the mythic clipping.

This all boils down to two things:
- How much power is enough to not worry about clipping anymore?
- Until one can afford such power, is there a way of marking a tab on the volume knob which reads "Guaranteed no clipping below this point"

Maestro, your thoughts?

A If you have ever cranked up your system for any reason, the chances are, you have heard clipping (it is hard not to turn it up when visitors come over for a demo. We've all done it.) It is a crackling, snapping sound, a little like the crackle you hear when lightning strikes some distance away while the stereo is on. That scratchy sound your brother heard was a variation of clipping, because when he turned the receiver off, the power supply still had some electricity stored which dissipated over a couple of seconds. It was enough to keep the sound going for a short time, and as it ran out, the demand on the power supply exceeded its storage, and miniature clipping occurred. But, he did not damage his speakers. When speakers are damaged from clipping, they usually just stop working altogether. The tweeter is the first to go. Sometimes, over extended use, the woofer voice coil starts to rub mechanically against the side of the magnet assembly, and it is a very obvious noise. But that is not necessarily the consequence of clipping. It could be just age and a sagging cone support (the support is called the "spider", and it keeps the voice coil centered).

You can avoid clipping in a number of ways, but every amplifier can be taken into clipping. Every one of them. One of the best ways is to use speakers with high sensitivity, e.g., 92 dB/w/m, and 8 Ohms nominal impedance (your mini-monitors are rated at 89 dB/w/m and 8 Ohms nominal). Audio Research makes a set of high output 8 Ohm speakers (their HO series), and one floorstanding HO model (the 312 HO) has a sensitivity of 97 dB/w/m, which means that a receiver with 70 watts/channel could drive them to the same level as a 560 watt amplifier with 88 dB/w/m sensitivity speakers! On a more realistic level, it means that your receiver could easily drive them to adequate levels with about 5 -10 watts/channel. In fact, the HO series was designed specifically for mass market receivers. I heard them at the San Francisco Hi-Fi show last year, and they sounded very good. They are expensive (about $1,200/pair for the top model), but if you like the one box solution that mass market receivers offer, this is an excellent way to go. I would get two pairs of the 312 HOs (one pair for the front and one pair for the rear, which is best for DD and DTS), and their center channel speaker, the CS 25 HO, which has a sensitivity of 94 dB/w/m.

As to the volume control setting, what I have found is that around 9 o'clock on the volume control knob is pretty safe for most components, 10 o'clock at the most. That is not really very loud, but it is loud enough to enjoy the movie. We REALLY tend to listen to home theater much too loud. Granted, gun shots and explosions are in real life, very loud. But those real life sounds, at those levels, can damage your ears too, just like a loud hi-fi system.

Speakers have more distortion than amplifiers, even at modest volume levels. A level of 10% distortion from a speaker is not unusual. But, just like amplifiers, the louder you play the speakers, the more distortion. Even with a high powered, high quality amplifier, a modest speaker will start to sound mushy when the volume is raised. This is the point to back off the volume control. I much, much prefer a clean sound at easy-on-the-ears levels, than true-to-life teeth gritting levels with distortion. The auditory receptors in our ears are easily damaged, and when the damage is done, it is permanent. High frequencies are particularly damaging to our hearing, so if you must turn up the volume, try turning down the treble control a bit. You could even put a piece of porous foam over the tweeter, on the outside of the grille. Anything to protect your hearing. High powered amplifiers and high-performance speakers don't mean that the only way you will get your money's worth is to shatter the wine glasses. It just means you can have cleaner sound at humane levels. This brings up another of Dr. J's rules of hi-fi enjoyment, which is, "Calm and clean is better than loud and mean."

Q I was just wondering what is the significance of the numbers of tubes in a preamp? Does it mean that the more tubes it has the better it is? I believe the different types/brands of tubes really affect the quality. What about the terms hybrid, dual triode, and single ended triode?

A More tubes do not necessarily imply better sound, but it will usuall mean more expensive. It depends on the function of the tubes in the circuit. The less tubes in the signal path, the better. Many of the tubes could be used for other purposes than amplifying the sound. Some tube amps use tubes in the power supply, and others use solid state devices. Some power amplifiers use tubes at the input stage and solid state at the output stage. A tube component with solid state parts for some of the functions is called a "hybrid". Triodes (they have a cathode, grid, and anode) arguably make the best tube components, but they don't have a lot of power. However, they are perfect for preamplifiers.

Tubed hi-fi components are going through a big renaissance right now. Mostly, the tubes are manufactured overseas in Russia, China, and other places. Some tubes are beginning to be manufactured again in the United States. For any particular tube, such as 300B, 12AX7, the sound differs a great deal depending on where it was made. That's why many tube amplifiers have options for tubes coming from different sources, and you pick the source. The price of the same tube also varies. A pair of 300B triodes can cost from about $80/pair up to several hundred dollars/pair. When I was a teenager, I built a radio transmitter that used a type 80 diode tube (cathode and anode, but no grid) in the power supply (the diode is used to "rectify" AC so that it becomes DC). We called them "Nickel 80s" because we only had to pay 5 cents for them. Brother, have times changed!

Power amplifiers use triodes, tetrodes (cathode, two grids, anode), or pentodes (cathode, three grids, anode). A dual triode is a tube with two triodes inside the same glass envelope. They can be connected in parallel or used as separate triodes. It isn't just for saving space. Since the two triodes are under exactly the same vacuum, they will perform more in an equal manner, keeping the signal handling similar. A single-ended triode is a preamplifier or power amplifier design in which both the positive and negative portions of the waveform are handled by one triode or a set of triodes in parallel. This contrasts with the push-pull design where one triode handles the positive portion of the waveform, and a different triode handles the negative portion. Single-ended preamplifiers and power amplifiers are operated in Class A, meaning that there is always enough current flowing in the output circuit that no additional current is needed to be "turned on", but rather, the necessary output current, depending on the input signal, is diverted to the speakers. This makes a Class A component very "fast", but also very hot and inefficient, because when the current is not being sent to the speakers, it has to be dissipated as heat.

� Copyright 1998 Secrets of Home Theater & High Fidelity
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