AUDIOCLINIC (Aug. 1996)

Resist Those Resistors

Q. My A/V receiver has a switch that must be thrown when using 4-ohm loudspeakers in the front channels. The receiver's directions say that I should use 8-ohm loudspeakers when using surround. Will I ruin the receiver by using 4-ohm speakers? Would putting 4-ohm resistors in series with these speakers save the day?

-Mark D. Mina, Costa Mesa, Cal.

A. You do stand a chance of damaging your receiver's amp section by using 4-ohm loudspeakers when the owner's manual calls for 8-ohm speakers. You can probably get away with it, if you don't drive the receiver anywhere near its maximum power. Also, it may be that the manual is indicating only that the surround-channel speakers must be 8 ohms. Double-check that before you do anything else.

Placing 4-ohm resistors in series with each loudspeaker will indeed give the amplifier an 8-ohm load it can be happy with.

But half your signal will be lost in the resistors, so you may be unable to get enough volume or may have to drive the amp so hard that it clips, perhaps damaging your speakers. You may also find that the bass sounds mushy or boomy because the resistors effectively increase the amplifier's out put impedance.

In the long run, it might be a better idea to replace your present loudspeakers with 8-ohm models or replace the receiver with one that can accept lower load impedances.

Adding Amplifier Level Controls

Q. I am going to bi-amplify my loudspeaker system with two tube power amplifiers. I need to add input level controls to these amps. What resistance should the controls have? And should I remove the 470-kilohm grid resistor from each input?

-Robert W. Clifford; Lancaster, Cal.

A. You might not need to remove the amps' input resistors. If you drive your amps from a solid-state crossover, the resistance of the required potentiometers will be so low that the 470-kilohm resistor will effectively be out of the circuit. You'll be able to keep the input resistors and use potentiometers that have about 50 kilohms of resistance for the level controls.

If you use a tube crossover, you might wish to maintain your amps' high input impedance. In that event, remove the grid resistors and use 470-kilohm pots. You should keep the cable runs between the wipers of the pots and the amplifiers' input grids as short as possible. If you hear diminished treble, especially when the controls are near their center positions, you probably need to use low-capacitance, shielded cables between the pots and the grids.

With either approach, be sure to buy audio-taper pots.

Effects of Oversampling

Q. What effect do sampling rates and oversampling have on digital audio's sound quality?

-Name withheld

A. Digital recording works by sampling a signal's voltage many times a second and then storing the numerical value of each sampled voltage. The number of samples per second sets the maximum possible frequency that can be recorded, which is just under half the sampling rate; the CD sampling rate of 44,100 times per second (44.1 kHz) allows frequencies up to 20 kHz or so to be recorded.

The recorded signal's sampling rate and frequency content cannot be changed. But most D/A converters work at higher sampling rates than the rate on the recording, mainly to enable use of digital filters.

The unfiltered output from a D/A converter contains the frequencies originally recorded as well as ultrasonic artifacts of the sampling process. This ultrasonic noise must be filtered out of the signal fed to your system, lest it overload amps and tweeters or cause other problems. Removing every thing above 22 kHz by conventional means without appreciably rolling off audio frequencies (20 kHz and below) requires a very steep analog filter, which can cause high-frequency phase shift and frequency response ripples in the audio signal. Over-sampling, on the other hand, enables use of a digital filter between the audio band and the much higher oversampling frequency, followed by a shallow analog filter. This approach makes it much easier to get flat phase and frequency response in the audio band. And 1-bit, or delta-sigma, converters require rapid oversampling in order to work at all.

An oft-repeated myth, by the way, is that oversampling improves error correction by reading each sample on the disc multiple times. This is completely untrue; oversampling is an entirely mathematical operation.

Turning a Subwoofer Off

Q. The receiver in my home theater system does not have a line output for a powered subwoofer, so I use a passive sub. But when I'm listening to music, I often prefer the sound without my subwoofer. Is there an easy way for me to connect the system so that I can easily turn the subwoofer on and off?

-Tom Gebbia, Lynbrook, N.Y.

A. If your satellite loudspeakers operate full-range, with none of their bass rolled off by crossover networks, all you need is a switch in line with the "hot" leads of the subwoofer's voice coil. If your sub-woofer has a dual voice coil, you will need a double-pole single-throw (DPST) switch for this; otherwise, a single-pole single-throw (SPST) switch will do. But get a switch that has a fairly high current rating (5 to 10 amperes will probably suffice).

If your satellites are fed from a crossover, you need a way to bypass the crossover as well as to turn off the subwoofer; just turning the subwoofer off will eliminate all bass.

Your switch will now have to do two jobs: It must divert signals from the crossover and subwoofer so that they feed the satellites in stead, and it must cut the connection between the satellites and the subwoofer.

You could do this with a double-pole double-throw (DPDT) switch and a double-pole single-throw (DPST) switch. Feed your amplifier to the center terminals of the DPDT, connect the terminals on one side of the switch to the satellites, and connect the terminals on the switch's other side to the subwoofer. Put the DPST switch into the line between the subwoofer and the satellites. Align the switches on some kind of panel so that both point the same way when you have them set properly; better yet, link them physically with a bar so that you automatically operate both switches simultaneously. It would be simpler still to use a four-pole double-throw (4PDT) switch, but they're hard to find.

Disappearing Left Channel

Q. Every so often, the left channel of my preamp cuts out. Sometimes I can fix this for a while by playing around with the controls or switching the unit on and off. The longest-lasting fix I've found is to unplug the cables between my preamp and my electronic crossover, plug them into the opposite channels, and then put them back as they were originally.

-Agim Perolli, White Plains, N.Y.

A. The problem might lie with either your preamplifier or your crossover.

But since the problem goes away when you unplug and re-plug your cables, I think it's caused by a defective cable, by a dirty or oxidized connector, or by loose connector contacts.

Unplugging and re-plugging cables tends to wipe dirt and oxidation off both plug and jack. It might be even better to dab a little contact cleaner on each connector and immediately plug that connector into its mating jack. Do this a few times with each cable.

To check for defective cables, wiggle each cable a bit while the system's working and see if the sound cuts in and out. If it does, you may well have a bad cable. Cables are most likely to develop intermittent connections near their plug ends. If you find an intermittent section, cut it out and solder on a new plug (or reattach the old plug to the new end, if that's feasible).

Intermittent operation can also be the result of loose contact between a plug's center pin and a socket. If that's the case, try other cables to see if your cable's plug has too thin a center pin. (This is more likely to occur with cheap plugs.) If all plugs are loose in the socket, you may be able to tighten the socket's center contacts by squeezing them with needle-nose pliers, from inside the preamp. (Be sure the preamp is off.) The skirts of RCA plugs can also become loose, but that's more likely to introduce hum, not cut the signal out.

Does Speaker Cable Age?

Q. A repairman recently replaced the pin plugs on my 10-year-old speaker cable with banana plugs. He told me that I should also replace the cable itself because it could have deteriorated over the years. Is this reasonable?

-Name withheld

A. I don't see any need to replace your cables. Cables do oxidize, but that's only a problem at their ends, where they are most exposed to air and where the oxidation can affect your connections. And the cable was presumably stripped clean at the ends when the plugs were replaced.

Loudspeaker Overload

Q. I have separate left and right front speakers but use my TV's speaker for the center channel. When I play movie sound tracks that have explosions, crashes, or other loud sounds, I hear a popping and cracking sound from the front. What causes this? And do I need a subwoofer to handle the low frequencies?

-Neal McIntosh; Front Royal, Va.

A. From what you say, it sounds like you are overdriving your front loudspeakers. The popping and cracking you hear is the speakers' voice coils striking their pole pieces when the signal demands that they move further than they were designed to. If you continue to overdrive your speakers, their voice coils will eventually be damaged.

This will cause audible distortion (especially at low signal levels) or, in extreme cases, total silence because of broken coil windings.

For the time being, you can prevent the problem by listening at lower levels. And if you're using a Dolby Pro Logic decoder, make sure you are running it in " Normal" mode, to keep as much bass as possible out of the TV's speakers. But, as you guessed, a subwoofer should cure the problem be cause it will enable you to feed less bass to your present speakers.

You'll also need a crossover that not only feeds bass to the subwoofer but also filters bass out of the signal that's being fed to your main speakers. Many subwoofers, especially powered subwoofers, have such crossovers built in.

If you are unhappy in other ways with your front loudspeakers' performance, this could be the time to audition other systems to find some you like better. Using speakers that have better bass response may or may not reduce your need for a subwoofer; how ever, if you still use your TV's speakers for the center channel, you're definitely better off with a sub.

Indeed, it is possible that the small TV speakers are the only ones being overdriven, in which case switching to a dedicated center speaker might on its own solve the problem while providing much improved over all sound quality in the bargain.

Two Center-Channel Speakers

Q. I'm setting up a surround system, and my equipment provides for one or two center-channel speakers. Even with two speakers, the center channel will still be mono, so what advantage could there be in using two?

-Name withheld

A. In general, best sound quality will be obtained with a single center speaker directly above or below the TV screen.

However, two speakers reproducing the same signal will sound like a single speaker placed midway between them-right in the middle of your TV screen, where dialog should seem to come from. With average sized TVs, mounting a single center speaker just above or below the screen will make the dialog seem to come from the right place.

But this illusion is harder to achieve with large projection TVs; that's where two center speakers might pay off.

In most installations there's not much room above or below the TV. So if you want to use a large speaker for your center channel (to match your main speakers, for in stance), you might find it easier to install one on each side of the TV.

Those people whose equipment does not provide for two center speakers can connect a pair in parallel to their center-speaker terminals. Hooking up two speakers in parallel will halve the load impedance (i.e., paralleling two 8-ohm speakers makes a 4-ohm load), so they must be sure that the resulting impedance is no lower than their receiver or amplifier allows.

Using Two Preamps

Q. I want to adapt my stereo system for home theater. None of the surround processor/preamps I've seen has phono in puts, but the excellent preamplifier I already own does have them. Would it make sense to keep my current preamp for its phono section, using a Y-connector to connect both my pre sent preamp and a new one to my main power amplifier? Feeding the old preamp into the new one would use up a high-level input, and the processor/preamp I want hasn't any to spare. I also think I'd get cleaner sound by feeding phono signals through one preamp instead of two. Could I also use Y-connectors to feed my CD player into both preamps? This would allow me to play CDs through my old preamp's DSP section as well as through the new surround preamp.

-John E. Gallagher; Goshen, Ind.

A. It is certainly reasonable to use your present preamplifier for its phono section and to use its DSP section with your CD player.

You could use a Y-connector to feed your CD player into two preamplifiers, as long as you switch both preamps to whichever in put the Y-connector is feeding. Otherwise, the preamp that you're not using will put a short across the CD player's output. That will kill the sound and might damage the player's output circuits.

Feeding your old preamp into your new one might not be as bad as you think it is. If you take the signal from your old preamp's tape outputs, you'll bypass that preamp's volume and tone controls (and most of its circuitry), so there should be little or no loss of sound quality. You could use your old preamp's line inputs as well, which would more than make up for the input you're using on the new processor/preamp.

Unfortunately, this approach would probably also bypass the old preamp's DSP section, which you say you want to continue using. So you'll need to take the signal from your old preamp's main output rather than from its tape loop. You'll then be able to use its volume and tone controls and its DSP section. If you still want to avoid running the old preamp through the new one, you will need a way to feed one preamp or the other to your amp. But you must not connect the two preamplifiers to the power amplifier via a Y-connector. If you use a Y-connector, whichever preamp you are using will see the other preamp's output section as a load; this could reduce output and might cause distortion. You'd be better off making or buying a switch box to connect the amp to either preamp alone.

Controlling Remote Speakers

Q. My house is prewired for audio, with stereo speakers in six zones. I want to use these speakers and will probably want music in several zones at once. What would be the best way to power this system at a reasonable cost? Should I buy a good stereo amp and a good external speaker selector? Or, since I've heard that the protection circuits on speaker selectors degrade sound quality, should I buy something like Niles Audio's Model SI-1200, a moderately priced 12-channel amp? If I do use a speaker selector, should the volume controls have protection circuits?

-Kirk Creswick; Phoenix, Ariz.

A. Using an external speaker selector and wall-mounted volume controls would be the most affordable way to go.

With separate volume controls in each zone, the listener can set his own sound level. (I prefer stereo controls; having separate controls for each speaker is an unpleasant complication.) At one time, I would have said not to do this, because volume controls were then resistive L-pads, which wasted amplifier power by turning it into heat.

Good modern volume controls are auto-transformers, which waste little power.

Although the protection circuits in good speaker switchers do use resistors and thus waste power, they should not, as you fear, noticeably degrade the sound. On the other hand, you need not switch the protection circuit in unless you're playing music in several zones. If you do so mainly for back ground music, then power limitations would probably not bother you. Since the protection circuit in the switcher will pre vent damage to your amplifier, you don't need additional protection in your volume controls.

The Niles Audio 12-channel amplifier has preset level controls for each channel, but those controls cannot be set remotely.

The listeners in any zone would have to be happy with the level they got, unless you used volume controls in each room.

(Audio magazine, JOSEPH GIOVANELLI, Aug. 1996)

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