AUDIOCLINIC (Jan. 1994)

Minimum Impedance Problems

Q. My receiver delivers 110 watts per channel into 8 ohms. Recently I purchased four new loudspeakers. One pair has an impedance of 8 ohms; the other pair, 6 ohms. I like to run all four speakers simultaneously. The receiver's instruction manual talks about either 12 ohms per speaker minimum or 6 ohms per speaker minimum. I have my speakers connected in parallel. Is that safe? Can I damage any of my components with my present wiring?

A. It seems to me that you can operate your equipment perfectly just as you now have it wired. If I understand you correctly, your manual says you can operate two pairs of speakers in parallel as long as the mini mum impedance per speaker is 6 ohms. One pair of your speakers meets that mini mum, and the other has a higher impedance than that.

If, on the other hand, the combined impedance of both speakers per channel must be 6 ohms, you'll have problems with a parallel hookup. In parallel, your loud speakers will have a combined impedance of less than 4 ohms.

Does your manual's reference to 12 ohms apply to serial connections? If so, I suspect that 12 ohms per channel is the lowest value recommended as the combined series impedance of your two pairs of speakers. In that case, your present loud speakers can run successfully in a series hookup. If the manual actually says each speaker's impedance must not be lower than 12 ohms (unlikely, as few speakers have impedances above 8 ohms), you have a real problem, as none of your speakers has such a high impedance.

If your manual is confusingly written, check with the service department of your receiver's manufacturer to see which hook ups are safe. Assuming both serial and parallel connections are okay, I recommend you operate your speakers in parallel because the damping is better.

If there is any doubt, don't drive your amplifier to maximum. If you run it at half power or a bit less, chances are you will minimize overheating and damage to the output stages. There is little audible loss of volume between full and half power.

"Pops" from the Power Lines

Q. I hear a "popping" sound from all speakers in my audio/video system. The sound is usually just a quick "pop" every 2 to 15 minutes, and I also hear it when a light switch is turned on. What is causing this sound, and how can I prevent or eliminate it? Will my components be damaged by these "pops"?

A. The "pop" you hear is caused by transient voltages on the power line. These transient spikes are often produced when a refrigerator or furnace starts and stops operating. As you have noticed, light switches often cause the sound.

I have never heard of such noise spikes being so loud as to damage equipment. But they are annoying, so you need to deter mine which of your system components is passing these spikes along to its output. Since all your speakers are reproducing the popping sound, look for components that affect all channels. Components that fit this description typically include A/V receivers, preamps, and surround decoders. If all your speakers are fed from the same amplifier, it's also a suspect.

To determine the actual source of the problem, try disconnecting one suspect component and running the system with out it. If the "pops" are still heard, reconnect the component and try disconnecting another one. Keep trying, working closer and closer to the power amplifiers, until the noise is gone. If you're left with only the power amps as likely culprits, check by substituting other amps for them.

Once you know which device is causing the problem, write to its manufacturer and ask what remedies he has for the problem. With some components, the solution is to place low-value capacitors across the feed back resistors, to narrow the bandwidth. Before you try that, let the equipment's maker recommend capacitor values.

Sometimes, especially in homes with old and undersized wiring, the problem can be cured by running a separate a.c. line from the fuse or breaker box for use with the audio system only.

You have observed that one or more light switches are sources of these "pops." If you get a noise spike when your furnace or air conditioner switches on, the relay contacts associated with your thermostat may be another source of the problem. For these sources, you may be able to reduce the level of the noise transients by putting RC circuits across the offending contacts. Such circuits consist of a resistor and capacitor wired in series across the contacts. Use half-watt or larger resistors, and capacitors with working voltage of at least 400 d.c. Try a 100-ohm resistor and a 0.1-uF capacitor for starters, but you may have to experiment to find what values work best.

It's not easy to install an RC network in a refrigerator or freezer, but you should be able to find noise suppressors that can be plugged in between such appliances and the a.c. line. Make sure these suppressors have sufficient power-handling capacity for whatever appliance you use them with.

Audio and AC Polarity

Q. Why do some high-end components have switches to change the polarity of the a.c. line? I can't see how changing the polarity of an a.c. signal can improve the sound quality.

A. Reversing the polarity of the a.c. power-line voltage entering a sound system will not improve the intrinsic audio quality of that system. On its face, it does seem ridiculous to think that by reversing the polarity of an a.c. voltage, the sound of an audio system can improve or deteriorate. However, there still may be some apparent sonic improvement. The power line has one side grounded, and changing the polarity of the power-line voltage affects the action of the ground in relation to the chassis. This can, in some situations, affect the amount of background hum that emanates from the loudspeakers. Even where hum is not apparent, it could be present to an ex tent where it will mask some nuances of the music. When the hum is reduced or eliminated (as a result of reversing the polarity of the power-line voltage or however it was done), the music will take on a more open quality.

CD Track Lockout

Q. Why are there presently no CD players that allow the user to "lock out" certain tracks on a particular CD so that they cannot be played? On any given CD, there is likely to be at least one undesired track Wouldn't it be easier to lock out one or two tracks rather than having to program the player for each CD so that only the desired tracks will be heard?

A. Quite a few current CD players offer such features as Favorite Track Selection, letting you. store the track programs for many CDs and automatically repeating those programs when you replay each disc. Check whether any model you're considering has the memory capacity to handle the number of discs and tracks you'll need. Philips and Philips-owned brands, such as Magnavox, were first with this feature, but other makers have it by now (often under a different name).

If all you want to do is lock out tracks on a one-time basis, look for a player with a "program-out" feature. This lets you pro gram the player either to play the tracks you want or to exclude the ones you don't want, whichever makes programming simpler and faster for a given disc.

Line-Voltage Conversion

Q. Last year, I had my receiver converted from 120-V operation to 220-V operation hey an authorized dealer in Paris. Is this in any way likely to degrade the performance of my receiver?

A. Inasmuch as you had the power transformer replaced by an authorized dealer, the correct replacement part was probably used. Therefore, I believe the receiver's performance will not have been de graded by the conversion.

CD Players, Amps, and Level Controls

Q. Do I really need a preamplifier be tween my CD player and my power amplifier? Can't I just use a high-quality ganged volume control between the player's output jacks and the inputs of my power amplifier?

A. Using a volume control between the output jacks of your player and the input terminals of your power amplifier will work fine. Just for the sake of completeness, some CD players are equipped with their own volume controls; such players will also work well for your application. However, I personally don't like this arrangement, because there are times when I would like to make tonal adjustments. Your arrangement makes no provision for this. A preamplifier also provides switching facilities and input/output connectors for other equipment in a system; your arrangement doesn't (although you could use a switch box).

Misaligned FM Tuner

Q. I have an FM tuner connected both to a dipole antenna and, via switching, to the local cable company. The cable system re transmits our local stations and provides a stronger signal than I can obtain using my dipole.

The tuner is still new, with less than 100 hours of use. I have noticed that the LED center-tuning indicator frequently shows that tuning a little higher in frequency may be required. If I move up 25 khz, the indicator shows correct center tuning, but the signal-strength meter shows a slightly weaker signal. This occurs whether I use my dipole or cable.

I have heard that atmospheric conditions can sometimes cause stations to drift. I've also heard that tuners can drift. Should I have my tuner serviced?

A. Atmospheric conditions have nothing to do with the frequency accuracy of an FM broadcast station. I cannot even imagine how such conditions would "fool" the tuner into a frequency error. I think the real problem is that the tuner's detector is not aligned properly. Let's say that it is supposed to be centered on 10.7 MHz. If, because of misalignment, it happens to be centered on 10.725 or perhaps 10.675 MHz, then the signal meter and the center-of-tuning meter readings will not agree. Depending on the bandwidth of the i.f. system in your tuner, this misalignment may produce some distortion on highly modulated signals.

(Audio magazine, JOSEPH GIOVANELLI, Jan. 1994)

= = = =