AUDIOCLINIC (Jul. 1986)

Bypassed Filter Capacitors

Q. I notice that many power sup plies use a 0.1-µF capacitor in parallel with a 10,000-µF filter capacitor. Why?

-Tom Unger, Gardena, Cal.

A. The 0.1-uF capacitor is there to suppress switching transients. so that they won't find their way into the audio circuits. Its use also tends to keep r.f. out of the power supply. Yes, the large filter is there, and you wonder: "How can this small capacitor work if the big one won't?" The answer is that the large capacitor is not a perfect capacitor at high frequencies. It possesses inductance. which is effectively in series with the r.f. being bypassed. reducing the amount of effective bypassing which will be accomplished by the filter capacitor at high frequencies.

Transient "Click" on Turnoff

Q When I turn my receiver off, a "snap” or "pop" is heard, and the protection indicator flashes for an instant. I wonder if this means that the amplifier is producing so much power that there is a chance of damaging my loud speakers. I've heard all sorts of opinions about this annoyance, ranging from "they do all do it" to "don't worry about it." What are your thoughts about this?

-Name withheld

A. I receive many letters of this kind.

From what I read and from my own experience, they don't "all do it." I have never heard of an instance where the "click" was so pronounced as to drive the power amplifier to a point where its protection circuit was activated. I have to believe that your protection circuit flashes not because of an overload. but because the operating voltages have fallen, leading to erroneous reading of that circuit.

Why would some gear act as you have described and other equipment be free from such noise? I have yet to figure it out. One manufacturer told me that much depends on the way the circuit grounds are laid out. I have noticed that many units which exhibit this set of symptoms are also subject to interference from external power-line transients. such as those produced by the turnoff of a refrigerator or the operation of a furnace.

I suggest that you check with the maker of your equipment or his representative to see if he has a solution. If you wish to experiment, and if you do not mind violating your amplifier's warranty. you can try wiring a resistor and capacitor, in series. across your amplifier's power-switch contacts. The capacitor should have a value of about 0.02 to 0.05 µF and be rated at 600 V d.c. The series resistor's value should be 50 to 200 ohms, and it should be rated at 1 watt. I have successfully silenced such problems. in some cases. with just this simple circuit.

Heads or Tails

Q. I was asked to send a cassette to someone "heads out and ready to roll." Can you tell me what the term "heads out" means?

-Steve Kranick. Loudonville. N.Y.

A. A cassette's beginning is said to be the "head" and its end to be the "tail." If we play a cassette to the end and do not rewind, we say that it is wound "tails out. If the tape has been rewound and is ready to play again, we say that is it wound "heads out." These terms are carry-overs from professional recording-studio practice.

Open-reel master tapes are always stored tails out. because the tape is wound more neatly on its reel or hub than it would be if it were rewound prior to storage. Wound tails out. there are no protruding turns of tape. whereas if the tape is rewound. there is a tendency for it to be more loosely packed, with some edges standing out from the pack. When the tape is placed in its container. these raised tape edges will be squeezed or folded down. This, in turn. causes permanent wrinkling of the tape and leads to less-than-perfect contact between the playback head and the tape.

People who request cassettes to be wound "heads out" need to be able to play them immediately, without waiting to rewind them.

Incompatible Equalization With Different Speakers

Q. I own a pair of speakers employing 5-inch woofers. I didn't like the way the tweeters sounded, so I disconnected them. To restore the lost treble I used an equalizer, and found treble boost settings which enabled me to restore the sound to "normal." In fact, I even prefer the sound from these woofers to some three-way speakers I own which cost hundreds of dollars more than the little systems. However, when I try these same equalizer settings with some of the other speakers, the music sounds awful. Why?

-Anthony Maul din, Lewisville, Tex.

A. There are a few factors operating here. First, your small systems use woofers which themselves are reason ably small. Thus, with some treble boost, these woofers are capable of producing reasonably smooth highs.

Your letter doesn't mention whether your other systems are still equipped with their tweeters. If so, before the equalizer is switched in, the treble is already more or less flat. If you then add the boost, the highs will be exaggerated. So the sound is likely to be "awful." Perhaps, in fact, you did remove the tweeters from your larger systems be fore introducing the equalization. In that event. your midrange driver may be of such a design that it is simply incapable of producing the full range of highs, even with treble added. Even if the driver can reproduce the frequencies, there may be a number of serious peaks and dips in the treble region that make the sound "awful." Or perhaps there is a low-pass filter associated with the crossover network which you should have disconnected.

With all of this, it is likely that, with your three-way speakers, you need a different EQ curve to bring about the smooth highs you obtained from your small speaker systems.

Matching a Preamp to a Power Amp

Q. My old tube preamplifier is specified to have an output impedance of 1 kilohm, and is designed to work best into a load of 100 kilohms or more. My transistorized power amplifier has an input impedance of 20 kilohms. The owner's manual for the preamplifier states that, in such a case, a modification can be made to the preamplifier to achieve optimum results, but gives no indication as to what this modification might be. Can you help? Is there an ideal ratio for impedances of interconnected audio components?

-Tom Unger, Gardena, Cal.

A. To make your preamplifier drive your power amplifier without loss of bass response. change its output coupling capacitor to a value five times higher than that which it now has. If you like, ten times its original value will also be fine.

The problem of bass loss can occur in a situation like yours because of voltage division between the preamp's output coupling capacitor and the in put impedance of the power amplifier. By making the value of the capacitor greater, the amount of such low-frequency loss is minimized.

The obvious question is, why didn't the maker use a large coupling capacitor in the first place? The answer is that he tried to prevent low-frequency pulses from finding their way into the power amplifier, where they would add distortion and perhaps damage the amplifier's output stage and/or the speakers. It is likely that. when the preamp was built, most power amplifiers had high input impedances.

As for ideal ratios between output impedance of the driver and input impedance of the load, if we are not discussing cartridges and loudspeakers, I usually suggest that the impedance of the device being fed should be ten times that of the driver. This is done so that a maximum voltage is transferred between the two devices (rather than a maximum of power, which would be transferred if the impedance between the devices were matched).

Intermittent Power Amplifier Operation

Q. Not long ago my integrated amplifier just stopped working. I took it to an authorized service shop, where I was told that it was working properly. Reinstalled, the amp worked properly for a few days, then cut off again. I checked the power and other circuit breakers with an ohmmeter, and there was continuity across each of them. What could be wrong?

-Elpidio Arocho, Newark, N.J.

A. I would venture that you may be operating the equipment somewhere near its maximum power output. Per haps the cutoff is the result of the protection circuit turning off the amplifier.

The next time this occurs, turn off the amplifier. Wait a minute and then turn it on again. If I am correct, all will be well.

Of course, to keep this problem from recurring, operate the equipment at a somewhat reduced volume.

What I have mentioned is certainly the simplest situation to deal with. But many other factors can bring about an intermittent failure of your equipment.

There may be poorly soldered connections, cracked circuit foils (which open when hot), dirty contacts on switches and controls, internal components which can cause problems if permitted to heat up, etc. The list is almost end-less, and intermittent conditions are usually the most difficult for a service technician to deal with.

(Audio magazine, Jul. 1986, JOSEPH GIOVANELLI)

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