AUDIOCLINIC (May 1986)

Intermittent Auto Amplifier

Q. My car amplifier does not operate properly. Sometimes I can turn it on and it is just fine. Other times it works for a few seconds, then turns itself off. At other times it does not turn on at all.

I have checked the voltage to the amplifier and to the switch wiring; it looked good. I also checked the speaker wiring for shorts and failed to find any.

The amplifier I am now using is the third one I have tried. The same condition occurred with the others. What do you think could be my problem?

-Martin Haase, Ellendale, N.D.

A. The fact that you've tried three amplifiers indicates that the problem lies in something other than the particular amplifier you are using.

You said you checked the voltage. Were those checks made while the amplifier was turned on and when the amplifier appeared to turn off? Did you make your checks against the ground point on the amplifier? Even though the voltage is normal when no current is drawn, it may drop when the unit is turned on; current demands may pro duce problems because of loose connections or defective switch contacts.

It is usually best to wire amplifiers directly to the battery and not to existing wiring (such as the cigarette-lighter connection). Appropriate fusing should be placed in the amplifier line.

There's one more check to make, which applies only if your car-stereo head unit has speaker-level outputs as well as (or instead of) preamp-level ones: Disconnect the speaker cables from the amplifier and connect them to the head unit instead. (I hope the routing of these cables allows this.) Operate the system in this way for a few days. If you find that the intermittent problem still occurs, the defect is not with the amplifier, but is due either to poor speaker connections or a defect in the head unit.

(Editor's Note: We also checked with a car-stereo specialist, Tony !gel, of Stratford Service in New York. He stresses the importance of checking ground connections, as intermittent grounds are a frequent cause of problems in car-stereo operation. He also suggests that the amplifier may have a thermal shut-off circuit, which could shut the amplifier off and then reset. Such circuits can be triggered by amplifier overheating due to load problems, such as intermittent defects in the speakers or speaker wiring. They can also be triggered by external heat sources such as the engine, if the amplifier is mounted 'on the firewall. The same can happen when a car is parked in the hot sun, but we suspect this is not a problem, most of the year, in North Dakota. We also checked with your amplifier's manufacturer, who says that your problem could be caused by a very low load impedance-presumably less than 2 ohms-or by intermittent shorts in the speaker connections. -I.B.)

Safe Timer Connections

Q. There are two a.c. outlets on my timer. I have an integrated amplifier plugged into one outlet and an equalizer plugged into the other. Connected to my integrated amplifier are a turntable, noise-reduction unit, and FM tuner. The instructions for my timer say it can supply 700 watts maximum. I have added up the wattages of the equipment plugged into the timer and arrive at 359 watts total power. I know this is much less power than the timer's instructions tell me is safe, but with all of this equipment being turned on/off at one time, and all funneled down into a single power cord (from the timer), is this arrangement really safe?

-Teddy Herbst, Swiftwater, Pa.

A. Yes, there is no problem with your wiring. The factor which limits the power that can be supplied by a timer is related to the switch contacts and what they can withstand by way of arcing. Believe it or not, the power cord can be readily "sized" to handle the 700 watts or more.

Even though the loads we are discussing are inductive, and though back voltage will cause arcing across switch contacts at times, designers are well aware that their timers will be used with such loads. As a consequence, added safety factors are usually included.

Feeding Turned-Off Equipment

Q. I frequently listen to my Compact Discs with headphones via my CD player's headphone jack. I am concerned about the 2-V output from the player feeding the AUX input of my preamplifier, which is turned off. Will any harm be caused by this? Similarly, when listening to LPs through my preamplifier's headphone jack, I keep the power amplifier turned off to prevent sound from reaching the speakers. Will any harm be caused by the preamplifier feeding its signal into a "dead" power amplifier?

-Russell S. Low, San Francisco, Cal.

A. Your preamplifier will not be damaged by signal feeding into its AUX input when the power is turned off. The same is true of your amp.

Along slightly different lines, you should be careful about the setting of your program-selector switch when using your player as you have described.

It was my sad experience to have had a preamplifier set to its phono position with a CD player connected and turned on. Like many preamplifiers, in puts which are not in use are shorted in order to prevent unwanted signal leak age. This player was a direct-coupled type, and the shorted input caused serious overheating of the player's output IC. I was within seconds of wrecking its output stage. (And it was a borrowed machine!!)

Distant Mike Pickup

Q. I'm interested in putting together a portable sound system-consisting of a microphone, a recorder, and an amplifier-to pick up the sound of voices at a greater-than-normal distance from the mike (20 to 100 feet). I have received conflicting advice from various dealers. One said I should use a PZM mike in conjunction with a small parabolic dish. Another said I would need a shotgun mike. Do I need an amplifier? If so, what type? Would a 20-watt guitar amp be good? Must I match the mike to the amp?

- Lawrence Charles, Westbury, N.Y.

A. I believe a shotgun mike is the simplest, and possibly the best, solution to the problem. Boundary micro phones such as the PZM might also work, though I have not heard of the PZM being used in conjunction with a dish. At the near end of your distance range, a cardioid, hypercardioid or even a figure-eight microphone might do the job. The unidirectional (cardioid and hypercardioid) types are preferred where interfering noises and room echoes are most likely to come from be hind the microphone, while the bidirectional (figure-eight) type is better where the interference would come from the sides.

If your purpose is recording, you do not need a power amplifier. An amplifier would be needed only if you were going to drive loudspeakers. If your recorder's microphone input does not have sufficient gain, you might need an external microphone preamp between the microphone and recorder. This could be either the preamp in a micro phone mixer (if you have a good, quiet one) or a musical-instrument "power booster." Shure, I believe, makes a fixed-gain phono preamp (for professional use) whose RIAA equalization can be switched out for "flat" response; this, too, might suit your purpose if you need more input gain. In any case, make sure the microphone's impedance is close to (and no higher than) that of the input it feeds. It may be necessary to use a high-quality impedance-matching transformer to make the two work well together.

If the microphone input is sensitive enough, however, do not use a booster. In fact, its use may degrade the signal by adding more noise than is present in the recorder's mike input.

Recording voices which are some distance from the microphone is fraught with other problems. If the recordings are to be made indoors, re flections from walls, floor and ceiling may render the voices unintelligible, even with highly directional micro phones. If the recordings are made outdoors, you stand a better chance of extracting information from them, un less there is a lot of background noise coming from the direction towards which you have the mike pointed.

I trust your reason for recording from a distance is not to record concerts or other performances without the per mission of those being recorded. This could lead to legal complications. In any case, the equipment required to make good recordings under such circumstances would be too bulky to be used surreptitiously.

(Audio magazine, May 1986, JOSEPH GIOVANELLI)

= = = =