Audioclinic (Q and A) (Mar. 1974)

by Joseph Giovanelli

Using a VU Meter to Read Power Output

Q. In two super-power, basic amplifiers, specifically, the Phase Linear 700 and Phase Linear 400, "VU meters" are used to indicate power output during complex musical passages. These meters are calibrated in decibels in much the same way as my Simpson 270-4 Volt-Ohm-milliammeter is. Is it possible to connect this meter to the speaker outputs to determine the output of the amplifier during complex passages? If this can be done,' what power level, in watts, does 0 VU refer to?

-Phil Saba; McLean, Virginia

A. It would be possible to use your meter to indicate average power output produced by an amplifier. The only catch is that the damping of the meter might be different from that of a standard VU meter. If it is, you will miss some of the peaks which should register, or you will read peaks which are higher than those which would have been obtained on a standard VU meter. VU meters are designed to have a particular characteristic of attack and "fallback" in order that an indication on one meter will be equal to that read on any other similar meter associated with the chain of equipment.

In any case, assuming that your meter has reasonably good dynamic characteristics, it should be possible to use it with any amplifier to indicate power output. You must watch out for the possibility of ruining the meter with excessive signal. (While my following comments deal basically with VU meters which require external multipliers, they more or less apply to your specific meter. Its internal multiplier may be used, but some additional external, variable resistance will be required for trimming.)

VU meters are designed to respond to signals of less than 10 mW. This is a very small amount of power. You would have to calibrate your amplifier in terms of voltage versus power output. You also need to know the ohms per volt of your meter. You would then have to provide a multiplier, similar to what would be contained in a VOM or VTVM, to fit the full-scale voltage of 0.707 volt which causes a standard VU meter to deflect to 0 VU. This voltage could either be made equal to the full power output of the amplifier, to a point which is 3 dB less than this full power or equal to perhaps the loudest listening level you might expect to use--whichever best suits your requirements. You would want to make a portion of the divider-or multiplier as you will--variable for exact calibration and interchannel balance.

Reception of Distant FM Signals

Q. I like to catch "FM skips." Please tell me what conditions produce these effects so that I can know when to look for them.

-Alan A. Durrenberger, Fort Walton Beach, Florida

A. "FM skips" are not heard at all times. They are the result of atmospheric phenomena. When listening to shortwave signals, it is often possible to hear signals which are a long distance from the receiving site. A listener who is much closer to the transmitter might not even hear this same signal; it has skipped over the closer receiving antenna. For this reason we often refer to distant-signal reception as "skip reception." FM reception is considered to be limited by the horizon.

Often, signals transmitted from much greater distances than the horizon can be heard, but these signals can be heard at all points between the transmitter and the most distant receiver, with no skipping. Despite all of this, it does appear that the term "skip" is becoming synonymous with the reception of any distant station, whether or not it has skipped over some receiving sites.

Foggy conditions up and down the coast and/or temperature inversions provide opportunities to look for "skip" stations.

If a meteor shower is likely, try to listen for distant FM signals then. Reception will be erratic. Identification of stations and their locations can be difficult.

Magnetic storms and auroras can cause an increase of distant-signal reception on the FM band, but not in all latitudes. Such reception is not likely in your area. It is likely in extreme northern and southern latitudes.

Look for weather fronts. The time just before the passage of a front is often good for the reception of distant signals.

Spring and fall months are best for such reception because the greatest temperature contrasts are present. These contrasts cause the greatest changes in atmospheric density. As we learned in high school physics, different densities result in diffraction of light rays. Exactly the same thing happens when radio signals (of high enough frequency) pass through atmospheric layers having different densities. This signal diffraction causes the signals to follow the curvature of the earth rather than being radiated out into space as would otherwise be the case.

There is always a certain amount of this diffraction, caused by dust particles and by random fluctuation in atmospheric density.

To take advantage of these minute opportunities, however, you need a high-gain receiver, as well as a high gain receiving antenna. You must aim such an antenna carefully, both in terms of azimuth and in terms of elevation.

This requirement calls for two rotators, one for conventional azimuthal alignment, and the other for elevation adjustments.

Unlike AM signals, distant FM signals can often be heard during daylight hours. Do not miss them by failing to check on reception during appropriate weather times during daylight hours.

Hum in a Receiver

Q. I have become aware of a problem in my receiver that may have existed for a long time before I noticed, but since that moment it is harder and harder to live with.

There is a slight hum in the speakers of both channels (also present with headphones), that persists regardless of input selected, volume, and which way the balance control is set. It has nothing, either, to do with tone controls which can be switched out entirely.

I took the unit to the local service shop. They tell me that the equipment conforms to specifications. It has not, however, improved since that wasted visit. I suspected the power supply or a problem with grounding.

Because it is next to impossible to hear the humming (which can get on your nerves if you can hear it) in a noisy showroom, the service people probably did not notice that there is any problem at all.

Will I be stuck until servicemen make house calls, or is there something I can do for myself?

-Steven L. Meyers, Buffalo, New York

A. Generally speaking, the earlier stages of a receiver have more filtering than the later stages because of the need to isolate these early stages from common coupling situations which would otherwise be present.

Thus, even when the main filtering of a power supply begins to fail, hum is present only in the later stages--those stages which come later on in the chain than tone and volume controls. Therefore, the amount of hum is not influenced by the setting of these controls.

An efficient speaker also adds to the difficulty. Your service shop might not have used efficient speakers. Hence, hum would not have been audible. There is a hum and noise specification for most pieces of equipment.

One such specification measures hum and noise with the volume completely turned down. When your equipment was checked, the technician might actually have performed this measurement rather than an aural test, and, if so, it could be that the equipment has some inherent, audible hum, at least with your speakers.

The replacement of power supply filter capacitors should eliminate the hum, assuming that the problem is not inherent in the design of the equipment.

(Audio magazine, Mar. 1974)

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