Audioclinic (Nov. 1980)

Current vs. Voltage

Q. I have never understood the statement and applied practice that, in the output stage of an amplifier, we are more interested in power than in voltage and therefore want a great flow of current rather than big changes of voltage. The way I see it is as follows: If power is the product of voltage and current, it does not make any difference which is increased. If we want a lot of current to flow through the loudspeaker, that is another story. Most present-day hi-fi systems have multiple speakers with crossover networks, and these net works incorporate capacitors in series with the voice-coil of the speaker. So what does the loudspeaker see? It would appear to see only the modulation voltage superimposed on the d.c. of the output stage. No current can flow through the capacitors.

The actual power is really that of the permanent magnet. The bigger the magnet, the higher the power. The greater the modulating voltage affecting the magnetic field, the greater the interaction. Obviously, there must be something wrong with my analysis.

-N. Brenes, Brooklyn, N.Y.

A. While an electrostatic speaker re quires considerable voltage at low cur rent in order to move its diaphragm, the conventional loudspeaker, employing a voice-coil, requires relatively low voltage at a comparatively high current. Both of these speaker types require power. The magnet in the speaker using a voice-coil does not supply power; that is the job of the amplifier to which the speaker is connected. The magnet supplies a steady field which is alternately aided and opposed by the variations in the magnetic field produced by the voice-coil.

The voice-coil is an electromagnet whose polarity reverses as the a.c. polarity reverses. This changing field is alternately attracted and repulsed by the fixed field of the permanent magnet in the speaker. Any electromagnet is created by what are known as "ampere turns." The number of these ampere turns is found by multiplying the number of turns in the voice-coil by the number of amperes flowing through the coil. The strength of the electro magnet is directly proportional to the number of ampere turns.

You stated that voltage is the important consideration because a capacitor cannot conduct and capacitors are used in multiple speaker systems. As a matter of fact, capacitors can and do conduct current. (One might say that current flows "around" the capacitor, rather than through it as is true of a piece of wire.) Usually we do not think about the amount of current taken through a capacitor because the circuits with which we are most familiar are those in which the capacitor feeds a high resistance which will not draw much current. The resistor does draw some current, however, even when it is as high as you would care to make it.

To illustrate the point, assume that we have an 8-ohm resistor fed by a capacitor which, in turn, is fed by a suit able amplifier. Let us assume further that a signal is applied to the amplifier.

In this example, we will assume that as a result of this signal, four volts appear across the resistor. If there is a voltage across this resistor, current must flow through it. Ohm's Law shows us that the current under these conditions is 0.5 ampere. By definition, one volt is that amount of electrical pressure which will force a current of one ampere through a resistance of one ohm. Accordingly, we can see that if we have a resistor and can measure a voltage across it, that voltage could not be present without forcing current through the resistor.

Again by definition, power is the amount of work being done. The work, in the case of a loudspeaker, is the movement of the cone against the resistance of the air, of the suspension system for the cone, and of the voice-coil, plus the heat produced in the voice-coil itself. Work, or power, is ex pressed electrically by multiplying the resistance by the square of the current.

It can also be expressed by multiplying the current by the voltage. Power can also be found by dividing the square of the voltage by the resistance. All of these methods will give the same answer, provided that all of the numbers are the same for all examples. No matter what method you use, you will see that, in our example, two watts of power are being consumed when we have four volts across an 8-ohm resistor.

Channel Imbalance

Q. I have a question regarding balance control settings. At present I must keep this control at approximately its two o'clock position for proper channel balance. I never had to set the balance control that far from its center position when I was using a pair of relatively inexpensive speakers. The problem has come about since up grading to more expensive speakers. Is this the result of some defects in the receiver so that it is not putting out equal power to the two speakers? Is it the fault of some malfunction within the speakers?

-Terry Greenberg, Brooklyn, N.Y.

A. Perhaps the balance control itself is not accurate. To obtain equal output from both channels it might have to be adjusted, apparently favoring the right channel.

It is possible that something has gone wrong in your receiver since re placing the speakers--this would sometimes lead to one channel producing more power output than the other when the balance control is in it's center position.

To determine whether the receiver is indeed producing equal power with the balance control centered, connect an audio oscillator into the auxiliary inputs of both channels simultaneously. Connect a dummy load to each output--with the speakers disconnected. Feed a small amount of signal from the oscillator, with the frequency set to approximately 1 kHz. Next, connect an a.c. voltmeter alternately across the left and right channel out puts. (There is no need to have more than approximately one volt appearing across each of the channel outputs.) Note the position of the balance control required for the receiver to pro duce equal output. Repeat this measurement using different amplifier volume control settings.

Of course the voltage output from the oscillator should be adjusted to compensate for these changes in the setting of the amplifier's volume control. The purpose of this latter set of tests is to determine whether or not the volume control tracks at different settings.

There is the possibility that you have more cable connecting one speaker to the amplifier than for the other. If the cable you have used is too light a wire gauge for the length of the run, you would lose more power in the longer cable than would be lost in the shorter cable. This would have to be over come by offsetting the position of the balance control.

Never use the very thin wire that is often sold as speaker wire. It is inadequate even for short runs. For a short run between amplifier and speaker, use at least No. 18 gauge wire. If the run is 25 feet or more, use No. 16 gauge wire. If the run is more than 35 or 40 feet, use No. 14 gauge wire. All of these wire gauges can be had in the form of "zip" cord.

It is possible that your two speakers do not produce equal output when the same amount of signal is fed to each. Correcting it would require that you set the balance control to favor the one having the least output. To do so, disconnect the speakers from their cables--leaving the cables still connected to the amplifier. Physically interchange the positions of the two speakers, exchanging the present right speaker for the left channel, and vice versa. If the speakers are at fault, you will have to swing the balance control to its ten o'clock position, favoring the left speaker rather than the right speaker.

What If It's Wide?

Q. I have noticed that several tuners feature switchable i.f. bandwidths. Is the ability to choose between receiving either strong, local signals with high sound quality (i.e. "normal" tuner) or distant, weak signals with low sound quality (i.e. "narrow" tuner) achieved at the expense of middle signals which tall into neither category? It seems that with a normal (non-switchable) tuner, some of these middle signals might be strong enough to achieve high-quality quieting, low distortion, etc., while in a switchable tuner the same signals would have to be received in the narrow (distant) mode, with its corresponding low fidelity, because the signal strength would not be high enough for the less sensitive, less selective wide (local) mode. Is my interpretation of the situation correct?

-Greg Jones, Ann Arbor, Mich.

A. The wide bandwidth i.f. system is applicable in any instance where interference is not a problem, whether listening to a weak signal or a comparatively strong one. Often the audible difference between the wide and narrow bandwidth is a subtle one, involving changes in separation and a slight rise in distortion (in the case of the narrow i.f. bandwidth). It is not a question of a tuner improperly receiving middle-distance signals.

Line-cord FM Antennas

Q. I am interested in wiring a linecord antenna into my FM tuner, which has the usual 300-ohm antenna terminals. Would this be more satisfactory than the simple wire hank that I am now using? (Reception is local only.)

-Name withheld.

A. Chances are that a simple folded dipole antenna will outperform either the wire hank or linecord antenna which you propose using. The linecord antenna does offer the advantage of compactness; there are no wires to string up on walls, etc.

You can try one of these linecord antennas quite readily. Wrap a turn or two of thin aluminum around the linecord--this piece of metal should be extended in such a way that it can be put under one of the screws that connects the conventional antenna to the tuner. I suggest that you cut a slot into this aluminum so that it will fit under the screw more securely.

Because there is a small amount of capacitance between the aluminum and the linecord, the cord can feed signals into the FM set. The linecord must be oriented for best reception of any given station. The linecord itself is the receiving antenna; the power line itself does not come into play here because it is essentially shielded within BX armor.

In The Long Run

Q. Which is more likely to cause a degradation of performance: A long run of cable between amplifier and speakers or an equally long one between preamplifier and amplifier?

Bill Nabor, Mission Viejo, Cal.

A. I prefer to have a long cable run between the preamplifier and power amplifier rather than a long run between power amplifier and loudspeaker. In the latter case, much power would be lost, and from my experience low frequency and quality of lows would both suffer dramatically.

In the former case, however, the worst that can happen would be a loss of some extreme highs. The output impedance of many preamplifiers is very low, often 100 ohms. Such a low out put impedance means that relatively long cable runs are possible between preamplifier and power amplifier with no noticeable high frequency loss.

(Audio magazine, Nov. 1980; Joseph Giovanelli )

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