AUDIOCLINIC (Jan. 1989)

Matching Power Amp Outputs to Loads

Q. You've written about this before, but I just can't seem to understand this business of not matching impedances between a loudspeaker and a power amplifier. Some amps can work with speakers having 2-ohm impedances.

-Tony Mauldin, Lewisville, Tex.

A. Contrary to popular belief, the impedance of the loudspeaker never matches that of the power amp-certainly not in the case of solid-state amplifiers, at any rate.

A power amp does have an output impedance, usually of about 0.1 ohm.

If we really wanted to obtain the highest possible power output from that amplifier, we would match impedance between it and the external load-in this case, the loudspeaker system. Remember: I am saying that this is so if, and only if, we are attempting to transfer the maximum power between the amplifier and its load.

The manufacturer knows that if a loudspeaker system with an impedance of 8 ohms is connected to the amp, the maximum power available to that loudspeaker will be, say, 100 watts.

If the impedance of the loudspeaker system is 4 ohms, the maximum amount of power available will be somewhat greater. Several variables enter into the picture, but we'll say that the maximum power at 4 ohms is 150 to 200 watts.

The reason more power is available at 4 ohms than at 8 ohms is that the external load is a closer match to the output impedance of the amp. Okay so far? We have not disagreed with the idea of getting more and more power as the external load drops closer to our 0.1-ohm amplifier impedance.

Even though we have placed a load of 4 ohms on our hypothetical amplifier, the match is not perfect. We are still a long way from 0.1 ohm.

Power is calculated by multiplying the voltage by the current. Most amplifiers are designed to maintain the same voltage output capability within the normal range of speaker load impedances. But as we come closer to matching impedance between the amp and the load, the current taken from the output stage increases.

Output stages have some d.c. resistance, and current drawn through a resistance is dissipated as heat. The more current drawn, the more heat, until the output transistors self-destruct. Somewhere between minimum recommended load and a true match of impedance between that load and the output stage, the stage will overheat and burn out.

The manufacturer knows from experience that his equipment can work safely into some given load, well above a true match between that load and the output stage. This is why this spec is included in the instruction manual.

Some output stages can withstand more current than others, which is why some equipment is capable of working into 2 ohms or even lower impedances.

However, for its own protection, no audio amplifier is built to work into a load whose impedance truly matches that of the amp's output stage.

Having a lower impedance than the speaker it drives also enables the amp to provide damping, and thus control the speaker's motion more precisely

Loudspeaker Destruction

Q. In my car I have two 10-inch sub woofers. These "subs" were made as complete enclosures. The bass, however, wasn't clean! I had what I thought was a brainstorm: Take the subwoofers out of the boxes they were in and have them installed on the rear deck. Well, I wrecked the "subs" the second I hit them with "real" bass.

Why do some speakers require an acoustic-suspension box while others don't? Some even have boxes with ports in them.

-Michael Van Kampen, Frankfort, Ill.

A. How a subwoofer is to be enclosed depends both on its free-air cone resonance and the stiffness of the cone's suspension. In the short space in this column, I can't give you complete guidelines on how to use the information properly. However, here are a couple of examples to help you understand what is happening: If the cone has low mass, its free-air resonance is around 30 Hz, and it is free to travel a considerable distance, then it should be mounted in a relatively small space. In this case, the enclosure should be rather well sealed against air leaks. When the cone attempts to move, air pressure builds up behind the cone, restraining it. You could think of the air as acting as a pneumatic spring. If the cone is not restrained in this way, it would travel beyond its design limits and its voice coil would be destroyed when it struck the pole piece. This is what happened when you removed your drivers from their enclosures and placed them on the rear deck. The trunk was a large enclosure, and insufficient back pressure was available to restrain the cone's motion.

If the driver has a rather high resonant frequency (say, 60 Hz), a high physical mass, and is not free to travel a great distance, it will operate best when mounted in a large enclosure.

Ports might even be helpful in such a situation. (These ports can be tuned to reduce the audible effect of the cone's resonant frequency.) On the other hand, by mounting this driver in a small enclosure, the back air pressure developed will raise the resonant frequency and reduce overall cone movement, resulting in poor sound.

Comparing Frequency Response

Q. Assuming all factors to be the same, what is the difference between subwoofer A, with a frequency response of 19 to 100 Hz (1.5 dB) and subwoofer B, with a frequency response of 19 to 100 Hz (4 dB)? Do these differences mean that if a sub woofer is played loud, it will not be able to reach down to 19 Hz? If subwoofer B is played lower than 4 dB, will it be capable of reproducing 19 Hz? Which subwoofer will be able to reach down to 19 Hz, whether I am playing my system soft or loud?

-Senen A. Silvestre, Cypress, Cal.

A. The dB figures specified in connection with these subwoofers' frequency response have nothing to do with the loudness at which these units deliver their rated frequency range. Instead, they describe how far response varies from flat.

The specifications given in your letter omit the arithmetic signs which usually accompany these dB figures. For instance, if subwoofer A were rated "19 to 100 Hz, ± 1.5 dB," its response would theoretically vary no more than 1.5 dB up or down over its entire rated frequency range. If it were rated "19 to 100 Hz,-1.5 dB," its response at its highest and lowest rated frequencies would be 1.5 dB below its response at some frequency in the middle of its range. One would have no way of judging how much, if at all, its response might rise at other frequencies.

In any case, subwoofer A, with 1 5 dB of variation, would appear to be much flatter than subwoofer B, with 4 dB. However, A's manufacturer could have used "± 1.5 dB," which implies a total variation of 3 dB, when his speaker actually measured "+0,-3 dB" that is, flat over most of its range but 3 dB down at its end points. If subwoofer B's response was listed as "-4 dB," there would actually be little difference between them, as far as specs are concerned.

Remember that specs are not the whole story. Use them to narrow your choice, but make your decision based on what your ears tell you.

Guitars and Hi-Fi Equipment

Q. I plug my electric guitar into my sound system. To obtain sufficient volume, I have to turn up the volume control much farther than for most other program sources. The guitar itself has no active circuitry, but occasionally I use an effects pedal, such as a distortion pedal. My guitar sounds much better through my high-fidelity system than it does when I play it through my guitar amp. Am I damaging my "good" speakers when I use the distortion pedal or straining my amp in any way?

-Lester Gong; Oakland, Cal.

A. It is unusual to hear someone say that his guitar sounds better through a high-fidelity system than through an amplifier designed for that application.

Guitar amps and their associated speakers are made to color the sound in ways which make an electric guitar sound like an electric guitar. A hi-fi system is designed, as far as possible, not to introduce coloration but to reproduce faithfully the sounds being fed into it.

The voltage produced by the pickups in your electric guitar is considerably lower than that which is produced by a tape recorder, CD player, or tuner. This lower voltage accounts for the fact that you must turn up the volume to produce acoustical power equal to that of your other program sources.

Whether your loudspeakers will be damaged depends on the amount of power the tweeters can handle: most speaker specs for power handling are for the system as a whole. The tweeter often cannot stand anything like the full power shown for the entire speaker system. Your effects pedal will cause your system to supply a larger than usual amount of power to the tweeter, a situation similar to adding a great deal of treble via your tone controls and an equalizer simultaneously.

==============

(Source: Audio magazine, Jan. 1989, JOSEPH GIOVANELLI)

= = = =