AUDIOCLINIC (Aug. 1992)

Power Dissipation and Output

Q. My amplifier is rated to deliver 330 watts per channel continuously into an 8-ohm load. This seems strange to me. There are eight output transistors in my amplifier--four per channel. I checked their ratings in a semiconductor guide. Each transistor is rated at a maximum dissipation of 150 watts, and four times 150 is 600 watts. As I see it, that should be the output per channel, not the 330 watts shown in the instruction manual for my amplifier.

-Name withheld

A. I can see how you arrived at your conclusions. You will notice that your transistors are shown as having a power dissipation of 150 watts each. Power dissipation refers to the maximum amount of power that can be wasted within each transistor in the form of heat. Notice that I said "within" a transistor. When rating power amplifiers, we are interested in the amount of power delivered from the transistors to the load, not the amount wasted within them as heat. And even when your amplifier is delivering its 330 watts per channel, the transistors are probably not dissipating 150 watts apiece-not if the designer left any safety margin to protect those transistors against burning out. In fact, dissipation probably remains under 150 watts per transistor even when the amp is feeding 4-ohm loads, which draw more current and therefore heat the transistors more.

Connecting Video and Stereo Systems

Q. I have a television set and a VCR and I would like to know how to connect these devices to my stereo system.

-Jim Haberlin, Nutley, N.J.

A. Not knowing details about your equipment, I can only provide a general answer--which should serve in most cases.

Most VCRs and many TVs have audio line outputs, which can be fed to any high-level input (such as AUX, tuner, or tape) on your stereo system.

Some TVs have two sets of outputs, one designed to feed a home entertainment system and the other de signed to feed a VCR. The difference is that the audio output to the VCR is usually at a constant level, while the level from the other audio output can be controlled by the TV's remote. If your stereo system has a remote control, you might prefer the "VCR" connections; otherwise, you'd probably prefer the controlled outlets. The audio output from your television should automatically match the video on the screen, whether you're watching an in coming program or playing a tape or videodisc.

If your TV does not have stereo line outputs, you can use the outputs from your VCR. When you watch a tape, these outputs will automatically feed the audio from that tape, but when you're watching an incoming program, you'll have to be sure the VCR is on and tuned to the same channel to feed appropriate sound to your stereo sys tem. When you're watching one show while taping another, you'll just have to listen through the speakers on the TV.

A VCR can be hooked into your stereo system's tape loop, just like an audio cassette deck. You will then be able to use it to record the FM stereo audio from TV/FM simulcasts along with the picture, if such simulcasts are still being done in your area. If the VCR is a Hi-Fi model, this type of hookup will also let you use it as an audio recorder.

If you're feeding audio from your TV, check your TV set's instruction manual for further details. You may have to select some option from an on-screen menu, and you'll probably have to switch the TV's own speakers off or turn them down when listening to TV sound through your stereo.

More About Noisy Volume Controls

This is just a quick note about your reply to a question in the January 1992 issue about a noisy volume control. I have found that problem is, as often as not, caused by an electrolytic capacitor (used to block d.c.) becoming leaky. The symptoms can be very similar, with the small d.c. component being modulated by the control rotation, causing noise. I recommend checking for d.c. around the control pot before replacing it.

-William Kindel, San Clemente, Cal.

(adapted from Audio magazine, JOSEPH GIOVANELLI, Aug. 1992)

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