AUDIOCLINIC (Apr. 1987)

Phonograph Cartridges and D.C.

Q. Transistorized preamplifiers use capacitors to block d.c. from the phonograph cartridge. I notice that vacuum tube preamps do not use such blocking capacitors. Why?

-Tom Unger, Gardena, Calif.

A. Blocking capacitors pass audio frequencies from the cartridge to the preamplifier, but they isolate the cartridge from the solid-state circuit's d.c. bias. Without them, some of the solid state preamp's bias could reach the cartridge. It is important to keep the bias from the cartridge because direct current can wreck a cartridge. Also, the cartridge's load on the circuit would change the bias voltage, which would certainly cause distortion and might even damage the preamplifier.

Vacuum tube circuits are another matter. There is virtually no d.c. voltage between the grid and ground. True. the grid is biased negative with respect to its cathode, but the cathode is usually raised above ground by the desired amount of bias voltage.

Power and Heat

Q. My solid-state receiver puts out 150 watts per channel, and its power consumption is said to be 550 watts.

Does this mean that the receiver uses this much power during the whole time it is turned on, or only as a maximum when the music calls for maximum power? If the receiver draws 550 watts continuously, does that mean the excess power must be dissipated as heat? And does the amount of heat have anything to do with the load on the amp?

-Jerry Wilce, Banks, Ore.

A. If your receiver operates in Class A, the amount of power it consumes will be more or less constant regard less of its power output at any given instant. Also, such an amplifier will run warmer than a non-Class-A unit having the same power output rating because the idling current is higher for a Class-A design. It is essentially true that any power supplied to an output stage which is not sent to the load (loud speakers) must be wasted as heat.

In other amplifier designs, the amount of power taken from the a.c. power line varies in accordance with the demands of the music. In your case, the 550 watts would be required only when the amplifier is delivering maximum power, probably only during transient peaks.

While power consumption is greatest at high power, power dissipation (heat) is greatest at moderate power levels. This created some consternation among amplifier makers when the FTC mandated that amplifier power tests be preceded by a warmup for one hour at one-third rated power, a condition producing very near-maximum heat. As a result, the test is more stringent than even the FTC intended.

Lowering the load impedance will also produce more output heat for a given input signal. The reduced load impedance draws more current from the output circuit. raising the power to the load, even though the output volt age remains fairly constant.

Monophonic Listening and Stereophonic Recording

Q. I have a stereo recorder, a stereo mixing console, and one monophonic amplifier. If I combine the mixer's main outputs to feed the mono amp when I monitor recordings, then the recordings made at the time are monophonic as well.

The only solution I have found is to unplug one channel's output from the feed to the amplifier. It is annoying to hear only one channel's signal, and it is also a nuisance having to unplug one channel each time I record a stereo program. Is there a way to wire the left and right channels to my amplifier and not have them affect the recording?

-David Robbins, Tacoma, Wash.

A. I know of no easy solution to your problem. The most logical solution would be to use a stereo amplifier.

Presumably you intend to get one at some point, or why would it matter to you if your recordings are monophonic rather than stereophonic?

If you have another mixer, you might use that to provide a monophonic mix to feed the amp. Connect two of the second mixer's inputs to the outputs of the first mixer that now feed your amp.

Set the second mixer so that both input signals go to the same channel, and feed that channel's output to your amp You could also build a passive mixing circuit. Assuming that your recording mixer has a low-impedance output, such as 100 ohms, connect a 10-kilo-ohm resistor to that mixer's left- and right-channel "hot" outputs. Then tie together the free ends of these resistors and connect them to your amplifier's "hot" input terminal. It will probably suffice to connect the amplifier's input ground terminal to the ground terminal of one mixer output. If it does not, it should probably be safe to connect both output grounds to the amplifier's input grounds, since your mixer's out put is apparently not a power-amplifier stage.

The resistors will probably not cause too much signal loss, though that depends on your mixer's output impedance and your amplifier's input impedance and gain.

Using this resistor network will some what reduce stereo separation in the mix fed to the recorder. If the loss of separation is too great, increase the value of the resistors.

Adding Antennas for AM

Q. Is there any way to use an external AM antenna with a tuner that pro vides no connections for one?

-Ben Shepherd, Margate. Fla.

A. If you have an AM tuner which is pretty good but lacks the provision for attaching it to an external antenna, you can often add one. Wrap a couple of turns of wire around the "cold" end of the ferrite rod which serves as the built-in antenna. You will have two free ends of this "link." Attach one end of the coil to chassis ground. Attach the other end to the "hot" conductor of a piece of coaxial cable, with the shield connected to chassis ground. You will perhaps wish to mount a terminal strip to make attachment of the coaxial cable a bit more rugged and neat.

Connect the far end of this cable to the end of a piece of wire perhaps 30 feet long. Keep this wire well clear of surrounding objects and as far from power lines and TV antennas as practical. This will keep noise from being picked up. You can also (but it is not usually necessary) attach a similar length of wire to the shield of the cable, and run it in the opposite direction to the main wire we just discussed.

(Source: Audio magazine, Apr. 1987, JOSEPH GIOVANELLI)

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