AUDIOCLINIC (Jun. 1981)

Station Break-In

Q. I am having a problem with r.f. interference involving connecting cords between my receiver and turntable. The receiver is one block away from the broadcast transmitter of the station causing the problem.

When in the phono mode, with cables connected, this FM station is clearly audible. When the cables are disconnected, no sound from the station is heard.

-Dan Scanlon, Detroit, Mich.

A. This kind of interference is not al ways readily removed. The quickest approach is to obtain the antenna from any discarded portable radio. Remove the ferrite rod, wrap all excess phono inter connecting cables around this rod, and secure the cables by tape or whatever means are at hand so that the coil will not slip off. This arrangement tends to work best at lower radio frequencies than those used to broadcast FM. It is still worth a try, however, because it is simple and does not require working on the innards of the receiver.

Another similar approach, while crude in appearance, has worked for me in similar cases, again without having to open up the equipment. Coil up all ex cess phono lead into as small a bundle as possible and wrap the entire bundle in aluminum foil. Ground this foil to the chassis of the receiver at one of the in puts to the phonograph.

If neither approach is successful, you will then have to work inside the receiver. First obtain ferrite beads, either from a portable AM/FM radio or a good electronics parts supplier. Slip the beads over the leads connecting the phono connector to the rest of the set. (This will be impossible in those instances where the connector is soldered directly to a p.c. board). The increased inductance provided by the ferrite beads may alone be sufficient to remove the interference.

The next possibility is to make up a "pi" filter using an Ohmmite Z144 r.f. choke and two 40-pF mica capacitors.

One filter will be needed for each channel, with the choke wired in series with the phono connector and the rest of the circuitry. The capacitors are run to ground with short leads from each side of the choke. For stereo, two chokes and four capacitors are required. If connectors are soldered directly to p.c. boards, you must first cut the foils to insert the choke. (But if you are not expert in working with p.c. boards, do not attempt this.) You may have to use somewhat higher value capacitors than those I have suggested, but I have chosen these values because they will have minimal effect on the audio performance of the system. Note that any increase in capacitance should be considered in terms of the overall capacitance your pickup must see.

It is often necessary for an equipment manufacturer to ground the phono connectors at some point in the circuit other than at the point on the chassis to which the connector is attached. While this arrangement may result in the elimination of ground loops and, hence, the reduction of a.c. hum, such a practice will result in an increase in possible r.f. interference.

To obtain good r.f. grounding and still maintain good audio performance, bypass the ground terminal of each pho no connector to the chassis with a 100 pF ceramic or mica capacitor, one for each connector. Use the shortest possible leads when installing these capacitors. Lead lengths which are too long will possess inductance and reduce the efficiency of the capacitor as a bypassing agent.

It may be that this ground arrangement, in and of itself, may remove the interference without resorting to "pi" filters and the like.

Mike L. Ement's Duration

Q. Is the life expectancy of electret condenser mike elements unlimited, or do they deteriorate with age?

-Tom Sherwood; Dayton, Ohio

A. Electret elements will deteriorate with time. Their output will gradually de crease. However, many years must pass before this becomes apparent. I have owned these microphones for more than 10 years and have not noticed any deterioration.

Cartridge Types

Q. What is the difference between a strain-gauge cartridge and a moving-coil cartridge?

- David Schermerhorn, Allentown, Pa.

A. A moving-coil cartridge is one in which the stylus moves a coil to which it is attached. This coil is in the presence of the field of a permanent magnet. The motion of the coil with respect to the magnetic field causes a voltage to develop, and this voltage is then amplified and equalized so that we hear a properly reproduced signal.

A strain-gauge cartridge is quite different. Here the stylus is attached to a semiconductor material which has the property of changing d.c. resistance in proportion to the amount of deflection, or bending, of the element. The resistance change will be proportional to the instantaneous position of the element, as moved by the stylus.

Connecting a strain-gauge cartridge directly to the input of a preamplifier does not produce any signal. The change of d.c. resistance, in and of it self, does not result in a voltage across the terminals of the cartridge so the preamplifier has nothing to amplify. To make such a cartridge produce meaningful output, it is necessary to apply a small d.c. voltage across this variable resistance element. The amount of voltage developed across the resistance will vary according to the instantaneous value of resistance. We now have a voltage which varies in accordance with the modulation impressed on the grooves, which is exactly what is required as the end product of any phono cartridge.

Again, as with a moving-coil cartridge, the voltage is small and must be amplified. Such a cartridge, however, requires less equalization to correct for the pre-emphasis placed on the discs being played.

This discussion of the operation of a strain-gauge cartridge is, of necessity, oversimplified; it does not account for necessary compensation for linearity of output voltage to instantaneous stylus position. It also does not account for the inability to change phase by merely reversing connections to terminal plugs, etc. We hope, however, that it gives you a basic feel for the nature of such a cartridge. When all of its particular eccentricities are taken into account, a strain-gauge cartridge is capable of incredibly wide-range, transparent reproduction.

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(Audio magazine, Jun. 1981, JOSEPH GIOVANELLI)

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