Audioclinic (Sept. 1982)

A Touchy Situation

Q. My stereo system consists of turntable, cartridge, tonearm, amplifier, tuner, and loudspeakers. Sometimes while listening to the tuner, I lose the sound when I touch the knobs or face plates of the electronics. The music comes back when I no longer touch the equipment. This does not happen when I listen to records. What is the cause?

-R. A. Buc, North Hollywood, Cal.

A. It sounds to me as though your FM signals sometimes disappear because of insufficient signal strength. My guess is that you are using an indoor dipole antenna. The combination of both your physical position with respect to this antenna and the capacitance to ground which comes into play when you touch the equipment produces the loss of signal. Using an outdoor antenna should resolve the problem, but at the least find a better location for the antenna you are now using.

Direct Noise Pickup

Q. I have been recording for a few years and have spent a considerable amount of money on my equipment. Being a musician, I have tried to find information on noise reduction for direct-line recordings from my guitar to my recorders. I have some knowledge of electronics and have attempted, on my own, to devise a system of filters to eliminate hum and noise. Although I have been moderately successful, I am fanatical about sound and have not yet attained the ultimate. Can you help?

-Darrell Francisco, Karn's City, Pa.

A. If you're getting noise and hum, it often comes directly from pickups, contact mikes and similar devices. Certainly no noise-reduction system of the kind most generally associated with this concept will work. (By this I mean that the processing systems by dbx or Dolby Laboratories won't help your particular problem.) Your task is to keep noise, now present, from entering the recorder, and the dbx or Dolby systems won't know the difference between the music and extraneous noise.

Obviously, your first attack on this problem must be to use the best possible pickups and mikes. Where possible, adjust these devices so that they can extract all available signal from the strings of the particular instrument you are using at the time. The better the coupling between the pickup and the strings, the higher the electrical output from the system will be. Because the ambient background noise will be more or less constant, this will mean an improvement in signal-to-noise ratio without taking any further measures.

You may find it best to use a mike to pick up the sound from your instruments, placing it in front of the speaker of the amplifier rather than wiring it directly into the recorder. I have found that this often reduces annoying hum.

It also allows you to record the sound of your instrument as it is heard by an audience, rather than the flat, often insipid sound produced by a direct feed into a recorder, and then later, played back on good speakers. Keep in mind that part of the character of electric guitars is the vibrato, echo and reverb which are often available only through the loudspeakers. The peaky quality of loudspeakers is definitely a part of the overall sound.

Beyond this, use good filters which can be set to roll off below the lowest frequency of interest and above the highest frequency of interest. A good graphic or parametric equalizer can often be used for this purpose. The two stereo channels can be connected in cascade, with the output of channel A feeding into the input of channel B. The actual program input, then, is the input of channel A; the actual, usable output is taken from channel B. Where any given band can be either cut or boosted some 12 dB, now, with the two devices cascaded, you have a total of 24 dB boost or cut. This is good enough in many instances for the kind of filtering we are discussing here.

With most electric guitars, the harmonic content is rather slight above the fundamental musical tones (except for some "fuzz"). By limiting high frequencies, some "buzzy" sounds and hiss can be eliminated.

Keep your instrument separated from the rest of the electrical equipment, insofar as it is possible, to reduce the likelihood of hum being induced into the pickups.

If you have a notch filter, the fundamental component of the 60 Hz can sometimes be eliminated with not too great a degradation of musical frequencies in this region. Even so, powerline voltage is not sinusoidal, thus it is often relatively rich in harmonics, which are hard to remove.

Weak Shielding

Q. I am experiencing a most perplexing problem with r.f. interference in my audio system. On the recommendation of a friend, I installed 13 foot long Belden 8421 spiral-wound shielded cable between my preamp and my power amp. Even when the volume control of the preamp is turned down fully, I receive a modulated signal from a local FM station whose frequency is 107 mHz. By substituting another cable, Belden 8401 (braided shield), which is four feet long, interference vanishes.

What mystifies me is why a shielded cable of low capacitance, such as Belden 8421, could pick up this interference in the first place. By the way, the 13-foot cables are terminated with Switchcraft RCA-type miniplugs which are supposed to be resistant to r.f. interference.

-Richard A. Links., San Francisco, Cal.

A. Basically, it isn't a question of one cable type vs. another, but rather of a 13-foot long cable vs. a 4-foot cable. You should always use the shortest length of cable possible if r.f.i. is a problem. The longer the cable with less than 100% effective shielding, the more area is exposed to the possibility of r.f. interference.

In stubborn cases of this kind, I suggest that you use Belden's Beldfoil cable. It will probably eliminate interference from this source. Its shield is a solid foil, not braided, and hence it doesn't have any openings through which r.f. can enter.

The Switchcraft connections are fine. With any RCA-type connector, however, the area exposed to r.f. is small relative to the area exposed by the braided shield of the cable. You may, thus, be able to get away with connectors of lesser quality.

(Source: Audio magazine, Sept. 1982; Joseph Giovanelli )

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