AUDIOCLINIC (Dec. 1986)

Hum in Phono Cables

Q. I am puzzled by a minor problem with my turntable. Any power cord within 12 inches of the phono cables causes an audible hum from my speakers. Power cords near other cables, such as those connecting preamp to power amp, tuner to preamp, etc., do not cause this hum.

Do my phono cables need replacing? I'd really hate to do that, because these cables are soldered directly to the turntable lug strip.

-Jerry Powley, Fort Worth, Tex.

A. Power cords produce a.c. fields which can be picked up by some cables and then passed into the audio circuits, where they are amplified and fed to the loudspeakers as audible hum. Because phono cartridges have very low-voltage outputs, phono input circuits have more gain than the other inputs in your system. As a result, any hum picked up by the phono cables will be amplified more than hum picked up elsewhere, and so will be more audible. Also, the phono system's impedance is likely to be higher than the combined impedance of a cassette recorder and AUX input, etc.

Audio cables are not completely shielded. This is why hum voltages can get into the cables in the first place. Do not replace your cables-just keep the power cords away from them, as stated in most turntables' instructions.

Type I and Type II dbx NR

Q. What is the difference between the dbx Type I and dbx Type II noise-reduction systems?

-Tim Schindler, Mechanicsville, Md.

A. Both are full-band 2:1 compression/expansion ("compansion") systems. Both boost treble during recording and impose a mirror-image cut in playback. The difference lies in the range in which the companding works.

The sensors that control the compander action of dbx Type I read all audio frequencies from 22 Hz to 21 kHz; those that control dbx II's com pander action read signals only be tween 30 Hz and 10 kHz, so they won't mistrack when used with imperfect re cording media. This band-limiting applies only to dbx II's control circuits, not its audio circuits.

Type I is designed for recording systems such as 15-ips studio recorders, which have the ability to record high level, high-frequency signals. Type II, because of its band-limited control circuits, is more forgiving of media such as cassettes, which have high-frequency headroom limitations, low-frequency head "bumps," and other imperfections.

Faulty Phono Muting

Q. I own a top-grade automatic turntable. At this time, however, the muting circuit is inoperative. In order to avoid possible speaker damage, I have to run to the equipment and turn the volume down during the "change cycle." Any advice?

-Bernard A. Du Pont, Putnam, Conn.

A. The mute is supposed to silence any output from the cartridge unless the stylus is on the record. This is accomplished by two sets of switch contacts, one set per channel. The contacts are usually flat leaves which touch together during muting. They are wired in such a manner that, when the contacts close, the output from the cartridge is shorted.

In your case, either the contacts never touch or there is dirt, oxide, or even grease on the contacts which prevents the shorting action. If the contacts simply don't close, the cure depends on the design of the leaves and upon the way they are pressed together. You will either have to bend them so they are closer together during "play" or you must adjust the position of a cam which works against the contacts. If there is surface contamination, a suit able contact cleaner should be able to fix the contacts.

Hum in Stacked Components

Q. I often see systems in which the individual components appear to be stacked one on the other. Shouldn't this produce problems? I remember that when I was dubbing eight-track tapes to cassette, the two decks were stacked. I could hear a hum during recording. In fact, that hum was re corded on my cassettes. When I separated the two decks, the hum was no longer there.

-Tim Schindler, Mechanicsville, Md.

A. An eight-track machine has a playback head, and this head can act like a transformer. It is, therefore, susceptible to hum fields, which can be produced by power transformers and motors. In your case, either the motor or power transformer in the cassette recorder was sufficiently close to the playback head of the eight track player for the hum to find its way into the head. Yes, heads are shielded against this hum, but shielding is never perfect, and some heads are shielded better than others.

With good shielding and judicious physical placement of hum-producing elements with respect to elements which tend to be sensitive to hum, it is possible to design components which will operate properly when stacked.

In summary, if a manufacturer designs a set of components to be stacked, he will have taken hum into account during the design process. If, on the other hand, you stack a random set of components, you run some risk of hum.

Interrelating Specs

Q. My stereo receiver has a frequency response of 20 Hz to 20 kHz and about 85 dB S/N for AUX signals; my cassette deck has a maximum frequency response of 20 Hz to 19 kHz and an S/N of 92 dB (with noise reduction). My turntable and cartridge have a signal-to-noise ratio of 78 dB and a frequency response of 10 Hz to 30 kHz. How do these specs interrelate? What is the maximum S/N and the maximum frequency response that could be obtained from the kind of setup I've described?

-Tim Schindler, Mechanicsville, Md.

A. The best S/N you can expect from a multi-component system will be that of the noisiest component that is handling the signal at the time (except when playing records, in which case performance is limited not so much by your equipment as by the background on the discs, which can be as low as 60 dB.) Assuming your receiver's S/N at the phono input is about 80 dB, you could expect an S/N of 78 dB, at best, when playing very quiet records, the limit being your turntable's S/N. When playing tapes made from low-noise sources, your best S/N would be 85 dB, limited by your receiver's S/N at its high-level inputs. In practice you'd achieve slightly worse figures than these, because even the quietest components would be adding some noise to the total; however, the added noise might not be audibly significant.

In frequency response, too, you will be limited by your system's weakest link-and perhaps by the sum of alt your components' frequency roll-offs, depending on how their manufacturers specify response. If response is specified flat within a fraction of a dB, then the effects of multiple components would be negligible; your response when listening to records would be 20 Hz to 20 kHz (limited by the receiver) and when listening to wide-range tapes would be 20 Hz to 19 kHz (limited by the tape). If response is specified in terms of the frequencies at which output is 3 dB down, however, the frequency errors would add up rapidly. If your cartridge, receiver, and deck were all 3 dB down at 20 kHz, say, then response when recording from your turntable and monitoring the tape would be 9 dB down at 20 kHz.

The Virtues of Noise

In the March 1986 issue, a question was published in "Audioclinic" from a reader who wondered why a faint hiss is audible even on CDs designated "DDD." I think it may actually be undesirable to remove every last vestige of wide-band noise from any digital format. In deed, it can be shown that a small amount of wide-band noise (called "dither") added to raw digital audio will not only mask but actually reduce the small amount of noise and distortion caused by quantization--if the dither meets certain mathematical criteria.

Quantization is a sequence of the smallest voltage changes which can be represented by a digital system, corresponding to changes of one count in the least significant bit. When used, dither is injected at a level which randomly modulates the system by at least one count, plus whatever audio is present, thus "blurring" the sharp, stepwise effect of quantization.

In actual practice, the noise in mixers, preamplifiers, and other nondigital studio equipment may provide this signal. However, as the noise floor of this equipment is further reduced (as a result of design improvements and the continued conversion to digital), it may be desirable to deliberately add dither to the signal. Dither can be added at any point between the studio and the CD player. At least one manufacturer, Carver, adds dither itself, via the Digital Time Lens circuitry in its players.

The sound of some CDs may be enhanced by this treatment.

If your reader has a Carver CD player, and if the Time Lens is switched in, this could account for the hiss.

-Karl Uppiano, Caldwell, Idaho

(Source: Audio magazine, Dec. 1986, JOSEPH GIOVANELLI)

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