TAPE GUIDE (Apr. 1989)

Right-Channel Problem

Q. When playing a cassette, my deck's left channel seems clearer than its right. After cleaning and demagnetizing the heads and checking all connections in my stereo system, the problem remains. I tried switching the interconnects between the deck and receiver--that is, left output of the deck to right input of the receiver, etc.--but then the left channel sounded poor.

The problem only affects the high frequencies. All my recorded tapes exhibit the same problem, although they sound fine on other decks. Do you have any idea what is wrong?

-Mike Cherepkai; Terre Haute, Ind.

A. It appears that the problem is in your deck's right playback channel. A variety of things could be responsible.

The right (lower) gap of the playback head may have worn excessively, causing treble loss. The right section of this head may have an internal defect, such as a partly shorted coil. There may be a defective transistor, capacitor, or resistor in the playback electronics of the right channel. Finally, a component in the playback equalization circuit for the right channel may have appreciably changed its value.

I doubt that you personally can do anything to remedy the problem, other than to take your deck to a competent service shop. Check with the deck's manufacturer, if possible, to find the names and addresses of the authorized service shops in your area.

Inputs and Impedances

Q. A tape deck was recently passed on to me. It has two inputs marked "Right Line Input" and "Left Line Input." What value is line impedance? I can arrange any source needed.

-Alexander Bell; Bala-Cynwyd, Pa.

A. Your question is not quite clear to me. Therefore, I will discuss the subject in general terms, quite possibly beyond your area of interest. Hopefully, you will find your answer some where in the discussion.

Line inputs are designed for relatively large signals, typically 0.5 V, and ranging from about 0.1 to 2V. In contrast, low-level inputs are designed for relatively small signals, typically about 5 mV (0.005 V), and ranging roughly from 1 to 50 mV. Low-level tape deck inputs are intended principally for dynamic or capacitor microphones, and similar types with relatively low output.

Low-level (mike) inputs are found on about one-third of cassette decks. In the case of receivers, low-level inputs are intended for moving-magnet pho no cartridges, while extra-low-level in puts (sometimes furnished) are intended for moving-coil cartridges having an output about one-tenth that of a moving-magnet type.

The line inputs of a cassette deck usually have a load, or input, impedance of 20 kilohms or more-perhaps up to 50 kilohms. Sources such as preamps (tape outputs), receivers (tape outputs), equalizers, noise-reduction devices, tuners, CD players, etc. typically have a source, or output, impedance of about 2 kilohms or less; 500 ohms to 1 kilohm is quite common.

The ratio between load impedance and source impedance should be about 10:1 or greater, in order to avoid distortion and significant aberrations in frequency response. Hence, even if the source impedance is fairly high as such impedances go (say, 2 kilohms) and the load impedance is a relatively low one (say, 20 kilohms), there should ordinarily be no problem of a mismatch.

To round out this discussion of impedance matching between sources and the components they feed, it may be mentioned that moving-magnet phono cartridges generally require a load impedance of 47 kilohms and a capacitance in the vicinity of 200 or 300 uF; moving-coil cartridges need about 10 to 100 ohms.

Taping 78s

Q. I am planning to purchase a cassette deck and to partially use it for taping 78-rpm records. Given the normal variation in starting, stopping, and getting up to speed, it seems to me that many difficulties loom ahead. Is there a reasonably simple method of taping these records?

-B. Carlat; Maplewood, N.J.

A. I see no problem. A tape deck usually starts up quite rapidly. Just after you place the cartridge stylus in the lead-in groove, start up the cassette deck. Many decks have a pause mode, which keeps the deck in record function, but the tape is not running.

Simply push the pause button to start up the deck or push it again to stop the deck. When one side of a disc has been recorded, push the pause button to stop the deck. If you make a mis take, you can erase that part of the tape which contains the error and try again. Every deck has a counter, which can help you to identify the section of the tape on which you wish to record, erase, etc. While the counter may not be 100% accurate, it is usually accurate enough for working purposes.

Why Not dbx NR?

Q. I have never seen any prerecorded tapes using dbx NR. Is there any reason for this?

-Dan Rasmussen; Marengo, Ill.

A. Dolby noise reduction is almost universally available in cassette decks with the least pretension to quality, whereas dbx noise reduction is avail able only in some of the better decks.

(This does not mean, however, that a deck must have dbx in order to fall into the "better" class.) Hence, prerecorded tapes favor Dolby noise reduction usually Dolby B NR, because many of the cheaper decks have only Dolby B and not Dolby C NR.

Editor's Note: Actually, there were some tapes issued with dbx encoding approximately a decade ago, about three or four dozen in all. There were also some LP records released as well. As I recall, the sound on some of these tapes and discs was pretty good. The problem the folks behind this introduction and use of dbx NR encountered was a sort of chicken and egg situation. Why, the big record and tape firm executives would question, should we put the money into equipment to make up tapes and discs in this format when Dolby NR is so much more popular, at least in tape? If the resulting tapes would be incompatible, they said, why should we pay any attention to this at all? As I understand it, the question of whether the resulting decoded tape (or disc) was quieter in playback was ignored in favor of these marketing questions. --E.P.

(Source: Audio magazine, Apr. 1989, HERMAN BURSTEIN)

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