Tuesday, April 28, 2015

Demagnetizing Dilemma: How Often Should We Demagnetize Our Cassette Tape Decks?

Though cassette tape deck manufacturers recommend cleaning record and playback heads every 12-hours or so, but how often should we demagnetize them to insure optimal performance?

By: Ringo Bones

Near the end of the 1970s, many hi-fi enthusiasts hailed the affordable high-performance cassette deck as the consumer electronic miracle of domestic audio. Unfortunately, there’s a topic that even today – way, way after the hi-fi cassette tape’s heyday – that sill remains nebulous and esoteric, as in the subject of cassette tape deck record and playback head demagnetization. Or more importantly, how often is enough to insure optimal function?

Until it is removed, a low-level of “permanent” magnetism builds up on playback heads in normal use and on record heads also if the record bias waveform is not perfectly harmonically pure or if the record bias is turned on and off too suddenly. Before the advent of cassette tape decks, owners of domestic open-reel tape decks often use dedicated hand-held demagnetizers to demagnetize the record, playback and even the capstans and tape paths. Unfortunately – unless you have the requisite skill and confidence to disassemble and reassemble the cassette tape deck you currently own – hand-held demagnetizers are a non-starter for use in cassette tape decks, especially car stereo units.

Over the years, dedicated electronic-based battery powered cassette tape deck demagnetizers – like the Nihonbashi ED-126 Electronic Demagnetizer and the Milty Magnet IX cassette tape deck demagnetizer – are my preferred accessories when it comes to demagnetizing cassette tape decks, including car stereo units. Electronic cassette tape deck demagnetizing units work by introducing a lightly damped 1-KHz alternating current waveform across the record / playback head of a typical cassette deck that usually lasts about one second and registers around +10 dB on the VU meters of tape decks for those who have them. Thus most electronic cassette tape deck demagnetizer manuals warning users to turn down the main volume of their amplifiers when using it since it will introduce a very loud 1-KHz squeak to their loudspeakers. Given that these electronic cassette tape deck demagnetizers are housed in a cassette tape shell shaped enclosure and usually powered by a single wristwatch battery, they are virtually foolproof to use even to a novice audiophile.

Most electronic cassette tape deck demagnetizers’ manuals recommend to using it to demagnetize your cassette decks heads after every 40 hours of use. During the 1980s, when cassette tape was the virtual de rigueur format of budding audiophiles, I tend to demagnetize my decks once or twice a month depending on frequency of usage. Other kinds of cassette tape demagnetizers use a tape-like strip magnetized with a series of alternating polarities – and virtually looks like a dedicated cassette head cleaning unit. There’s even a cassette tape deck demagnetizing unit that uses rotating permanent magnets to demagnetize the record / playback head of your cassette tape decks. Besides the electronic cassette tape deck demagnetizer units, the other types were, over time, rejected by cassette tape audiophiles because they are either ineffective – the tape strip type – or does more harm than good – i.e. the type using rotating permanent magnets.

Whatever the magnetic flux source, the working principle behind cassette tape deck demagnetizers is that the fluctuating magnetic field must gradually weaken until it fades out altogether; otherwise it will leave behind residual magnetism, which is precisely what you are trying to eliminate. Thus the use of electronic cassette tape deck demagnetizing units of a lightly damped 1-KHz alternating current waveform whose voltage dies out slowly over one-second duration.

In a properly operating cassette tape transport, the AC bias generating during the recording process tends to demagnetize separate record and erase heads so they normally are unlikely to need additional degaussing or demagnetization. Combination record / playback heads may be similarly protected. Tape guides and the playback heads are the parts most likely – from a theoretical standpoint – to become magnetized. In my experience as an audiophile and amateur musician for 27 years, I’m not sure that any type of demagnetizer can successfully demagnetize a capstan, but I’ve seldom found a capstan that needed it.

Signs of playback head magnetization include a noticeable increase in tape hiss. If left untreated, the cassette tapes you are listening to may suffer an irreversible loss of high frequencies. The loss will be less likely if you are using metal particle – Type IV – tape, whose high coercivity make it inherently more difficult to erase than other tape types. An FM radio station I used to intern back in 1987 has a DJ that tend to let 100-hours or more to go by before demagnetizing the station’s cassette tape decks. If you wait until hiss builds up or highs are erased, it’s too late. So I tend to demagnetize ever 30 to 40 hours or so back then. But some cassette tape deck manufacturers – Teac amongst them – have recommended against demagnetization. Part of their reasoning may be the danger of inadvertently magnetizing parts in the tape path, particularly if one uses the most aggressive hand-held demagnetizers – probably stems back during the days when Teac still makes those wonderful sounding open-reel tape decks that we still covet until this day.      

2 comments:

  1. Thank you for the very informative post. So it is your feeling that the Nihonbashi or Milty are safe, effective options? Where do you recommend one find either of them?

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  2. I have an older audio-buddy who bought his Nihonbashi ED 126 Electronic Demagnetizer back in 1986 - surprisingly, it still works until this very day.

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