Cassette Deck Magnetization

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Any measures to improve cassette playback quality can be useful when transferring these medium quality audio sources to digital files.
Demagnetisers are sometimes used in an attempt to improve cassette playback quality, but their value is open to debate.

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Demagnetizers are sometimes used on cassette decks, but they are rarely needed, and usually don't produce enough field strength to demagnetise.

Head magnetisation used to occur sometimes on early decks using DC erase, for 2 reasons.

  1. The tape magnetised by DC erase had a very slight tendency to magnetise the record/play head(s) over time
  2. The metal the heads were made from was relatively easy to magnetise

DC erase has not been used on anything but cost-cutting decks in decades. However permanent magnet erase is found on some modern low cost decks, and does effectively the same thing as DC erase.

Tape head technology has come a fair way, and modern head materials are not as vulnerable to magnetisation as was once the case. This also means they aren't as susceptible to demagnetisation, and a demagnetiser is not normally capable of either magnetising or demagnetising a cassette deck head.

Demagnetiser Test

If you really do need to demagnetise a tape head, a hand-held tool does not normally produce enough field strength to do so.

Studios use much larger higher power demagnetisers to produce enough field to erase tapes. These items weigh several kilos and may consume 500w or so. Demagnetising a tape head requires greater flux density than for erasing tapes.

Magnetising a head takes more flux density than recording a tape. This has to be the case for all tape heads, otherwise they'd magnetise themselves in normal use. So if your demagnetiser can't erase a tape completely, you can be sure it won't demagnetise any tape heads.

Two retail demagnetisers were tested in 2006 and found to cause no data loss on floppy discs, and to be unable to erase a ferric cassette tape recording.

Reel to Reel

The situation is different with open reel decks, because many of these date back to the 1950s and 60s, and use more easily magnetised head materials.

Connecting a multimeter to the erase head can tell you whether it uses AC or DC erase. The better AC erase decks should not suffer any tendency to magnetisation.

If you have a DC erasing deck, the deck will normally have more quality problems than simply the DC erase itself. It is possible to convert these decks to AC erase if really desired, but almost no-one would want to use a sub-optimal open reel deck for regular recording today.

8 Track

Most 8 track decks don't record, so have no erase system, and thus nothing to magnetise the head. Magnetisation is unlikely in these decks.

For magnetisation to occur would require:

  1. A susceptible head
  2. Use of DC erase
  3. A lot of recording use

Magnetisation is also unlikely in recording decks where the record function is not used a lot.

8 Track demagnetisation cartridges

The demagnetisation cartridges sold in the 1970s are ineffective. They used small permanent magnets and lack the necessary field strength.