| Rule number 1--always, Always, ALWAYS
use two separate tape decks for playback and recording! Now, if
all you do is play tapes, fine--make sure the record jacks on
the tape deck aren't connected at all. If all you do is record
(you never play tapes), make sure the play jacks on the back of
the tape deck aren't connected.
This problem isn't new at all. In fact, it's existed since tape
decks were first available. All it takes is a one-time mistake
and SCREECH! ...you've blown out your speakers. Take a look at
the diagram below.
This shows a typical sound system with a single tape deck being
used for both playback and recording. During playback, no problem
exists. When recording, the mixer sends the audio signal to the
tape deck. At the same time, you'll find the same audio signal
present at the tape deck output (play out) jacks. This signal
goes back into the mixer. When it does, the signal is re-amplified
and is again sent to the tape deck to be recorded. Now, for all
practical purposes, electricity travels at the speed of light.
Audio signals are electrical (when traveling through wires), and
so they also travel at the speed of light. This electrical feedback
loop created through the mixer and tape deck can and will turn
into a howling, screeching tone or set of tones immediately. How
immediate? Well, depending on the levels on the mixer and tape
deck, it could be anywhere from instantaneous (from the split
second you hit record), to a second or two. In either case, there
isn't enough time for you to react to shut the noise down before
it blows up your speakers.
Feedback created in such instances drives the power amplifiers
to their full output. The huge amount of power then drives the
voice-coils of the speakers (the horn drivers and woofers) so
hard they heat up instantly, burning themselves out. Speakers
are made to be able to withstand the high temperatures they themselves
create, but full power from an amplifier in the form of feedback
can actually make the glue holding them together boil!
An attempt to be cost-effective and purchase only a single cassette
deck actually turns out to be a much more costly route. Speaker
driver replacement isn't inexpensive. Even if a new set of drivers
can be purchased at a reasonable cost, you must also consider
the cost of having someone bring in the scaffold or lift truck
so they can get up to the speakers and do the needed repairs.
So what's the solution? Simple--buy (and use) two tape decks.
One tape deck should be dedicated to recording only, the other
for playback only. Take a look at the diagram below--here you
can clearly see there's not circular feedback path as there was
in the previous example. This feedback loop problem isn't limited
only to cassette decks, but DAT machines, too. Since typical CD
players only play (and don't record), they aren't at risk.
There are people who say that they'll be careful and they'll
be sure to always mute or otherwise turn off the playback channels
on the mixer. True, this is a solution for the single-deck setups.
However, it's not a wise solution. Human error comes into play
here. What if you forget (all it takes is one mistake)? What if
someone else is using the system and thinks they want to record
something, not knowing of the problem they may cause?
Please, play it safe--use two tape decks if you both record and
playback cassette tapes with your sound reinforcement system.
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