Hi all,
I'm new here, and for starters I have a really noobish question:
Why exactly is it impossible to connect HiFi Speakers to a computer
(except because of the fact that they don't have a mini-jack plug 
(at least mine don't) )?
Well, it isn't impossible. In most cases, though, you won't get
enough sound to be useful.
Loudspeakers require that a significant amount of power be delivered
to the drivers (the "speakers" themselves mounted in the cabinet) in
order to create a useful amount of sound. Typically, a HiFi speaker
is driven by an amplifier that can deliver a maximum of tens to
hundreds of watts. For most hi-fi speakers, average listening levels
run in the range of a few watts. Speakers have a relatively low
"impedance" (measured in ohms)... this means that they draw a fairly
large amount of current for any given amount of voltage. 8-ohm
speakers are quite typical. Stereo receivers and amplifiers are
designed to drive these low impedances, with enough voltage and
current to deliver their rated power.
Most computer sound-card outputs do *not* include a speaker amplifier.
They aren't designed to deliver large amounts of power, or to drive
low impedances. They are typically designed to drive efficient
headphones (drawing a fraction of a watt, at impedances of 32 ohms on
up), or to drive "line-level" devices (amplifier inputs or
"self-powered" computer loudspeakers, drawing a tiny fraction of a
watt with impedances of thousands of ohms).
In short, computer sound-card outputs are "wimpy" - they don't have
strong enough electronic "muscles" to deliver the power needed to move
a heavy loudspaker driver's cone back and forth to make sound.
Many computer loudspeakers have built-in power amplifiers, required to
boost the line-level signal and drive the computer speaker properly.
Most hi-fi loudspeakers do *not* have a built-in amplifier... there
are some exceptions, but most do not.
I know it's possible to connect a computer to a HiFi system (by
plugging it into "Line In"/"AUX"), but I would like to be able to do it
without the Hi-Fi system, only with the speakers.
To do that, you'd need one of two things:
- A hi-fi speaker with a built-in amplifier (they do exist), or
- A new computer sound-card with a built-in speaker-level power
amplifier. Once again, these do exist, but they're not very common,
and typically can't deliver more than a couple of watts per
channel.
I guess what I really want to know is what kind of a 'converter' is
there inside the Hi-Fi system..? Does it modify the signal from the
computer, or is it merely a small circuit just adapting the
amperage/voltage/impedance/whatever..?
It's an active amplification stage (or several stages). It takes an
incoming signal with only a few milliwatts of power (a typical
line-level signal is 2 volts peak-to-peak into 5k ohms or so, which is
less than a thousanth of an ampere), boosts the voltage upwards by a
factor of 10 or more, and is capable of driving the speakers with
currents of several amperes. In short, the amplifier is delivering
thousands of times more power to the speaker, than it's taking in from
the computer's sound-card output.
The signal coming out of the amplifier looks like a "big brother" of
the signal going in... the waveform should have the same shape (if it
doesn't, the amp is distorting it) but it's larger in both voltage and
current.
The additional power is drawn from the building's AC power mains. Not
all of the power pulled from the mains ends up going to the speaker...
there's a significant amount of waste in most amplifier designs, and
this ends up being dissipated as the heat which warms up the amp.
--
Dave Platt AE6EO
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