First, everything Lull said is right on track as usual. But let me add a
few things.
You wondered about impedences and why some are 8 and some are 4. Well, the
reason for this has to do more with tradition than anything else. Car audio
speakers are almost ALWAYS 4 ohms, the only exception to this can be found
in modern automotive subwoofers which will often have dual (or even
quadruple) voice coils and by wiring these in certain ways (parellel,
series, or some combination of both as is the case with my two dual voice
coil subs) you can achieve a particular desired impedence to maximize the
power your sub-amp can produce.
Home speakers on the other hand are almost ALWAYS 8 ohms. I have been into
car audio for nearly 30 years and I have never taken the time to find out
why the difference, perhaps an EE can offer an explanation as to why the
impedences differ. As I said before, however, my guess is it is simply
tradition. Home speakers and home amplifiers are designed for 8 ohms (yes,
I know some high-current home amps can accomodate 4 ohms), and car speakers
and car amplifiers are designed for 4 ohm impedences (although again some
high current car amps can accomodate two, one, or even 1/2 ohm impedences).
Always remember what impedence exactly is: it is a measure of electrical
resitance measured in ohms. A direct short-circuit would have an impedence
of 0 as there is no electrical resistance at all if one wire touches another
(for instance). The higher the resistance, the more difficult it is for
current to flow through the circuit. So you will never run into problems by
chossing a speaker with a higher impedence than what an amp is rated for
(you just won't get as much current flowing, in other words watts at a given
voltage). However, choosing a speaker with a lower impedence than what an
amplifier is rated to handle CAN create problems as (like a direct short)
you are allowing much more current to flow through the circuit (you are
IMPEDING the flow of electrons less), and too much current can end up frying
an amplifier (I learned this the hard way about 20 years ago with an old
Rockford Punch 75).
Amps typically have three wires to connect them, a positive that should run
directly to the battery, a negative which should be grounded to the car's
chassis, and a remote turn on wire, or REM as it's often reffered to. Your
pink third wire is most definately this REM wire. Now a typical car audio
head unit has a blue REM wire that goes out from the HU and attaches to the
amp, crossover, EQ, or any other device in the system. Basically, when the
HU (head unit) is turned on it sends 12 volts through this REM wire which
tells the amplifier, to turn on (remember, the amp should be connected
directly to the battery so in a sense it is always connected to it's
positive and negative).
Now, if you want to connect your iPod directly to your amp, you will need
some way to turn the amp on and off because this function is typically
performed by the HU via the blue REM wire, which it sounds like you will not
have. HOWEVER, there is a simple workaround. You just take your power
wire, split it before it gets to the amp, have half go to the regular B+
positive power supply, and the other half attach to a toggle switch that can
be mounted on the dash and then run the other end of that to your pink REM
connection on the amp.
The only inconvience is that you will have to turn your amp on and (most
importantly if you don't want to run down your battery) off with the toggle
switch.
But rest assured this works fine as I have done it for several of my
friend's cars as they saw absolutely no point in having a HU as all they
listened to was their portable MP3 player. You would just have to get into
the habit of always remembering to turn the toggle switch on or off.
Now there is another way if you want to eliminate the need for a toggle
switch. You can connect the amp's REM wire to the car's ignition wire (in
most car's, it's the red one behind the dash, the yellow wire behind the
dash is usually an "always on" 12 volt power supply). What this will do is
turn the amp on EVERY TIME the key is turned (whether the iPod is on,
playing, or even connected). The benefit here is that you never have to
worry about accidentally leaving the amp on all night if you forget to
switch the toggle. The downside is that the amp is always on when the key
is turned whether you are playing the iPod or not.
Anyway, longer answer than I planned, as my wife always says, I can be a bit
wordy. Hope some of this helps.
MOSFET
"Doug McLaren" wrote in message
...
Just picked up a used LA Sound Redondo 80 amplifier. No instructions,
of course.
Looks like it's rated at 35w x 4 at 2 ohms, or 20w x 4 at 4 ohms.
What would happen if I used 8 ohm speakers? Just less power? The
release of magic smoke?
I haven't checked my speakers, but the last time I messed with stereos
(decades ago!) 8 ohms was the standard. Has this changed?
I plan on powering it with a deep cycle 12v battery and feeding it
with an iPod directly to the line inputs.
Would it get upset if I put the inputs in parallel and the outputs in
parallel so that two channels drove each speaker for more power? I
assume I need to make sure the polarities of the wires all match, but
beyond that, it should work, no?
It's got two big power wires -- red and black, it's obvious what
they're for -- but there's also a smaller pink one. What's that for?
My guess is that the red and black can be wired directly to the
battery, and the pink one can be wired to the car stereo or ignition,
to be hot when the amplifier should turn on. Or is it something else?
Thanks in advance ...
--
Doug McLaren,