Here are some thoughts on the amplifier-resistor-speaker question. I am
not an expert, but I am perhaps half-an-expert. I'll let you decide if
that is better or worse than someone with zero knowledge. I have built
a lot of speakers including designing crossover networks. I have 12
speaker systems in my living room, and another 8 speaker systems at
other places in my house. Last fall several science students, faculty,
and I built 24 speakers (12 pairs) for ourselves and the students. Here
is a fun picture of the crew and the speakers.
www.bluffton.edu/~edmistonm/speakers.jpg I'm the guy on the far right.
I am working on reducing the "inner-tube" around my waist... something
that appeared a few years ago and doesn't want to go away.
(1) If the amplifier is relatively new (last 10 to 15 years) it probably
has protection circuitry that will shut it down if it gets too hot or
senses too much current in the output stages. Therefore, you can
probably hook the 4 ohm speakers to the amplifier, start at low volume,
and slowly increase the volume to listening levels.
I have an Onkyo receiver that is about 10 years old. It says not to use
speakers less than 6 ohms, but I have had speaker combinations all the
way down to 2 ohms. Only once, when I was trying to rattle the windows,
did the protection circuitry shut-it down. Reducing the volume brought
it right back to life.
Of course I will deny having said this if you destroy your amplifier.
But solid state amplifiers today are way more rugged, and they have good
overload protection, compared to amplifiers 30 years ago. Many
amplifiers are integrated preamps feeding integrated power amps (rather
than discrete transistors) and these integrated power amplifiers are
well protected.
(2) If you're afraid to try it, or you try it and it keeps cutting out,
you can put a 4-ohm resistor in series with the 4-ohm speaker to get an
8-ohm combination. This is done all the time. For a good speaker
system it is rare that the amplifier connects directly to the speaker.
The woofer at least has an inductor in series, and probably has a
resistor/capacitor in parallel (called a Zobel network). The Zobel
network evens out the woofer impedance (which is not constant) and helps
with damping. The midrange and tweeter typically already have resistors
in series with them because they usually have higher efficiency than the
woofer, and they need to be attenuated. Most speakers I have built have
anywhere from 4 ohms to 10 ohms in series with the tweeter to balance it
with the woofer. I do not think having a low resistance to the
amplifier is as important as some have stated.
If you do use a 4-ohm series resistor, it would be best if it is a
non-inductive wire-wound resistor. Wire-wound to take the power, and
non-inductive so it does not have more impedance for the high
frequencies.
If you need 10 watts dissipated in the speaker to make it as loud as you
want, your amplifier will have to deliver 20 watts... 10 for the
speaker, and 10 for the resistor (approximately, because the speaker is
not 4 ohms at all frequencies). I presume your amplifier can do this,
but you clearly cannot use 4-ohm carbon-film resistors that can only
handle 1/4 or 1/2 watt. 4-ohm 10-watt non-inductive resistors are not
hard to find and are not expensive. However, larger wattage
non-inductive are a bit harder to find. You could buy 8-ohm 10-watt,
and parallel them to get 4-ohm 20-watt, etc.
(3) If you are interested in speaker building, check out
www.partsexpress.com This is where I buy most of my speaker stuff.
They also have a lot of info and some links available at their site.
They sell a lot of nice speaker things at fair prices, they are fast,
and pleasant to work with.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu