Every
amplifier is designed to handle a certain load. All
amplifiers can handle a higher resistance (load), but
they will produce less output. Most quality
amplifiers can also handle a lower resistance. A good
rule of thumb is not to go below 2 ohms or above 16
ohms. With stereo power amps, always avoid running 1
speaker cabinet one side and two or three on the
other. This goes for any unbalanced number of
speakers on one side. The ohms are bound to be messed
up and you could damage that channel or the whole
amp.
How to bridge an amp?
First, make sure that the amplifier you have is
bridgeable. If you try to bridge an amp that is not
designed to be bridged, you can damage the amp.
Basically, when you bridge an amp you use the +
terminal from one channel and the - terminal from the
other. The instructions that came with the amp should
describe which terminals to use. If not, look for
some indication next to the speaker terminals on the
amp. Sometimes, there will be lines connecting the
two terminals you should use to bridge the amp, as
shown on the right.
What
happens when you bridge an amp?
Theoretically, the output should be 4 times the power
of a single channel on the amp. A 50Wx2 amp bridged
would act as a 200Wx1 amp. Unfortunately, you only
see about 2/3 to 3/4 of this power. This number is
usually the same as the combined power of the 2
channels at half the load. Take the following example
of a 2 channel stereo amplifier:
50x2 stereo at 4 ohms
75x2 stereo at 2 ohms
150x1 bridged at 4 ohms
Unless
stated otherwise, this amp would be unstable below 2
ohms stereo or 4 ohms mono bridged. Attempting to use
an amplifier at an unstable load will damage the amp.
What
is bi-amping?
Bi-Amping refers to using different amplifiers (or
different channels of the same amp) for the low and
high frequencies in the same speaker. Usually, the
mid and tweeter are driven by one amp, and the woofer
is driven by more powerful amp. This allows you to
purchase a high quality low power amp for the highs,
and a more powerful amp for the lows. With the proper
pre-amp you can also have more control over the bass
output.
In DIY
audio, bi-amping has even more advantages. Low pass
crossovers for woofers require very large inductors.
These inductors are basically very long coils of
copper wire which can have a very high resistance.
Using an active crossover before the amplifier
removes the need for these inductors. Bi-amping also
removes the need for any circuits to fix problems
caused by different sensitivities or impedances
between drivers.
If a
speaker is capable of bi-amping, then the plate on
the back of the speaker will have 4 binding posts: 2
+ terminals and 2 - terminals. Both + terminals and
both - will have a piece of metal connecting them
together. To bi-amp the speaker, remove the metal
piece. Then, use the top 2 terminals for the high
frequency amp, and the bottom two terminals for the
bass amp.
What
size speaker cable should I use?
Cable sizes get fatter as the gauge number gets
lower, i.e. 12 gauge is bigger around than 18 gauge.
Different cables will have different line
capacitances, which can cause the cable to act as a
low pass filter.
As a rule of thumb...
Under
250 watt amp running 20 feet of cable or
less...18 gauge should be fine.
Over
250 watt amp running 25 to 75 feet... 16 or 14
gauge should be used.
Higher
powered amps running long distances to speakers
should use 14 gauge or lower.
Don't
let that music store guy sell you 150 ft. 20 gauge
speaker cables! You may be temped to use typical
electrical cable instead of speaker cable to save
money. The copper in electrical cable is of less
quality than speaker cable, and will be prone to
rust. Also, the insulation isn't of the same quality
as speaker cable.
