11.3 How do I drive more than two speakers with one stereo amplifier?
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This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
11.3 How do I drive more than two speakers with one stereo amplifier?
One amp can drive many speakers. However, there are two limits
to this practice. The first is that you can overheat or damage
an amplifier if you drive too low of an impedance to loud
listening levels. Avoid loading any amplifier with a lower
impedance than recommended. Adding two speakers to one amp
output loads that output with half the impedance of one speaker.
(See also 11.2 above)
The second is that with tube amplifiers, which are uncommon
in today's common system, it is important that the speaker
impedance and the amplifier output impedance be well matched.
When driving two or more speakers from one amp output, always
wire them in parallel, rather than series. Series connection,
while safe in terms of impedance levels, can hurt sound quality
by raising the impedance that the speakers themselves see.
Also, when different speakers are wired in series, amplifier
voltage will divide between the speakers unevenly, because
different speakers have different impedance-versus-frequency
characteristics.
Many amplifiers have connectors for two pairs of speakers. In
general, these amplifiers also have a speaker selector switch.
Most amplifiers connect speakers in parallel when both are
selected, although some less expensive ones will wire the
speakers in series. It is common for these amplifiers to require
8 ohm speakers only, because the amplifier is built to drive
either 4 or 8 ohms, and two sets of 8 ohm speakers in parallel
loads the amplifier like one set of 4 ohm speakers. It is
almost always safe to connect one set of 4 ohm speakers to
an amplifier with two sets of outputs, provided that you
NEVER use the second terminals for any other speakers.
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