wjm (03/24/83)
The most important thing about speaker wires is to make sure that they
are sufficiently large to carry the amplifier current to the speakers
without adding a significant amount of additional resistance (which wastes
amplifier power and decreases the damping factor). Damping factor is important
for good bass response since woofers have a nasty habit of "running away" or
"ringing" when driven by a low damping factor amp. There are several ways
amps can control the speakers to prevent this effect, the first is Kenwood's
Sigma Drive circuit which includes the speaker cables in the amp's feedback
loop and requires 4 wires to each speaker - one pair to drive the speakers and
a second sense pair to tell the amp what's going on at the speaker terminals.
The more common method (used on my Hafler 220, among other amps) is for
the amp to sense the "back EMF" (the voltage generated by the moving woofer
cone) at its terminals - this only requires 2 wires to each speaker BUT
for it to work properly the speaker wires must have sufficiently low resistance
that the voltage at the amp output terminals is a reasonable approximation
of what's at the speakers.
The bottom line is that 18 gauge is the minimum gauge speaker wire
that should be used, and only for short runs (less than 8 feet, with 8 ohm
speakers, and with a low power amp). For longer runs, higher power, or 4 ohm
speakers, one should use 16, 14, or even 12 gauge. I'd recommend that the
total resistance be kept to under .1 ohm (this works out to 8 feet with 18
gauge, 12 feet with 16 gauge, and 20 feet with 14 gauge). Note to emrath ..
you have to double the resistance shown in a wire table, since the
current must flow through the speaker cable resistance twice, once from amp
to speaker and then back from speaker to amp ... Thus the resistance of
a 100' 18ga speaker cable is 1.277 ohms (@ 68F) which is 14% of the total
cable + speaker impedance of 9.277 ohms (if one assumes the speaker impedance
is a pure resistance - which it usually isn't and if the speaker impedance
is complex, the cable resistance will form an even larger portion of the
real part of the impedance) which is far too high - 1 out of every 7 watts
put out by your amp is being dissipated in the speaker cables!
Emrath also touches on an interesting issue, how much amp power is
required - the answer is, it depends upon the efficiency of your speakers,
how loud you like your music, and how much headroom you want for peaks.
I agree that under normal levels (I don't like my music too loud) .5 to 2
watts is all your amp is sending to the speakers, but what happens when
a cannon goes off in the famous Telarc '1812'? I also like to listen to
digitally mastered recordings and don't want my amp to clip on the peaks
so I've got a 115 W/ch (RMS both channels driven @ 8 ohms 20Hz - 20KHz)
amp with about 3 dB of dynamic headroom. (It also gives me the freedom
to get less efficient speakers if I decide to do so.)
For most cases, I'd say 50 W/ch RMS is reasonable, but if you listen to
digitally mastered recordings or dbx encoded material or have inefficient
speakers, 100-150 W/ch may be more appropriate. Of course, if you have
a high powered amp, it is essential to protect your speakers (by fusing
them) from over-exuberant use of it.
End of Flame *** Bill Mitchell
Bell Labs - WHwjm@whuxl.UUCP (MITCHELL) (07/26/84)
<gulp> I must disagree with the article that regarded the use of standard electrical wiring (specifically armored cable) as overkill. Sure, its a bit more expensive to install (although not much more than any other 14 gauge wire - since the major component of the cost is the copper wire) but when you're running wires through walls, where someone may put a nail through it sometime in the future, the mechanical protection is worth it. Another alternative is to put standard hi-fi wire in conduit, but that's more expensive and labor-intensive (especially if the walls are already in place. Also, the use of standard hi-fi wire (without conduit) may not comply with the local electrical code - there are strict power limits in the National Electrical Code for "Class 2 and 3" control and signalling circuits, which are the types of circuits that can use hi-fi type wire under some circumstances (read the Code or the Code Handbook (available from most libraries) for the specific details). Bill Mitchell (whuxl!wjm)
wjm@whuxl.UUCP (MITCHELL) (11/12/84)
I agree with the concern about internal wires in speakers. Although they are much shorter than the amp-speaker cables, they may be much smaller and contribute significant amounts of resistance to the circuit. However, sometimes fine wires are part of a speakers crossover network- the resistance is designed in - so remove he fine wires with care. Bill Mitchell (ihnp4!lcuxc!wjm)