Crane Song AVOCET II - Manual (Page 41)

Polarity is not Phase
This is still a confusing topic, perhaps because people are too timid to say
polarity when they mean it.
The polarity of a loudspeaker refers to whether the driver moves outward or inward with positive-going
signal, and can be corrected by a simple wire reversal. Remember that phase means relative time ;
phase shift is actually a time delay. The so-called phase switches on consoles are actually polarity
switches, they have no effect on the time of the signal! Sometimes this is referred to as absolute
phase , but I recommend avoiding the use of the term phase when you really mean polarity . If two
loudspeakers are working together, their polarity must be the same. If they are separated by space, or
if a crossover is involved, there may be a
phase difference between them, measured in time or
degrees (at a specific frequency). I have a pair of Genesis subwoofers with separate servo amplifiers.
There are three controls on the crossover/amplifier: volume (gain), phase (from 0 to 180 degrees),
and low pass crossover frequency (from 35 Hz to over 200 Hz). Notice there is no high pass
adjustment. The
natural approach to subwoofer nirvana assumes that your (small) satellite
loudspeakers have clean, smooth response down to some bass frequency, and gradually roll off
below that. It’s logical to use the natural bass rolloff of the satellites as the high pass portion of the
system and to avoid adding additional electronics that will affect the delicate midrange frequencies.
So we use a combination of lowpass crossover adjustment and subwoofer positioning to fine-tune the
system.
A good subwoofer crossover/amplifier usually provides more than one method of interconnection with
the satellite system. The best is the one which has the least effect on the sound of the critical main
system. I prefer not to interfere with the line level connections to the (main) power amp feeding the
satellites. If your preamplifier does not have a spare pair of buffered outputs, I recommend using the
speaker-level outputs of the main power amp. The
Genesis provides high-impedance transformer-
coupled balanced inputs on banana connectors designed to accept speaker-level signals. Connect
the main power amp’s output to the sub amp’s input with simple zip cord with bananas on each end.
No real current is being drawn, so wire gauge does not have to be heavy. Double-bananas make it
easy to reverse the polarity of the subwoofer, a critical part of the test procedure. Some subwoofers
use a 12 dB/octave crossover, others 18 or more. Interestingly, for reasons we will not discuss here, a
12 dB crossover slope requires woofers that are wired out of polarity with the main system. My subs
use a 12 dB slope, but to make it easy on the mindless, the internal connections are reversed, and
you’re supposed to connect “hot to hot” between the main power amplifier and woofer amplifier. Leave
nothing to doubt-we must confirm the correct polarity.
You have to sit in the “sweet spot” for the listening evaluation. If your subwoofers have an integrated
amplifier, you’ll need a cooperative friend to make adjustments. Since the Genesis amplifier is
physically separate, I was able to move the subwoofer amplifiers to the floor in front of the sweet spot,
and make my own adjustments. Here are the two test CDs:
1. The Mix Reference Disc, Deluxe Edition, MRD2A, available from Music Books Plus , or any
source of 1/3 octave filtered pink noise. Track 71 contains full bandwidth pink noise, and tracks
11 through 41 use multi frequencies in 1/3 octave bands.
2. Rebecca Pidgeon, The Raven , Chesky JD115, available at record stores, high-end stereo
stores, or from Chesky Records .
I recorded Rebecca’s disc in 1994. Track 12 is Spanish Harlem, which has a slow, deliberate acoustic
bass part that makes it easy to identify notes that “stick out” too far and covers the major portion of
the bass spectrum. This record has never failed to reveal the anomalies of different rooms and
loudspeakers in several years of use as a musical reference. The ear is better with instant
comparisons than absolute judgments, and this test relies on our ear’s ability to make comparisons.
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