Why Care About Phase

From detritu–(at)–x.netcom.com Mon Jun 1 10:43:04 CDT 1998
From: detritu–(at)–x.netcom.com(Lord Valve)
Newsgroups: alt.guitar.amps
Subject: Re: speaker wire reversal on re-issue 4×10 bassman
Date: 1 Jun 1998 05:52:01 GMT
X-NETCOM-Date: Sun May 31 10:52:01 PM PDT 1998
Xref: geraldo.cc.utexas.edu alt.guitar.amps:108085

In <357237F1.2FB--(at)--oncentric.net> Robert Hinson
<6bq5el8--(at)--oncentric.net> writes:
>
>to aga gurus, in g. weber’s first book he has a section on the 4×10
>bassman and says you need to reverse the polarity on the speaker wires
>to the current speakers (alnico mojo/eminence) because the signal from
>the output section is inverted with respect to the input signal from
>your guitar. this part i understand (because of the gain stage type
and
>arrangement) but why then wouldn’t the same be true of any amp using
>similar preamp gain structure (marshall, vox, etc.–they all invert
the
>signal with respect to the input)? he says what you want is the
speaker
>cone to move forward when signal is applied or when you test the
>polarity with a 9v battery. as far as i know, the current alnico
>eminence speakers move forward when given the battery test. the only
>difference is the orientation (left or right) of the speaker terminals
>when compared to the orientation of ceramic magnet speakers. if a
>speaker moves forward when given either signal or the polarity test,
why
>the hell should you reverse the wires? i thought maybe this was just
>true with the old jensens which weber says move backward so that is
why
>one would have to make the wiring change. lord valve, trem, do you
guys
>have the real scoop on this? maybe there’s something about speaker
>construction i don’t understand or that gerrie left out. thanks rh

Lord Valve Speaketh:
“Absolute” phasing – whether a single speaker (or an array of
speakers) moves inward or outward when presented with a positive-going
transient – is really a matter of taste. Some people (I happen to be
one of them) can hear this; of those who can, only around half express
a preference for one ‘phase’ over the other. My own take on this is
that I don’t care much, unless the program material has a lot of
kickdrum in the mix and it’s LOUD. In that case, I prefer to have the
‘absolute’ phase be correct. Note that this is really a pretty
haphazard process, as (in a large PA system) the signal will pass
through many dozens of stages of preamplification, EQ, summing,
buffering, etc., and many discrete stages such as the mixer, outboard
processing (including compression, EQ, filtering, time-based effects,
etc.) electronic crossover, power amplification, and so forth. Some of
these stages/devices will invert signal and some won’t. If any of
these devices use balanced line connections, the ‘absolute’ phasing
will also be at the mercy of the guy who wired the connecting cables.
There is really only one way to determine whether an entire system is
“in phase,” and that requires test equipment. “Relative” phasing is
much more important…if you have two speakers wired electrically
out-of-phase in the same cabinet, you’ll be looking at a pretty drastic
drop in low-frequency performance. Having two arrays (like the left
and right speaker stacks of a PA system) out of phase with each other
is more subtle…the folks in front of the right stack will like what
they hear, as will the folks in front of the left stack. The folks in
the middle will be listening to some pretty squirrelly cancellation
effects, though. As far as your Bassman goes, feel free to try it both
ways. You can’t hurt anything by running the speakers out of phase
with respect to the input signal, so if you actually find that you
prefer one phase over the other, go for it. I have a customer who had
me put a phase-reverse switch on his Twin; he likes it “in” on some
tunes, and “out” on others. Why don’t you try it yourself and post the
results.

Lord Valve
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