From: jch@alexander.cs.umn.edu (J.C. Hamlin)
Newsgroups: rec.audio.car
Subject: Re: Dual Voice Coil Subwoofers & Bridging
Date: 26 Jan 93 23:33:04 GMT
Article-I.D.: news2.C1HIrs.1F2
Organization: University of Minnesota

In article <1993Jan26.162519.24496@csc.ti.com>, ajones@dadd.ti.com (Alan Jones) writes:
|> Can someone briefly explain to me when to choose a dual voice coil subwoofer
|> versus a regular single coil?  It seems to me that they make these for folks
|> who want to install only a single sub, correct?  And when you do this, you
|> simply connect both left and right channels to the sub, right?

You can connect two amp channels to each of the two voice coils as normal,
or you can connect only one amp channel to one voice coil and forget about the 
other, or both in parallel, or both in series, or bridge the amplifier into 
one voice coil, or bridge it into both in parallel (very low impedance),
or both in series.  In other words, just treat the two voice coils as two 
diffierent woofers -- the impedance figures for the voice coils will be given.
The rest of this stuff has nothing to do with DVC specifically, but can 
be used to describe bridging (what the salesman might have been getting 
confused on).

|> I'm confused because I was shopping this past weekend for subs and found a
|> place which is clearing out it's Earthquake subs.  They're selling the EQ-12
|> for ~$60.  (Anyone out there have one of these??)  The salesman there said 
|> that if I hooked up the left and right channels of my amp the sub would tear
|> itself apart.

(snicker).  Those damn sales droids, often they don't know what they're talking
about.

|>  Instead, he said I need to add a gizmo to one of the channels
|> which would change the phase by 180 degrees such that both channels are in
|> phase.  Does this make sense?

Well, 180 degrees "out of phase" is a bad combination.  Phase means time, 
and it is measured in seconds, not degrees.  Out of phase means the 
same signal reaches your ears at two different times (like two speakers 
with the same signal a little ways apart).  What you do mean is reverse 
polarity.  The reason why these two terms are often confused because 180 
"degress" out of phase (makes sense only describing a fixed 
wavelenth/frequency signal) and reverse polarity are identical for a sine wave.

You could simply reverse the polarity of the second driver by flipping the 
connections to the terminals.  That would cancel the signal from the 
first voice coil and the woofer wouldn't move (assuming a mono bass 
signal).  The other thing that salesman could have been confusing a DVC 
woofer with is bridging an amp, where you need to use an inverted phase 
signal on the other line -- bridging can be easlity diagramed:

Bridging an amp requires inverting the phase of a signal of one channel 
of the amp without inverting the polarity.  Most newer amplifiers have 
this ability built in via a manual or automatic swtich.

A normal (non-bridged) signal looks like this:

Left:                          Right:                   
/\/\/\/\/\/\/\/\ positive      /\/\/\/\/\/\/\/\ positive
________________ ground	       ________________ ground  

Notice the ground is constant, and the signals are in phase.  If you just 
connect to the left + and right + you get this signal:

/\/\/\/\/\/\/\/\ positive (from left positive)
/\/\/\/\/\/\/\/\ ground   (from right positive)

Which doesn't do you any good because the potential difference between 
the two is constant (resulting in no sound).  So, you need to invert the
phase of one of the channels.  Under normal circumstances you want the left 
channel to be in phase with the right (otherwise you get cancellation), 
but this is a case where you want to invert one use them out of phase.

/\/\/\/\/\/\/\/\ positive (from left positive)
\/\/\/\/\/\/\/\/ ground   (from right positive)

Notice how the difference in amplitude is two times that of the original.
This results in a four times increase in power into the same impedance--
which requires the amplifier to be stable at that current output which 
is identical to each channel having half the load (half the original 
impedance) or you have to use a speaker with twice the impedance of normal 
(i.e. from 4 ohm to 8 ohm driver if you want to bridge an amp is not 2 ohm 
stable). 

The rated power theoretically increases by four times, and the THD at 
rated power will be a little higher (so that they can get that figure).  
The damping factor will also change if the impedance per channel changes
(since it is measured at 4 or 8 ohms per channel).

Hope that helps those of you who were a little underinformed about what 
bridging was.  If I've got any gross technical errors here, feel free to 
correct me.

J.C. Hamlin
Automotive Sound & Security by Hamlin, Minneapolis, MN
jch@cs.umn.edu
Newsgroups: rec.audio.car