reversed signal polarity?

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Hey,

Im pretty much an amateur when it comes to car audio. I have a four channel amplifier, components, tweeters and a 12 inch sub. The sub is connected to two bridged channels of the amplifier.

What confuses me is that the sub is wired out of phase i.e the positive terminal of the amplifer is connected to the negative terminal of the sub and vice-versa. The installation was professionally done and I've asked the guy who did it, and he says it is correct. It sounds good but i'm just curious to find out why the sub must be wired like it is.

With thanks,
Gareth
 
it has to do with phase....

to make things simple, consider a bass wave as a wave in the ocean, but many, say every 2 feet there is a wave crest....

along with the mids and hi's there are also these 'wave crest'

to make everything hit you at the same time, all of these crest have to hit you at the same time, the peak of the wave....they all have to meet at the same time for perfect unison.

you can place the negative/positive polarities on either the top or bottom of the wave...

the way to move the polarity from the top or bottom is to switch the wires back and forth until you find that point of equilibrium.
and the waves hit you at the same time.
 
G_a_r_e_t_h said:
I do want everything to be in phase.
Im still struggling to understand the concept though.
Reversed polarity would be equivalent to a 180 degree phase shift, right?
I understand that there may be some phase shift but why 180 degrees?

Thanks for all your replies.

Gareth

The R and L channels basically have the same referance. This would be a + of one channel tied to the - of the other channel. For this reason, they are initially 180 degrees out of phase. So by bridging, you are tying the OTHER + and - terminals together (these are technically in phase) which sum the outputs together giving you twice the signal.
 
G_a_r_e_t_h said:
I do want everything to be in phase.
Im still struggling to understand the concept though.
Reversed polarity would be equivalent to a 180 degree phase shift, right?
I understand that there may be some phase shift but why 180 degrees?

Thanks for all your replies.

Gareth


Because a 180 degree shift is a mirror copy of the original (a 360 degree shift is the exact same as the original). In other words, when you create a 180 degree phase shift, you are essentially flipping the wave upside down. Reversing the terminals on the amp accomplishes the same thing.
 
Gareth wrote:
the sub is wired out of phase i.e the positive terminal of the amplifer is connected to the negative terminal of the sub and vice-versa.

Look carefully at the amp. Typically both positive (+) outputs are used to connect to one speaker in bridge mode. It uses two amplifiers. One of the amplifers is non-inverted. That amplifier will connect to the speaker positive (+). The other amplifier takes the input and inverts it. ...So the signal is the same but flipped. As someone said before. The inverting amplifier "positive" output (+) will connect to the speaker minus (-). If you have the owner's manual for the amp, check it out and see if there's a diagram that agrees with what the installation tech said.

I would think that the subwoofer should be in-phase with the mid-bass drivers at the crossover frequency. Play something with plenty of bass and connect both ways. When the mid-bass and the subwoofer are in phase, there will be greater acoustical output (louder) because the signal from two driver are adding and not subtracting. There are varying degrees of phase addition and subtraction, but this sounds like broad strokes troubleshooting. It should be pretty noticeable with the right music material. Perform the experiment back and forth several times. The right way should become apparent.
 
mrshow4u said:
Gareth wrote:

Look carefully at the amp. Typically both positive (+) outputs are used to connect to one speaker in bridge mode. It uses two amplifiers. One of the amplifers is non-inverted. That amplifier will connect to the speaker positive (+). The other amplifier takes the input and inverts it. ...So the signal is the same but flipped. As someone said before. The inverting amplifier "positive" output (+) will connect to the speaker minus (-). If you have the owner's manual for the amp, check it out and see if there's a diagram that agrees with what the installation tech said.

That's true, but typically the inverted channel's + is labelled '-' (in fact it's so typical that I've never seen or heard of an amp that's not labelled that way), and that's the one you use as the negative terminal when connecting the sub.

Therefore, when looking at the terminals on the amp, the two labelled "bridge" will be a - and a +, and those correspond to the - and the + for the sub. In reality though (as you pointed out), both of these terminals are +, but the one labelled '-' is the + from the inverted channel.
 
Crossovers are what cause phase shifts. I'm not 100% sure on the details, but some XOs cause a 180 deg. phase shift at the XO frequency. So when you hook everything up "in phase," let's say subs and mids, one signal will be out of phase due to the XO. You would then want to reverse the phase on one of the drivers (either on the high side or the low side of the XO frequency) so that you would get constructive interference instead of destructive interference.

Recently, I played around with my head unit settings, and I realized that my sub and mids were out of phase. Now I have the sub phase flipped 180 degrees, and it sounds a lot better. I have this setting built into the head unit. Seems like you don't, so reversing the phase of the sub would be the way to do it.
 
xplod1236 said:
Crossovers are what cause phase shifts. I'm not 100% sure on the details, but some XOs cause a 180 deg. phase shift at the XO frequency. So when you hook everything up "in phase," let's say subs and mids, one signal will be out of phase due to the XO. You would then want to reverse the phase on one of the drivers (either on the high side or the low side of the XO frequency) so that you would get constructive interference instead of destructive interference.

A 2nd order filter will cause a 180 degree shift in the stop band, but at the crossover frequency (which is really what matters) it will only be a 90 degree shift. But, the lowpass for the sub will create a -90 deg shift at the xover freq, and the highpass for the mid will create a +90 deg shift at the xover freq, so you have a 180 deg difference and your comments do apply. That's only if you have a 2nd order filter for both your HP and LP though, that's common for the HP but most LP filters built into subwoofer amplifiers are 4th order, so things aren't necessarily that simple.
 
xplod1236 said:
What is the phase shift on an 18 dB/oct XO? I know I'm bringing this thread somewhat off topic by throwing in my questions, but it's still relevant to the thread.

It's 90 degrees per order in the stop band

18dB/oct is 3rd order, so it has a 270 deg phase shift in the stop band. +270 deg for a highpass, -270 deg (which is the same as +90 deg) for a lowpass.
 
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