Can subs change resistance?

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blazinred99

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Joined: January 19, 2006

Location: United States

Posted: October 31, 2006 at 9:09 PM / IP Logged

I have a 12 inch pioneer sub, 4 ohms, 150 watts rms bridged to a Dual amp that is 175x1 at 4 ohms.

it worked fine until one morning it just quit. it worked fine the night before and had turned up maybe a little too much.

whats happening is the amp keeps going in to protection when barely turned up so i hooked up to just one channel so it would technically have 50 watts and it works fine but not as loud as before. Also it seems louder than 50 watts would. But its not as loud as when bridged

my main question is if a sub gets to much power can its resistance change. Or why does it go into protection.

The Amp

The Sub

Steven Kephart

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Posted: October 31, 2006 at 9:39 PM / IP Logged

Sure it can. It can change from 4 ohms nominal to infinite resistance if the coil cooked open or a tinsel lead broke. I suppose it is possible to produce a direct short too, but that is much more unlikely. That probably isn't what is going on with your system. I would guess the problem to be either an improper gain setting/bass boost turned on, improper wire size used, a poor ground, or corrosion at the battery terminal. Was the amp professionally installed? What size power and ground wires were used? How was the gain set if not professionally installed? Was a bass boost on the amp turned on? Is the ground connected to bare metal?

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boulderguy

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Posted: October 31, 2006 at 9:48 PM / IP Logged

Sounds to me like you cooked your sub the other nite and yes, that can change resistance. Partially anyway. When you re-wire it to single channel the amp's not seeing the same current demand & therefore doesn't go into protection the way it does bridged.

The simple test is to wire up a new sub or other speaker to the bridged amp & see if the same thing happens. My guess is that'll solve the problem.

blazinred99

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Joined: January 19, 2006

Location: United States

Posted: October 31, 2006 at 11:36 PM / IP Logged

i put the amp and sub in a different car and it acted the same so i know the wiring is ok.

The sub sounds good now on one channel, it doesn't sound blown but i dont really know what a blown sub would sound like.

When bridged with the bass boost off and the gain barely up it goes into protect. I would adjust it till it when into protect and i got about a quarter turn. so I'll live with it a little quieter till i find a new sub. thanks for the help.

boulderguy

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Posted: October 31, 2006 at 11:51 PM / IP Logged

Good luck, shouldn't be an expensive fix.

Understand that a cooked speaker isn't necessarily a blown speaker. It could be any number of creative adjectives - borked, fried, toasted, you get the idea. What I mean is that a speaker can sustain damage from overload w/o being "blown" to the point of total thermonuclear annihilation.

If you get inside one you'll see it's just a really thin wire wrapped around a paper cylinder with current pumped thru it. Those wraps can melt, short, break, fray, any number of things that all sound different.

master5

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Joined: October 10, 2006

Location: United States

Posted: November 01, 2006 at 12:06 AM / IP Logged

I noticed in certain instances the the Re (dc resistance) of a subs coil(s) can change. This does not neccesarly mean the sub is damaged but of course if it's supposed to read close to 4 ohms and it reads 9ohms, 1000ohms, 1.3ohms, 0 ohms etc. you can safely assume the voice coil is damaged. A very low reading is a short, a very high reading is a break.

Now as far as when a sub is actually operating the resistence changes constantly, hence why it is called impedance, since you can't physically measure the resistance of a device in operation, the impeadance given as a specification is more or less an average.

I am in the habit of measuring the DC resistance of every sub before I install it. I find in general the DC resistance is a little lower then the rated impedance, but by an insignificant amout (ie 3.7 ohms for a rated 4 ohm)

Since you state that the same problem exsits even if in a different car still hasn't identified the problem. As with any problem such as you are having the FIRST thing I would do is use a DMM and check the voltages AT the amp terminals. Place the meter in DC on the proper range to measure 12v. Put the negative probe on the ground terminal of the amp, positive on the 12v+ terminal of the amp. If it is not at least 12v you have to start tracking down where you are losing the voltage. If it is at least 12v, start the vehicle and take readings as the volume is turned up. If the voltage is dropping, once again your problem is power/ground related.

If the volatge remains stable enough as volume is increased there is a problem with impedance. At that point if the sub's dc resistance is measuring within specs I would suspect an amp probem..it happens, see it all the time. I have a pile of amps laying around where only one channel will play a sub, but when bridged to specs will go into protect. The easiest test would be substitution. Try a different sub or amp and see what happens.

Keep us posted.

killer sonata

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Posted: November 01, 2006 at 12:16 AM / IP Logged

do you have your gains set correctly?

Steven Kephart

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Location: Oregon, United States

Posted: November 01, 2006 at 1:53 AM / IP Logged

master5 wrote:

Ok, here's the lay-down of DCR/Re/DC Resistance as well as nominal and actual impedance. Impedance is the total resistive load produced by the resistance, inductance, and capacitance in an AC circuit and it changes with frequency. The Re (the technical term for DC resistance) is only part of the impedance, which is why it is always lower than the impedance. If you graphed the impedance with frequency, you will find a curve like this one: http://www.adireaudio.com/Home/Images/ExtremisZ.gif Speakers are rated at what is called a nominal impedance. What that is is the lowest impedance the driver will reach. Many times these are rounded off to a nearest value, which means that not all 4 ohm subs will provide the same load on an amplifier.

aznboi3644

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Joined: May 01, 2006

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Posted: November 01, 2006 at 3:38 PM / IP Logged

Nicely stated Steven

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Can subs change resistance?

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