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Mikael Abdellah 5th November 2006 11:39 AM

Car woofers destroys normal amplifiers?
Im not really sure about how I should describe the problem but I will give it a shot.

A friend of mine explained to me that most car subwoofers use a shorter voice coil so if a very big power pulse is sent to the woofer the voice coil falls out of the magnet field before the speakers reaches the mechanical excursion limit. (Im not sure I got it roght but I hope you get the point). When the cone is going back into the magnet field again, it sends a MASSIVE pulse back to the amplifier. Cheap car amplifiers supposedly handles but big, powerful and well constructed PA amplifiers break down. Can someone guide to more information about this phenomenon? How should I construct or modify a solid state amplifier to have 150% control of the cone and at the same time handle the return pulse?

richie00boy 5th November 2006 11:55 AM

Utter nonsense in several respects, don't worry about it.

The only thing even remotely similar to what your friend is on about is simple phase (voltage) lead caused by inductive voicecoils, thereby making larger than normal stresses on the output devices.

Mikael Abdellah 5th November 2006 11:59 AM

I have seen it happen with my own eyes so somewhere there has to be some truth to it. I witnessed a failry low budget 12" woofer blast several ouput transistors on a Ram Audio BUX 3.4 wich is a very powerfull amplifier.

AndrewT 5th November 2006 12:37 PM

what happens to the voice coil impedance when it partially leaves the magnetic gap?
Does the impedance move towards the resistance of the voice coil.

Car speakers are often 4ohm but many of the bigger ones are 2ohm. The DCR of a 2ohm speaker could be 1r0 to 1r5.

The low impedance would load the amp severely.

I think it is much more likely that it is not the "CAR" speaker that is the problem but the "LOW" impedance load that is the real problem.

BHTX 5th November 2006 12:57 PM


I thought this was a joke at first..LOL.

Anyway, I'd have to agree with richie00boy 100% on this one. Your first post (or both of them) are complete nonsenese. The entire statement is completely false and makes no sense whatsoever.

In fact, as the overall quality of the vast majority of automotive subwoofers has increased over the last several years or so, a select few have become VERY popular for use in DIY projects in hi-end home theater setups (ie Adire, Ascendant Audio, TC Sounds, Sound Splinter, etc etc). When the correct mobile subwoofer is implemented properly, fantastic results can be achieved, to say the least...especially when taking price/performance ratio into consideration (as with most DIY projects).

As for the "12 inch woofer blast several ouput transistors"...that obviously wasn't the case. The amplifier had to have failed for other reasons than the fact that a woofer designed by a car audio manufacturer was connected to it.

The only thing that COULD be a limiting factor is the 4 ohm load (sometimes 2 ohms, or 1 ohm, or even less). But that pro amplifier you mentioned certainly should have handled just about any load thrown at it.

In a polite manner, I suggest you attempt to inform your friend that he's been misinformed.


RAM Audio BUX 3.4:

Continuous Average Power RMS, 1kHz, 1.0%THD+N
@ 2 ohms 2x 2900W
@ 4 ohms 2x 1750W
@ 8 ohms 2x 970W
Bridge @ 4 ohms 5800W
Bridge @ 8 ohms 3500W


clem_o 5th November 2006 01:05 PM


Originally posted by richie00boy

The only thing even remotely similar to what your friend is on about is simple phase (voltage) lead caused by inductive voicecoils, thereby making larger than normal stresses on the output devices.

What about back-emf?


Leolabs 5th November 2006 01:05 PM

Theorytically,maybe it's true.Pratically,never heard it at all.

Burnedfingers 5th November 2006 01:27 PM

I'm not familiar with the Ram Audio amplifier so I will withhold comments about the quality of the amplifier.

I will comment on personal experience here however. I have used car woofers on occasion and have caused them to have excessive movement to where the X-max has been severely exceeded. With an amplifier with sufficient number of outputs that was capable of driving a low impedance load no problems were experienced. I have built subwoofer amplifiers and have driven 2 0hm or lower loads with them without any problems.
Even a slight amount of back emf that would be encountered is not going to pose any problem.

No, car woofers do not destroy normal amplifiers. Been there done it as they say.

Burnedfingers 5th November 2006 01:29 PM


Continuous Average Power RMS, 1kHz, 1.0%THD+N
Ok, what are the power figures when driving low frequency loads?

These power figures are for 1K sine wave.

After looking at the web page I can tell you that these wouldn't be considered for any large format commercial system at all. They might work for light DJ work.

AndrewT 5th November 2006 01:55 PM

now that we are really into commenting on the amplifier.
Here's mine.

My usual method of assessing an amp's output capability is to compare power values into varying loads.
My preference is for a tripling of power delivered when the load changes from twice the intended speaker impedance to half the speaker impedance.

So, if I were choosing the Ram, the 8r power = 970W and the 2r power = 2900W the ratio is 2.99 That, to me, indicates that this is a good 4ohm amplifier. But there is no information to assess it's low impedance ability nor whether it becomes unstable on low impedance loadings.
How hot did the output stage become? No information!
How long was it running for?
How much current was being delivered?
Was it oscillating?
Was the fan turning or blocked?

There is more that we don't know than we do know.

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