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Why "daily" systems should not count on impedance rise


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#1 sundownz

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Posted 10 August 2009 - 10:25 AM

I know it is said a 100 times a day... "with impedance rise you are going to be over 1 ohm anyway" when talking about a half ohm load or lower on a daily system.

Sure... if you constantly played individual sine waves one at a time. But, is that what music is?

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http://www.audiograp...odamplifier.htm

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"The upper graph illustrates the conventional way of measuring the loudspeaker impedance.
The lower graph illustrates the dynamical approach.

We took a commercial ’off-the shelf’ loudspeaker and did a standard impedance plot for it. We swept the frequency from 20 Hz to 20 kHz, measured the input voltage and current, and calculated the impedance.

However, instead of using a sine wave input signal, we used a square wave. The reason for this is that square waves consist of a large amount of sine waves, as does music. The square wave is of course not an equivalent of music, but for this test it was an easy way of showing that a complex signal (not just a simple sine wave signal) may make the load, from the amplifier point of view, very low.

If you study the graph resulting from the test, you will probably agree that not only is it necessary to check the amplifier’s behavior for resistive, capacitive and inductive loads – the amplifier should also be checked for loads with lower impedance than the nominal impedance of the loudspeaker.

This proves to be very important, since a loudspeaker with a nominal impedance of 4 ohms will sometimes have an actual impedance of 1 ohm or less. The PowerCube helps you perform testing of an amplifier, taking all these load attributes into consideration. "

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Keep that in mind when you want to run your amplifier at a super low impedance "daily"
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#2 sundownz

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Posted 10 August 2009 - 10:26 AM

QUESTION FROM "atsaubrey":

Since Sundowz brings up the issue, I would love to hear his take on low impedence loads period and how it effects things. Like, effciency, longevity, SQ, heat and electrical systems. I see it everyday on this forum, people asking can this or that amp run at .67 or .5 ohms although it is only rated at say 1ohm. Since the manufactuer stated the thread maybe people will listen. So Jake will you answer those questions? Jake at what impedence do you run your own personal gear at on a daily basis?


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MY ANSWER:

If you look at a data sheet you can see that as a transistor gets hot it has a lower current rating... running an amp at a lower impedance increases the current passing through the transistors, heating them up, and progressively lowering their ability to handle the current until something fails.

Dropping an amp to 1/2 ohm doubles the output current (up until the amp just can't put out more at least) and makes everything less efficient so the power supply in turn draws more current, often times inducing more voltage drop which calls for MORE current to maintain the same output level - again, turning into a cycle if it continues until something fails.

Many amps are robust enough to handle this pretty well - but the more you push the amp and the more clipping that goes on you get closer to realizing the impedance chart I posted above for a square wave which can drop down VERY low. That sort of thing can destroy even a very robust amplifier design when you are already running at 1/2 or less of the rated impedance load.

It all started with SPL competition... where you play ONE sine wave at a known impedance so you can get away with a really low nominal load. That is all well and good, but music is not a single sine wave - you do not know what the impedance curve can do and it CAN drop below the nominal load, unlike "common knowledge" on the forums may tell you.
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#3 98GMC

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Posted 10 August 2009 - 11:23 AM

Very informative, Thank You!

Edited by OldSkool_08, 10 August 2009 - 11:24 AM.

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#4 c_b_

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Posted 10 August 2009 - 11:48 AM

i have alot of learning ahead of me . . .

does all of that explain why we should not run amps any lower than the manufacturer's lowest resistance?
perhaps people are running amps at lower ohms thinking that "impedance rise" will keep them at a safe resistance without any other worries? and this thread explains why that is not a wise decision.

thank you.

Edited by c_b_, 10 August 2009 - 12:03 PM.

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#5 sundownz

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Posted 10 August 2009 - 11:58 AM

Right. The "theory" of impedance rise assumes music is just like playing one sine wave after another and it is not.
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#6 KU40

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Posted 10 August 2009 - 12:22 PM

Thank god you posted that. I'm so tired of hearing about impedance rise BS and wanting to run amps below rated.
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#7 CrazyKenKid

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Posted 10 August 2009 - 03:17 PM

wow! a lot of info there to absorb lol!! thanks for posting that it cleared things up! i guess i learned something new today! score two points for the good guys (sundown)
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#8 shizzzon

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Posted 10 August 2009 - 03:30 PM

and on top of all that, DO NOT get drawn into the hype about "getting your amp modded to handle lower ohm loads".

All that is doing is basically removing the protection circuit from prohibiting the amp from shutting down so u can run it DIRECTLY into destruction.

Ask DB-R about people who mod amps just to turn right around and send em to him to fix because of their stupidity.


I saved someone else over the weekend from doing such a wiring configuration.

For some unknown reason, he has 2 Dual 2 ohm subs wired to 2 ohm on a sundown 3000D.

The subs he is using needs a lot more power than that load can offer but i was tuning his car and found out he has a 2.96x rise at his peak note.

Wiring his subs down to 0.38DCR would allow him to just surpass a 1 ohm load on the amp.

I told him you can do it but you gonna need more battery...

Very next thing he said.... "you think it be alright if i were it like that for daily?"

I answered NO before he even finished the sentence.

I already know that rise DROPS lot lower in places that usually don't sound as loud to the ear.

You can be pullin only 800w out of a 3000w at your loudest frequency or small range there but once u start listenin to areas that dont sound that loud, don't be surprised that the amp starts puttin out over 2000w in that area and it's not that loud at all. That's where crossovers come in handy.
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#9 bigmike

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Posted 10 August 2009 - 03:50 PM

This should be posted on every forum!!!!
I/m guilty of this, But now I know better.

Thanks Sundown!!!
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#10 loud by design

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Posted 02 September 2009 - 10:01 PM

Good post very helpful.
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#11 jbizzle

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Posted 18 November 2009 - 10:33 PM

bump for a good thread

Edited by jbizzle, 18 November 2009 - 10:33 PM.

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#12 corrie3232

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Posted 21 November 2009 - 10:08 AM

repost it for all the knuckle heads out there
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#13 Don Ganso

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Posted 15 March 2010 - 10:15 AM

This might have just saved me a lot of stress. Excellent info!
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#14 Moparbass28

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Posted 23 May 2010 - 11:39 AM

Image no longer works. Would like to see the graph.
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#15 sundownz

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Posted 31 May 2010 - 02:01 PM

Unfortunately the graph seems to be gone.
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