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Old 14th November 2002, 08:46 PM   #1
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Default extreme novice on amps, speakers

ok, on another forum, actually a car audio forum, someone asked about how one would go about increasing the impedance the amp sees from a speaker (i don't think that's the best explaination of it but...)

basically, someone said to just hook a 4ohm resistor in series with the speaker. I disagreed with that idea. beyond the practicalities of the heat from the resistor, I said that because impedance is variable across the freqency domain (that is the right word?) that hooking up a fixed resistance would be goofy.

then i thoght back to the "output impedance" of an amplifier. I am assuming that it is somehow related to the impedance of the load to prevent a similar mismatch problem, or made to be extremely small compared to the load. or am i looking at this wrong?
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Old 14th November 2002, 09:33 PM   #2
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A- Resistors come in a variety of wattages to deal with heat. I've no idea what would be needed in a car system but one could be hypercautions and use a 25W sandcast. Cost is under $5.

B- At any frequency adding the 4 ohm resistor in seriies would just raise the impedance of the speaker by 4. Or if you were to express it on a graph L vs. F it would simply shift the curve up by 4 ohm.

If I'm mistaken, about this someone will correct me I'm sure.

A more interesting question is why would someone want to do this? To make the speaker more "efficient"? I expect the sound quality is going to suffer.
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Old 14th November 2002, 10:56 PM   #3
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Humm, all this will do ( simplistically) is waste power from the amp by heating the resistor, is this a case that someone had a dodgy car amp that would only drive 8 Ohm? I personally don't see the point...
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Old 15th November 2002, 12:03 AM   #4
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Tried it once just for fun. D'ont know why but the sound quality suffer lot when I've done it. really not a good idea as it's going to wast half of the energy.

But I can still not understand the extreme lost in sound quality. The resistor was ok.
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Old 15th November 2002, 05:44 PM   #5
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well, kinda missing the point a little.

the exact problem someone had was a 1ohm tweeter and a 4ohm amp. now i know the tweeter won't need uber-power. someone suggested a series resistance.

I said it would not be good, because the impedance of a speaker is not constant when you look at freqency. i haven't checked to see if phase would do anything to my reasoning. in any case i asked basically, how is amplifier output impedance different from a series resistance.

i ran winISD and adding a series resistor altered freqency response.
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Old 15th November 2002, 07:48 PM   #6
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often when you are told an amp is for 4 or 8 Ohms, it has to do with when the amp will melt. For example, If am amp is rated for only 8 ohms and you hook it up to a 4 ohm speaker, the four ohm speaker will draw twice as much current (this is why an amp is generally rated at a higher power for a 4 ohm speaker). If the amp supplies (or tries to supply) the required current the transistors in it will get very hot. When they get hot bad things start to happen. For your case the amplifier will need to supply 4 times the current it would for a 4 ohm speaker. I will say that just hooking up the 1 ohm speaker will not blow the amp but if the volume gets high look for fire, I would not suggest hooking it up and trying to keep the volume low.
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Old 15th November 2002, 08:19 PM   #7
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ok, i'll try this over.

i explained how the question came up, and that part was answered instead. so i re-asked and got another question answered. so now i ask my question for a third time.

I want to know how the output inpedance of an amp differs from a series resistance before the speaker. also, what effect does changing output impedance of an amp have on the sound that would come out of the speaker.

i thank all who have posted, but i feel my question is either not clear or being avoided, despite the line:
"in any case i asked basically, how is amplifier output impedance different from a series resistance. "

maby i shouldn't have stated i was a novice, because it seems like the questions you're answering are easier than the question i asked.
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Old 15th November 2002, 08:31 PM   #8
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You are corect in that this is not such a simple question. The way I understand it, the output resistance of an amplifier works the same way as a resistor in series with the speaker, usually it is seen as desirable to minimize this so as to get closer to the ideal amplifier. This resistance is lowered usually by using additional output transistors. The bad news is that if the amplifier uses feedback (and they almost all do, especially in cars) this resistance is inside the feedback loop. If you add enough resistance inside the feedback the amplifier will change to where it is supplying current instead of voltage.
So you may ask "what does all this mean?" and my answer is I'm not sure. Here is a link to some information which you may find useful http://sound.westhost.com/z-effects.htm
Hope this gets us back on track.
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Old 16th November 2002, 02:33 AM   #9
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ahh, i understand better. this explains what i say predicted in winISD. also, it helped me understand damping factor.
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