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measure ohms?

Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=82108
Printed Date: May 09, 2024 at 3:40 PM


Topic: measure ohms?

Posted By: yohinan
Subject: measure ohms?
Date Posted: August 27, 2006 at 1:54 AM

1.) Where is the correct place to measure ohms with a meter? Or no matter where you measure it should all equal the same i.e. any V/C or wired connection on the amp?

2.) When an amp is bridged the amp sees this as the ohm load being cut in half. For example say your speaker/speakers are wired to be 4 ohms in series or parallel. Now when you bridge it is this considered to be running 2 ohms or it's still considered 4 ohms but the amp sees a two ohm load?

I think thats all for now and thanks again. I cant seem to figure these out in the reading I have been doing.




Replies:

Posted By: Flakman
Date Posted: August 27, 2006 at 2:11 AM

With question 1, there are others that would better answer. I believe that because it is impedence and not resistance, an ohm meter cannot measure correctly on a speaker.

Question 2: The amp will see the total load of the speakers that are connected. If the speakers are connected in a way that presents a 4 ohm load, that is what the amp will see regardless of bridging. Bridging is not a function of the speakers, but the mechanics of combining both channels of a stereo amplifier to create one single channel. Basically taking the positive of one channel and the negative of the other to create one output. Most amps can only handle a load of 4 ohms when bridged.



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The Flakman
I feel strange. I have deja vu and amnesia at the same time.

John | Manteca, CA




Posted By: Steven Kephart
Date Posted: August 27, 2006 at 1:55 PM

Flakman is correct that you cannot easily measure the impedance presented to the amplifier, just the DC resistance.  To understand why, you must know what impedance, and from that nominal impedance is.  Impedance is the total resistance produced from resistance, inductance, and capacitance in an AC circuit and changes with frequency.  Nominal impedance is a rating that suggests the lowest point this impedance will go, and is usually rounded off.  Here's a link to an impedance curve for an Adire Audio Extremis 6.8: https://www.adireaudio.com/Home/Images/ExtremisZ.gif  As you can see, the impedance changes quite a bit with frequency, with a large peak at the resonant frequency of the driver (fs).  Now since DC resistance is a part of the impedance, it will always be lower than the nominal impedance of the driver.  So measuring it can get you an idea of what the actual nominal impedance is.  So if you measure .8 ohms, then most likely the nominal impedance will be around 1 ohm.  A 3.2 ohm measurement will probably be a 3 ohm load.





Posted By: stevdart
Date Posted: August 27, 2006 at 4:26 PM
"...A 3.2 ohm measurement will probably be a 4 ohm load."  Steven didn't catch that typo.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.




Posted By: Steven Kephart
Date Posted: August 27, 2006 at 8:40 PM
Good catch Steve.  Thanks!




Posted By: yohinan
Date Posted: August 28, 2006 at 10:59 AM
Ok so I am a still a little confused. So if I have a speaker @ say 8 ohms or even 4 ohms and it's running bridge that the amp, is that considered 4 ohms and 2 ohms respectively? Since the amp sees a load of half when bridged. Or is this just a known and it in reality does not change the ohms? Just trying to make a final decision in an amp selection and figure out exactly how to calculate ohms when running an amp in bridged mode.




Posted By: Flakman
Date Posted: August 28, 2006 at 11:42 AM

No. If you have 1 sub @ 8 ohms, that is what the amp sees regardless of bridging or not bridging. Bridging has nothing to do with the impedence of the speaker.

If you already have subwoofers, let us know what you have and how many. We can then offer ways to hook them up and the amp to drive them.



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The Flakman
I feel strange. I have deja vu and amnesia at the same time.

John | Manteca, CA




Posted By: kfr01
Date Posted: August 28, 2006 at 1:33 PM
Yohinan:

Flakman is right. I think the twist that might be confusing you is this:

Many amps are 2-ohm stable in stereo mode, but only 4-ohm stable in bridged mode.

The amp isn't "seeing" the speaker load differently, but bridging the amp causes the amplifier to draw as much current as in 2-ohm stereo mode. Too much current destroys amplifiers. Thus, while most amplifiers are 2-ohm stable when run in stereo mode, they are only 4-ohm stable in bridged mode.

My advice:

If you want to run one sub, don't mess around with bridging, buy a 2-ohm stable class mono amplifier and be done with it.

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New Project: 2003 Pathfinder




Posted By: aznboi3644
Date Posted: August 28, 2006 at 3:45 PM
kfr01....class mono amp?? or class D???




Posted By: kfr01
Date Posted: August 28, 2006 at 3:49 PM
Sorry, I was typing too fast, it seems. Yes, I meant to say "class D mono amp."

Thanks for the catch, aznboi.

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New Project: 2003 Pathfinder




Posted By: INSTALLER_MSS
Date Posted: August 30, 2006 at 1:24 AM
if a speaker is 4 ohm, then the amp will see 4 ohms whether its bridged or not.  if it's 8 ohms, then it'll see 8 ohms, bridged or not

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"If a man made it, another can modify it...it just takes some thinking."
"If you ask questions, you're a fool for 5 minutes; if you don't, you're a fool for a lifetime."




Posted By: haemphyst
Date Posted: August 30, 2006 at 8:15 AM
From an earlier post of mine...

I'll save you having to read the whole thread (but you can if you'd like posted_image ):

I hear a lot of people saying that in bridged configuration, the amp channels are "sharing the load". Wouldn't sharing the load make the load lighter for each channel, in effect making the amp "see" a higher impedance? Answer: Yes. That's what "sharing the load" is. If you have one truck, producing 100 horsepower, towing a load that takes EXACTLY 100 horsepower to keep it moving, and you add an additional IDENTICAL truck to SHARE that load, the trucks each only have to produce 50HP, right? Do half the work, each, right? Answer: Again, yes. THAT is "sharing the load". An amplifier doing half the work would be equivalent to loading it with half the load, or twice the impedance, i.e. 8 ohms, vs. 4 ohms.

What actually happens when bridging an amplifier, is you are now putting TWICE THE VOLTAGE across the load, which will cause the load to pull more current, twice the current, to be exact. This makes the amp produce 4 times the power of a stereo load. The additional CURRENT demand (2X the stereo current demand) is why the amplifier "sees" twice the load, i.e. half the stereo impedance.


The load does not change, people are exactly correct in saying this. If you have (2) 4 ohm voice coils in parallel, that is a 2 ohm load - period. The amp is still loaded with 2 ohms whatever the configuration. ELECTRICALLY, due to the reasons described above, it "sees" a lower impedance across the terminals.

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."




Posted By: stevdart
Date Posted: August 30, 2006 at 7:33 PM

"Splitting" or "sharing" the impedance load across two channels is a paradox due to our natural inclination to place a higher weight, or value, to a higher number.  Just like the paradox involved with wire gauges, where 0 is substantially heftier than 2 (you just wouldn't think it if you didn't know), the number of ohms of impedance causes the same reverse-logic way of thinking.  The result comes out right when you think in terms of splitting 8 ohms across two channels to give each one 4 ohms, but that result actually comes out that way because of a much more complicated process (explained by haemphyst).

Split 8 into two...if you speak of that using just numbers and leaving out the "load" part, it is correct.  It becomes just a simple math division, and the answer is 4.  And it is the easiest way to explain it to someone trying to grasp all these elemental concepts in order to get their subs pounding.

Is it important for ME to understand that the speaker load is not actually split between two channels?  Yes, it is.  Is it important for professionals in the industry to know that?  Yes, it is.  But how that information is relayed to a relative novice is another thing.  The term KISS comes to mind.  posted_image



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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.





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