Impedance Question - AVS Forum
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post #1 of 9 Old 04-07-2009, 03:29 PM - Thread Starter
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Hi All,

I have a quick impedance question: I just bought new speakers (PSB Image) and the specs on all five of them state the following:

Impedance:
6 ohms nominal
4 ohms minimum

I'm wondering how to set up my receiver (Onkyo TX-DS797). There are two settings on the unit:
1 - Minimum: 6 ohms
2 - Minimum: 4 ohms
The owner's manual for the receiver says the following:

"If the impedance of all speakers are between 6 and 16 ohms, select '6 ohms.' If the impedance of even one speaker is between 4 and 6 ohms, select '4 ohms.'"

Can anyone provide a bit of guidance?

Thanks a lot,
Ben

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post #2 of 9 Old 04-07-2009, 03:40 PM
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As I understand it, you should pay the most attention to the nominal rating of 6 ohms, and set your receiver accordingly at 6 ohms. All speakers' impedance fluctuates during use and many that are rated at 8 ohms can occasionally dip a couple of ohms or so.

Steve

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post #3 of 9 Old 04-07-2009, 05:05 PM
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Quote:
Originally Posted by fireman325 View Post

All speakers' impedance fluctuates during use and many that are rated at 8 ohms can occasionally dip as low as 2.

Do you have any examples of that? A 8 ohm rated loudspeaker dipping to 2 ohms that is.

Yes they may dip a bit below 8 ohms, but something that goes down to 2 should NOT be rated at 8 ohms

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post #4 of 9 Old 04-07-2009, 05:13 PM
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Quote:
Originally Posted by Ivan Beaver View Post

Do you have any examples of that? A 8 ohm rated loudspeaker dipping to 2 ohms that is.

Yes they may dip a bit below 8 ohms, but something that goes down to 2 should NOT be rated at 8 ohms

Good point. But many 4 ohm speakers have dipped lower. Currently, the "4 ohm" Triad Gold LCR dips to 3.2 ohms at one frequency. In the past, the Apogee Scintilla, rated at either 3 or 4 ohms, presented a toxic 1 ohm load at 20 Hz, and only a few amplifiers could handle it. If a speaker dips to 2 ohms at any frequency, it isn't an 8 ohm speaker.

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post #5 of 9 Old 04-07-2009, 05:36 PM
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Quote:
Originally Posted by Ivan Beaver View Post

Do you have any examples of that? A 8 ohm rated loudspeaker dipping to 2 ohms that is.

Yes they may dip a bit below 8 ohms, but something that goes down to 2 should NOT be rated at 8 ohms

Quote:
Originally Posted by Paul Scarpelli View Post

Good point. But many 4 ohm speakers have dipped lower. Currently, the "4 ohm" Triad Gold LCR dips to 3.2 ohms at one frequency. In the past, the Apogee Scintilla, rated at either 3 or 4 ohms, presented a toxic 1 ohm load at 20 Hz, and only a few amplifiers could handle it. If a speaker dips to 2 ohms at any frequency, it isn't an 8 ohm speaker.

OMG!! What a typo! Thanks for catching that guys. That's not what I meant to type, but I guess it's what I get for trying to do several things at once. Original post corrected. Tanks again!!

Steve

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post #6 of 9 Old 04-07-2009, 07:46 PM
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Ben, this lower setting does not optimize the receiver for handling the speakers with a lower impedance rating, as some imagine. This is basically a safety-related measure intended to limit the possibility of overheating, pursuant to UL requirements. This limits the voltage which is available on the power supply rails. When following Ohm's Law, when voltage goes down, so does current and power. Yes, this will reduce the possibility of overheating, but does it at the expense of maximum performance.

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post #7 of 9 Old 04-07-2009, 08:56 PM
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Quote:
Originally Posted by fireman325 View Post

All speakers' impedance fluctuates during use

I don't think that speaker impedance changes during use. It is what it is. I think what you mean to say is that impedance is a function of frequency and, at some frequencies, it probably dips lower than the quoted "nominal" (single) value. The problem is that there is no standard way to compute the nominal impedence. It's too wishy washy - how can it be that there are no 7 ohm speakers?

Ed
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post #8 of 9 Old 04-07-2009, 09:29 PM
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Quote:
Originally Posted by ekb View Post

I don't think that speaker impedance changes during use. It is what it is. I think what you mean to say is that impedance is a function of frequency and, at some frequencies, it probably dips lower than the quoted "nominal" (single) value. The problem is that there is no standard way to compute the nominal impedence. It's too wishy washy - how can it be that there are no 7 ohm speakers?

Ed

A much better explanation. Thank you.

Steve

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post #9 of 9 Old 04-08-2009, 11:10 AM - Thread Starter
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Hi All,

Looks like I should go with 6 ohms. Thanks a lot for all of the responses and information!!

Ben

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