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Speaker cab impedance

Postby Dr. B » Sun Aug 16, 2009 7:22 pm

Hi,

Someone just gave me a speaker cabinet - 1x15".
I think it's probably from a PA system but I was thinking of using it for my Peavey 100W head at rehearsals, to save me lugging a 4x12 around.
Now, the Peavey has selectable output impedance - 4, 8 & 16 ohm.
The 1x15 had nothing on it, so I measured the impedance - 6 ohm!
So do I set it to 4 or 8?

Thanks.
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Re: Speaker cab impedance

Postby GaryM » Sun Aug 16, 2009 7:53 pm

4.
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Re: Speaker cab impedance

Postby Dr. B » Sun Aug 16, 2009 8:40 pm

Thanks very much!
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Re: Speaker cab impedance

Postby zenguitar » Sun Aug 16, 2009 10:42 pm

Is it a valve or solid state head?

And you have to show some caution when measuring resistance with a multimeter. The meter usually measures the DC resistance but speakers are generally rated on AC resistance. The DC resistance is usually lower than the AC resistance. Most 8 Ohm speakers have a DC resistance between 6.5 and 7.5 Ohms.

Check the frame of the 15" driver, you may find some extra info printed there.

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Re: Speaker cab impedance

Postby Dr. B » Mon Aug 17, 2009 6:13 am

It's a valve head.

Interesting point about the AC impedance.
I'll see if I can break into the cabinet - it looks pretty solidly put together!
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Re: Speaker cab impedance

Postby ef37a » Mon Aug 17, 2009 9:11 am

That's an 8Ohm driver.

Virtually all 8Ohm "non guitar" LF units read about 5.6R DC regardless of size! So set the amp for 8Ohms in fact always set valve amps equal to or higher than the load i.e. a 4R speaker on a 16R tap will do little harm, 'tother way, arcing bottles!

Transistor amps never have impedance taps (well ok, Quad 50E!) but never go below 4Ohms unless the book says you can and some very old amps will pop below 8.

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Re: Speaker cab impedance

Postby Dr. B » Mon Aug 17, 2009 9:34 am

Brilliant.
Thanks for the info.
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Re: Speaker cab impedance

Postby zenguitar » Mon Aug 17, 2009 11:37 pm

Cheers Dave,

While I am very confident with my guitar stuff, there are too many gaps in my amp knowledge for me to give hard advice. And while it's good to know I was in the right ball park, it's very helpful to have you here.

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Re: Speaker cab impedance

Postby ef37a » Tue Aug 18, 2009 6:36 am

Well,thank you Andy, one does ones best!

The questions about speaker/amp impedance crop up with great regularity, especially on forums more "guitar centred" shall we say. Much confusion seems to range but in fact the underlying principles are not that hard to grasp being mostly about Ohms' Law!

One should also be warey of "common knowledge". For instance it is often said that a transistorized power amp will drive an open circuit safely. 99.99% of the time with modern, sophisticated, often IC designs this is true but some older or less well specified circuits (as found in guitar amps!) will in fact burn out the Zobel network on the output stage if the load is lifted under heavy drive.

Most modern audio equipment is pretty bombproof and forgiving and a little forethought and care, when advancing levels for instance, will keep kit (and ears!) undamaged.

Rock on chaps,
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Re: Speaker cab impedance

Postby zenguitar » Tue Aug 18, 2009 11:09 pm

I was doing OK until I got to the bit about burning out the Zobel network... is that related to the cold war Cambridge spies?

LOL

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Re: Speaker cab impedance

Postby ef37a » Wed Aug 19, 2009 5:09 am

Zen,
I could explain it further but then I'd have to kill you....


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Re: Speaker cab impedance

Postby zenguitar » Wed Aug 19, 2009 11:45 pm

Ahh... The Cambridge Audio Conspiracy...

nuff said

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