Dr. B
Joined: 27/03/05
Posts: 59
Loc: Huddersfield/Sheffield
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Speaker cab impedance
#762898 - 16/08/09 07:22 PM
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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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GaryM
Joined: 06/11/08
Posts: 618
Loc: Dundee, UK
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Re: Speaker cab impedance
[Re: Dr. B]
#762902 - 16/08/09 07:53 PM
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4.
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Dr. B
Joined: 27/03/05
Posts: 59
Loc: Huddersfield/Sheffield
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Re: Speaker cab impedance
[Re: GaryM]
#762914 - 16/08/09 08:40 PM
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Thanks very much!
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zenguitar
active member
Joined: 05/12/02
Posts: 7621
Loc: Devon
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Re: Speaker cab impedance
[Re: Dr. B]
#762934 - 16/08/09 10:42 PM
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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. Andy 
-------------------- When the going gets weird, the Weird turn Pro.
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Dr. B
Joined: 27/03/05
Posts: 59
Loc: Huddersfield/Sheffield
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Re: Speaker cab impedance
[Re: zenguitar]
#762961 - 17/08/09 06:13 AM
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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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ef37a
Joined: 29/05/06
Posts: 5629
Loc: northampton uk
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Re: Speaker cab impedance
[Re: Dr. B]
#762998 - 17/08/09 09:11 AM
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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.
Dave.
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Dr. B
Joined: 27/03/05
Posts: 59
Loc: Huddersfield/Sheffield
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Re: Speaker cab impedance
[Re: ef37a]
#763007 - 17/08/09 09:34 AM
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Brilliant.
Thanks for the info.
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zenguitar
active member
Joined: 05/12/02
Posts: 7621
Loc: Devon
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Re: Speaker cab impedance
[Re: Dr. B]
#763217 - 17/08/09 11:37 PM
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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. Andy
-------------------- When the going gets weird, the Weird turn Pro.
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ef37a
Joined: 29/05/06
Posts: 5629
Loc: northampton uk
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Re: Speaker cab impedance
[Re: zenguitar]
#763241 - 18/08/09 06:36 AM
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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,
Dave.
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zenguitar
active member
Joined: 05/12/02
Posts: 7621
Loc: Devon
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Re: Speaker cab impedance
[Re: ef37a]
#763589 - 18/08/09 11:09 PM
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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 Andy
-------------------- When the going gets weird, the Weird turn Pro.
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ef37a
Joined: 29/05/06
Posts: 5629
Loc: northampton uk
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Re: Speaker cab impedance
[Re: zenguitar]
#763618 - 19/08/09 05:09 AM
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Zen,
I could explain it further but then I'd have to kill you....
Dave.
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zenguitar
active member
Joined: 05/12/02
Posts: 7621
Loc: Devon
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Re: Speaker cab impedance
[Re: Dr. B]
#763904 - 19/08/09 11:45 PM
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Ahh... The Cambridge Audio Conspiracy... nuff said Andy
-------------------- When the going gets weird, the Weird turn Pro.
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