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Tech: Testing correct speaker/cab Ohms ???

GUEST,punkfolkrocker 28 Feb 10 - 02:44 PM
pdq 28 Feb 10 - 02:55 PM
Bernard 28 Feb 10 - 03:16 PM
Richard Bridge 28 Feb 10 - 06:57 PM
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Subject: Tech: Testing correct speaker/cab Ohms ???
From: GUEST,punkfolkrocker
Date: 28 Feb 10 - 02:44 PM

I need to know how to easily and safely test used guitar cabs and speakers
to ensure they are functioning at their correct Ohm rating.

Please anyone know how or can direct me to online beginner level tutorial ?

Is it possible to just plug some inexpensive test meter device into the cab sockets
and get a quick reliable reading.

eg, if a 4x12 has sockets and switching for 4/8/16 Ohms
then I need to confirm that all selected settings are working correctly.

..same question also for testing the contacts on individual speakers ???


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Subject: RE: Tech: Testing correct speaker/cab Ohms ???
From: pdq
Date: 28 Feb 10 - 02:55 PM

Remember that impedence is not the same as resistance.

Also, all measurements must be done with at least one terminal disconected, as in "out of the circuit".

A common multimeter will give you a resistance reading which is always less than the impedence.

A single woofer, measured out of the circuit, will probably measure about 60% of its impedence.

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Subject: RE: Tech: Testing correct speaker/cab Ohms ???
From: Bernard
Date: 28 Feb 10 - 03:16 PM

It's too complex for measurement without the correct equipment. A multimeter isn't adequate.

You need to find someone who has an impedance bridge which generates a tone such as the Toa ZM-104 (rather old and analogue - a digital equivalent is more accurate), but at around £200.00 it's a tad expensive for one-off use.

I have one, which I use mainly for measuring 100v line circuits.

If the loudspeaker chassis isn't marked up, then you may get a fair approximation using a multimeter by measuring a known loudspeaker and comparing the reading for the unknown one with it... but it's rather hit and miss.

If a cab is 'switchable', it will have an auto transformer doing the 'switching', and it's possible that someone may have driven it too hard, causing the varnish on the windings to break down. I would guess that's the only problem you're likely to find other than an open circuit driver.

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Subject: RE: Tech: Testing correct speaker/cab Ohms ???
From: Richard Bridge
Date: 28 Feb 10 - 06:57 PM

I find a multimeter gets me close enough - a fiver from Maplins. Use on each speaker while the speaker is out of circuit.

I have built a 2 x 12 that used two 16 ohm drivers and could be switched using a DPDT switch (no transformer) between (8 ohm with horn)/(two stereo 16 ohm channels)/(8 ohm without horn).

A 4 x 12 can be simply switched (without transformer) 4 ohms/16 ohms if it uses 16 ohm drivers or if it uses 4 ohm drivers, but you can't do anything useful of that sort with 8 ohm drivers.

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