The Multimeter Cannot Lie.....right?

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masque

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OK got a pretty weird occurrence. Here is the long version of this story. Please read on though as it is interesting and I could use some opinions. I have a rectifier 4 x 12 loaded with four 30 watt V30's rated at 8ohms.

The back plate on the cab says 8ohm stereo and 4 ohm mono. I've owned this cab for over 20 years.....I bought it new. Never had any reason to question any of that.

Recently I've started looking into one of the KHE amp/cab switching units and I want to try to get all my cabs working at 8ohms so I won't have any mismatches when switching on the fly. So naturally, before just delving in and rewiring the cab, I ran a speaker cable out of the mono output and put a multimeter set to read OHMS on it......

to my huge surprise it read 8ohms. it was jumping between 7.4-8.5. "that can't be right" I said to myself. I double checked.....same thing. Then I moved over to the 8ohm stereo inputs and became even more baffled......they each read between 3.5-3.9 ohms.

So, regardless of what the backplate says, I have to trust the multi-meter........but I will admit that I am a bit stunned that plate is wrong in comparison to how it is actually wired. I understand that mishaps can happen in the factory, and I love my Boogie stuff, but have any of you experienced anything like this with a brand new cab? Thankfully, I have always been running the right kind of mismatch to avoid damaging the amp.
 

Eevil

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Hello.

Multimeter doesn’t lie about the resistance it is measuring. The impedance is not the same as resistance (google - my English is not good enough to explain the difference)

But the readings you’re getting are normal for 1x 8ohm mono - 2x 4ohm cab :mrgreen:
 

mark1406

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The are a few factors to consider where using a multimeter. What brand of meter is it and how accurate is it. Did you make a good contact with the meter leads?

The other factor which doesn’t effect guitar amps is the frequency range of the meter and the input resistance of the meter. With modern meters this isn’t a factor, but if you are using a very old meter like an AVO 8 or a Simpsons meter, then you have to consider these things.

Regards

Mark
 

bandit2013

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Look at the specifications for the speaker. Celestion V30s have a DC measured resistance of 7.3ohms. This is at 0Hz. Impedance is a measure of the resistance + the reactance of the voice coil. If you measure with a frequency driven meter that value should be much closer to the 8 ohm impedance.
 

G-Point

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Your DC measurements are rather real, and it means the backplate is wrong.
In most cases, the loudspeaker's DC resistance is some 10...30% lower than the average impedance. DC resistance is rather constant, but impedance depends generally on frequency, and on a number of different other factors (enclosure, resonances, room, etc.).
V30 is noted 7.3 ohms DC resistance, but approximate orienting impedance is noted as ~8 ohms. It does not mean the real impedance is exactly 8 ohms at any frequency, it is just orienting an average number taken from different mid-frequencies.
Two V30 connected parallel (2x12) provide ~4 ohms impedance for each stereo pair/side, but both such stereo pairs connected in series for mono (2x 2x12) provide ~8 ohms impedance. Or both ~4 ohms stereo pairs connected parallel in mono (all 4 loudspeakers parallel) provide ~2 ohms.
An alternative is 2x12 pairs first connect in series for ~16 ohms impedance on each stereo side. And then both ~16 ohms pairs connected parallel in mono provide ~8 ohms impedance again, or connected in series (all 4 loudspeakers in series) provide ~32 ohms impedance.
 

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