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Post subject: FM 412 SPEAKER DRIVER IMPEDANCE
Posted: Fri Feb 26, 2010 1:36 pm
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The cabinet is 4 ohms, but I'm wondering if each individual driver is 4 ohms or other, as I'd like to change the impedance to 8 or 16 ohms.


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Posted: Sat Feb 27, 2010 5:59 pm
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If you cannot find the spec. for the cab, and if it is not printed on the back of each speaker, you can isolate one of the speakers (remove the leads) and put an ohm meter across the tabs. This will measure resistance, not impedance (it will not include the reactance due to the coil), but you will be able to tell if it is 4 ohm or 16 oms, as the resistance reading will be slightly less than overall impedance of the speaker. There will be no doubt as to whether it is 4 ohm or 16 ohm.

My guess is that each pair of two 4 ohm speakers are wired in series and then the two 'sub' circuits are wired in parallel. (In other words series parrallel).

Now on to the question of whether or not it would be wise to re-wire the cabinet. (I can't answer that for sure.)

If 4, 4 ohm speakers are then wired in series the result is 16 ohms, but an 'open' failure of any of the 4 gives no sound and infinite impedance.

Current through each speaker doubles in comparison to the series parallel and voltage drop across each is halved. I don't know what that would do to performance??? Nor do I know the risk (if any) it poses to the amp.

Most of the multiple speaker cabs that I am aware of have some parallel circuit in them. (2X12's are usually 2 ea. 8 ohm speakers yielding an overall 4 ohms; or 2 ea. 16 ohm speakers yielding 8 ohms. For 4X12's, series parallel connections of 4 ea. 4 ohms, yielding 4 ohms; or 4 ea. 16 ohms yielding 16 ohms) There must be a design reason for always using a parallel connection.

Perhaps someone with a better understanding of speaker cabinet design will now respond since I threw in my limited understanding (Did it because noone else responded!)

As a matter of interest, why do you want to do this?

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Posted: Sat Feb 27, 2010 7:32 pm
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The specs are four 16ohm speakers in a parallel configuration. Art

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Posted: Sat Feb 27, 2010 11:34 pm
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Thanks Art. That's what I was hoping to hear, as it allows me to wire the cabinet in series/parallel for 16 ohms.

Garry[/quote]


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Posted: Sat Feb 27, 2010 11:46 pm
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RVM lead asked [As a matter of interest, why do you want to do this?]

I appreciate what you wrote, RVM. I guess I could have given a bit more info in my post. I don't own the cabinet, but am considering buying one; hence, I can't make any measurements on the speakers. I just prefer the sound of 16 ohm cabinets when driven from a tube amp, as the low resonant peaks and upper frequency range yield a higher impedance to the head (ie. a little more bottom and top boost).

Thanks again,
Garry


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