Confused about impedance and wattage.

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Confused about impedance and wattage.

Post by Shimmer »

I recently bought the Vox MV50 Clean. It is rated at 50W for an 4 ohms load, 25W for 8 ohms and 12.5 W for 16ohms.
I bought it with the little BC108 speaker which is rated a maximum of 25W. So far so good.

But if I wanted to get a second BC108 speaker and run it in parallel so that I am getting the full 50Watts, will that be a problem with that cab? I mean will each speaker now be getting 50Watts ? (and therefore receiving more than it can handle?).

Or is it as simple as each still only gets 25W and 25+25 = 50W?
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Re: Confused about impedance and wattage.

Post by jeremyb »

How many ohms are the 25W speakers?
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Re: Confused about impedance and wattage.

Post by Shimmer »

Ah - forgot to say, they are 8ohm
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Re: Confused about impedance and wattage.

Post by Candeevr4 »

The BC108 is 8 ohm, so to get 50wrms you need to have a box that parallels the signal. They in theory will be getting 25wrms each, but usually speakers can handle more than rms, ie i run 120rms into my fronts in my car which are rated at 50rms. ;)

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Re: Confused about impedance and wattage.

Post by Shimmer »

The BC108 is set up to do that (It has parallel connectors on the back.)
I guess I was worried that if I hook two up like that , that each would be getting more than the 25W they are rated for as the amp is now putting out 50W.
But I guess they still get 25W each?

(Showing my lack of knowledge here)
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Re: Confused about impedance and wattage.

Post by Candeevr4 »

Yes this is correct the power will be split.

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Re: Confused about impedance and wattage.

Post by RectifiedAmps »

Speakers wired in series each get the full wattage coming out of the amp. Speakers in parallel split the wattage between them. As mentioned, speakers are usually rated for RMS, not max output and amps are usually advertised for max output (probably more than is actually produced), not RMS.

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Re: Confused about impedance and wattage.

Post by Shimmer »

Thanks for the replies. Sounds like I'll be on safe ground if I connect them in parallel then.

What I still don't understand is how the Vox mv50 'knows' whether there is a 4ohm or 8 ohm load attached - I mean there is no switch to select between 50w or 25w output. It just does it . Like magic. Or physics... :|
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Re: Confused about impedance and wattage.

Post by Conway »

Shimmer wrote:What I still don't understand is how the Vox mv50 'knows' whether there is a 4ohm or 8 ohm load attached - I mean there is no switch to select between 50w or 25w output. It just does it . Like magic. Or physics... :|
Just a minute... what's that switch next to the speaker output then?!

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Re: Confused about impedance and wattage.

Post by Shimmer »

I've got the Clean and it doesn't have that switch. And from what I read, on the AC/Rock models, the impedance switch is more about matching the "voicing" rather than the output selection
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Re: Confused about impedance and wattage.

Post by Candeevr4 »

The clean doesnt have that switch it has an attenuation switch. The head should detect the impedance, its just like car speakers/subwoofers. ;)

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Re: Confused about impedance and wattage.

Post by Shimmer »

Candeevr4 wrote:The clean doesnt have that switch it has an attenuation switch. The head should detect the impedance, its just like car speakers/subwoofers. ;)
So does the amp always push out the full 50W power when at maximum volume, and then that gets converted to 25w if it detects 8 ohms etc...?
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Re: Confused about impedance and wattage.

Post by Candeevr4 »

Shimmer wrote:
Candeevr4 wrote:The clean doesnt have that switch it has an attenuation switch. The head should detect the impedance, its just like car speakers/subwoofers. ;)
So does the amp always push out the full 50W power when at maximum volume, and then that gets converted to 25w if it detects 8 ohms etc...?
In short yes.

Think of the speaker as a hose, the smaller the hose, the higher the impedance, limiting the amount of power it can produce.

If you lower the impedance the hose gets bigger and allows more flow, therefore more power.

There are other things at play here, box rise etc but we dont need to go there.

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Re: Confused about impedance and wattage.

Post by Shimmer »

Candeevr4 wrote:
Shimmer wrote:
Candeevr4 wrote:The clean doesnt have that switch it has an attenuation switch. The head should detect the impedance, its just like car speakers/subwoofers. ;)
So does the amp always push out the full 50W power when at maximum volume, and then that gets converted to 25w if it detects 8 ohms etc...?
In short yes.

Think of the speaker as a hose, the smaller the hose, the higher the impedance, limiting the amount of power it can produce.

If you lower the impedance the hose gets bigger and allows more flow, therefore more power.

There are other things at play here, box rise etc but we dont need to go there.
Ah - that is much clearer! Thanks.
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Re: Confused about impedance and wattage.

Post by Shimmer »

And I just found this cool site which is helping my understanding a little bit more
https://geoffthegreygeek.com/speaker-im ... ier-power/
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