Make an 8ohm speaker into 16ohms with an 8ohm resistor in series?

[Gravity]

So could I do this and still have the amp and driver function properly/safely?

 

I ask because, if I build the single 15" guitar speaker cabinet, I'd like to be able to use it with a few different amps.

 

One amp is a Mesa Boogie Subway Rocket combo. It has a tap for a 4ohm and 8ohm load. I would run the combo speaker (8 ohm) and the additional 8ohm 15" ... no problem.

 

The other combo is a Crate VFX5112. It has an impedance switch for either 16 or 8ohms. The speaker in this combo is a 16ohm.

 

Would I be OK just running the mismatched load (8 and 16 in parallel) at 5.33 ohms with the output tranny's impedance switched to 8ohm?

 

Or, would it be possible to wire in a switchable 8 ohm resistor, in series, with the 8 ohm 15" speaker?

 

I'm assuming it would have to be a very beefy resistor.

 

 

 

But .... what do you think?

[psalt_phallow]

the resistor will work but it will be very inefficient I'd just run with the impedance mismatch and not go full blast on the volume

[Pilot]

Or as I mentioned in a recent post, use a speaker auto-transformer to match the impedance. That way you won't lose any power.

 

Bryan

[Gravity]

Can you give me a link to a prefered autotransformer?

 

 

How do you wire them up ?

 

 

Thanks.

[Pilot]

Dunno any links but you could use one of transformers used in PA distribution or make one yourself. Get an old power transformer of about 50 watts, remove the laminations and rip out the windings. Fill the bobbin with 18 gauge wire and replace the laminations. Schematic attached. You can use it either way - 8 to 16 or 16 to 8.

 

Bryan

[Web Surf]

Beefy resistor in series : Will have to be atleast 1/4th the power rating of the speaker

 

will affect damping ratio ie bass will sound uncontrolled and not "crisp n tight"

 

 

 

Speaker transformer : will also be beefy

May eat up low frequencies

 

 

If you series up the internal 16 Ohm and the external 8 Ohm to get 24 Ohms and drive that at the 16 Ohm setting, there is no way you can blast the speakers or the Amp. Maybe this is a safe option. Ofcourse you will output less total power.

[Pilot]

Should have mentioned that the tap in the winding shouldn't be at the centre but 29% from the top for 8 ohms. A centre tap will give you 4 ohms. This is because the impedance ratio is equal to the square root of the turns ratio. You shouldn't have any problems at low frequencies if the core is big enough, and high frquencies should be OK because you're working at low impedance and there won't be a lot of capacitance and leakage inductance .

 

Bryan