Speaker/Amp Ohm Match

[marnold]

I was thinking of getting a Jensen MOD to breathe some like into my Peavey Blazer 158 that my son is using. The amp says 15W into 4 ohms. I assume then I'd get a 4 ohm speaker. The odd thing was that the Blue Marvel in there is listed as 3.2 ohms. I didn't think it was a good idea to run an amp with a speaker with a lower ohms rating. Am I right or confused as usual?

 

[tunghaichuan]

I was thinking of getting a Jensen MOD to breathe some like into my Peavey Blazer 158 that my son is using. The amp says 15W into 4 ohms. I assume then I'd get a 4 ohm speaker. The odd thing was that the Blue Marvel in there is listed as 3.2 ohms. I didn't think it was a good idea to run an amp with a speaker with a lower ohms rating. Am I right or confused as usual?

The 3.2 ohm rating is probably the DC resistance of the coil. If you have a digital multimeter, set it to ohms and measure the terminals. It should read about 3.2 ohms. In fact if you measure any 4 ohm speaker, it should measure about 3.2 ohms.

I've seen this on old Champs, the amp has a 4 ohm output, but the speaker is marked 3.2 ohms.

At any rate, 3.2 ohms to 4 ohms is not that much of a mismatch, I wouldn't sweat it. What you have to worry about is running a 2 ohm load on a 4 ohm output.

tung

 

[Tele-Dave]

The 3.2 ohm rating is probably the DC resistance of the coil.

3.2ohm DC resistance is average for a 4ohm speaker. 6.3ohm DC resistance is average for an 8ohm speaker. Notice the word "resistance" which has a different meaning than "Impedance".

Not trying to sound too technical, but:
Impedance describes a measure of opposition to an alternating current.
Factors determining inpedance are voltage, inherant resonance of the moving coil, magnetic flux density, plus the amplitude of the alternating current. All these factors are averaged to determine optimum DC resistance of the voice coil to achieve a stated impedance.

 

[Tele-Dave]

I didn't think it was a good idea to run an amp with a speaker with a lower ohms rating.QUOTE]

Solid state amps are more critical of impedance mismatch. Tube amps "usually" more forgiving. A tube amp designed to run with 8ohm speaker will usually just just start distorting at high power levels when the tubes go into saturation when they can no longer put out more power to drive the lesser impedance. A solid state amp will usually burn out the transistors while trying to output the extra power to drive the speaker at less resistance. The transistor amp sees less resistance which leads it to think it is putting out less power, when it is really putting out more. A tube amp can only put out so much power, and then thats it. It usually results in a little shorter tube life if run at high power for extended lengths