Amp watts and speaker ohms?

David Doggett · Posted 6 Nov 2003 11:03 am

Is this right? If you have a 100 watt amp into 2 ohms for four 8 ohm 10" speakers, then it would put out 50 watts into 4 ohms for two 8 ohm 12" speakers, and would put out 25 watts into 8 ohms for a single 8 ohm 15" speaker.

And would the former combinations sound a lot louder and have more headroom than the last combination?

If you run the amp from its two speaker jacks into two 8 ohm speaker cabinets, is that also 4 ohms and twice the output compared to going from a single jack into a single 8 ohm cabinet?

If a speaker comes in 8 ohm and 4 ohm versions, does the 4 ohm get more output from the amp and sound louder?

If you have an ohm selection switch on the back of the amp, what happens if you select 4 ohms for an 8 ohm speaker cabinet, or 8 ohms for a 4 ohm cabinet? Does this change the loudness?

Thanks for any advice.

[This message was edited by David Doggett on 06 November 2003 at 11:07 AM.]

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

Depends.

If you are talking solid state, then yes, the logic is correct (a power amp rated at 100watts into 2 ohms, would be 50 at 4 ohms, 25 at 8 ohms etc)...

Tube amps, however, are different in that the output transformer must be matched to the speaker load... in other words, if the amp has a setting for 4 or 8 ohms, and you have it set for 4 ohms, you better have a 4 ohm load, and vice versa.. ANY mismatch when dealing with tube amps will cuase less power, faster tube failure, and sometimes catastrophic events (SOMETIMES, THAT IS.. some transformers are overbuilt and cn take a little mismatch... however when erring with tube amps, its' always safter to run a lower impedance speaker than what the amp is set for -- in other words if you have yoru amp set for 8 ohms, a 4 ohm load would be safter than a 16 ohm load)..

David Doggett · Posted 6 Nov 2003 12:25 pm

Gino, I thought I had heard the opposite - that it would be okay to run an 8 ohm speaker with a 4 ohm amp, but not good to run a 2 ohm speaker with that amp. As for my other questions, does the perceived loudness and headroom correspond to the amp wattage output through the selected speaker combination?

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

Again, with solid state it's safter to run an 8 ohm load on an amp designed with 4 ohms

solid state by nature HATES a short circuit condition (0 ohms), and loves higher impedances (many ohms)... in other words, a shorted speaker cable will fry a SS amp, while forgetting to plug in the speaker cable will not harm the amp

Tube amps by nature love impedances approching a short, and hate open circuit conditions. If you forget to plug in yoru speaker cable on a tube amp, you can kiss your tubes and or output transformer goodbye... but if your speaker cable shorts, chances are things will be fien (fender amps were designed that if you forgot to plug in yoru speaker, it would short to save the output transformer)..

With a solid state amp, the lower the impedance load you give the amp, the louder it will be. With a tube amp, ANYTHING other than the prescribed impedance load will be less volume.

The more speakers(or speaker area) will mean more volume all else being equal (in otherwords, if a 10" speaker with a 95db sensitivity was compared to a 15" speaker with 95db sensitivity, the 15 ohm speaker will be louder)... 4 10" speakers will be louder than a single 15", due to the increased volume of air they can displace just by the cone moving say 1/2"
an example (r = radius of the speaker or 1/2 of the diameter, pi = 3.14) the displacement would be pi x r-squared x the distance the cone travels
four 10" speakers = 4 x 3.14 x 5 x 5 x .5 = 157 cubic inches displacement if the cone moves 1/2"
two 12" speakers = 2 x 3.14 x 6 x 6 x .5 = 113.04 cubic inches if the cone travesl 1/2" and
one 15" speaker = 1 x 3.14 x 7.5 x 7.5 x .5 = 88.3125 cubic inches displacement if the cone travels 1/2 inch (2 15" speakers would give 176 c.i. and so on)...

so in other words, the 4 10" speakers would be louder, in general and would give the 'appearance' of more headroom, since the amp would have to be driven roughly half as loud to get the same sound pressure levels... however for pedal steel, I would consider 4 10" speakers for E9 or lap steel, but not for C6... the 12" speakers would be better, however, the 15" speaker does tend to sound the best all around for pedal steel by most opinions.

*NOTE* The math done is hypothetical, since the figures would assume a completely FLAT speaker, not taking into consideration the depth of the basket, and actual displacement of the cone etc, so the numbers MAY not be as dramatic if you would use actual examples and did all the complex math involved...but for this discussion, this is 'good enough'

[This message was edited by Gino Iorfida on 06 November 2003 at 01:01 PM.]

C Dixon · From: Duluth, GA USA

Gino,

Very respectfully I must disagree on tube amps. The load should always be equal to or HIGHER than the source.

You are correct on SS amps. In fact the ONLY limiting factor on SS amps is the max current carrying capability of the output transistors. This is why the industry standard is; don't go less than 2 ohms.

Reason is very simple. 2 ohms is fast approaching a dead short and from this point on down, the current flowing in the output transistors rises exponentially into catastrophic amounts thus blowing them in short order.

carl

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

The reason I stand by my statement on tube amps is the following (of course the perfect load is always the ideal condition)...

1) the transformer doesnt have an impedance of it s own but actually reflects the load of the amp (speakers) to the tubes. Too high of a load and you are approaching an open circuit condition.. if too high, the transformer can and will store the voltages, and a flyback effect will happen, 9 out of 10 times when this happens, the transformer is toast. In a short condition, the transformer will not be able to flyback, however, the tubes can in essense melt down...

in other words, neither condition is ideal, but from a repair point of view, a set of tubes is cheaper than a set of tubes + an output transformer...

a bit of trivia, though, MOST of the bigger fender amps (Twin, deluxe,-- pretty much all except the princeton, and vibroluxe which have wimpy transformers, and the Super which is already designed for a 2 ohm load) can handle a 100% mismatch... in other words, you can safely run a 4, 8 or 16 ohm load on that blackface bassman head that is set for 8 ohms... however in any case, the output will be reduced if you use ANYTHING other than the 8 ohm recommended load....

in other words, just use the right load and be done with it

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