Crate powerblock speaker impedance missmatch

[rexindigo]

Hello, I am new to this forum so hi to everybody. After heaving around tube Amps  for years and years my band decided to go for digital modeling and minimal equipment. This works very well in the rehearsal room, but ended as a fiasco (do you have this word in english?) on the last gig. We went for no floor monitoring only sidefills, and they dropped out. So I decided to make my own monitoring for the next gig and got myself  a used crate powerblock wich is 2x 75 watts at 4 ohms. The very small powerfull 5 1/2" i have built into my small floor monitors  are labeled 50 watts rms at 8 ohms. As far as i understand the impedance missmatch will lead to a reduced power, so i hope that in this case the speakers are safe? Please excuse my poor english, any comment on this topic is welcome, greetings rex

[Roly]

Hi and welcome.

>fiasco (do you have this word in english?)

We certainly do;

"A thing that is a complete failure, esp. in a ludicrous or humiliating way.
failure - flop - washout - fizzle"

We had a major fireworks display here in Melbourne, (Aust) a while back.  It started okay, then something went wrong and all the fireworks fired at once - a half-hour display reduced to ten seconds; a very spectacular fiasco - so I'm sorry to hear that.   


Your amp is rated at "2x 75 watts at 4 ohms".  Amplifiers generally, and solid state particularly, have an output limited by voltage.  As the maximum voltage will be the same, so the power in to twice the load should be half.

75/2=37.5 watts

This is less than the rating of your speakers (and assuming that rating is realistic mounted in the cabs they are in) your speakers should still be under-driven and safe.

This applies if the duty is basically PA and under-driven, that is not allowed to clip too much.  If these were for direct instrument reproduction, such as a guitar amp that is often hard driven into clipping/limiting, then you need to be more cautious.  Presented with a speaker that isn't specifically intended guitar direct guitar use, I would assume a rating of 50 watts clean un-clipped, and say 25 watts if being driven by a heavily clipping amp.

HTH

[Big Bruce]

Your Power Block is actually adjustable for either 75 watts X 2 or 150 watts - (I'm taking this information straight from the Owners Manual, which you should be able to easily access on-line).  The ratings read:  75W RMS @ 10% THD per side @ 4 ohms; OR
150W RMS @ 10% THD @ 8 ohms.

You change the power setting via a "Mono Bridge" switch that is on the rear panel of the head.

You're welcome.

[J M Fahey]

The very small powerfull 5 1/2" i have built into my small floor monitors  are labeled 50 watts rms at 8 ohms.

I don't think those are "Guitar watts" and suspect a *high* possibility of blowing them to pieces.

Get 1 or 2 real guitar speakers.
Pullouts from , say, Peavey/Fender/etc. are available for cheap at EBay, Craigslist (say, U$20/25 each) when owners upgrade to a better speaker and have no use for the old one.
In any case use the amp in "Stereo" mode, and connect 1 speaker to each speaker out.

PS: actually telling us where you live may produce a more accurate response.