Gettin a new amp

[rivethead]

i'm looking for a new amp. i want an amp that can be loud and aggressive but yet gentle and firm at times. i want to go with a stack also. i'm looking at the behringer ultrastack bg412s. i really like that cab but behringer dont make heads so i really dont want say a marshall head with a behringer cab. i thought about getting a power amp instead of a head. what do you guys think? should i keep looking for a good head or just go with a power amp? also, is there anything else that anyone might think would be a better fit for me. i'm also on a bit of a price range. i really dont want to go over the $600-700 range. thanks everyone

[rivethead]

also i forgot to ask. do power amps interchange with all cabs or would i need a special one. i doubt that i would but i'm just making sure. i was looking at a qsc rmx850 which is only $300. it sais that it delivers per channel: 200W at 8 ohms, 300W at 4 ohms, and 430W at 2 ohms. how do i determine the ohms in my cab if my cab is 400W. thanks

[Lordathestrings]

Any power amp can drive any cab, as long as the nominal impedance (Ohms) is within an acceptable range. The power handling capability of a cab is something you will see listed on the spec sheet. There is no direct relationship between the impedance and the power a speaker can handle. If your cab is rated to handle 400 Watts, it should be OK with a QSC rmx850 amp, as long as it has an impedance higher than 2 Ohms. Its unusual to find a cab with less than 4 Ohm impedance, so again, its probably OK.

[fuhrysteve]

i would be VERY CAREFUL if you are running any speaker below 4 ohms, you can blow the speaker's voice coil quite easily with a very small amount of distortion or DC current due to the low resistance, you could also, in some cases, cause the amp to overload and overheat too, i did that once with a power amp, and it's in the shop right now getting the output transistors replaced. so BE VERY WARY DUDE!

[rivethead]

i'm not quite sure i understand everything about ohms. if you could explain to me what its all about, that would be great.

[friskynibbles]

it's been years since my physics classes...
ohms are units of resistance...
so if you're only running 4, you could overload those speakers (cook 'em basically)
if i'm right...
-Daniel

[fuhrysteve]

yeah, ohms are units of resistance, if there is little resistance, the sound can easily drive the speaker. with more ohms, it is harder to blow the speaker, as the speaker has more resistance to the signal. if you're running at low ohmage, you therefore have to be careful, because little resistance means some signal might be strong enough to just plow right through your speaker and blow it for you. now doesn't that suck? just be careful.

[Lordathestrings]

This topic got some coverage Here (http://www.guitartricks.com/forum/showthread.php?threadid=3383)

[rivethead]

how can determine how many ohms my speaker has? is it in the specs?

[Lordathestrings]

If it isn't in the specs, you can get a rough approximation by measureing with an ohmmeter. An 8 Ohm cab will show as about 5 Ohms on a meter.

[fuhrysteve]

Originally posted by Lordathestrings
...a rough approximation by measureing with an ohmmeter. An 8 Ohm cab will show as about 5 Ohms on a meter.

if you calibrate your ohm meter, and have a good one, it should show up at 8 ohms though... it's always worked accurately for me. i think it may also depend on wierd stuff like elevation or something too.

[Lordathestrings]

The DC resistance (which is what an Ohmeter measures) is usually lower than the AC impedance. Remember I said the two are not interchangeable. The windings of the speaker voice-coil add inductance to the circuit, which usually makes the impedance higher at audio frequencies.