Questions for people who know power amps and lots about Ohms.

[Diatenshi]

I need to power my 400 watt 16 ohm mono cab you see.

The questions on power amps, has any one EVER used one from ebay? I see alot of decent looking ones for around 250-300$ which is unfortunatly what my budget is allowing.

The questions for the Ohms masters are as followed. If you have a 16 ohm cab that handles 400 watts. You want to run about 300 16 ohm wats to the cab so as not to over load it, correct? OR what I have heard is, since alot of amps only run 8 ohms, if you run double the power through 16 ohms, it will work well also? So instead of 300 watts of 16 ohms, try 600 watts of 8 ohms?

Would that work well? I have got alot of different answers, but none actually pertaining to the 8 vs 16 ohm question. If anyone can please help, I would appreciate it, I cant seem to find a 16 ohm power amp.

I know The Dark Wolf said his 1400watt behringer power amp works well, but thats 1400 watts of 4 ohm. Not even close to 16. Since its such high power though, does it even out? like 1400/4 = 350 watts in to a 16 ohm?

Stupid ohms.

Thanks and sorry in advance for the bother : P
Dan

 

[noodles]

Welcome to the "gotcha" world of amplification.

Amps are generally rated in RMS*, which is the average amount of power they put out. Peak spikes can be up to triple this number. Speakers, on the other hand, are generally rated at peak power. This is why the Marshall halfstack you see at the store consists of a 100w head and a 300w cab.

So, if you 300w power amp, you really don't want to run it into a single 400w cab. You can risk blowing the speakers. However, the good news is you'll probably never hit a 900w spike, because that's only going to happen if you're running the amp at full volume.

Here is a good rule of thumb to remember regarding impedance:

-If the impedance of the amp is lower than the impedance of the load (what you're plugging it into), then you are safe. This is an inefficient way of running the amp (it will never achieve maximum power), but it poses no threat of damage to the amp. So, a 100w 8ohm amp, run into a 16ohm load, is more than likely going to put out a very safe, inefficient 50w.

-If the impedance of the amp is higher than the impedance of the load, you risk destroying the amp. You are causing the amp to work much harder than it is designed to do. So, if you plug that same 100w 8ohm amp into a 4ohm load, you will more than likely be trying to place a 200w demand on your amp. I don't think I have to explain what this will do to your amp's output transformer.

The only other thing you need to know about is running multiple cabs. Check the documentation for your amplifier, because it should tell you if the multiple speaker outs (most mono amps have two, most stereo amps have two per side for a total of four) are in series or parallel. If this is not listed, contact the manufacturer. If they are in series, you add the impedances of all your cabs together. If they are in parallel, you average the impedances of all your cabs.

Hope this helps. It really isn't as complicated as it seems, but it may help if you describe your specific application, so we can better help you out.

*Root Mean Sqaure. If you're not a math buff, don't worry about it.

 

[Popsyche]

Welcome to the "gotcha" world of amplification.

Amps are generally rated in RMS*, which is the average amount of power they put out. Peak spikes can be up to triple this number. Speakers, on the other hand, are generally rated at peak power. This is why the Marshall halfstack you see at the store consists of a 100w head and a 300w cab.

So, if you 300w power amp, you really don't want to run it into a single 400w cab. You can risk blowing the speakers. However, the good news is you'll probably never hit a 900w spike, because that's only going to happen if you're running the amp at full volume.

Here is a good rule of thumb to remember regarding impedance:

-If the impedance of the amp is lower than the impedance of the load (what you're plugging it into), then you are safe. This is an inefficient way of running the amp (it will never achieve maximum power), but it poses no threat of damage to the amp. So, a 100w 8ohm amp, run into a 16ohm load, is more than likely going to put out a very safe, inefficient 50w.

-If the impedance of the amp is higher than the impedance of the load, you risk destroying the amp. You are causing the amp to work much harder than it is designed to do. So, if you plug that same 100w 8ohm amp into a 4ohm load, you will more than likely be trying to place a 200w demand on your amp. I don't think I have to explain what this will do to your amp's output transformer.

The only other thing you need to know about is running multiple cabs. Check the documentation for your amplifier, because it should tell you if the multiple speaker outs (most mono amps have two, most stereo amps have two per side for a total of four) are in series or parallel. If this is not listed, contact the manufacturer. If they are in series, you add the impedances of all your cabs together. If they are in parallel, you average the impedances of all your cabs.

Hope this helps. It really isn't as complicated as it seems, but it may help if you describe your specific application, so we can better help you out.

*Root Mean Sqaure. If you're not a math buff, don't worry about it.

A little edit there!

Also, as Dave so eloquently put it, this applies to any amplifier with an output transformer, which nowadays is a tube amp. I haven't seen a solid state amp with an output transformer used anywhere but paging systems. ALmost any solid state amp will run a 16 ohm load.

You can run with a 50 watt amp if it is loud enough for you. You really don't want to be in a room with a guitar amp putting out a continuous 300 watts! That would be very painful!

 

[Diatenshi]

Well, I had an amp that did 8 ohm stereo at 75 watts per channel, and it had almost no balls at all. since its a 400 watt 16 ohm cab, which many have verified that it can shake the shit off your walls with low end chunk, what power at what ohm should I run, since most power amps dont do 16 ohms?

I want it loud yes, but I want it to be ballsy as hell. I want my 7th string to make a statement you can feel

One more thing - !!

Since the cab has a switch to do either 8ohm stereo or 16 ohm mono, for low end metal I would want mono right? Because in stereo if 2 speakers play one thing and 2 play another, that messes up the air flow, kinda like having one subwoofer connected to neg and pos, and the other connected to pos and neg. It messes up the flow in the cab if they both play something different.

Thats why I got a straight cab instead of a slant, so the internal air flow compresses and decompresses all at the same time with each speaker, so you get max "hit" with each speaker pulse?



------------------------------Another edit
Have been doing alot of reading on alot of power amp sites which say you want a power amp thats capable of double your speakers RMS, because you need an amp with alot of headroom so you dont under power the speakers.

Grr....I hate speakers haha. I just want it to hit like a mofo when I want it to.

 

[hirah]

a tube amp from 50 to 100 watts is more than loud enough for just about any situation.
solid state i would go for between 400-600 watts. but why?
i use a mesa poweramp at 15 watts into a 5150 cab . i play on 1.5 - 2 and it's loud as hell.

 

[Popsyche]

------------------------------Another edit
Have been doing alot of reading on alot of power amp sites which say you want a power amp thats capable of double your speakers RMS, because you need an amp with alot of headroom so you dont under power the speakers.

Grr....I hate speakers haha. I just want it to hit like a mofo when I want it to.

That is for PA speakers that are not reproducing a distorted guitar. Guitar speaker cabinets are a different animal. WHat amp are you using?

 

[Diatenshi]

Actually, it is for a guitar cab. I play a 7 string. And the 150 watt amp I have seems to have no bass response. Thats the problem I am having. I dont want it just loud. Many who own the same cab agree that it can shake the paint off your walls with low end.

All I am looking for is how to make this damn thing heavy. >_< It shouldent be as hard as it is becoming : \ Maybe I just need a bass amp.