Live sound question part two

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rockironwebb

senior newbie caveman
With two outputs per channel on a two channel power amp, that is 200watts per channel at 4 ohms. Is each 1/4in output 200 watts at 4 ohms? Or does it get split up if I am hooking up two speakers on one channel? There is also a single red and black wire terminal.
 
If you hook two 4 ohm speakers to one side in series, that will make the overall impedance 8 ohms and reduce your power to the 120ish watt level.

If you hook the two 4 ohm speakers in parallel, that reduces the impedance to 2 ohms which will try to draw too much power from the amp and probably burn it out--only the better amps can handle a 2 ohm load and, even on the expensive ones, it's often not recommended to operate that way constantly.
 
If an amp is rated at 200w at 4 ohms per channel, then that is what each speaker output will deliver to a 4 ohm speaker.

If you hook up two speakers to one channel, what you get depends on how you hook them up, and what ohms each has.

Do you know the ohm rating of each of your speakers?

Let's say that your speakers are rated at 8 ohms (not uncommon).

If you hook them up in series, the combined rating is 8 + 8 = 16 ohms. This is okay, but you will be getting less power out of your amp (the higher the ohms, the less the power).

If you hook them up in parallel, the combined rating is the inverse of 1/8 + 1/8. 1/8 + 1/8 = 2/ 8 = 1/4. The inverse of 1/4 = 4. So with two 8 ohm speakers hooked up in paralle the amp will see 4 ohms, which it will be very happy with and deliver its 200 watts.

However, if the speakers are rated at 4 ohms each, one speaker will get you the 200 watt rating. Two in series will give 8 ohms; again okay, but will result in less power. Two in parallel will cause the amp to see 2 ohms. This will give more power . . . but unless the amp is very robustly built, you could end up with fried output circuitry or blown fuses.
 
Two identical 8 ohm speakers connected in parallel to one channel of an amp that puts out 200W into 4 ohms will get 100W.
 
My 2-channel MosValve power amp has 2 outputs per channel and you can only use one of them at a time with a 4 ohm load. Connecting a 4 ohm cabinet to both outputs of one channel will produce a 2 ohm load at that output and cause lots of problems. I'm not sure if all amps with 2 outputs per channel are built this way but I'd do some research into that particular amp before trying such a set up.
 
Not sure what the crappy speakers specs are, I am going to replace them. I am trying to get a firm grasp on the whole watt,ohm,series,parallel thing before I buy anything. I've got the amp manual, so I'll stare at the diagrams and figure out how the two outputs on a single channel are wired. I am gonna guess that it is parallel. Thanx for the help. I am the only one in my band that likes to get the facts and figure shit out. Everyone else will just plug anything into anything and keep doing it till it works, or it blows up. Since I own the majority of gear I have had to get Hitler with everyone so they have to consult with me before fucking with my gear.
 
It would be helpful to know the make/model of the amp to be sure but it would be extremely odd if the two outputs on a channel were not parallel.
 
Bobbsy said:
If you hook the two 4 ohm speakers in parallel, that reduces the impedance to 2 ohms which will try to draw too much power from the amp and probably burn it out--only the better amps can handle a 2 ohm load and, even on the expensive ones, it's often not recommended to operate that way constantly.
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If an amp is rated at 2 ohms it should be able to run into 2 ohms with no problem.
My Mackie 808 powered mixer is rated at 2 ohms for instance and it'll run wide open all day into 2 ohms ........ I've done it so I know for sure.
I have a Crown power amp that's rated to 2 ohms and the same thing.
Basically you shouldn't go lower than the amps rated at and plenty of them won't go lower than 4 ohm.
So 4 ohms is a good safe minimum load most of the time if you don't know for sure.
But even that can be iffy. An Orange amp (most of them anyway) isn't rated for less that 8 ohms which seems silly to me but there iut is.
OTOH my old Ampeg tube amps have switchable imedance matching and they all go down to 2 ohms.
 
rockironwebb said:
Since I own the majority of gear I have had to get Hitler with everyone so they have to consult with me before fucking with my gear.
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:laughings:

Not disputing Lt. Bob's experience (he was there, I was not!) but I avoid running amps at 2 ohms, even ones the manufacturer says can handle that low a load. I was told by someone who should know that you are just "too close for comfort."

What happens when the impedence load is too low, is the amp gets rather hot and components burn out.

Multiple outputs on a single channel are almost always parallel outs.
 
it is an old Peavey cs400. There is another thread on the newbie forum with a photo, I just posted it a couple of days ago.
 
For those asking, it's a Peavey CS400 from the picture he posted. I found the manual (and posted a link)--it's rated down to 4 ohms only.

Regarding the issue of using amps rated at 2 ohms actually AT 2 ohms, I agree you should be able to. However, next time you do, feel the amp and see how hot it gets (I've tried this with offerings from Crown, Lab Gruppen and a few others.) The all get VERY hot when pushed at 2 ohms. I have to think this must affect the service life even if it doesn't cause an immediate failure, and there is a lot of anecdotal evidence to back this up.

Perhaps it's okay to work that way, particularly if you're not driving things hard--but I like the analogy of a car engine. Just because it says in the manual that it red lines at 6000 rpm, doesn't mean it's good for the engine to drive on the freeway in 2nd gear at 6000 rpm for any distance....
 
Bobbsy said:
For those asking, it's a Peavey CS400 from the picture he posted. I found the manual (and posted a link)--it's rated down to 4 ohms only.

Regarding the issue of using amps rated at 2 ohms actually AT 2 ohms, I agree you should be able to. However, next time you do, feel the amp and see how hot it gets (I've tried this with offerings from Crown, Lab Gruppen and a few others.) The all get VERY hot when pushed at 2 ohms. I have to think this must affect the service life even if it doesn't cause an immediate failure, and there is a lot of anecdotal evidence to back this up.

Perhaps it's okay to work that way, particularly if you're not driving things hard--but I like the analogy of a car engine. Just because it says in the manual that it red lines at 6000 rpm, doesn't mean it's good for the engine to drive on the freeway in 2nd gear at 6000 rpm for any distance....
Click to expand...
well, I'm not arguing per se but here's my perspective: ..... certainly you're running the amp at its' limit but if it's rated at it it should be able to do it and I've done this 5-7 nights a week for 40 years and have never had a problem ..... have never blown up an amp. Not ever ...... not once.

Now to be sure ..... I have enough brains to not run the amps where the meter's pegged constantly. I will always turn an amp down if it's pegged ..... you're not getting anything extra at that point ...... just more distortion and compression. So I don't mind the meter bumping up against its' limits but I never allow it to just be pegged there. A newbie might not recognize that.

Something else ..... I ALWAYS have a good sized fan on all my gear ..... regardless of whether it has its' own little dinky internal fan or not.
My Mesa Mark V has built in fan but if you ever come up at any of my gigs and look behind it you'll see a little 10" fan blowing right across the tubes and trannies.
Same thing with my Mackie and same thing with my power amps.
I ALWAYS have enough fans to keep them cool to the touch.
It's a bit of an extra PIA to carry around 3 or 4 fans but I feel it's worth it.
 
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