Ohm and impedance calculations question

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distortedrumble

distortedrumble

all up in yo grill!
i slept right through this part in class or something....if an amp is 100 watts at 4 ohms and the cabinet is 8 ohms. thats a 20% loss of power leaving the cabinet producing 80 watts. now if the cabinet is 16 ohms then that would leave it producing 40 watts.

right?

the idea is a 20% loss of power per every 4 ohms greater than the amp's ohms

someone let me know if I'm correct here please
 
This entire impedance/resistance thing drives me nuts. On PA amps it's common to see something like:

560W per channel @ 2 ohms, 450W @ 4 ohms; or 275W @ 8 ohms and 1,100W @ 4 ohms bridged

Stereo Mode: 4 Ohms: 500 Watts RMS x 2
Stereo Mode: 8 Ohms: 300 Watts RMS x 2
Bridge Mode-Mono: 8 Ohms: 1000 Watts RMS

Stereo: 280W @ 8 ohms, 450W @ 4 ohms, and 700W @ 2 ohms. Bridge mono: 900W @ 8 ohms, 1,400W @ 4 ohms.

I just went to MF and randomly copied some specs from the PA amps... When I went to the guitar amps, the few I looked at did not have similar information. You might try to seem if the vendor's site has that information.
 
My understanding (and this won't be quite right) is that the wattage output of an amplifier is the output transformer's max voltage squared divided by the speaker impedance.

This would imply that an amp would produce twice the wattage into 4ohms than it does into 8, and the reason it doesn't is due to speaker inefficiencies.
 
lol i've been to the manufacturers websites...i did that before even asking but to no avail. i think what i need to do is find a scientific formula to calculate that I think what I have is right though
 
Basic Electronics:

Power (watts) = Voltage * Current (amps)

Current (amps) = Voltage * Resistance (ohms)

So what does that mean - If you double the resistance (ohms), you will cut the power in half.

If you have a 100 watt light bulb and a 50, the 50 will have double the resistance of the 100.

Now, what does this have to do with amps, well not a whole lot UNLESS you use the same poles for the 2,4, and 8 ohm and also don't change any switches on the amp. It is NOT recomended to do that though.

So now to the real world, guitar amps and PAs. Each amp will be set up differently. They will normally have different plugs or poles for each of the different speakers. If there is only one set of poles or plug, they will provide a switch for you to adapt different cabs. The basic reason for this is that if you cut the resistance in half, you will double the voltage (or current) and the tube or driving transistor may not be set up for that. Another words, you will probably smoke the amp. So you really need to look at the specs for each amp and see what you can get out of it with your particular setup.

Damn, that one hurt my brain......Time for a beer.
 
I'm glad I'm not the only that has a problem with all of this. I'm fairly comfortable with just about any tech issue (computers, guitars, mixing, etc.). But for whatever reason I get totally lost on this impedance/ohm thing.

We have several amps at our church. The specs all list these number at various ohms, stereo vs. mono, bridged... I'm embarassed to say that I really don't have a clue what we really have!!!!!

Plus part of the problem is that with PA's you end up having a number of cabinets - in our case the original "designer" set us up with two EV 8 ohms cabs for each channel of a stereo amp (total of 4 cabs) , and then we have an 8 ohm sub on the each channel of the other stereo amp.

I just turn everything up and mix until it sounds right :D God does the rest :p
 
Ok, I am going to hurt my brain again. Let's cover total resistance and how this relates to speakers. In these samples we have 2 speakers (SP1 and SP2). Each speaker has a + and - terminal. The resistance for each speaker (8 ohms), will be R1 and R2.

To hook the speakers up in parallel, you take the + power from the amp, and hook that up to the + side of SP1 and SP2. Take the - power from the amp and hook it up to the - side of SP1 and SP2.

To hook the speakers up in series, you take the + power from the amp and hook it up to the + of SP1. Take the - from SP1 and hook it up to + side of SP2. Take the - side of SP2 and hook that to the - side of the power from the amp.

Now to calculate total resistance. If the 2 speakers are hooked up in series, you simply add up the resistance. 8 ohms from SP1 and 8 from SP2 = 16 ohms. To calculate the resistance in parallel you multiply R1*R2 and divide that by R1+R2. Another words, 2 8 ohm speakers in parallel are (8*8) / (8+8) = 4 ohms.

Now the standard way they hook up cabs internally will be 2 sets of 2 speakers each set in parallel, then the 2 sets hooked up in series. Another words, with 8 ohm speakers used, each set will have 4 ohms resistance and add the 2 set together to get 8 ohms for the cabinet.

Now if you have 2 - 8 ohm cabs, you can hook them up in parallel or series. So if you hook them up in parallel, you get 4 ohms total. In series you will get 16.

One last note. You should always keep your loads (speaker cabs) the same. You should not run a 4 ohm cab and hook that up in parallel with an 8 ohm cab. If you do that, the 4 ohm cab will be twice as loud as 8 ohm cab. It will be doing twice the work. Besides the fact that you would come up with 2.67 total resistance and I don't think anyone makes an amp to handle that.

Hope that helps
 
I was just on the randall and marshall web sites and thier 4 x 12 cabinets are both 4/16 ohm mono and 8 ohm stereo. why do they lump the 4 and 16 ohm together? is the 4 ohm technically 66% louder then?
 
They have 4 - 12 inch - 4 ohm speakers in those cabinets. They are wired as descibed above in my last post 2 parallel sets wired in series. When running all 4 in series, it gives you 16 ohms. That is what the switch does. Or you can run 2 sets in series and it gives you 2 - 8 ohm sets.

They are not necessarily louder when in 4 ohm mode, the reason they do that, is more to be able to match your amp setup. It all depends on what amp you are running and how much power you run into them.

Maybe all of this is very confusing to some of you, but the most important thing you need to remember is MAKE SURE YOU MATCH YOUR SPEAKER CABS TO YOUR AMP. If you don't match your amp, you will fry either the amp or speakers. It will not be louder hooking up 4 ohm speakers to the 16 ohm port. Well, actually it will be for the 2-3 seconds it takes for the amp to smoke in front of you.
 
Hmmmm?????

ok i get some of this ,i don't mean to hijack the thread but i have a question about ohms also ....i have a peavey classic 50 with an ohm switch 4-8 what should it be on and will it hurt or help to switch it from either 4 or 8 ,will one produce more grunt or is this for use with an extension speaker only, again i'm sorry to hijack but thought it would be silly to post a whole new thread when ya'll were already on th subject ...thanks
 
you can leave it on 4 and be ok..(this is what I've gotten so far) this all has really made me think though. but what has me at odds here is when juststartingout said they arent neccesarily louder in 4 ohm mode....if my amp is 4 ohm output and my cabinet is 4 ohm then it should recieve the full amount of watts from the amp vs. having a 4 ohm amp and a 16 ohm cabinet
 
oh yeah and i had a peavy classic 50 one time....i loved the hell out of it
 
Re: Hmmmm?????

lesterpaul said:
ok i get some of this ,i don't mean to hijack the thread but i have a question about ohms also ....i have a peavey classic 50 with an ohm switch 4-8 what should it be on and will it hurt or help to switch it from either 4 or 8 ,will one produce more grunt or is this for use with an extension speaker only, again i'm sorry to hijack but thought it would be silly to post a whole new thread when ya'll were already on th subject ...thanks
Click to expand...

PLEASE READ THIS. IF THEY DON'T MATCH, YOU WILL ABUSE THE EQUIPMENT AND POSSIBLY RUIN IT, FUCK IT UP, TRASH IT.

It depends on what speaker you are using. If you are using an 8 ohm speaker, set it to 8. If a 4 set it to 4.

IF THEY DON'T MATCH, YOU WILL ABUSE THE EQUIPMENT AND POSSIBLY RUIN IT, FUCK IT UP, TRASH IT.

The reason it produces more grunt, is that you are over driving the speaker.

IF THEY DON'T MATCH, YOU WILL ABUSE THE EQUIPMENT AND POSSIBLY RUIN IT, FUCK IT UP, TRASH IT.

And this point is very, very important.

IF THEY DON'T MATCH, YOU WILL ABUSE THE EQUIPMENT AND POSSIBLY RUIN IT, FUCK IT UP, TRASH IT.

OK, now that I got that out, you really do need to make sure you match the impeadence(ohms). The circuity in the amp depends on the proper speakers being attached. I really don't want anyone out there killing an amp when it could be prevented. There are effects to create a little more grunt in your sound, use them. If it's hook up correctly, the amp and speakers will last a lot longer. When you hear that someone blew a speaker, a lot of times it's because it was not hooked up correctly.
 
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well now! if that didnt get the message across then juststartingout will be subject to bitch slapping people that dont get it. i got one more question for you juststartingout.....what would be your method of extracting info out of saddam?
 
Re: ...................

distortedrumble said:
what would be your method of extracting info out of saddam?
Click to expand...

That's easy, make him listen to rap!!

But anyway, if you get a good deal on an amp and/or cab that cannot match, just rewire the cab. It's not that hard to do.
 
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Silent for a week, and then I have to come along and throw a new curveball! I have a Johnson Millenium Stereo 250 head, which runs 125 watts a side...I get the math (even though my brain is tuned to helping my daughter make change for a dollar and whats "quater past two") BUT I have a question about stereo heads. The head adjusts to the ohms of the cab so I don't have to worry about setting a switch, my question is it seems that all the 4X12s I have found run at 8ohms stereo, 4 mono (which seems to validate the math). Here comes the part that hurts my head...

Will I get "More" running two 4ohm Mono 4X12s then running one Stereo 8ohm 4X12? The common sence would be that you get less because the head is pushing 8 speakers in stead of four, but based on resistance it seems to be the same work load...and if its the same workload, then 8 speakers is more than 4...which would be sonicly "more" (is sonicly a word?)


Mox
 
Y'all are leaving voltage out or your calculations.


All amps send some sort of DC voltage to the speaker.

To caculate the power you need to use Ohm's Law.

Someone got close to this in an early reply.

These are the scientific equations you are looking for.

R = Ohms = resistance
P = Watts = power (roots means squared assumed)
I = Amps = current
E = Volts = electromotive force

To solve for watts:
E x I = P or E square / R = P or I square x R = P

To solve for amps:
E / R = I or P / E = I or square root of (P / R) = I

To solve for volts:
P / I = E or I x R = E or square root of (P x R) = E

To solve for resistance:
E / I = R or E sqaure / P = R or I square x P = R
 
Mox said:
The common sence would be that you get less because the head is pushing 8 speakers in stead of four, but based on resistance it seems to be the same work load...and if its the same workload, then 8 speakers is more than 4...which would be sonicly "more" (is sonicly a word?)
Click to expand...
Yes Max, 8 speakers will be louder than 4 and you will using the same amount of power. Basically, each speaker will work half as hard, but you have twice the speaker area. The reason they are louder is that the speakers are more efficient at lower power. A speaker is a mechanical device. As such it needs time to move the speaker. That is easier to do (low power) if you don't have to move it as far (high power).

I guess that's the easiest way to explain it.

Personally, I perfer to use one cab over two. I like to drive my tube amps a little more with less sound. I may even try a 2 speaker configuration just for that reason.
 
The thing about different impedences can't be broken down to a simple rule because every amp is different and speakers are too. When they say a speaker is 8 ohms.......that's what's called a nominal resistance. But impedence...which is what the amp 'sees' in operation varies all over the place depending on frequency. It's not unusual to see a speakers' impedence go as low as a couple of ohms and as high as 300 or so depending on what frequency you put thru it.

So what happens is that at low ohmages...the amp needs to put out more current. That bit about power doubling at half the impedence only works if the amps' power supply is capable of putting out the extra current. If it can't.....then you'll get distortion or worse, something will blow.
Your old tube amps (like all the old Ampegs that I use ) have different taps on the transformer to match it to different loads. But as long as you set it correctly for the load.....it won't put out more into 4 ohms than it does into 8 ohms. That's because the different taps are to match it to the load and as long as it's matched....it's gonna put out generally the same output because in both cases it's matched to the load.

Nowadays........a lot of amps don't have different taps......they just give a minimum. Go below that at your own peril......however; I've been doing this a very long time and in my experience.....virtually any amp will handle down to 4 ohms with no problem.
Going to a higher impedence will not damage an amp......it will just not output as much and probably have increased distortion.
Lastly......all these things are more critical as you run the amp closer to it's maximim output.
 
well after some revisions on purchasing a head. I've gone with the marshall MG100HDFX its 100 watts and the 2 outputs on on the back say minimum 4 ohms. my cabinet says 16 ohms mono and 8 ohms per channel stereo. my assumption there is that in stereo I'll be getting 80 watts. i guess 80 is enough. this is these ohm calculations are what make me take a second look at some heads when people give them bad reviews about the amp catching on fire and what not. i start to wonder if they just did what i did so many times before and take a head and cab and connect them together with the extra guitar cable lying around and went for it. i will say I've been fortunate enough not to have a situation where things catch on fire.
 
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