Ohms Law

[SilverlineDiscos]

Hi guys,

I was thinking about this the other day and wondered if anyone could shed any light on the subject.

I was just wondering what the score is exactly with regards to Ohms and watts. Currently I have a pair of 200W Cabs and a 200W a side AMP at 8 Ohms. The speakers are also 8 Ohms so everything is matched nicely.

Now, what happens if you buy a pair of 500w speakers which run at 8 Ohms and the AMP runs at 250W at 8 Ohms BUT 500W at 4 Ohms??

How does the amplifier work this out and what do you do to get a perfectly matched AMP with your speakers?? I get very confused with this so could someone please sort my confusion out once and for all??

I have another question regarding AMPS and speakers, but that's for another rainy day ;)

Many thanks in advance,
Adam

[Tom]

If the amp delivers 500 watts at 4ohm per channel and the speakers are 500 watts each with a resistance of 4ohm also, then all is good and work correctly.


I'm not that hot on the electrics on how the amp determines the resistance. I think it might be the voltage or something, I don't know. Lol.


Hope this helps.

[SilverlineDiscos]

Hi Tom,

Thanks for the reply. So in other words, make sure you match the resisitance with the speakers and power rating and you should be ok?

I was looking to buy some bass bins at some point to go with my 200w 15" cabs. I'd need to buy one hell of an AMP to run them wouldn't I?

[SilverlineDiscos]

Just had a thought, if I bought some bass bins that ran in 8Ohms, along with my other 8Ohm cabs, would that then run in 4Ohms??

[James]

i've had this ohms thing explained to me by dj's, technicians, shop owners, experts on forums, & i still don't get it.

i wish there was a lehmans terms way of explaining it in a couple of words

[Cj_The_Dj]

Just had a thought, if I bought some bass bins that ran in 8Ohms, along with my other 8Ohm cabs, would that then run in 4Ohms??

yes that should be fine,

but as other people on this forum would recommend is 2 different amps one for each pair. and use an active crossover.

[sleah]

OK, it's complicated but I'll try to make it easy:)

Something you are very unlikely to do is run speakers in series. That's when you connect the neg of the first speaker to the amp, then the pos of that speaker to the neg of the 2nd speaker, then the pos of the 2nd speaker to the amp.
When connected like that you simply add up the ohms.
Soooo... 2 @ 8ohms=16ohms.

Most likely is you will be connecting in parallel. That's when you connect both speaker pos's to the amp and both negs to the amp.
Then you divide by 2 (well, sort of!)
Soooo... 2 @ 8ohms=4ohms
and.... 2 @ 4ohms=2ohms
also... 4 @ 8ohms=2ohms
but.... 3 @ 8ohms=2.6ohms

Some top quality amps can handle 2ohms, but most only 4ohms. You can safely go higher, so you could have 16ohm speakers on just about any amp.

When an amp says 250w @ 8ohms / 500w @ 4ohms, if you have 4 cabs @ 8ohms each, connect 2 to each side of the amp, normally just plug the 2nd set in to the back of the 1st set, you will have the magic 4ohms per channel:D

Hope that helps

[Jiggles]

1/Rt = 1/R1 +1/R2 so on and so forth :D To work out Resistance in parallel. and Rt= R1 + R2 + R3...

[SilverlineDiscos]

Hmm... I like the idea of having two amps for two sets of speakers. You'd get a much better sound rather than pushing a amp to run four speakers. Only trouble is, two amps and a crossover is going to get costly, and I'm not made of money!!

I'm really just thinking futureproof. I mean as I said earlier, I have a pair of 200W 15" cabs already, and I have a Stereo AMP to run them. Ideally I'd like to get an Disco Power AMP which is more suited to the job. The current AMP I have now is not 19" rackmount and is as heavy as a house.

So I think for the future, I might just buy a couple of lower power amps, run them with a active crossover and then buy a couple of bass bins.

Make sense?

[Tom]

If you plan to run more than one speaker per channel you need to make sure the cabs are working in Series or parallel.


Here it it in it's simple form.


Series

2ohm + 2 ohm = 4ohm

4ohm + 4ohm = 8ohm

8ohm + 8ohm = 16ohm

and so on.


Parallel

16ohm + 16ohm = 8ohm

8ohm + 8ohm = 4ohm

4ohm + 4ohm = 2ohm.

You can mix and match between series and Parallel but you dont really need to know any more than that really.

In most cabs you will need to use parallel.


You need to be carefull if you wish to use 200w rms speaker top at 8ohm and a 500w rms bass bin at 8ohm. Yes this will give you 4ohm, but you will be chucking a load of power to the 200w speaker which is might not be able to cope with.

You best bet really is to use seperate amps. This also gives you the security if an amp decides to die. If you run a whole pa off one amp and it dies then its end of party and go home, unless you have backup.

With two amps, if the bass bin amp dies, you still have sound from the speaker tops, so the party can still continue.

[Tom]

Hmm... I like the idea of having two amps for two sets of speakers. You'd get a much better sound rather than pushing a amp to run four speakers. Only trouble is, two amps and a crossover is going to get costly, and I'm not made of money!!

I'm really just thinking futureproof. I mean as I said earlier, I have a pair of 200W 15" cabs already, and I have a Stereo AMP to run them. Ideally I'd like to get an Disco Power AMP which is more suited to the job. The current AMP I have now is not 19" rackmount and is as heavy as a house.

So I think for the future, I might just buy a couple of lower power amps, run them with a active crossover and then buy a couple of bass bins.

Make sense?

As I said, two amps is better than one.

Best to go rackmount really.

Maby have a look at behringer amps. Quite good budget amps.

[SilverlineDiscos]

That's VERY wise advice Tom, many thanks. I think the best best would be to get two amps as I am in a position where I can do either option.

Good thing is, I could pick a couple of low power amps for about the same price as a high power amp. I've been looking at the price of them and I can pick up a 300W per side amp for about £200 ish.

How much are crossovers??

[Tom]

That's VERY wise advice Tom, many thanks. I think the best best would be to get two amps as I am in a position where I can do either option.

Good thing is, I could pick a couple of low power amps for about the same price as a high power amp. I've been looking at the price of them and I can pick up a 300W per side amp for about £200 ish.

How much are crossovers??


The behringer amps are a little more than that and I think would be better for you as they would have more power if you decide to get better speakers at a later date.

As for crossovers. I run the Behringer Ultradrive. Fantastic bit of kit for the money. It has 3 inputs and 6 outputs. You can set up the crossover, limiter, eq and mony more to each output channel.

But having said that, I think behringer do a 3 and a 4way eq. Maby have a look at those aswell. :)

[SilverlineDiscos]

Hi Tom,

Would this (http://www.djstore.com/item/processing/behcx2310.htm) crossover do the job?

I also looked at the Behringer AMPS, they seem quite expensive but I guess it's worth the extra for the future.

Any other recommendations?

[Cj_The_Dj]

Have a look at the T.amp similar to the c mark

[Tom]

Hi Tom,

Would this (http://www.djstore.com/item/processing/behcx2310.htm) crossover do the job?

I also looked at the Behringer AMPS, they seem quite expensive but I guess it's worth the extra for the future.

Any other recommendations?

Yes, that will do the job. :)

It only has the one sub output, so you can either run two 8ohm subs as 4ohm in bridge mode from an amp, or get a xlr splitter lead so it goes from one female xlr to two male xlr connection. One for the left channel of the amp and one for the right channel. This will still be mono, which is what you want as bass is best being mono, not stereo.

Behringer might seem expensive to you but it's classed as a budget brand. You could spend thousands on an amp, but for our line of work, we dont need that kind o power or amp (depending on size of gigs you do).

Maby have a look at here http://www.djkit.co.uk/category.php?id=40 where you can select the amount of watts and find how much an amp would cost.

You dont have to buy from here, just using it as a guide. :)

[SilverlineDiscos]

Well, for a hall that holds about 500 people, a 2K system is a good enough guess for any DJ. I mean, I plan to undertake weddings and such in the future, which of course depend on the event may require that little bit extra sound.

I think for now, 500w tops and 500w bass cabs might be a good start. That would give me a 2K setup in total which I think is enough? This way, I would be aiming to purchase two 500w a side @ 8 Ohms AMP and a crossover.

Is my logic making sense, or am I being dim in some areas? I'm just trying to get all this in my head, so that I know what I'm doing when the time comes.

Cheers for all the information by the way, it is greatfully appreciated! :)

[Excalibur]

Hi guys,

I was thinking about this the other day and wondered if anyone could shed any light on the subject.

I was just wondering what the score is exactly with regards to Ohms and watts. Currently I have a pair of 200W Cabs and a 200W a side AMP at 8 Ohms. The speakers are also 8 Ohms so everything is matched nicely.

Now, what happens if you buy a pair of 500w speakers which run at 8 Ohms and the AMP runs at 250W at 8 Ohms BUT 500W at 4 Ohms??

How does the amplifier work this out and what do you do to get a perfectly matched AMP with your speakers?? I get very confused with this so could someone please sort my confusion out once and for all??

I have another question regarding AMPS and speakers, but that's for another rainy day ;)

Many thanks in advance,
Adam

Re the highlighted text, post#6 on here may help. ( Or it may not. :D )
http://www.mobilediscodirectory.co.uk/forums/showthread.php?t=10463


Right, let's have a go at Bins and Tops, eh? I'll use my rig as an example.
Tops are 350w RMS, 8ohms
Bins, 350w RMS, 8ohms also.
Amp(s), 500w RMS at 8ohms, 750w at 4ohms.

One amp feeding tops as full range cabs, the theory is that since we rarely get a continuous 500w signal, the cabs can cope with that safely. This is the way Peavey explain it.

One amp, with two pairs of tops in parallel. Second pair plugged into first pair.
As explained above, 4ohms is the impedance of this rig, so 750w SHARED EQUALLY between the cabs, so 375w per cab. Still safe. :D

One amp, subs and tops. Tops fed from crossovers in subs.
Now it's getting a bit more controversial. According to Peavey, the impedance is still 8ohms. Imagine the subs and tops as being all in one box.
So 500w shared, but not necessarily completely equally. The subs tend to work harder. I have a spare amp which overheats when driving subs. :(
The advantage of this is that both cabs don't have to reproduce as wide a range of frequencies, which gives clearer sound.

Two amps, subs and tops, with active crossover.
The crossover splits the frequencies, before they get to the amps. Each amp/cab is only producing the narrower frequency range, so good news again, good sound. Power is same as first example.

That's as simple as I can get away with. There's more to it than that, but that's more than enough to be going on with.
One important point, if you intend to go with the last example, it's widely accepted that you need twice as much power for the subs as the tops. Therefore, if you are happy with your existing amp and tops, you only need a crosover like you linked to, and an amp and subs of around 500w .
This amp's well regarded.
http://www.thomann.de/gb/tamp_ta1400_amplifier.htm

[SilverlineDiscos]

Blimy, thanks for all that info Excalibur! Quite a good read actually.

To make this clear, does your rig for example have 1 or two amps? I think I grasp the concept now, just need to put something together to make sure I understand.

As I said, not looking to upgrade anytime soon, but 'when' the time comes, I want to make sure I get this right. If not, it's an expensive learning curve!! :D

[DeckstarDeluxe]

Would you not be better off invested in higher powered tops that can handle more bass and have abit more of a "punch". 200w seems quite low.....?

[SilverlineDiscos]

Hi Deskstar,

The cabs I have at the moment are 200w speakers. They were given to me for free and I use them as my main speakers, simply because they are the only disco-type speakers I have. I am unsure as to the exact power they produce but I know they have 200w Eminence bass drivers. They are 2-way and they use a compression driver.

I did a 13th birthday party on saterday and they were plenty powerful for the hall :)

I will consider using them as tops when I move up to a bass bin + tops setup, but for now, they suit me just fine while I get things going.

Edit: here is a picture of them with a very convenient pint in front of them so you can see roughly how big they are...

Link (http://silverlinehosting.co.uk/disco/DSC00109.JPG)

[SilverlineDiscos]

Right,

I've been looking at various amps and speakers and just want to clear this up once and for all. I have selected for the option to use two amps, two top cabs, two bass bins and a active crossover.

Here is the gear I've come up with, so if anyone would be kind enough to tell me if I'm right (hopefully!)

Right, for the tops: I found these (http://www.thomann.de/gb/jbl_jrx_115.htm). They should provide some nice clear sound, and enough to fill a good sized hall or function room.

For the bass cabs, I've found these (http://www.decks.co.uk/products/kam/extreme18sub). I think 500w is enough again for a good sized hall or room.

For the tops, I thought this (http://www.thomann.de/gb/tamp_ta1400_amplifier.htm) amp might be a good one.

For the bass bins, I thought the same AMP as above would work? Perhaps something more powerful?

For the active crossover, I thought this (http://www.thomann.de/gb/behringer_cx2310_super-x_pro.htm) would work.

Please let me know if I'm wrong. I'd like to get this in my head properly. I'm not necessarily going to buy that equipment, but I was to make sure I get the right concept.

Many thanks,
Adam

[Sapphire Disco]

If I were you I would get a pair of subs approx 350w and a new 500w per side Amp, use passive crossovers in the subs also make sure subs are 98db min.
You may think this is too powerful for your 200w tops but it will be fine because the subs will be taking the strain and your tops won't need to work as hard, probably not even as much as they do now on their own.

Keep your old amp as back up and later on you can get some better 300w tops without having to worry about your amp & subs matching.

[Excalibur]

Blimy, thanks for all that info Excalibur! Quite a good read actually.

To make this clear, does your rig for example have 1 or two amps? I think I grasp the concept now, just need to put something together to make sure I understand.

As I said, not looking to upgrade anytime soon, but 'when' the time comes, I want to make sure I get this right. If not, it's an expensive learning curve!! :D
Yes. :D :D :D :D Read again mate, it says in each example how many of everything. ;)


Would you not be better off invested in higher powered tops that can handle more bass and have abit more of a "punch". 200w seems quite low.....?
He doesn't need to rush. If it ain't broke, don't fix it. I ran happily for many years with a pair of Peavey Eurosys 3's, same spec effectively. ;)


If I were you I would get a pair of subs approx 350w and a new 500w per side Amp, use passive crossovers in the subs also make sure subs are 98db min.
You may think this is too powerful for your 200w tops but it will be fine because the subs will be taking the strain and your tops won't need to work as hard, probably not even as much as they do now on their own.

Keep your old amp as back up and later on you can get some better 300w tops without having to worry about your amp & subs matching.

Sensible advice.

[SilverlineDiscos]

Thanks for all the advice guys, I cannot thank you all enough :)

Sorry to drone on, but the equipment I mentioned earlier, am I right in the matching specs?

Also, diamondgeezer, were you suggesting I buy two amps or just one? If only one, surely there would not be enough power for all 4 speakers?

Thanks in advance,
Adam

[Tony Scott]

Thanks for all the advice guys, I cannot thank you all enough :)

Sorry to drone on, but the equipment I mentioned earlier, am I right in the matching specs?

....Thanks in advance,
Adam

The JBL's, t-amp and x-over are all fine but consider these: http://www.thomann.de/gb/the_box_t18.htm for the subs, they will go lower and louder and would match the JBL tops much better than the KAM 'toys'! :)

[SilverlineDiscos]

Thanks Tony!

They look a bit basic, but it's whats inside that matters! :)

[Sapphire Disco]

Thanks for all the advice guys, I cannot thank you all enough :)

Sorry to drone on, but the equipment I mentioned earlier, am I right in the matching specs?

Also, diamondgeezer, were you suggesting I buy two amps or just one? If only one, surely there would not be enough power for all 4 speakers?

Thanks in advance,
Adam

My advice is don't buy big heavy 18" subs 15" will be fine and think about the weight if you work on your own.

Yes one amp is fine as long as the quality is responsible, with the passive crossover in the subs you are still running at 8:r: even though there are 4 speakers.

[Tony Scott]

My advice is don't buy big heavy 18" subs 15" will be fine and think about the weight if you work on your own.

If you can't manage 37kg's then you should give up IMHO!...My mid-hi's are nearly that! :D

[Sapphire Disco]

If you can't manage 37kg's then you should give up IMHO!...My mid-hi's are nearly that! :D

I've just got some 27kg subs 15" compact and they make life much easier and yes they do the business and are very punchy.

[Tony Scott]

I've just got some 27kg subs 15" compact and they make life much easier and yes they do the business and are very punchy.


Not interested in punch, that's what my kick bins are for......I want 18", 32hz rumble!!!! :D :D :D

[Excalibur]

The JBL's, t-amp and x-over are all fine but consider these: http://www.thomann.de/gb/the_box_t18.htm for the subs, they will go lower and louder and would match the JBL tops much better than the KAM 'toys'! :)
Agreed, but I'm curious as to why there's no 15" version? :confused:


My advice is don't buy big heavy 18" subs 15" will be fine and think about the weight if you work on your own.

Yes one amp is fine as long as the quality is responsible, with the passive crossover in the subs you are still running at 8:r: even though there are 4 speakers.
Agreeed on both counts.


If you can't manage 37kg's then you should give up IMHO!...My mid-hi's are nearly that! :D

How can I put this tactfully? :confused:
HSE guidelines mean that 37Kg is a two man lift. ;)
The " Less is more " concept appears to be gaining popularity on here, even unto the speaker department. :eek: Personally, I can't see it myself, but "Chacun a son gout " :D :D :D :D :D :D

[Tom]

If less is more, then my small compact sub design from 30hz up to 100 is perfect for the job then. Lol. Downside is that you will need 4 of the things just to get around 130db. Needs about 2000 watts to get there also. :( bit of a bummer but would rather move smaller subs which I can hide under the deckstand than have 1 large cab which looks out of place.

People will be asking, where all the bass is coming from. Haha. Lol

[sleah]

If less is more, then my small compact sub design from 30hz up to 100 is perfect for the job then. Lol. Downside is that you will need 4 of the things just to get around 130db. Needs about 2000 watts to get there also. :( bit of a bummer but would rather move smaller subs which I can hide under the deckstand than have 1 large cab which looks out of place.

People will be asking, where all the bass is coming from. Haha. Lol

Apart from frequency response, it's about sensitivity.

A small cab may have a sensitivity of say 130db@1m (400w) and a large cab 127db@1m (400w).

The smaller cab will actually be louder.:cool:


But I bet you knew that;) :p :D :D :D

The figures I've used are pulled from the air and not accurate by any means, but illustrate the point.:D

[Excalibur]

Apart from frequency response, it's about sensitivity.

A small cab may have a sensitivity of say 130db@1m (400w) and a large cab 127db@1m (400w).

The smaller cab will actually be louder.:cool:


But I bet you knew that;) :p :D :D :D

The figures I've used are pulled from the air and not accurate by any means, but illustrate the point.:D

Not disputed at all, cos I've witnessed it. ;) However, loudness is subjective, cab size is absolute. Pile 'em high. :D :D :D :D :D :D

[sleah]

Not disputed at all, cos I've witnessed it. ;) However, loudness is subjective, cab size is absolute. Pile 'em high. :D :D :D :D :D :D

So very true:)

Even cab size & design can affect the overall loudness, not just frequency response, all though it's sort of the same thing. If a cab has a good response at frequency X, then it means it's louder at that frequency....

My heads about to explode:p:D:D

Not that most DJ's care about such things....
Pile 'em high and crank it up:cool:
It it's not very sensitive.... turn it up louder:p:p:p

TURN UP DA BASS:D:D:D:D:D:D:D:D:D

[Tony Scott]

...However, loudness is subjective, cab size is absolute. Pile 'em high. :D :D :D :D :D :D


So very true:) .....Not that most DJ's care about such things....
Pile 'em high and crank it up:cool:
It it's not very sensitive.... turn it up louder:p:p:p

TURN UP DA BASS:D:D:D:D:D:D:D:D:D

:lol:

[Tom]

So very true:)

Even cab size & design can affect the overall loudness, not just frequency response, all though it's sort of the same thing. If a cab has a good response at frequency X, then it means it's louder at that frequency....



I did make a post about that but took it off as I feel it was off topic and would get a slap on the wrist for it. lol.

But you are correct though.

For anyone who would like to know more about this, check out hoffmans iron law. That will explain whjy you can't get small cabs that are 200db spl and can go down to 1 hz. lol

[rob1963]

I really can't get my head around this Ohms law thing.

Could somebody explain how the same amp will put out roughly a third more watts into a 4 ohm speaker than into an 8 ohm one?

It's a bit like saying you have a 1.2 litre engine in a particular car which becomes a 1.6 litre engine if you take it out & put it into a different car.

:shrug:

[Jiggles]

Less ohms means that the amp has less to push against (Resistance). Think of it as a pipe. The resistance is the diameter of the pipe. high restistance = very thin pipe. low resistance = smaller diameter but not as small as the other one.

[rob1963]

Less ohms means that the amp has less to push against (Resistance). Think of it as a pipe. The resistance is the diameter of the pipe. high restistance = very thin pipe. low resistance = smaller diameter but not as small as the other one.

That helps...thanks Callum :D

[Jiggles]

That helps...thanks Callum :D

That's the best way to think of electricity imo :D