Watts & Ohms - Confused

loctite

Please can someone shed some light on this for me.

I have played guitar and bass for years but never really paid any attention to what I was playing with.

I'm looking to invest in a proper cab and head but I'm baffled by the Ohms/Watt relationship. Can someone please shed some light.

What is the relevance of the ohms on the speakers..... does it restrict what heads that you can use.etc...... complete beginner when comes to amplification.

nialldabass

Speakers have a resistance which is the ohms rating

They also have a power rating which is the watt rating

When conecting an amp and speaker the ohm rating is the most imprtant

Most amp heads will allow you to switch ohm rating 4ohm 8ohm 16ohm

And you have to make sure the ohm rating is set for your speaker cab
especially true with tube amps as they become part of the circuit, a mismatched amp and speaker cab can fry your amp, wreck your speakers, or set your house on fire :eek:

Now thats the scarey bit out of the way, its all very straight forward and all bought amps and cabs will be clearly marked, make sure you use proper speaker cables and never use a guitar cable, or more scarey fires and stuff
Sorry I thought I was finished with the scarey stuff:D

Now if its a fifty watt head lets say, then you need a fifty watt and above speaker, although you could put a 10 watt speaker and and turn up to 1
as long as the ohm rating is correct, or a 1000 watt as long as the ohm rating is correct

But Im being silly, Its all about the ohms, the power handling comes second
There is of course alot more to think about if you want to get really anal, how you want your amp to drive your speakers . how much clean headroom and so on. But that should cover your question

Paolo_M

To start
Wattage rating is the amount of power a speaker can take from the amplifier before burning up.
Ohms are a measure of the "load" a speaker pesent to the amplifier. Think of this as like gears on a bicycle. The gear must be right or the bike wont move or you'll burst a vein pedalling and getting no where. Mst amps are designed to work in the 4, 8 and 16 omh range.


Wattage and ohm rating requirements are different for a solid state or a valve amp.


SS:
The ohms do not really matter much here.
SS amps adapt well to different speaker loads (load refers to ohm value) and usually have no trouble with no load ie no speaker.
They do delivery different amounts of power to different speaker loads.
A SS amp will deliver more power to a 4 ohm cab than a 16 ohm.

Wattage follows the SS amp rating. Once you buy a cab raed the same as the amp you'll be fine. SS amps do not deliver more power than they are rated at. This is one of the reasons people think "valve watts are louder than SS watts", which is utter BS by the way.



Valve amps:
Ohms are abolutely critical here. You must match the cab to the amp.

Valve amps are very poor at adapting to different speaker loads and, instead of regulating themselves like a SS amp, they blow themselves up tring to deliver the same power.
Luckily most valve amps allow you to adjust the ohm setting between 4, 8 and 16 allowing you to use most cabs.
Many quality valve amps are perfectly capable of handling a mis-match but unless you've spent more than €1k you probably don't have one of these.

Wattage is also different. A 100 watt valve amp will put out up to 170 watts when turned up fully. Again part of the reason people think valve amps are louder.
You need to buy a cab that is at least 1.5 times the power rating of your amp if you intend turning it up.
For example a Marshall JCM800 is putting out 100 Watts when the master volume is at just below 4.


Universally speaking:
Niall's spot on; use proper seaker cable
Different speakers have different efficiencies. One speaker migt put out 103dB with 1 watt from the amp, another might put out 93dB which is half the volume.
Different speaker have very different tones, and can completely change your amps charactor. Make sure you research your speaker selection. The are a bunch of good speaker comparisons on uTube so you can get a feel fr the differences.
Not all cabs are built equally. A poorly built cab, even when loaed with quality speakers, will still sound farty and weak especially at higher volumes.

gotBass

loctite wrote: »

Please can someone shed some light on this for me.

I have played guitar and bass for years but never really paid any attention to what I was playing with.

I'm looking to invest in a proper cab and head but I'm baffled by the Ohms/Watt relationship. Can someone please shed some light.

What is the relevance of the ohms on the speakers..... does it restrict what heads that you can use.etc...... complete beginner when comes to amplification.

I posted this a while back for someone with a bass amp/cab scenario
in addition the valve /solid state thing - valves louder on a WATT vs WATT output, not going to tell you why, but its to do with how valves perform when driven hard.


Just incase you are going out buying amps cabs or combos.

An extreme example would be putting a 500w head into a 1x10 inch speaker.
(Yes I know you would most likely blow it up) but this is only an example.
electrical signals get turned into sound by the speaker. If the speaker is only small it generally cannot move enuf air to make soundwaves. the amplifier is making loads of electric signal but is limited by the 1x10.

however if you had a 100w amp going into a 2x10, the speakers can take the power from the amp and convert it to more sound. Therefore the lower powered amp and more suitable speaker setup would sound louder.

For any newbies out there thinking that huge powered bass combos are better, there are other considerations like cab configuration, speaker efficiency etc., but more on that at another time. the simplest lesson to learn is this, and is based around separate amp and cab setup. I'm not going into huge detail on this, as it is a can of worms....

Your amp output is rated A Watts at B ohms.
Your speakers are rated at C Watts and D Ohms
If D is less than B
( speakers have lower impedance than amplifier can handle) the amp will run hot and possibly blow. If D is bigger than B things are ok.

If A is bigger than C, this can be ok, but make sure that you are not way over. the amp can be run at low volume and will be ok. If the Amp is run at full volume it has higher output than speakers can handle, and you get farty noises on top of your nice clean bass sound. ( not nice and could blow speakers)

If A is less than C then the speakers can handle everything that the amp can throw at them, but running the amp at full belt may cause it to get damaged.

to summarise, if getting separates:
Get an amp with a higher watt output and lower impedance rating than your speakers, and avoid running it a 100% volume output.
If in doubt ask about it here or the guy in the shop where you are buying should have a clue.

bytey

you do know you can get 500 watt 10 inch speakers, yes ?

the speaker will not limit the amp becasue of speaker size and amount of air pushed.

it will limit the amp

1/ if the speaker ohms are higher than the amps rated ohms
( ie 8 ohm amp into a 16 ohm speaker )

2. if the amp is 100 watt but the speaker is 10 watt , you can only barely turn on the volume, or you will blow the speaker- this is in a way limiting the amp, manually.

the only thing limited by the speaker will be frequecny response in the low end
as the speaker gets smaller

or freq response in the high end as the speaker gets bigger.

taytothief

watts an ohm

gotBass

Hi Bytey,
When I wrote my original post it was intended for someone who had little knowledge on the subject of matching speakers and amps. I attempted to give them a simple rule of thumb whereby the amp won't blow fuses or speakers at a gig. Also what happens when the speaker is suited better to the amp. There are misconceptions out there about amps and speakers, and how watts equate to actual volume.

I understand the principle of maximum power transfer and frequency response characteristics, as do some on this forum ( including yourself), but some are very new to the area, and are not really technical.
Perhaps we should produce a sticky covering the basics of the subject, using plain english, for the young and upcoming amongst us?

Sandvich

It's surprisingly hard to find a speaker cable in music shops here. Apparently people tend to use instrument leads ;/

nialldabass

Its hard to find most things in this country, and when you do you pay twice as much, but if anyone out there is using a guitar cable for a speaker cable on a tube amp then I wish them well, takes about 10 mins research online to realise how stupid this is.
Not so important with solid state amps, but the cables are different for a reason

eoin5

taytothief wrote: »

watts an ohm

Yore ma is an ohm

Maybe Paolo M or other Eoin might know this. Is there a strong correllation between DC resistance and actual resistance? I know theres a decent difference in general but could you rely on it to guess what the actual resistance is? A 16ohm g10 vintage measures 12.33 DC but an 8ohm measures 7.47. Would this be an indication that in general you would get a relatively larger difference the higher the impedance or is it just down to manufacture?

nialldabass

gotBass wrote: »

Hi Bytey,
When I wrote my original post it was intended for someone who had little knowledge on the subject of matching speakers and amps. I attempted to give them a simple rule of thumb whereby the amp won't blow fuses or speakers at a gig. Also what happens when the speaker is suited better to the amp. There are misconceptions out there about amps and speakers, and how watts equate to actual volume.

I understand the principle of maximum power transfer and frequency response characteristics, as do some on this forum ( including yourself), but some are very new to the area, and are not really technical.
Perhaps we should produce a sticky covering the basics of the subject, using plain english, for the young and upcoming amongst us?

I think a sticky would be a great idea, in plain english mind
A do's and dont's maybe and then get more technical explaining reasons
Cause I get lost once you get into the nitty gritty, and i think its un nessasary for most people:D

120_Minutes

nialldabass wrote: »

Its hard to find most things in this country, and when you do you pay twice as much, but if anyone out there is using a guitar cable for a speaker cable on a tube amp then I wish them well, takes about 10 mins research online to realise how stupid this is.
Not so important with solid state amps, but the cables are different for a reason

Been using a guitar cable on a vintage marshall for years. No ill effects. Gave up on speaker cables when they'd go missing after gigs and I couldn't replace them.

Oh and I know why they're better, I'm aware of the difference, just saying is all...

nialldabass

120_Minutes wrote: »

Been using a guitar cable on a vintage marshall for years. No ill effects. Gave up on speaker cables when they'd go missing after gigs and I couldn't replace them.

Oh and I know why they're better, I'm aware of the difference, just saying is all...

You do realise how dangerous it is though?
So its not a question of better its about being safe . You can make a cable in ten minutes. The OP is asking about Ohms and Watts and linking amps and cabs, I think Im giving sound advice, why risk it ? You not only risk your amp but the sound coming out of it will not be as good, and these are facts. Should be cheaper than guitar cable aswell so I dont understand why shops do not carry them. But like I said, good luck to you, and I really mean that, I just would not risk it on any of my gear, vintage or modern.

"While a genuine speaker cable of good quality delivers the amp’s output equally to the speaker’s positive and negative terminals, the unequal design of a guitar cord — one thin insulated signal wire, surrounded by one very wide shield — introduces a lot of unwanted capacitance to the connection, which can create an impedance mismatch between amp and speaker. On top of this, the thin positive wire in a guitar cord is of a much lower gauge than you would normally want to use to make an output connection, especially from a powerful amp. The impedance mismatch will in itself strain your amp’s output tubes and output transformer, and will severely choke and dull down your tone as a result; if that thin “hot” wire inside the guitar cord gets too hot and shorts, you could blow your output transformer entirely, necessitating an expensive repair".

Not my words but sums it up nicely

montec

Hey lads,
thanks for the great information above.
Like a lot of people, I've been playing music for a long time without actually knowing a great deal about the scientific end of things.
The question I have is directly related to the information given above.
Basically, I just want to know the correct and optimum set-up for my head and cab.
I've included all the information I can find about the amp and cab below:

Amp:
Carlsbro 50 Top Evolution. Valve Amp.
Valve mode: Pentode (50W) or Triode (25W)
Output Power:
50W RMS 16 Ohms

100W Peak
8 Ohm Total Load
1X8 Ohm or 2X16 Ohm


Speaker Cabinet:
Classic 412
Stereo
16 Ohm
Right
2X160W Stereo

or

Mono
320W
4Ohm
Left.

Thanks for your help.

gotBass

Firstly do not run the cab in 4 ohm mode.
since the lowest output from the head is 8 ohms.

I'd run the head at 16 ohms into a 16 ohm cab.
or 8ohms into 16 ohm cab.
I run my Bass head (4 ohm) into an 8 ohm cab and its' fine.

don't crank the amp to 100% though it may run too hot, as the cab looks like it can take everything that the amp can output and more( assuming that all numbers quoted are RMS) The volume level should be fairly hefty from the sounds of this. Also Protect your ears they'll be more important to you in the long run.

Like everything here it's experimentation.
I'm a big fan of driving preamp stage fairly hard and going easy on the power stage.

montec wrote: »

Hey lads,
thanks for the great information above.
Like a lot of people, I've been playing music for a long time without actually knowing a great deal about the scientific end of things.
The question I have is directly related to the information given above.
Basically, I just want to know the correct and optimum set-up for my head and cab.
I've included all the information I can find about the amp and cab below:

Amp:
Carlsbro 50 Top Evolution. Valve Amp.
Valve mode: Pentode (50W) or Triode (25W)
Output Power:
50W RMS 16 Ohms

100W Peak
8 Ohm Total Load
1X8 Ohm or 2X16 Ohm


Speaker Cabinet:
Classic 412
Stereo
16 Ohm
Right
2X160W Stereo

or

Mono
320W
4Ohm
Left.

Thanks for your help.

montec

Thank you very much gotBass!