ohm question

[spencer.b]

So I'm no expert with ohms etc but am I right in thinking if you're using one cab and a biggish head a 4ohm cab would be getting more power out of your amp,if so then why are most 4x10 and 2x12 8ohm,I'm thinking of getting something like an aguilar 212 to use on it's own so am I better off getting a 4ohm one if I can find one?

[skidder652003]

they're quite often 8ohm so you can pair them up and go down to 4 ohm if you fancy playing bigger venues. For most pub gigs one cab is enough so a 4 ohm cab is fine, to be honest an 8ohm cab on its own is also fine as the actual difference in volume isn't all that great, but the real experts will be along soon enough....

[grandad]

A 4ohm cab will draw twice the power from the amp as an 8 ohm cab. The loudness will be noticable but not twice as loud.

Is it loudness you're after?

[spencer.b]

Yeah I'm after loudness .I do all sorts of gigs but one of my regulars is 3 piece wedding band where we take our own pa with no sub. Another is a trio with cigar box guitar, bass and drums , sometimes there is decent pa but sometimes there isn't and I have to fill the room and the bass is quite prominent in the songs
I've got a matamp 410 which does the business but I can't be bothered carrying it as I gig 4ish times a week
Thinking of a aguilar db212 but maybe the 4ohm is a better idea IF I can find one?

Edited April 25, 2016 by spencer.b

[warwickhunt]

A sensitive 8ohm cab will be as 'loud' as a 4 ohm cab with less sensitivity... added to which more drivers/displacement means you will think you are louder. You don't say what power your amp is but with cabs of equal sensitivity you might have to have your amp turned up a fraction more for the 8 ohm cab... things are never as straightforward as 'X' v 'Y'!

[spencer.b]

I use an orange ad200b which I love the tone of but I also have an aguilar ag500dc which I'll take if need loads of volume

[Dad3353]

[quote name='spencer.b' timestamp='1461624730' post='3036327']
I use an orange ad200b which I love the tone of but I also have an aguilar ag500dc which I'll take if need loads of volume
[/quote]

You will never on this Earth need more volume than a 200w valve amp can put out,be it into 2, 4, 8 or 16 ohms. An Aguilar 500 will fill most halls, too. I wouldn't worry too much about ohmage with those amps; think more in terms of sensitivity (db/watt..?), spl (how much noise it can make..?) and portability/practicality.
Just my tuppence-worth.

Edited April 25, 2016 by Dad3353

[spencer.b]

Sorry for being dense but could someone explain sensitivity in laments terms
So is the general gist that if I downsize my cab then the ohmage isn't critical cos both my amps are powerful enough to put as much power into the cab as it needs?

[spencer.b]

This has got me thinking maybe my orange is a bit knackered!!

[Dad3353]

[quote name='spencer.b' timestamp='1461625706' post='3036332']
Sorry for being dense but could someone explain sensitivity in laments terms
So is the general gist that if I downsize my cab then the ohmage isn't critical cos both my amps are powerful enough to put as much power into the cab as it needs?
[/quote]

Depends on the venue, really, but, as an example, we use a Hiwatt 200w valve amp and it's inconceivable that it be turned up full whack; the walls would need plastering.
Sensitivity..? Layman's terms..? Imagine a billiard cue and a billiard ball. Touch the ball and off it goes. Now, replace the ball with a basketball. Use the same power in cueing; the basketball will hardly move.
A more sensitive cab will give more sound than a less sensitive one, given the same power input (the cue, remember..?). There are many reasons for this, but it's not necessary to know all about it. Usually, a 'reference' power of 1 watt is given (the cue push...) and the db is the amount of sound produced for that 1 watt input. One may produce 95 db (the basketball...) another 105 db (the billiard ball...). The higher the db figure (for the same power in...) the more sensitive the cab.
All well and good, but a very sensitive cab that can't handle enough power to fill a room is no good for a rock band. You also need to know how much sound (spl...) the cab can produce at its maximum. Some will state 95 db, others 102 db or whatever.
Be warned, of course, that these are just numbers. 102 db of lousy sound is not better than 95 db of quality tone. The real test is the ears, hearing the cab, preferably in context. Go to see bands playing in the same vein and see what they use..? Borrow (or hire...) a cab and try it for a couple of weeks..? Don't just go by figures.
Hope this helps.

Edited April 25, 2016 by Dad3353

[Bill Fitzmaurice]

[quote name='grandad' timestamp='1461621353' post='3036286']
A 4ohm cab will draw twice the power from the amp as an 8 ohm cab. The loudness will be noticable but not twice as loud.
[/quote]Yes, and no. With the same amp settings the 4 ohm cab will be 3dB louder, which is noticeable, but not significant. But at the full power capacity of the cab they'll have the same output. There's really no advantage to a 4 ohm cab unless your amp is rated at less than half what the cab is.

[Dad3353]

[quote name='spencer.b' timestamp='1461626632' post='3036342']
This has got me thinking maybe my orange is a bit knackered!!
[/quote]

Here's a copy of a post of mine from September last, concerning our Hiwatt...

[color=#800080][font=helvetica, arial, sans-serif]There are two versions of this amp; 6 x EL34 and 4 x KT88. Both are very powerful; I have a preference for the KT88, simply for the clean headroom. Not best suited for O/P valve 'grit', as these valves are very hard to 'bend'. Clean, chiming, natural... Rare indeed, so I cheat and use a DR205 PA head instead. Different tone stack, 6 I/Ps (I've used it at a French Bash with several basses and guitars plugged in at once...), but the same awesome power.[/font]
[font=helvetica, arial, sans-serif]An anecdote..? Yes..? OK, then, but you'll have to be patiently seated...[/font]
[font=helvetica, arial, sans-serif]I got the amp from a young lad in Kingston, Surrey. I popped over with our youngest to collect it, on foot; my elder brother picked us up from the ferry in a [/font][font=helvetica, arial, sans-serif]Jaguar[/font][font=helvetica, arial, sans-serif]. Parking in Kingston is not easy; we found the street, but had to park quite a fair way further down the street. Big Brother stayed in the car whilst we went to see the amp. The bloke brought us through to his back kitchen, where the amp was sitting atop of a 4x12 cab. He brought out a bass to plug in, and we twiddled about for a few minutes, the young'un on bass, and me turning the dials. All seemed to work just fine. The fellow then said "I'll show you that it's working well..." and fetched his Telecaster guitar. With no further warning, he plugged in, turned up the juice full belt and let out a power chord. I, and our youngest, leapt back about a yard and a half; we were stunned. We let the amp cool a bit, popped it into its flight case and carried it out to the car, about 500 yards down the street. Blinking heavy, are [/font][font=helvetica, arial, sans-serif]Hiwatt[/font][font=helvetica, arial, sans-serif]s, but I digress... Elder brother had a concerned look on his face, and asked us what in the name of blazes had happened. He'd been quietly listening to the radio in the [/font][font=helvetica, arial, sans-serif]Jaguar[/font][font=helvetica, arial, sans-serif] when he heard this almighty crash from down the street. The sound had carried through the house, down the street and into a closed car, enough to make [/font][i]him [/i][font=helvetica, arial, sans-serif]jump, too..! Oh how we laughed...[/font]
[font=helvetica, arial, sans-serif]Loud..? Yes, these amps can get very, very loud. We don't play that way, and don't use tone controls anyway (straight 'plug'n'play', old school...), but the headroom is heavenly, right up to the sky. Excellent amps; wonderful for bass, but heavy. Very, very heavy.[/font][/color]

[color=#282828][font=helvetica, arial, sans-serif]Just sayin'.[/font][/color]

Edited April 25, 2016 by Dad3353

[spencer.b]

Thanks dad , I get sensitivity now , nice one. I'll check the figures , tbh I've owned or gigged most things , loved my berg hds never should of sold them and thinking about it they didn't seem to get more out the amp together at 4ohm than my matamp at 8ohm ( different cabs though I )

[grandad]

http://barefacedbass.com/technical-information.htm - Good information here if you have the time to read it.

With so many variables to consider it has to come down to trusting your own ears. The sensitivity is just another static measurement under certain or more often uncertain conditions. So how to try out other cabs before shelling out any cash is the problem.

[taunton-hobbit]

Just thinking aloud, but I wonder if the Orange might benefit from a bias/general checkover?
These are a very good amp, should be easily capable of shattering eardrums.......

[spencer.b]

I do find my amps loud enough I just thought if I'm gonna downsize my cab would the ohmage be critical but now I know otherwise

[Conan]

[quote name='spencer.b' timestamp='1461622832' post='3036313']
Yeah I'm after loudness
Thinking of a aguilar db212 but maybe the 4ohm is a better idea IF I can find one?
[/quote]

The main disadvantage of a 4 ohm cab is that if you find yourself requiring more volume (or even needing more height, so that your cab is more audible to you yourself), you can only add another cab if your amp can safely run down to 2 ohms. Not that many do.

The Aggie may well be a good solution, but an 8 ohm version will not be noticeably quieter and still gives the option of adding a second cab if you need/want it.

I'd go for the 8 ohm version

[EliasMooseblaster]

This has got me wondering again: I recently bought the most delightful Bergantino 2x12 from another upstanding member of this parish, and in my haste to give it a spin, I immediately connected it to the 8ohm output on my Little Bastard - force of habit from my previous, single-driver cabs. It was only when I got it up to the studio and was about to connect it to my larger CTM-100 that I realised the sticker on the back said "4ohms." Whoops.

Fortunately the LB has not since exploded, so I presume no permanent harm has been done, but I understand that the point of matching these resistances is to make sure there's enough load up against the amp output. Would I be right to conclude, therefore, that this mistake wouldn't harm my amps or cab, but running the 2ohm output into a 4ohm cab might have done?

[Conan]

[quote name='EliasMooseblaster' timestamp='1461683062' post='3036715']
Would I be right to conclude, therefore, that this mistake wouldn't harm my amps or cab, but running the 2ohm output into a 4ohm cab might have done?
[/quote]

I don't think amps have outputs with different impedances, do they?

My understanding is that the output from the amp is in Volts, and that depending on the impedance presented by the speaker cab, that voltage creates a certain amount of movement in the speaker that we perceive as sound. More voltage = more sound.

The socket on the back of the amp indicates the minimum (or even recommended) impedance of any cab connected to it...

I believe all of the above to be true. That doesn't mean that it is, of course!

Edited April 26, 2016 by Conan

[EliasMooseblaster]

[quote name='Conan' timestamp='1461683551' post='3036722']
I don't think amps have outputs with different impedances, do they?

My understanding is that the output from the amp is in Volts, and that depending on the impedance presented by the speaker cab, that voltage creates a certain amount of movement in the speaker that we perceive as sound. More voltage = more sound.

The socket on the back of the amp indicates the minimum (or even recommended) impedance of any cab connected to it...

I believe all of the above to be true. That doesn't mean that it is, of course!
[/quote]

It sounds like we're thinking along similar lines - obviously the cab itself provides the 2/4/8ohm resistance that the amp's output voltage 'sees,' but I assumed there would be some sort of resistance between the output transformer and each of the output sockets to tame the output voltage to an appropriate level for that load.

[4stringslow]

My understanding is that it's important to match impedances with valve amps, which is why they often have some form of impedance selector switch/jumper to enable them to be used with different impedance cabs. Too high or too low can be bad news with a valve amp.

With solid state amps, matching is less important as long as you observe the minimum specified cab impedance. You'll lose power output with higher impedance cabs but it won't do any actual harm.

[grandad]

A valve audio amplifier has a high output impedance. In order to drive a low impedance load such as a loudspeaker, a step-down transformer is placed between the amplifier and the load. 2, 4 and 8 ohm speakers require different size secondary windings for a correct impedance match. These may be switched or use separate terminals.

A SS audio power amplifier has a very low output impedance. No step-down transformer is required and the stated minimum speaker impedance, usually 4 ohms and occasionally 2, depends on power and thermal considerations.

[Merton]

[quote name='Conan' timestamp='1461683551' post='3036722']


I don't think amps have outputs with different impedances, do they?

My understanding is that the output from the amp is in Volts, and that depending on the impedance presented by the speaker cab, that voltage creates a certain amount of movement in the speaker that we perceive as sound. More voltage = more sound.

The socket on the back of the amp indicates the minimum (or even recommended) impedance of any cab connected to it...

I believe all of the above to be true. That doesn't mean that it is, of course!
[/quote]

Mr MooseBlaster's amps are all valve though so have different transformer tappings for 8, 4 and (in the case of the CTM100) 2 ohms.

Valve amps are different from solid state - going too low on the impedance is *generally* not quite as bad because of the way the current and voltage is delivered and soaked up. With solid state amps the lower the impedance the more current the output transistors try to pass. It gets to a point where they can't keep passing more current so they eventually go pop.

Edited April 26, 2016 by Merton

[mcnach]

[quote name='spencer.b' timestamp='1461618578' post='3036245']
So I'm no expert with ohms etc but am I right in thinking if you're using one cab and a biggish head a 4ohm cab would be getting more power out of your amp,if so then why are most 4x10 and 2x12 8ohm,I'm thinking of getting something like an aguilar 212 to use on it's own so am I better off getting a 4ohm one if I can find one?
[/quote]

the difference in volume will not be that big between the 4ohm and the 8ohm cabs, so I'd go for the 8ohm one. It gives you the option of adding a second cab later on if necessary and THAT will give you a noticeable increment in volume

[grandad]

I agree with getting an 8 ohm cab and have the option of adding another.