Ohm's law

Quote:-

Can I assume that my amp sees the Nbls as half the resistance & therefore an easier load to drive??
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Nothings that simple Matt. A speaker that is rated at just 4 ohms is harder to drive than a speaker that requires 8 ohms.

The NBL is quite a demanding speaker and most people say that it requires a 500 before it really comes on song.


Regards


NB

Matt

Please ignore the previous three replies. You are correct. A 4 Ohm speaker is easier to drive than an 8 Ohm speaker.
My 110 can produce 110 watts into a 4 Ohm load, it will only produce about 75 Ohms into a n 8 Ohm load.

Fatcat

quote:

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Originally posted by Nick S:
it still means the amp is having to put twice as much current through the 4 ohm speaker to get the same effect.

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Nick,

I believe that it is the current drawn (rather than the voltage put out from the amp) which drives the voice coils of the speaker.

I suspect that a low-impedance speaker may be able to draw a lot of power out of a very powerful amp. If this were not the case, I'm sure that most speakers would be in the 6-8 ohm range.

cheers, Martin

E-mail:- MartinPayne at Dial.Pipex.com

By reducing impedance you get a higher sensitivity. Reducing impedance from 8 ohm to 4 ohm gives an increase in sensitivity by 3 db (I think). It looks nice on paper but requires an amp that is capable of delievering current in to a low impedance. So an 8 Ohm 87db loudspeaker might be easier to drive than a 4 Ohm 90db one.(if the amp doesn't double the current when impedance is halfed)

[This message was edited by bjorne on MONDAY 23 June 2003 at 22:59.]

Isn't the rating RMS, ie. unlike real music?
Thus it's an indication rather than an absolute.
(Kind of like a car engines maximum torque.)

quote:

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Please ignore the previous three replies. You are correct. A 4 Ohm speaker is easier to drive than an 8 Ohm speaker.
My 110 can produce 110 watts into a 4 Ohm load, it will only produce about 75 Ohms into a n 8 Ohm load.

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I assume that's a joke

Based on the laws above, if R reduces by 1/2 from 8ohms to 4ohms, then power should rise by a factor of two, from the 75 watts to 150W (both channels summed).

It cannot do this, only managing 110W, because it's having to work much harder (source more current) that the PSU cannot cope with, hence the internal supply voltage drops, reducing the 'V' bit above.

Patrick's reply is correct.

The complication comes when you consider those 4 ohm, or 8ohm figures are nominal loads, and not maintained over the entire audio band.

At certain points in the band, the impedance could be MUCH lower (some as low as 1 or 2 ohms), or MUCH higher making things more complex. I wouldn't expect Naim speakers to display radical dips in impedance though.

A good amp helps to negate the differences.

Andy.

Matty boy,

Trust you are now out of pie turbulence and into clear blue skies.

Let's call a spade a spade. What you are really asking is : "should I sack the current board of directors in favour of a shiny new triumvirate of 500's ?", to which the answer is a hearty "YES". Can I take the liberty of suggesting that after you have craned them black beauties into your shopping cart you maximise the opportunity and forklift in a 552. Oh, and you will be needing a Supercap and Snaxo and it would be rude to leave out the CDS3.

It may seem a bit overkill, but as explained above, custard tarts take an awful lot of driving.

As Maxi Me BSc (in Physics I should add) I would like to say that over simplification is the order of the day.

For dc systems V=IR and P=IV through a resistor. (Wow 11 years later its all coming flooding back, I'm an accountant these days)

ac through resistors, inductors and capicitors gets a lot more complex, including 'complex' maths with the square root of -1 (Uh, doesn't even trickle back to me)

Even so the correct upshot is that as nominal impedance (average resitance as a function of signal frequency) drops the current requirements increase. Amplifiers struggle to deliver current. Therefore lower resistance/impedance is harder to drive.

Using Google to check I spelt Impedance correctly I came across this: Acoustic Impedance
See I told you we were over simplifying

Seth
Is Naim the only way forward?
Ahh, Ahh! I'm burning!!

Hi Folks,

Interesting thread this one. So why does impedance vary with frequency? Seems that impedance seems to drop in the lower frequencies with a lot of speakers making them difficult to drive.

Cheers, Jens

Matt,
Don't bother with the maths, all the above is B.S. (except Maxi Me) or uncomplete. If you really want to understand, go back to school and have a degree in electrical engeneering.
Believe your ears, you'll find 135s or a new 250 as the minimum to drive NBLs.
4,6 or 8 ohms rating doesn't mean anything. It doesn't take into account the really important factors making a speaker an easy load or not: what is the minimum impedance, is the impedance mostly inductive or capacitive, is the impedance constant with frequency or not...
If you could find a 10 watts power amp, with very high current capacity you could drive NBLs in a small room with enough dynamics. NBLs are quite a easy load, you can make them sing with a Nait5, but... The limiting factor is not the power but the quality of the amp. NBLs are easy to drive but very revealing, they need a very good source and preamp (nothing to do with power).

Jens,
Drivers don't have a constant impedance: they typically have a min impedance at DC, a peak at their resonnant freq, some time going back lower than DC, and a slow rise at high freq. The tunning of the enclosure will affect the impedance: A well made transmition line will have a constant impedance, a closed box will have 1 peak at the resonnant frequency of the enclosure, bass reflex will have 2 peaks apart the tunning freq of the port...Passive cross-over will seriously affect the impedance curve, that's their job: their impedance rises when you want to attenuate a driver output. Enclosure's vibrations will create ringing in the impedance curve. It is economically impossible to do a constant impedance loudspeaker, and even if economically possible, the efficiency would be very poor.

Emmanuel

All opinions are my own, and reflect those of the organisation i work for, even if not stipulated.

[This message was edited by Manu on TUESDAY 24 June 2003 at 05:03.]

[This message was edited by Manu on TUESDAY 24 June 2003 at 05:13.]

Cheers guys for your replies!!

Interesting stuff, Patrick is poss to show the power rating diff between what the 180 & 250 would put into 4 ohms using your equation. Incidently it would also be interesting to find out the same for the 500 beast!


Ok so basically it's probably a good idea to increase the amount of current the amp can deliver, hence the 250,135 & even 500 prompts.

I'll wait for the nbl's to run in before I seriously judge the sound I,m getting ..but things are sounding fairly acceptable at present

My aim is to eventually go active with three olive 250's ... this upgrade process will probably go something like ;

1)Change 180 for 250

2)Add supercap to 82..whilst retaining the hi for the snaxo.

3) Aquire over time two more 250's of similiar vintage..& of course a snaxo 3-6 and lets not forget the two extra runs of Nac a5!!

Mad Boy Matt

Voltage is like 'pressure' and current is like 'flow rate', to make a plausibly useful analogy.

CDPs and phono preamps produce signals that appear as voltages varying in time. There's an assumption that the cones of a loudspeaker will follow the voltage, say at +1v they stick out 1cm, at -1v they withdraw 1cm and pro-rata inbetween. Too many volts and they hit the endstops and sound nasty.

An ideal power amplifier is a voltage source. The voltage on the output is a constant multiple of that on the input regardless of what load is connected. The lower the load the more current that must flow to sustain a given voltage.

If one were to plot maximum output voltage of an amp against load impedance the curve would start out near zero into very low loads then rise towards a plateau that represents the input voltage as the load impedance rises and less current is required. A NAP110 can do 35W into 8 Ohms, which is about 16 volts, but into 4 Ohms it drops to 55W which is only 14.8v.

In general better amps sustain their 'constant voltage' characteristic to lower impedances, but eventually they all must give up.

FWIW Naim generally name their power amps with the total power available into 4 Ohms. So a NAP140 can do 70 Watts per channel into a 4 Ohm load, NAP250 125W, NAP135 135W and presumably a NAP500 250W. I'm sure the convenient numbers are a complete accident...

Paul

My advice would be to get a 52 or CDS2 in there first and go for 135s or wait for some NAP300s to hit the used market. A couple of years from now they should be going for c.£2500 and Nap500s less than £4K

quote:

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Why do they do this when most speakers are 8 ohms ?

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You'd probably have to ask them, but I'd guess it's a number that reasonably describes the relative capability of each amp.

And 'NAP250' has a nice ring to it.

Paul

For the near future I'm sticking with the olive stuff so I quess the cds2 & 52 are an option ....later on ...... I'll probably go for either a super or 250 first!

Sorry Bosh I'm not going over to the dark side just yet

Mad Boy Matt

Thanks James and Paul to enter this post, you`ve explained things much better than me.
I was quite upset by the completly false assumptions made at the begining.

Guys, if you don`t know, don`t speak.

Emmanuel

All opinions are my own, and reflect those of the organisation i work for, even if not stipulated.

And furthermore, unless you've got an active system, the load is that of the crossover, not the speaker.

Paul, BSc(Hons) (also Electronics, UMIST, early 80's, remember damn-all!)

quote:

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Originally posted by Matt worlock:

I have recently replaced my 8 ohm speakers with Nbls that are rated at 4 ohms. Can I assume that my amp sees the Nbls as half the resistance & therefore an easier load to drive??

Or doesn't it work quite like that??

Any advice greatly appreciated

Mad Boy Matt

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Matt, it doesn't quite work like that.

The measurement of loudspeaker impedance is not so simplistic to explain. Some of the postings so far have touched on certain elements of the explanation, but your question has yet to be fully answered, nor will it be here. Ohm's law does not tell the whole story.

First off, you cannot measure a loudspeaker's impedance with an ohm meter. An ohm meter measures DC resistance. Amplifiers do not make DC, they make AC. If your amplifier is making DC (clipping) you have a problem. DC would be the equivalent of zero Hz, which does not exist in the audio spectrum.

In order to accurately measure a loudspeaker's impedance, you need an AC ohm meter which is a very specialized piece of test equipment. It can be had as a stand alone piece, or part of a larger test piece (distortion analyser, etc.)

Nominal impedance is "supposed" to be the lowest value seen by the amplifier from 20Hz-20kHz, therefore, a 4 ohm speaker would never dip below 4 ohms load at any frequency. It is up to the manufacturer to specify this and to be responsible for it's accuracy. Some brands are better at this than others. The industry is very blase and glib when determining specifications. Imagine, the spec for frequency response is (traditionally)always rated +/- 3dB. It is therefore perfectly acceptable for a loudspeaker to have a frequency response which is, for a rediculous example, +3db at 60 Hz, -3dB at 61Hz, +3db at 62Hz, -3dB at 63Hz, etc. It probably doesn't happen, but the industry say's it is perfectly acceptable.

Music is dynamic, and a loudspeaker is a dynamic load, so the impedance of a loudspeaker will change at every frequency. We don't listen to pure tones. That's why an AC measurement is required, and the test is performed as a "sweep" from lowest (20Hz) to highest (20kHz) to achieve the impedance graph. The most crutial area for the loudspeaker designer is in the low frequency range, as the frequency drops the power required for linear output increases. It is therefore important that a loudspeaker be a very stable impedance load at low frequencies. The high frequencies are also important, but this is more an issue of skillful drive unit design as a tweeter draws very little power and operates over a much wider range of frequencies.

Not all amplifiers increase or double their output into 4 ohms as opposed to 8 ohms. It depends on the topography of the design. Some amplifiers actually have lower output at lower impedance loads. Amplifiers are measured into a fixed or "dummy" load, which is static, driven normally by a pure tone of 400 Hz or 1kHz. This is how the manufacturer comes up with the numbers.

Some loudspeakers are developed to become 4 ohm or 8 ohm speakers by design. Others are rated after the design is completed and the graph is charted (happenstance). Some are intentionally rated improperly to falsefy their rating, the idea being to trick the customer who thinks a 4 ohm loudspeaker will play louder than an 8 ohm loudspeaker. This is one of the most "dark" areas of the hi fi business, as it is not even that well understood by many of the people who actually produce commercial loudspeaker products.

When a company designs and builds their own drive units (or designs and has a reputable manufacturer build their drive units) the behaivior of those drive units can be accurately controlled within their passband, and a very simple crossover network can be used. Epos loudspeakers are a good example of this, simple first order crossover, 6dB per octave, one component in the "network". Easier to predict and control performance and impedance. When drive units are bought "off the shelf" from different suppliers, they must be brought together and "married" through a complex crossover network. If you have ever seen the crossover for the B&W 801, you have seen a very complex network. More difficult to control performance and impedance.

An amplifier and a loudspeaker from the same manufacturer are a pretty good bet to work properly together. A combination of products from different manufacturers known to many users (like Naim and Neat, Naim and Royd, etc.) to be very good together are also a safe bet. It's when you take unknown quantities of components and start putting them together that strange things start to happen.

Sorry for the long post. It doesn't even go into some of the many other dimensions of crossover design and amplifier compatibility. It is just the more important ones, I believe.

The woman just hung up on me when I laughed hysterically at fatcat's post.

As for prowla - my deepest sympathy for your scars from studying at the most godawful university ever. The only way I can think to improve it is to burn it to the ground!

Somehow this thread makes me want to become a physics teacher

Andrew

there are 10 types of people in this world... those who can read binary, and those who can't...

James

I said "And furthermore, unless you've got an active system, the load is that of the crossover, not the speaker".

You correctly replied with what I meant to say "The electrical load seen by the amplifier is a combination of the crossover and the driver units attached to it".

Paul

(I'm almost motivated to go and buy a book on the topic!)

Andrew

"The only way I can think to improve it (UMIST)is to burn it to the ground!"

Erm - I don't think concrete burns very well...

Paul