how ohms affect amplifiers and speakers/

fguihen

im new to this whole hi fi game. i got a set of decent tannoy mercury m2 speakers with an impedance of 8 ohms. im looking at a sony amp, but it works at 4 ohms. what difference will this make? will it give me much lesser sound quality??

here is the amp

http://www.peats.com/cgi-bin/shop/db.cgi?view=1&id=5898&type=6&path=10x4199x4335

the speakers get a mention on this site:

http://store.teleconsystems.com/Details/d_Merc2.htm

andy1249

The fact that the sony is designed to work into 4 ohms and your speakers are 8 will not affect sound quality to any degree noticeable.

That rating on the amp tells you what the amp is capable off in terms of driving a load , any load less than this , ( you dont really get less than 4 , but lets imagine the amp was rated for 8 and your speakers were 4 ! ) , would cause the amp to overdrive and result in distortion , in some cases possibly even damage , however most amps these days would cut out before that happened.

So to sum up , the Ohm rating of the speakers is the load the amp will be driving , as long as the load is the same or higher than the amp rating, all should be well .

Incidentally , I am currently using a Sony STRB790 and mine has a switch for 4 or eight ohms !! Check yours , it may have the same.

Hope this helps

Mad Mike

Andy1249 is on the ball - you can happily drive an 8 Ohm speaker from a 4 Ohm amplifer - However you cannot drive a 4 Ohm speaker from an 8 Ohm aplifier.

The lower the impedance in Ohms the more current the speaker will draw from the amplifier. Every amplifier has a minimimum impedance that it can drive (representing the maximum output current). Using a speaker with a higher impedance will work fine - you just won't be using the full power of the amplifier.

For the techie minded it should be pointed out that the impedance rating of an amplifier is not the actual output impedance of the amplifier - The amplifier itself generally has a very low output impedance. The concept of impedance matching for maximum power transfer does not apply in this case.

Sparky

Just think simply that an ohm is a unit of resistance, the higher the ohm of the speaker, the more resistance there is. So in the case of 8 ohms the amp will drive it happily because it can take the current. Then take say for example 2 ohms, since its less of a resistance your amp will give more current which the amp cant handle.

ZENER

It should be noted that a speaker with an IMPEDANCE rating of 8 Ohms only presents this value across an unspecified (in most cases) range of frequencies. For example at 50Hz the speakers may present a load of 2 ohms to the amplifier while at 10KHz it could be 20 ohms. Matching a speaker to an amplifier is all about (technically speaking) finding a speakers whos impedance characteristics closely match those of the amplifier that will be driving it in the hope of realising a flat frequency response across the range. This also applies the other way around too, an amp will have an output rating based across a certain frequency range, it won't be flat from 20 - 20K.

Complicated business this hi-fi stuff but suffice to say that "in general" match the impedance of both speaker and amp as closely as possible, use at least a 1.5mm CSA cable for interconnecting. Most important of all, listen to the music - not the equipment, i.e. if you're listening to the equipment too much then it's getting in the way of your music and isn't doing its job.

EDIT: One other thing: Resistance IS NOT the same as Impedance !!

My tuppence worth !

ZEN

Cinemaworks

Unless your protection circuit is cutting in on your amp, you should not change this switch.

the dynamic capability of your amp/receiver will be reduced if you do...

most good quality amps have decent power supplies and heat transfer that will drive a 4 ohm load without any problems at normal listening levels as movie soundtracks are not at a constant level all the time...

considerations for using 4 ohm speakers are:

1) Plenty of air around the receiver and not covering the air vents..
2) Dont play with the ohm selector switch unless your amp is cutting out...
3) dont drive your amp at constant high volumes (no matter how good your home cinema amp is)
4) give me a call if you are worried...:D

Thanks Adam

Sparky

ZENER wrote:

Resistance IS NOT the same as Impedance !!

Well resistance is taken as a pure resistance and is constant, were as impedance is the combination of resistance, inductance and capacitance and varies as the frequency changes, plus is measured in ohms. So I guess thats what you mean.

ZENER

Sparky-s wrote:

Well resistance is taken as a pure resistance and is constant, were as impedance is the combination of resistance, inductance and capacitance and varies as the frequency changes, plus is measured in ohms. So I guess thats what you mean.

Close enough I guess, but it's important to realise that when talking about reactive loads like loudspeakers, which contain pure resistance, capacitive reactance and inductive reactance (R+XL+XC) and amplifiers which are designed to work into loads such as this.

Sorry for being pedantic but a true hi-fi buff would have a stroke here.:)

ZEN

Sparky

I would of thought it was: Z = √R²+(XL-XC)²
Where Z is the impedance and will change as the inductance and capacitance change.

Will I start on resonance

ZENER

Sparky-s wrote:

I would of thought it was: Z = ?R²+(XL-XC)²
Where Z is the impedance and will change as the inductance and capacitance change.

. . . or to be more precise in this application and context,

Z = √[R²+(XL-XC)²]

Where Z is the impedance which changes with frequency. Which is why the nominal impedance quoted for loudspeakers only applies over a certain frequency range in between resonance peaks.

Will I start on resonance

Will I start on phase relationships ?

ZEN

Sparky

Yes do, ive yet to do it.

mathias

Or to be even more accurate , seeing as how inductance and capacitance for any particular system will in all likelihood be fixed in the speakers and cables ,

XC = 1/2 fc

and XL = 2fl

where the only variable in any fixed speaker system is the frequency , or the frequency range over which the system will work , which is usually the audible range , from say about 10 hz to 20000 hz, Reactance both inductive and capacitive will change in step primarily with Frequency.

Audio systems though , are nearly always tested with a 1khz sine wave at 1v- peak to peak , so that will be the basis for the Ohm rating on any particular set of speakers.

Apologies but the question marks above are supposed to be pi , boards doesnt seem to recognise the symbol font !!

ZENER

Loudspeakers can have 2 or 3 impedance peaks across the frequency range 20 - 20K, these are caused by resonance and vents or tuning ports. Between these peaks, which may occur at 20 to 50 Hz and 2 to 5KHz (caused by voicecoil XL increasing with F), there is an area where the speakers "nominal impedance" is found which - typically 8 ohms @ 200Hz for domestic hi-fi speakers.

1KHz is normally used for testing distortion figures of amplifiers or determining SPL from speakers.

All these problems are largely solved by bi-amping and getting rid of crossovers.

ZEN

Sparky

Cheers, Ive done AC theory and Im doing the speakers thing aswell, both are realitivly simular. ie: the peak to peak, RMS Hz etc.