Speaker impedance?!? More confusion...

[Marcw]

Ok, I know I should already know this but I'm gonna ask some more advice. All my speakers - B&W DM602 S2, CC6 S2, Kenwood CM7ES Surround (next in line for replacement!) have nominal impedance ratings of 8 ohms, the B&Ws state minimum impedance 4.3 ohms.

My amp, a Yamaha DSP-A592 (getting on a bit, have an external DD5.1 processor though so it's not all bad!) has an impedance switch on the back which can be set for the speakers to have an impedance of 8 ohms, or 6 ohms. Currently it's set to 8 ohms, is that right? Does anyone know what difference it would make if it were set to 6 ohms? Could either setting damage my speakers?

What a minefield!!!

Cheers,

Marcw.

[Bapapapa]

If they are all rated at 8ohms then leave the amp set to 8ohms. To be honest though, I'm as confused with all this resistance business as you are, but as long as you don't crank the amp to it's limits then your amp/speakers will be fine whichever setting you use.

:nuts:

[ToffeeCrisp]

Stolen from What HiFi:

A low impedance draws a high current flow from the source, while a high impedance draws a little. This means that speakers with a low impedance (lower than 6-8ohms) are more difficult for an amplifier to drive.

[Bapapapa]

ToffeeCrisp - I know that. But I still don't understand the implications of it.

:nuts:

[ToffeeCrisp]

This should help :)

What Hi-Fi info on sensitivity (http://www.whathifi.com/scripts/sadbuttrue.asp?id=39)

The harder the speakers are to drive, the more strain they put on the amp, and the less power it has to give some umphhff for those big noise swings I suppose.

[Bapapapa]

The harder the speakers are to drive, the more strain they put on the amp, and the less power it has to give some umphhff for those big noise swings I suppose.

Yeah I know that too. Maybe I'm just thick because I still don't understand the technicalities of it.

:nuts:

[Marcw]

I guess then that if I set my amp to 6 ohms and my speakers are 8 that I could potentially be driving them too hard? Does that make sense based on the quote from What Hi-Fi?

[Just]

Bap, imagine impedance as a tray of treacle. I want to roll some ballbearings (the signal) through that tray.

The impedance (treacle) is either thin and watery (16ohms) or thick and gloopy (4ohms). The effort it takes to push the ballbearings through the treacle increases if the treacle is thicker (lower impedance).

While that sounds like you should just go for the highest impedance speakers going, having some resistance gives the amp 'control' over the speakers to some extent and therefore improves the sound characteristics.

A comparision for this part would be power assisted steering, while it makes it easier to steer a big car, it also reduces the 1 to 1 control relationship between the driver and the car.

Let me guess - you already knew that;)

[Just]

Originally posted by Marcw
I guess then that if I set my amp to 6 ohms and my speakers are 8 that I could potentially be driving them too hard? Does that make sense based on the quote from What Hi-Fi?

Yes, the amp would be expecting to drive something that is harder to drive than it really is. The implications of this could be damage to the speakers. If the speakers say nominal is 8 ohms all round, then this is the setting. The max and min impedance rating can be somewhat ignored.

[Marcw]

Everyone,

Thanks very much - question answered. I love these forums, you can get pretty much anything answered here!

Marcw.

[Bapapapa]

Nice analogy, Just. But you see that's what confuses me - how comes the thicker the treacle, the lower the resistance? Surely it should be the other way around?

:nuts:

/me hides from all the EEs laughing & pointing

[Just]

I dunno - I was always crap at Physics......

[Roy]

If I remember my elctronics days properly, and I don't so anyone who wants to contradict me feel free ;)

It's all to do with the amount of current drawn by the speakers.. a lower impedance draws more current from the amp, this begins a cascade effect that eventually leads to clipping. I think the damage to your amp is caused when it overheats too much..

The higher the impedance the easier the amp can drive the speaker as it does not have current "dragged" out of it, avoiding the cascading effect.

I'm off to see if I can find my books...:)

[DVDWotcha]

Guys it's all about balancing.

Amplifiers are designed with output load in mind. Internally there is a parallel load applied accross the output terminals of the amp. In the case of an 8 ohm speaker there would be an 8ohm internal load. This equal balancing ensures maximum power delivery to the speaker driver. If you unbalance the amp by using a speaker with a lower impedance you have to supply more power to the speaker driver. This can melt an Amps power amp circuits.
If on the other hand you use a speaker with a higher impedance (say 16ohms) the amp will not be able to provide enough power to drive the speaker, this leads to distorsion and speaker damage. (similar but not the same a clipping - which also damages speakers).

So this is why your amp has a 6ohms switch, to balance it with a 6ohm speaker, but as you have 8 ohm speakers leave it on 8ohms !!

[Neon]

This is how it really is:

Power amplifiers have a very low output impedance (around 0.1 ohm or less) and act as a constant voltage source. So, all things being equal (i.e. power supply and output stage up to it), they will produce the same voltage across the speaker terminals for any given input voltage and volume setting.

(Incidentally, the term "damping factor", which manufacturers often quote, is the load impedance divided by the output stage impedance. So, for a (nominal) 8 ohm load and an output stage impedance of 0.1 ohm, the damping factor will be 80).

Four volts RMS (say) across 16 ohms requires 0.25 amps (dissipating 1 watt). Four volts across 8 ohms requires 0.5 amps (2 watts). Four volts across 4 ohms requires 1 amp (4 watts).

I am surprised that your amp is switchable for 6 or 8 - the difference is marginal - I have seen 16/8.

The intention is, as has been stated, to protect the output stages in the event of too much current being drawn, which may result in clipping and/or damage.

The only complication is that speaker impedances are nominal. An 8 ohm speaker may actually vary between 4 (or lower) and 16 (or higher) ohms at various frequencies anyway.