Ohm - does it matter

J

Jayv79

Guest
i have recently purchased the Sony STR-DB790 and Tannoy FX5.1 home cinema kit.

I have delayed the setup because of conflicting ohm differences....

The speakers state a nominal impedance of 6ohm?? what ever that means, but the amp only has a switchable 4 and 8 ohm?

are not the speakers compatiable with the amp?

help!!
 

KraGorn

Novice Member
In a nutshell, and not necessarily a completely accurate scientific explanation (before the pedants chime in ;) ), and 'ohm' is the unit of resistance, analagous to 'volts' as the unit of electrical potential. Another term for 'resistance' is 'impedance', though they differ in detail in practice you can consider them interchangeable as far as amps and speakers are concerned.

As long as the 'ohm' rating of the speaker EXCEEDS that the amp. is expecting everything will work fine, but NEVER connect speakers with a lower resistance than the amp. needs as you risk blowing the speaker.

So, select the 4 ohm setting and you'll be fine, the only noticeable thing would be if you ever got hold of 4 ohm speakers they'd sound a bit louder with the same setting on the volume control .. but electrically a 4 ohm amp into a 6 ohm speaker is perfectly safe.
 

Reiner

Active Member
are not the speakers compatiable with the amp?
The funny thing is actually that the figures stated are not representative of the true impedance nor technically correct, they are merely a figure for comparision (compability) purposes.
E.g. amps do not have an impedance as such, the figures just mean that the amp is compatible with 4 to 8 Ohm speakers.
Since 6 Ohm speakers fall inbetween it's no problem at all, in fact the manual will tell you to choose the 8 Ohm setting if the speakers have an impedance of 8 Ohm minimum or switch to 4 Ohm if the speakers have an impedance between 4 and 8 Ohm.

Actually you could even use the 8 Ohm setting with 4 or 6 Ohm speakers, as long as you don't drive the system too hard it's possible; the switch will in fact half the current to the power amp stages to protect it from overheating.
Most modern AV amps have a protection circuit which will shut down the power amps but there is still a risk that it starts clipping and that will kill your speakers. So better go with the manufacturers recommendations and choose the 4 Ohm setting - with the exception of less headroom you should not notice any tonal differences. (During normal operation you require a few watt only, not the full / max. power)
 

Apollo11

Active Member
I am using a Pioneer amp with Kef front speakers, the amp has 2 impedence settings: 4 - 6 Ohms & 8 - 16 Ohms, the speakers claim to have a nominal impedence of 8 Ohms, I checked this with a multi meter and got a reading of 4 Ohms, Im not really sure if this is because the multi meter is measuring the resistance at 'DC' ?, I have the amp set at 8 Ohms but am wondering if I should switch to 4 - 6 Ohms ?.
 
B

bobbypunk

Guest
i was always told as a rough guide that 2 ohms either way would be unlikely to cause any significant problems.
I'm happy to have this torn apart by those in the know, but it sounded a reasonable rule.
 

eviljohn2

Novice Member
Originally posted by bobbypunk
i was always told as a rough guide that 2 ohms either way would be unlikely to cause any significant problems.
I'm happy to have this torn apart by those in the know, but it sounded a reasonable rule.
Those in the know say it's reasonable around the 8ohm mark, but not in the 4 ohm region.

It's a 25% difference for 8ohm nominal but 50% different for 4ohm nominal.

In general, cheap speakers will be fine with 8ohm settings whilst more expensive ones need the 4ohm settings.
 

KraGorn

Novice Member
I don't see the problem selecting 4 on the amp and knowing you're safe, rather than selecting 8 and taking any sort of risk. :confused:
 

Reiner

Active Member
i was always told as a rough guide that 2 ohms either way would be unlikely to cause any significant problems.
I'm happy to have this torn apart by those in the know, but it sounded a reasonable rule.
A speaker certified by the German DIN norm says it should not drop more than 20% below it's nominal impedance, so that's 3.2 Ohm for a 4 Ohm speaker.
There is no upper limit as this usually doesn't cause a problem and 'normal' speakers often reach up to around 20 Ohm.
(E.g. Dynaudio Contour 1.3MkII is considered a 4 Ohm speaker but the impedance varies between 3.5 - 14.4. Ohm)

In general, cheap speakers will be fine with 8ohm settings whilst more expensive ones need the 4ohm settings.
I don't see any base for this rule, expensive doesn't equal 4 Ohm and cheap doesn't equal 8 Ohm.
The setting is to protect the amp from being overloaded and is impedance dependent, not price dependent.
If in doubt select 4 Ohm, as Kragorn says that's the safest setting you can choose.
 

eviljohn2

Novice Member
Originally posted by Reiner
I don't see any base for this rule, expensive doesn't equal 4 Ohm and cheap doesn't equal 8 Ohm.
The setting is to protect the amp from being overloaded and is impedance dependent, not price dependent.
If in doubt select 4 Ohm, as Kragorn says that's the safest setting you can choose.
Hence the "In general" bit ;)
In general, larger and better quality motor assemblies have a lower impedence.

But as has been mentioned numerous time, if you're unsure set it to 4ohms.
 

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