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Speaker Impedance (3 Ohm) Query?

whimsical

Standard Member
Hello All,

I'm hoping someone can advise regarding a technical query.

I'm looking at some 2 1/2 way speakers that appear to have a minimum impedance of 3 Ohms (sensitivity of 90-91db if that is significant). I am also looking at a Marantz PM8003 amplifier that is rated at:
Power Output (8/4 Ohm RMS), 70 / 100W.

My question therefore, will 3 ohm speakers be OK for the aforementioned amp? Not sure how much of an additional load this represents!

Many many thanks​
 

Rich9600

Established Member
What speakers are they? 3 Ohm seems very low! What is their average or nominal impedance? Is this 3 Ohms the only figgure quoted?
I have the PM7003 with Monitor Audio RS6's (NOMINAL 6 Ohms) and the combination sounds great. The PM7003 is very similar in terms of it's output (70W at 8 Ohms, 100W at 4 Ohms), the PM8003 is just a tad more refined with slightly better components and the associated price hike.

It should work, but if they are out of the recommended range of the amp it is unlikely to sound as good as with speakers in the optimised range. Lower impedance speakers draw more power and sensitivity is a measure of how loud they are for a given power input. It would follow that 3 Ohms would draw something in the region of 110W... I can't remember the EMF equations from physics lessons so this is just a guess!

I would think that this would mean that there would be a greater risk of overpowering the speakers as they would draw more power. This could lead to speaker damage, particularly if listening to a dynamic piece at high volume.
 

whimsical

Standard Member
Thanks for the input Rich9600. The speakers I am looking at are actually a DIY design (looking to put woodworking tools to use):

CNO 2.

The notes next to the impedance summary state that a 25-35 watt valve amp will drive these adequately but I was of the understanding that valve amp power is not the same as chip amp output. The Impedance stated is minimum. Would nominal impedance mean average?
 

Rich9600

Established Member
That looks like an interesting project. It appears that they don't actually know the final impedance as there appears to be a couple of crossover options. Each is likely to give a different overall impedance.

It seems like the speakers are designed to be able to be driven by lower powered class A valve amps. The Marantz is a class AB, from what I can remember, and seems much more powerful than the speakers require. It would surely drive the speakers with no problem.

However, I wonder if you may get a better sound out of a lower powered valve amp...

I've no idea what to suggest... Except, build the speakers and try a few amps with them. I'm sure somebody on this forum will have a better idea than me!

Good luck. It looks like it will be fun!:smashin:
 

BlueWizard

Distinguished Member
What the designer of these speakers say is -

"System impdance = 3 ohms minimum, the price to pay for going 2½-way. No problem for solid state amps or 25-35 watt valve amps.
Even my 8 watt 300B SET amp can run these speakers at normal listening level, which tells we can never fully predict how difficult a load a speaker will be without actually trying."


Keep in mind that the Re or DC resistance of any driver or speakers is usually about 0.75 of the nominal rated impedance. So it is common for 4 ohm speakers to drop to 3 ohms, just as it is common for 8 ohm speakers to drop to 6 ohms.

I think you can safely consider these 4 ohm speakers. These are 2.5-way speakers, in the 0.5-way where both 8 ohm woofers are in parallel, you have a nominal 4 ohms. The one 0.5-way bass speaker fades out slowly as is typical of this design, so it is still having some effect above the drop frequency, but overall, again, I think you can consider these 4 ohm nominal speakers.

The designer implies in the quote above, that he has had no problem driving the speakers with common low powered amps. So, I think you are safe. If this was a problem, or if this required a special amp, I think the designer would have come right out and said that.

Steve/bluwizard
 

whimsical

Standard Member
Bluewizard, Rich9600, Sincere thanks for the advice!!!

Will consider these speakers 4 ohm (and not a problemto drive) as suggested.

Thanks again!
 

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