Electronics > Projects, Designs, and Technical Stuff
Audiophile help please - Ohms and power
DW1961:
If you have an amp rated at 100 watts RMS total @ 4 Ohms (but can power 4-8 Ohm speakers), if you plug 8ohms speaker into it, do you get 50 watts total in reality?
I thought a 4 Ohms speaker would be more sensitive than an 8 Ohm speaker, thus requiring less power for the same volume. I've been reading what sounds like it is the opposite. If a 4 Ohms speaker is less resistance, wouldn't that make it easier to drive with less power?
Thanks again.
DaJMasta:
The answer is not really, if you take the basic ohms law V = IR and the basic calculation for power P = VI, you'll find that to get a given power you need a combination of current and voltage applied. So with a lower rated resistance of your speaker, there needs to be more current applied by the driver, and with a higher rated resistance it needs more voltage to get the equivalent power dissipated by the speaker in either case.
Since your driver says it can drive 4 or 8 ohm speakers, it can likely put 100W into either (since it would be sort of fudging its specifications if it can't!), but for any given amplifier, you're going to be limited by your driver supply current when driving lower impedance (resistance) speakers and you're going to be more limited by your drive voltage capability when dealing with higher impedance speakers.
It ends up being finding an optimum point on a few curves given your driver circuit and your load (the speaker), and usually the maximum output of the driver comes along with some distortion, so there's also probably a THD concern when driving at the edges of your driver's output ratings.
So more generally, a speaker rated for however many watts should sound equivalently loud as another speaker with a different rated impedance properly driven with the same power - though depending on speaker construction and choice, there can be some difference in efficiency of turning the electrical power into vibrations in the air given the physical construction of the given speaker.
Psi:
The speaker is just a load.
If you put a 4 ohm load across a PSU that's outputting 12V you get 36W
If you put a 8 ohm load across a PSU that's outputting 12V you get 18W
The 4 ohms speaker draws more current for the same voltage.
So a 4 ohm speaker will pull more power from the amp and be louder than an 8 ohm speaker.
Of course there is a limit, the amp must be able to supply the current.
You can't put a 1 ohm speaker on the amp and expect even more power. Unless the amp is rated to drive 1 ohm speakers
DW1961:
--- Quote from: Psi on July 30, 2020, 07:41:24 am ---The speaker is just a load.
If you put a 4 ohm load across a PSU that's outputting 12V you get 36W
If you put a 8 ohm load across a PSU that's outputting 12V you get 18W
The 4 ohms speaker draws more current for the same voltage.
So a 4 ohm speaker will pull more power from the amp and be louder than an 8 ohm speaker.
Of course there is a limit, the amp must be able to supply the current.
You can't put a 1 ohm speaker on the amp and expect even more power. Unless the amp is rated to drive 1 ohm speakers
--- End quote ---
Okay, so if you run identical speakers except for ohms, then the lower Ohm speakers would put out the same volume with less power needed?
Psi:
--- Quote from: DW1961 on July 30, 2020, 07:48:23 am ---
--- Quote from: Psi on July 30, 2020, 07:41:24 am ---The speaker is just a load.
If you put a 4 ohm load across a PSU that's outputting 12V you get 36W
If you put a 8 ohm load across a PSU that's outputting 12V you get 18W
The 4 ohms speaker draws more current for the same voltage.
So a 4 ohm speaker will pull more power from the amp and be louder than an 8 ohm speaker.
Of course there is a limit, the amp must be able to supply the current.
You can't put a 1 ohm speaker on the amp and expect even more power. Unless the amp is rated to drive 1 ohm speakers
--- End quote ---
Okay, so if you run identical speakers except for ohms, then the lower Ohm speakers would put out the same volume with less power needed?
--- End quote ---
Nope.
Volume is power and power is volume.
You cant get free power or free volume.
If you run two identical speakers except for ohms one will use more power and be louder and one will use less power and not be as loud.
You can get more efficient speakers which are louder than other speakers at the same power.
But this is not ohms related, this is because they are made out of lighter materials so need less power to move air.
Both speakers are getting the same power, but in one of them more of that power is converted into sound.
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