# Speaker Ohms rating

Discussion in 'Electronic Basics' started by Rene, May 16, 2004.

1. ### ReneGuest

I understand that speakers come in different flavors; one of the flavors has
to do with the ohms rating of the speaker. For the most part, it looks like
the majority of the speakers come in 4 and 8 ohms.

My question is, what is the benefit between having an 8 and a 4 ohms
speaker? I know that you must match the speaker to the amplifier but why so
many ohms ratings? The only thing that I could come up with is that the
higher the ohms rating the better the sound quality that I will be able to
get from the speaker, otherwise there would be no purpose in having the
different ohms ratings, right?

Any help is appreciated.

2. ### The Al BundyGuest

The lower ohm rating how higher power output is. In cars 4ohm speakers are
used, even 2ohm speakers sometimes to make alot of noise...

Al

3. ### John PopelishGuest

In the days of tube amplifiers, where they had to have a matching
transformer between the tubes and the speaker, the transformer could
be wound to drive just about any speaker impedance, as needed. So the
impedance selected tended to favor whatever was best from the
speaker's standpoint. 8 and 16 ohm speakers were very common in this
era because this impedance range gave a good compromise between mass,
ruggedness, manufacturability, magnetic gap thickness, etc. When
direct coupled solid state amps were introduced, this impedance range
worked pretty good, as long as the power supply voltage could be
optimized to match the voltage requirements of the speaker. But
automotive applications are most cost effective if you can use the 12
volt battery as the supply, instead of converting it to some higher
voltage.

From the power formula P=V^2/R it is pretty obvious that a +- 6 volt
swing applied to an 8 ohm speaker is not going to deliver much power
(something like 4 watts peak after transistor saturation drops and 2
watts RMS). Even if you bridge the speaker between two out of phase
amplifiers to approach 12 volts in either direction you only get about
12 watts peak. So they either had to step the battery voltage up or
lower the speaker impedance to increase the power. They do both.

4. ### ReneGuest

John, thanks for responding but I would like to ask you two more question if
it's ok!

So why not make all of the speakers 4 ohms? I am sure that it wouldn't be
more expensive or a big deal to design a home stereo system that uses 4 ohms
(I would think). I am sure even production cost would be lower if you have
to produce only one type of speaker, right?

Also, can a 4 ohms speaker be created that could deliver the same amount of
decibels and clarity of any 8 ohms speaker that could be created?

5. ### tempus fugitGuest

Well, it's not really about clarity at all. A lower impedance (i.e., ohms)
speaker will have less resistance (or impedance) to current flow. Therefore,
more current will flow through the speaker, thus delivering more power.
The downside to this, of course, is that the amplifier must be able to
source (supply) this amount of current. So, if you have an amp rated at 25w
into 8 ohms, it will deliver more power into a 4 ohm speaker, and even more
into a 2 ohm speaker. However, it may not be capable of sourcing the amount
of current required to power a 2 ohm speaker, and burn out.

Most home speakers are 8 ohms, and I suspect that no standard has been
established simply because companies are designing their gear with
compliance in mind. No one is going to design a stereo amp for use at 16
ohms when all the speakers are 8 and vice versa. That being said, most home
stereo amps will be able to power 4 ohm speakers, but won't easily go down
to 2.

6. ### John PopelishGuest

Probably so. But these things have a certain inertia. And physics
haven't changed. The compromises that made 8 ohm speakers a good idea
before still apply. But it is not much off the sweet spot to make 4
ohm instead of 8 ohm.
I don't see why not. But what I don't see might sneak up and bite me.

7. ### Jan PompeGuest

It's fine for home stereo systems where the voltage of the power supply
can be tailored to fit the requirements of your output stage but when
supplied with a 12 volt battery it starts to become messy when you want
high power output - as in a car.

8. ### benchGuest

no, no, no,
putting a 4 ohm speaker instead of an 8 ohm one will *not* deliver more
power. What will happen is that the current will increase and this will
cause more voltage to be "wasted" on the internal resistance of the
amplifier and the voltage on your speaker will actually drop and hence
the power on your speaker will drop (V^2\R). This is known as the maximum
power
transfer theorem and can be prooved with simple calculus mathematics.
(well, I may be wrong, but I am sure if I am someone will correct me)
So you need to put exactly the speaker ohm rating which is specified.

9. ### John PopelishGuest

If we assume that feedback holds the output voltage constant, then the
speaker current will go up when you lower the speaker resistance.
Audio amplifiers do not usually operate with an impedance match
between amplifier and speaker, but act as low impedance voltage
sources. So if the amplifier is capable of holding the voltage the
same, changing an 8 ohm speaker to a 4 will double the current and by
P=I^2*R, the power will also double. The losses in the amplifier will
also increase. But assuming the amplifier can deliver the increased
current while still operating as a voltage source and can handle the
extra heat, this is fine.

In reality both of these assumptions are problematical. If you use a
speaker with a resistance lower than specified, you will usually
increase the distortion (as the amplifier current limits at the peaks
and thus, is not a voltage source, so the peaks sag) and the output
stage overheats and possibly is destroyed, eventually.

10. ### Rich GriseGuest

AFAIUI, that's not true any more. In the days of TOOBz, and mongo
output transformers, yes, you had to impedance match for pretty much
the reason you've cited - but these days, they make power amps with
an output impedance that's almost negligible. Transistors _do_ have
their place, after all.

Cheers!
Rich

11. ### Glenn GundlachGuest

Sir, I respectfully disagree. The amplifier will certainly deliver
more power else why does it run hotter? It appears you're assuming the
output voltage will 'droop' under the higher current. Any halfway
decent amplifier has a fixed gain (just a big opamp) so the lower ohms
will double the current doubling the power.
Henry Kloss used that 'trick' on the Smaller Advent loudspeaker in the
early '70s. The woofer was mass loaded to reduce the system resonance.
The mass reduced the efficiency so the impedance was dropped raise the
power level. A 'down' side of the reduced impedance is the damping
factor drops because the resistance of the wire becomes a larger
factor. (damping is the ratio of the actual amplifier impedance/load
impedance) Phase Linear claimed damping of 1000 on the old 700 watt
amp. That amp couldn't possibly work into the implied 0.008 ohm load
to 'match' the impedance to the load.

Now if you're working with a transformer coupled amplifier, the power
will not double because you use a different tap on the secondary.
However, these are definitely the minority.

Sorry my 2 cents turned into a nickel.
GG

12. ### Kevin AylwardGuest

It was *never* true for power amps. For example, the choice of
transformer in tube/valve amps has nothing whatsoever to do with
obtaining a power match in the sense of the maximum power transfer
theorem. Obtaining a "maximum" match is pretty daft. You would throw
away half the power.
Misleading, at best, incorrect at worst. Impedances for power amps are
chosen based on the maximum current and voltage capability for the
output transistors. Even for RF amps one designs the load so that it is
something like Vmax/Imax of the output devices at the device output.

Its obvious that if the transistor can take 100 volt, and only 100ma,
then to get its *real* max power of 10W requires a load of 1k. This load
needs to be transformed to say, 8 or 4 ohms.

This maximum power theorem is usually churned out wily nilly with no
idea of just what it means. For the most part, it is meaningless. The
"maximum" power of an audio amp with 0.01 ohm output resistance at 50
volts would be 62.5KW, clearly irrelevent.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

13. ### John FieldsGuest

---
Considering that an audio amplifier is a voltage source with a very
low output impedance and since power, in watts, is equal to:

P = ---
R

If you were to connect an 8 volt source to an 8 ohm load, you'd get:

E² 8²
P = --- = --- = 8 watts dissipated in the load
R 8

If, however, you were to connect the 8 volt source to a 4 ohm load,
you'd get, instead:

E² 8²
P = --- = --- = 16 watts dissipated in the load
R 4

So, by halving the load resistance you double the power it dissipates.

14. ### benchGuest

well, it seems I was wrong, thanks for the corrections.

15. ### CholsGuest

I have one question here.
Back in my engineering study days, we had to design an amplifier using
op amps for the first stage and equalizers, and then use power (though not
too big) transistors for the output stage, attaching a heat sink as needed
to prevent it from overheating. We had to drive a fixed load, that is,
deliver a maximum power without a given distortion.

But, and here's the trick, we also had to design the amplifier with a
protection so that if we short circuited the output stage, and we had to
prove this, the amplifier would NOT burn. I don't remember how exactly we
did it, but it was some sort of current controller in the output stage that
would shut off the power transistors if conducting too much current. It
worked (well, some in the class failed to work, beautifully turning the
transistors bright red and then pufff.... ;-) but the question is...
Aren't these short circuit protections added to comecial amplifiers as a
standard? :-o

16. ### John PopelishGuest

Just as with your class, there are a wide variety of protection
schemes with varying degrees of ruggedness. Production amplifiers
range from just poor designs to very cost reduced designs to well
proven and tested designs to absolutely iron clad. bullet proof, money
is no object designs.

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