Axiom Home Page Forums General Discussion Technical Questions Ohms--matching amplifiers and speakers

OK, the message board may have given me "Frequent Flier" status, but I'm really still a Newbie...........So, can anyone help me understand the concept of matching amplifiers to speakers by virtue of how many Ohms they can "handle"?

Thanks!

This will probably be far more info than you need but that's what happens when you ask an EE about Ohm's law.

The "Ohm" rating you see is a reference to the resistive load of the speaker. The lower the rating, the less resistance and the more power delivered, assuming current and voltage remain the same. The basic equation that governs this is Ohms Law:

V = I * R

Where V refers to the voltage potential between the two terminals, I refers to the amount of current flowing through the load and R is the resistance. Power is defined as such:

P = I * V

Plug the first into the second and you get:

P = I^2/R

As such, the lower your resistance, the more power you'll draw from the system. The electronics in your receiver have a set ability to deliver power. If you try and pull too much, you'll fry the components. Thankfully, all receivers (at least all of which I'm familiar) have internal circuitry to prevent the receiver from trying to output too much power. As such, most will simply turn off if you attempt to pull too much power from it.

That's why it's a good idea to look for a receiver that's rated to supply to such a small load. However, most receivers out there don't bother to spec it in their documentation but will provide to a load of this size with ease. When in doubt, ask.

Regards,
Josh

Thanks, Josh--I knew it was just a matter of dusting off that pre-med physics. It really makes perfect sense.

Jon

My pleasure. Any excuse to type out equations!

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It really makes perfect sense.

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Except that: P = V^2 / R (or P = I^2 * R)
Sorry to pick nits, Semi...

At any rate, the max power output of a solidstate amp is basically limited by its swing voltage (rather than the current) that can never go beyond the power-supply rail voltage. So, for a given max swing voltage (which is higher in a more powerful amp), the lower the load ohms (impedance), the higher the max power. This is as long as the amp can tolerate supplying the larger and larger output current into the speaker.

As shown in the equation, if the output current is truly a non-issue for an amp, then the max power at 4-ohm and 2-ohm load should be twice and four times that at 8-ohm load, respectively. Some expensive power amps, such as the Krell KAV series, actually meet this relationship, indicating that they use a very well-regulated power supply.

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Except that: P = V^2 / R (or P = I^2 * R)
Sorry to pick nits, Semi...

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Mother fuc...

Quite right. That'll teach me to blaze through the equations without actually thinking about what I was saying.

Yeah, P=I²R. Don't want to get Ohm hot at us.

You know it's a party when people start making engineering puns.

It's like being in college again.

Coming back to Doc's original question, if you solve the same equation for I, you get:

I = sqrt[P] / sqrt[R]

This means that the lower the load ohms, the more current the amp has to pump out for a given power output. More current means more heat&stress born by the power transistors in the amp. That is why the amp may eventually engage its thermal or overcurrent shutdown circuit when driving low ohm speakers very hard. Some speakers actually have impedance values dip drastically below its rated ohms, at certain frequencies. Those speakers, especially if they are less efficient and thus gobbling up lots of power, can cause shutdown problems on less tolerant amps.

Luckily, Axioms' impedance curves behave pretty well in general (no huge dips in the power-hungry bass frequencies), according to the NRC Canada measurements available online. Plus, the Axioms are very efficient speakers, leading to the claim that they are "very easy to drive'!


Impedance curve for the M22 (rated 8 ohm):


Impedance curve for the M80 (rated 4 ohm):


Source: www.soundstagemagazine.com

Damn you're good Sushi...

Now what does all that mean in newbie english?

Jim, let me give it a try. The expression of Ohm's law which is probably most in point with Doc Jon's question is I=E/R(i.e. current equals voltage divided by resistance). This shows us that if the voltage isn't reduced, when the resistance of the speaker is 4ohms rather than 8ohms the current will be doubled.If the receiver can't handle this and is in danger of overheating its protective circuit will cause a shutdown. To prevent this many receivers have a "4 ohm" switch on the back or setting in the setup menu. This reduces the voltage which can be supplied and thereby the maximum current flow,without a shutdown. Since it also reduces the maximum power available this setting should only be used when experience has showed it to be absolutely necessary.

Then, despite knowing all the equations,sometimes mysterious shutdowns plague even powerful THX certified(which certification includes testing with low impedance speakers)receivers such as early samples of sushi's Pioneer 45TX. Apparently Pioneer has now corrected what may have been an overly-sensitive protective circuit and now just about anything goes.

Oh, well--I *thought* it made perfect sense. Now I'm back to being confused! But don't worry, Sushi, I think I've got the basic concept.

theaterdoc

Holy impedance, Batman--I can't believe the avalanche of technical dialogue started by my innocent question!

theaterdoc

Thank you very much. I crave to learn this stuff inside and out. That was newbie enough for me. So now I understand the purpose of the ohm switch on back of my 1300. The lower the ohms, the lower the output of watts per channel rating of a receiver/amp?

Yes, Jim, in restricting the voltage and current, the maximum power available is also reduced when the switch is set to 4 ohms. So, regardless of what speakers are being used, the 4 ohm position shouldn't be set unless the receiver has been shutting off because of low-impedance speakers being driven.

Although it is way too late for me to be up and i've had about 2 bottles of a truly fine wine, i am a touch confused myself nowif not incredibly idzzy.
When ohms decrease, power output usuallly increases (preferably by double from 8 to 4 ohms.)
Can you clarify your last statement?

Point of note, i actually got my Onkyo 797 up to 85 on the absolute scale this evening.
Decibels measured 110 on the ol sound meter. (approx. 2.5m from centre speakers).

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The lower the ohms, the lower the output of watts per channel rating of a receiver/amp?

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jbzn, I think JohnK got you correctly. But just in case... if you meant by "the lower the ohms" the ohm rating of the speakers (not the position of protection selector switch on the back of a receiver), in fact the opposite is true.

"The lower the speaker's ohm rating, the higher the max power (watt per channel) that a receiver can pump out to the speaker" -- as long as the receiver does not shut itself down.

Sorry for the confusion, but English is my SECOND language, and it's been hard enough for me. The Newbiese would be my THIRD... LOL

Chess, by now you may have had the third bottle of that wine and are on the floor, but if you can read this, we're talking about two different 4 ohm situations. You're correct that if sufficient voltage and therefore current was available more power(theoretically double) would be available into a 4 ohm speaker as compared to a 8 ohm speaker. However, I was speaking of the 4 ohm SWITCH POSITION which limits voltage and thereby current and allows for less maximum power to any given speaker than when in the 8 ohm position.

Thanks for the clarification.
Not only was i reading something again in the wee hours of the morning but add to that a copious amount of wine, port and imported beer.
I need more sleep.

You're english is great. I just needed the explanation brought down a notch to my beginners level. Keep on passing the good information.

Can I hijack this thread and bring it down again to a practical level?

I was just about to connect my new QS8 speakers (surrounds) to my system, only to find out that they are 6 ohms and my receiver has this ugly sign on the back that says "8 ohms minimum impedance" !!!!!

Can I connect them anyway, or is this a "showstopper"?

Thanks for any comments.

Miguel

What receiver? I assume it has a fault circuit that will merely turn it off if you draw too much power.

My receiver is a Rotel RSX-1055.

Asuming it had this feature, would it be ok to connect them?

Thanks

Miguel, sometimes it's the company attorneys, not the engineers, who put those ugly signs on the back. All modern receivers have protective circuits to prevent overheating, but your 1055 should handle your QS8s with ease.

Hey, thanks for the response, John.

BTW, the fact of not having hooked up the surrounds yet had a nice side effect - I have been experimenting all weekend with 2 channel music using just the M22s and the subwoofer.

Boy.... those fronts can SING! They are just amazing. The staging, the clarity, the separation. I was listening to some orchestral music (specifically a beautifully recorded set of Rossini's overtures) -- how about being able to pinpoint the exact location of the TRIANGLE in the middle of a full-orchestra "fortissimo"? Amazing.

Kudos, Axiom. You made my day today, that's for sure.

Are you sure it say 8 ohm minimum? I checked the back of my Rotel RSX-1065 and it say Speaker impedence 4 Ohm. Which I didn't even notice before.


Saturn

Also I had 6 ohm speakers connected to my fronts for 6 months without any issues.

Saturn

Yeah, that's what it says. 8 ohms minimum. But it's a 1055, not a 1065. Maybe they're different in this?

However I've been assured by several sources that I'll be fine with my 6 ohms surrounds.

Thanks for the reply.
Miguel

"You're english is great."

Sorry, I couldn't resist.
I'm leaving now.