• New Patreon Tier and Early Access Content available. If you would like to support AVForums, we now have a new Patreon Tier which gives you access to selected news, reviews and articles before they are available to the public. Read more.

Speaker Impedances?

Sawz

Standard Member
At the moment I own a Yamaha AS500 integrated Amplifier which is rated at 120wpc at 40hms and 85wpc at 8ohms (it has separate switches for flicking between the two impedances on the back). Currently this is coupled with a set of Teufel Ultima 40 Floorstanding speakers rated at 4ohms (so obviously I have the amplifier set to the LOW - 4ohm minimum switch).

However today I have a set of Wharfedale XARUS 5000 6ohm Floorstanders being delivered. I know that I should set the amplifier to HIGH - 6ohm minimum, but I was wondering what actually would happen if I used the LOW setting? Not just sound-wise, but also the electronics behind it too?

I understand that 4ohm speakers can be used with an 8ohm amp, only they will draw a higher power (so you have to watch out for the amp overheating). But I just wondered what would happen using either of the two settings with these Wharfedale speakers on my amplifier?
 

Wilseus

Active Member
It would do no harm if you set it to LOW, but it would do no good either. AFAIK all that switch does when set to LOW is limit the amount of power the amp can supply, to prevent overheating. So all you'd be doing is limiting the maximum power of your system.
It would probably not do any harm to the amp if you drove 4 ohm speakers on the HIGH setting, so long as you didn't go too mad with the volume control.
I do not believe the position of the switch makes any difference to the sound.
 
Last edited:

PSM1

Distinguished Member
The general advice with AV receivers is to leave the switch set to the high or 8 ohm setting even with 4 ohm receivers. This is because the switch limits the current the amp can produce and this can decrease sound quality. The switch does nothing that careful use of the volume control can not do so there are no real gains as long as you can be trusted with the volume but you can lose some sound quality. With 6 ohm speakers I would definately have it set to the high setting.
 

Sawz

Standard Member
Ah cheers for the responses guys! All of that makes sense actually, so yeh I'll leave the amplifier on the High setting regardless of speaker, but just be more controlled with the volume!
 

BlueWizard

Distinguished Member
Just so we are clear, you are NOT planning to use BOTH sets of speakers on this amp ... RIGHT?

The amp will happily drive either speaker just fine. But, very very few amps will drive both pair of speaker together. The combined impedance is way too low.

As to the Impedance Switch, I agree with others. Switching to 4 ohms limits the output power to make the amp run a bit cooler. Far better to leave the switch at High Impedance, but that requires that you use your amp wisely. But for any sane, yet loud, volume, you shouldn't have any problem. It is only when you cross the boarder from Sane to Insane that problems arise. But then, that is always true. What you abuse, you break.

Steve/bluewizard
 

Sawz

Standard Member
Yes I'm only using one set of speakers at a time, my Teufel speakers are going to be repaired at the moment anyway... A screw inside the cabinet keeps vibrating! These Wharfedales are a cheaper, more robust pair of speakers that I can take to Uni whilst my nicer Teufel speakers await me at home!
 

Rikard

Standard Member
I understand that 4ohm speakers can be used with an 8ohm amp, only they will draw a higher power (so you have to watch out for the amp overheating). But I just wondered what would happen using either of the two settings with these Wharfedale speakers on my amplifier?

Erhm, it´s the other way around! a 4 ohm amp will drive any speakers from 4 ohms and upwards (higher load impedance = lower load stress).

If you set the amp to 8 ohm mode but load it with 4 ohms it´s more likely to end up with burnt output transistors. If you plan to use 6 ohm speakers, keep the amp in 4 ohm mode to be safe.

:)
 

BlueWizard

Distinguished Member
4ohm speakers can be used with an 8ohm amp

There is no such thing as an 8 Ohms amp. Amps have their power rated at 8 ohms so that the power of various amps can be fairly compared. If you test the power to a lower impedance, the power comes out higher. But the true limiting factor on real power is the Power Supply Voltage.

For example, here is the voltage limit on a 50w/ch and a 100w/ch amp (power to 8 ohms) -

50w = 20 volts
100w = 28 volts

If we assume the amps have sufficient power supplies, then to a 4 ohm load the 50w amp appears to be a 100w amp, but the voltage is still limited to 20 volts.

So, some manufacturers would inflate their power ratings by simply saying 100w/ch with no qualifying impedance. So, the government and audio users got together and set a standard for rating power in Hi-Fi amps. This is sometimes referred to as FTC or RMS. In this standard, power is always measure to an 8 ohm load, so that all amps can be fairly compared to each other.

Now, while most real Hi-Fi components must adhere to this standard, they can post or print other power ratings; rating to a range of impedance and Dynamic power in addition to the standard 8 ohm power.

Most quality amps can handle impedance loads in the range of 4 ohms up to 16 ohms, which covers a vast majority of speakers out there. As the impedance goes down, the amount of current goes up, and correspondingly, as current goes up, heat goes up. If the heat gets too high, the amp will (hopefully) shut down to protect itself.

So, the fact that the amps power is rated to 8 ohms, does not make it an 8 ohm amp. It simply makes it an amp. Most amps can handle loads from 4 ohms up to 16 ohms.

Now the High/Low impedance mode switch found on Yamaha, and other amps, it there to reduce heat for low impedance loads. But this is something of a cheat. From what I've been able to gather, it simply drops the voltage on the output stages, in effect, lowering the potential power, and thus making the amp run cooler.

I have a Yamaha amp and always use the HIGH setting on the Impedance switch because it gives the most uncompromised sound. But it does require that I use the amp with a degree of common sense, which I have always done. I've never blown a speaker or an amp, and I've thrown some raging parties in my time. Though sadly that was years ago.

Just a few thoughts.

Steve/bluewizard
 
Last edited:

Rikard

Standard Member
Well, amps are actually limited to a certain lowest load impedance, so I think it´s fair to consider an amp with a specified minimum load impedance of 8 ohms, an "8 ohm amp". It shouldn´t be used with lower impedances as this could potentially kill its output transistors as these will be conducting a larger output current, in turn meaning more heat dissipation. The thing with different settings for load impedance is just a cheap way for the manufacturer to offer slightly higher output power in a higher load impedance as the supply voltage can be increased as long as the amp is not loaded excessively.

:)
 

BlueWizard

Distinguished Member
OK, Rikard, you've got a fair point. But an amp that is limited to speakers no lower than 8 ohms, is not likely to be a very good amp. Though I'm sure there might be exceptions. I think a few of the Sony amps are limited to 6 ohm speakers, which makes me suspect a weakness in the power supply.

But most common amps can safely handle a load as low as 4 ohms, and unless I am mistaken, and I'm pretty sure I'm not, Yamaha amps are 4 ohm rated as are a few other amps.

NAD amps are certainly 4 ohm rated, but if you check the specs you will find the power rated down as low as 2 ohms, which indicates a pretty strong power supply.

So, yes, you do have a valid point, if an amp's impedance is limited to 8 ohms, then one could consider that an 8 ohm amp. But generally, the "8 ohms" simply represents the impedance that the amps was test under in conformance to some testing standard.

Steve/bluewizard
 

DodgeTheViper

Moderator
Onkyo Receivers are rated to 4 ohms. In fact the switching is either 4 or 6ohms, with 6ohms setting catering for 8ohms also
 

Rikard

Standard Member
OK, Rikard, you've got a fair point. But an amp that is limited to speakers no lower than 8 ohms, is not likely to be a very good amp. Though I'm sure there might be exceptions. I think a few of the Sony amps are limited to 6 ohm speakers, which makes me suspect a weakness in the power supply.

But most common amps can safely handle a load as low as 4 ohms, and unless I am mistaken, and I'm pretty sure I'm not, Yamaha amps are 4 ohm rated as are a few other amps.

NAD amps are certainly 4 ohm rated, but if you check the specs you will find the power rated down as low as 2 ohms, which indicates a pretty strong power supply.

So, yes, you do have a valid point, if an amp's impedance is limited to 8 ohms, then one could consider that an 8 ohm amp. But generally, the "8 ohms" simply represents the impedance that the amps was test under in conformance to some testing standard.

Steve/bluewizard

I competely agree with you, an amp that is rated to minimum 6 or 8 ohms is most likely not very good as an amp (there are always exceptions though). The weak current handling capabilities strongly suggest it cannot fully cope with heavily reactive load conditions, such as excessive phase shift loads (as most speakers tend to be). All parts of the amp are put to more stress with a lower load impedance, but I would be most worried about the output stage itself, and maybe not as much the power supply. It can usually handle short term current peaks without any problems, but a weak output stage is almost certianly going to limit the top currents, causing audible distorsion, especially with very short term peaks within a cycle that can cause the stage to burst into oscillation temporarily.

:)
 
Last edited:

Nawty

Active Member
All this assumes that a speaker is a perfect load, which it isn't - the impedance of the speaker varies with frequency and can vary massively, some 8ohm speakers will easily drop to 2ohms at some frequencies (often near the frequency the port is tuned to). This is why some amps can power some speakers better - the cope with the impedance changes much better.
 

DodgeTheViper

Moderator
What is a better amp, is it designated by design and quality components ?
 

Nawty

Active Member
Well, better is a largely subjective matter but in this case I will say it's an amp that has sufficient current supply to cope with the wild drops (and rises) in impedance without it affecting power output.

Also, damping factor and THD are proportional to impedance so those would be considerations too.

Naturally, that may not sound better to your ears with your system in your listening room...
 

BlueWizard

Distinguished Member
All this assumes that a speaker is a perfect load, which it isn't - the impedance of the speaker varies with frequency and can vary massively, some 8ohm speakers will easily drop to 2ohms at some frequencies (often near the frequency the port is tuned to). This is why some amps can power some speakers better - the cope with the impedance changes much better.


Within limits you are right. Most commonly an 8 ohms speaker will drop down to about 3/4 of its rated impedance, though it could go up to 10 times the rated impedance. I would be extremely rare for an 8 ohms speaker to drop down below 4 ohms though.

However, because of cabinet loading, there are some B&W speakers nominally rated at 8 ohms that can drop as low as 3.2 ohms at certainly frequencies. But this is rare. Usually this lowest impedance dip is just above and/or just below the resonance frequency. Oddly the resonance frequency is the point there the impedance is highest.

Keeping in mind that a common 4 ohms speaker is likely to drop to about 3 ohms, so a normal good consumer amp should be able to handle the load. Again, the real enemy is heat, heat caused by current, which in turn is cause by low impedance. In a speaker with a complex load, the impedance rarely lingers at its lowest, and therefore is less likely to general enough heat to cause the amp to shut down.

However, low end bargain amps probably don't have sufficient power supply or cooling reserve to take overly complex or low impedance loads.

Likely if an amp has it's power rated at 4 ohms continuous, it has sufficient power supply and cooling to handle all but the most complex loads.

Steve/bluewizard
 

BlueWizard

Distinguished Member
What is a better amp, is it designated by design and quality components ?

I would say it is first, the power supply. If the supply has sufficient reserves, then the amp probably sound pretty good. The very best design can't cut it if the power supply can't keep up with demand.

Second, I would say is cooling or heat dissipation. If it can't get rid of heat, that is going to stress the components.

Thirds, quality of components and design. If you look at Marantz amps, most of the high end consumer amps have the same 70w/ch. So we assume in the more expensive versions, the money went into better design and components of higher quality with tighter tolerances.

But even the best theoretical design doesn't always sound the best. Sometime design theory and actual sound are at odds with each other. Though among experienced designers, this is less of a problem.

Steve/bluewizard
 

Rikard

Standard Member
OK, Rikard, you've got a fair point. But an amp that is limited to speakers no lower than 8 ohms, is not likely to be a very good amp. Though I'm sure there might be exceptions. I think a few of the Sony amps are limited to 6 ohm speakers, which makes me suspect a weakness in the power supply.

But most common amps can safely handle a load as low as 4 ohms, and unless I am mistaken, and I'm pretty sure I'm not, Yamaha amps are 4 ohm rated as are a few other amps.

NAD amps are certainly 4 ohm rated, but if you check the specs you will find the power rated down as low as 2 ohms, which indicates a pretty strong power supply.

So, yes, you do have a valid point, if an amp's impedance is limited to 8 ohms, then one could consider that an 8 ohm amp. But generally, the "8 ohms" simply represents the impedance that the amps was test under in conformance to some testing standard.

Steve/bluewizard

An amp that is rated also for a lower impedance, 2 ohms for example, does not only indicate a proper power supply, but more importantly a stable amp design with proper phase margin. Driving a low impedance load is difficult from all aspects, but the most tricky one is how to avoid oscillation. That´s where a good amp differs from a poorly designed one.

What is a better amp, is it designated by design and quality components ?

I would say mostly the design. The components themselves are usually only directly responsible for sound quality when placed directly in the signal path at places where no NFB is involved. As soon as NFB is involved any signal errors and deviations are reduced by the amount corresponding to open loop gain divided by closed loop gain. Of course there are exceptions, and that´s where it´s important
to use quality components, but actually you can make a fantastic amp using just crap components! Or put in other words, a poorly designed amp will never sound great just by replacing components with "high end" stuff. And the other way around, an excellent circuit design is likely to sound great no matter what components you use. (within limits)


And one more thing, an amp that measures perfectly well in all respects will sound just as the input signal which is in my world how it should be. Some people prefer the sound from a certain amount and kind of distorsion added, which is ok of course. But one thing is certain, an amp that sounds awful has some issue that will show up on some of all the different measurements that can be done. There is no way you can fool all of todays measurement methods, and I do disagree with anybody claiming that a zero distorsion amp would sound anything less than perfect.

:)
 
Last edited:

The latest video from AVForums

Fidelity in Motion's David Mackenzie talks about his work on disc encoding & the future of Blu-ray
Subscribe to our YouTube channel

Full fat HDMI teeshirts

Support AVForums with Patreon

Top Bottom