Speaker impedance vs amplifier impendance

[Miks]

Hello! Lately I've been struggling to wrap my mind around impedance and wattage. I have 4 ohm 90 watt rated speakers and amplifier rated at 8-16 ohms and 50 watts per channel. I have searched and searched for information but i can not make it sound logical. If amplifier wattage for 8 ohms is 50 watts then at 4 ohms it will be 100 watts. So speaking from numbers the amplifier will be able to support the speaker. Then why everywhere i search it says that i should not overdrive the amplifier cranking the volume up too much. Why if the amplifier at 4 ohms is 100 watts and the speaker is 90 watts. Like where is the missing link that would finally make it clear why i can or can not cross impedances and watts?

 

[thesteve]

So power is the product of current and voltage. And the current drawn is inversely proportional to the resistance.
Which means that as resistance decreases, current increases.
You said as much in your post. The thing is though, the spec of your amplifier is what it is.
If your amp could deliver 100 watts at 4 ohms, the manufacturer would have said so. It can't.
It can safely deliver 50W into 8 ohms or more. At 4 ohms it strains the amp and draws twice the current at a given volume level.
You probably won't damage anything, but don't expect more than 50 clean watts. If it were that easy everybody would buy 4 ohm speakers and have twice the power.

 

[tuberadios69]

If the amp doesn't state it can do 4-ohm speakers, i would punt. It may not have an adequate power supply for that impedance. What is the model and make of your amp? That info would help.

 

[JoeESP9]

The first thing to acknowledge is, speaker wattage ratings are worthless and useless. You can safely use virtually any amplifier with any speaker as long as you pay attention to the sound.

If you turn it up too loud, and it sounds strained you're either overdriving the amplifier or the speakers. In both cases the solution is to turn it down.

As for amplifier wattage ratings, very few amplifiers will double down wattage. Those that do will tend to be large, heavy and expensive. Receivers need not apply and most integrated amplifiers are in the same category.

 

[bowtie427ss]

amplifier rated at 8-16 ohms and 50 watts per channel.

Does it have provisions for more than one set(pair) of speakers?

 

[Lavane]

You could put your amp into thermal shut down using 4 ohm speakers. I have a 80wpc integrated amp that will go into thermal shutdown using 4 ohm speakers. Even at lower volumes. It never does using 8 ohm speakers.

 

[bowtie427ss]

If his amp has parallel A + B speaker outputs, it will handle a single 4 ohm pair of speakers.

I have a 80wpc integrated amp that will go into thermal shutdown using 4 ohm speakers.

If your amp is rated for 4 ohm use, I would opine that either your 4 ohm speakers have an impedance curve that swings moderately below 4 ohms in places, or your amplifier has inadequate thermal dissipation. Is it vintage? has the thermal compound dried out?

Not all 4 ohm speakers present the same load to an amplifier. At the same time not all 80 watt integrated amplifiers handle a 4 ohm load with the same ease.

Assuming all components are in good health and meet designed spec, it really depends which amplifier and which speakers.

 

[mcgjohn]

also depends on how nasty an impedance drop or absolute low point, the speaker has at a given frequency. Some 4 ohm speakers are pretty easy to drive, while others have some wild swings in the plot of impedance vs. frequency.
some speakers also have a large impedance peak at a given frequency which is a different challenge.

 

[Lavane]

If his amp has parallel A + B speaker outputs, it will handle a single 4 ohm pair of speakers.



If your amp is rated for 4 ohm use, I would opine that either your 4 ohm speakers have an impedance curve that swings moderately below 4 ohms in places, or your amplifier has inadequate thermal dissipation. Is it vintage? has the thermal compound dried out?

Not all 4 ohm speakers present the same load to an amplifier. At the same time not all 80 watt integrated amplifiers handle a 4 ohm load with the same ease.

Assuming all components are in good health and meet designed spec, it really depends which amplifier and which speakers.

The power supply was recapped along with new thermal paste. I tried several different 4 ohm speakers. After a short time it would go into thermal shutdown.
I've had other amplifiers not rated for 4 ohm speakers did ok, but ran hotter than
They should.

 

[bowtie427ss]

The power supply was recapped along with new thermal paste. I tried several different 4 ohm speakers. After a short time it would go into thermal shutdown.
I've had other amplifiers not rated for 4 ohm speakers did ok, but ran hotter than
They should.

The one hard and fast rule of audio, exceptions.

 

[Lavane]

I've just never thought it a good idea to use 4 ohm speakers if the manufacturer states not to, or only lists 8 or 16 ohm in the manual.

 

[50nstillhifi]

Throwing in another "element of consideration".. your music material ? You did not mention what that is and I am thinking it would certainly give a measure of the demand put on your speakers. Also, there is the dynamic range that is quite different from the nuances of spec related to your amp and speakers, including the impedance.

The speakers have a crossover, typically it is rated as well and holds the value near the larger driver, the woofer. If you going to drive your speaker system into loudness, absent the amp and speaker ratings, that crossover will either support this or not. Well.. you can damage some great speakers ignoring this rating, particularly without circuit protection.

Use the impedance and proper matching to at least get you into a "safe" level of operation. From there you can then adjust up/down how you can tailor your sound or power (watts) to your speaker system.

To ignore this, is undoubtedly very risky for either your amplifier, speakers, or both.

 

[bowtie427ss]

I've just never thought it a good idea to use 4 ohm speakers if the manufacturer states not to, or only lists 8 or 16 ohm in the manual.

Many times on the unit itself it specifies only 8 or 16 ohm speakers when the unit has parallel switching for A + B sets of speakers. The unit itself is safe for 4 ohm operation.

But, to avoid any confusion or mistakes that would be detrimental to the unit, the manufacturer will not mention the use of 4 ohm speakers even though the use of one, and only one pair at a time would be within the unit's safe operating range.

It's that exception thing again. I don't think any two manufacturers really spelled it out the same.

I like the way they did it with the SX-1250, pretty clear that you can only run one pair of 4 ohm speakers at a time. It also has a unique switching mechanism whereby you may only play any 2 pair at a time. but, not all 3.

 

[rothwellaudio]

If amplifier wattage for 8 ohms is 50 watts then at 4 ohms it will be 100 watts.

By that logic the amp will deliver 200 watts into 2 ohms and 400 watts into 1 ohm. Into a short circuit the power would be infinite!!
It doesn't work like that. Amps will fail (or go into protection mode) when presented with an uncomfortably low impedance.

 

[62caddy]

If amplifier wattage for 8 ohms is 50 watts then at 4 ohms it will be 100 watts.

That would be true in a perfect world but the sad reality is that few (and usually only high end) amplifiers are designed with such capability.

A 4 ohm speaker draws twice the current of an 8 ohm therefore the theoretical voltage the amplifier would be able to develop (while remaining within distortion spec) would be halved leaving you with roughly the same "50 wpc".

If only 8 and 16 ohm ratings are provided by the manufacturer, it should be assumed the unit isn't designed for 4 ohm operation.

 

[whoaru99]

In a perfect world all amplifiers would operate as a perfect voltage source*, meaning they would be capable of producing the same output voltage into any load, thus double in power for each halving of impedance.

However, very few amps come close to being a perfect voltage source as others have mentioned. Most amps operate in the scheme of voltage source though, increasing output to some degree more or less, as impedance drops.

*unless, of course, you want a current source amp for a particular use case

 

[crazy-in-az]

The lower the ohms, the heavier the electrical load on the amp. Some amps will handle low impedance loads with ease, some will struggle a bit. Still others will either go into "protection mode" to keep themselves and the speakers from getting toasted, or will simply play along until they die in a puff of blue smoke.

Each amplifier is different, and if the manufacturer does not expressly state that it can handle 4 ohm loads, then it probably can't. It's better to search for a 4 ohm rated amp and be safe then risk losing both the amplifier and speakers, possibly even the upstream components as well.

 

[spark1]

In a perfect world all amplifiers would operate as a perfect voltage source*, meaning they would be capable of producing the same output voltage into any load, thus double in power for each halving of impedance.

However, very few amps come close to being a perfect voltage source as others have mentioned. Most amps operate in the scheme of voltage source though, increasing output to some degree more or less, as impedance drops.

*unless, of course, you want a current source amp for a particular use case

Even with a perfect voltage source, to really double its power output into halved impedance in real-world use, wouldn't the amp need to be designed with enough output devices and heat sinking to accommodate the doubled current (and associated heat)?

The cost of these things, and the cost of a highly regulated power supply, explain the difference between the ideal amp and real-world amps. The closer an amp is to the ideal, the greater its cost...and each step closer becomes exponentially more costly.

 

[MaxP]

I plan on using my new JBL 580s with my Pioneer SX 5590 (essentially an SX 1250), despite the 6 ohm rating. Am I pushing my luck? I plan on running them alone.

 

[whoaru99]

Even with a perfect voltage source, to really double its power output into halved impedance in real-world use, wouldn't the amp need to be designed with enough output devices and heat sinking to accommodate the doubled ..
.

If it were a perfect voltage source the implication is it would inherently have the capability... else it wouldn't be (a perfect voltage source).