Why 4 Ohm Loads Stress Your Amplifier

The Infinity's are rated nominal 6 Ohm, max 150 W.

Your Sansui 6060 receiver seems to be rated 40 wpc at 8 Ohm:
http://www.radiomuseum.org/r/sansui_stereo_receiver_6060.html

The Marantz is rated at 38 wpc at 8 Ohm, and 48 wpc at 4 Ohm:
http://www.classic-audio.com/marantz/2238b.html

IMO, both are underpowered to really drive the Infinity's to their potential. Since Marantz is rated to 4 Ohm, it is probably a better choice. Just do not crank up the volume, or you will damage the receiver, the speakers or both...
I don't have any receiver yet, but I can pick up the Sansui 606 or marantz 2238b for a good price. What vintageish receiver would you recommend to drive my infinity's?
 
And how can I know if speakers will go below 4 ohms? Sorry for asking so many questions, Im new to this :p

They probably will at some point, what matters is what frequencies they do it at, how far they drop, and how long they stay there.

Speaker ratings are "nominal" or stated numbers, not gospel. They are supposed to represent something that really can't be represented very well by one number. Some speakers are hard to drive, some are easy, which has been a recurring theme here in this thread.

I know it's a crappy answer, but the answer is always "it depends." It depends because all amplifier testing is done with purely resistive loading, not real dynamic speaker loads. "It depends" because speaker manufacturers do the same "ballparking" and some of those loads can be really wicked at times. An example from 25 years ago was the Infinity Kappa 9. Infinity rated it at 4 ohms but it had a dip to just over 1 ohm and other nasties. In short, it was a pig to drive. I walked into a hifi shop at that time where the sales crew had just smoked an Adcom 555 amp at very modest volumes due to the incredibly difficult load. The step down models were not particularly difficult, but did reward a stiff amplifier.

Best to avoid budget amps into "4 ohm" speakers. Most vintage stuff will be fine from midline on up, but modern stuff, not so much, especially AV receivers. A moderately difficult speaker load will leave them dead and lifeless sounding.

Then again, this is why there are sites like this with guys who can help you out.

BTW, IMHO, avoid the Carver TX stuff, in my experience they are not very good performers. I've never been too impressed with Carver amps/preamps, and that series was even less so in my experience. The dynamics that Carver did typically have just wasn't there with them. There are a ton of good amps out there to choose from.
 
Rules of thumb becoming obsolete

There is no way to over emphasise the point that the single number X?ohms is meaningless.
Speakers aren't resistors.It's a bit like listing a SUV's land speed and assuming it's the same across water.
What I am finding challenging is that so many of my amp opinions/prejuideces are being knocked down. Amps should be heavy and run hot after 20-30 min. of on time. So- how did the older Exposure stuff do what it does ? Cold runnin -great sounding ? Worse -borrowed a pair of Anthem M-1 amps for the weekend a couple of months ago. These are really light just plain small fro a 1kW amp-run cool , will drive low impedance loads and are probably the best sounding amps I've heard in a couple of decades.Very dissapointing -since this means I actually have to pay some attention to class D or so called digital amps-they are worth listening to to seperate the wheat from the chaff .Much easier when i could dismiss the entire group as inherently nasty sounding.
 
How does rail voltage figure into the equation if there is no current limiting in an amplifiers design?

Scenario A

Amplifier A has 90 volt rails and an output stage that is capable of a maximum of 700 wpc into 1 ohm without being damaged.

At maximum output the amp produces the following output:

  • 100 wpc into 8 ohms
  • 200 wpc into 4 ohms
  • 400 wpc into 2 ohms
  • 700* wpc into 1 ohm

*Amplifier A is damaged when it attempts to output 800 wpc into 1 ohm.




Scenario B

Amplifier B has 45 volt rails and an output stage that is capable of a maximum of 700 wpc into 1 ohm without being damaged.

At maximum output the amp produces the following output:

  • 50 wpc into 8 ohms
  • 100 wpc into 4 ohms
  • 200 wpc into 2 ohms
  • 400 wpc into 1 ohms

Amplifier B can safely handle speaker loads as low as 1 ohm at full power.


In the real world would these scenarios be true?
 
As soon as speakers are hooked up, those numbers don't mean much at all. Theory.

As for design, it's going to depend on how many devices you have to share the load/ditch the heat, and how efficiently you can do it.
 
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load duration

The other issue is duration of low impedance -a speaker that occasionaly dips to 1 ohm may not be much of a problem. To me -the arguement for amps that double down is that there is at least a chance it may produce powerful -accurate bass and again -maybe- also sound pretty clean while it's doing that. As a reasonably lazy service tech I had a strong preference for amps that were stupid proof via protection circuits. Do the relays in particular used as part of these circuits always age gracefully ?? Maybe not so much?
The big thing to me is the original thrust of this thread was the amps (HT recievers in many cases) that are labeled as not being ready to drive low impedance speaker loads. When i encounter this labeling on the product -I REALLY believe it. And frankly have a prejidice against products so labeled.
Worth noting that most modern amps will sound awful on music well before they "blow up". Now if drugs ,alky or dinasoar footfals are involved -may be hard to tell at what point a amp is getting distressed and that is a problem.
 
I wonder if anyone makes a home or pro audio amplifier that actively varies the voltage rails in response to the load impedance for the optimum power output over a range of impedances like the JL Slash series of car audio amps do?

That would be nice to have as the amp could then deliver the same power output over a wide range of impedances and be better able to handle real low impedances.
 
Wow, this thread is huge, and full of info. May as well throw in my experience with this topic. I've been interested in this subject ever since I got my Polk SDAs. They taught me that just because an amp says it will drive 4ohm loads, that doesn't mean all 4ohm loads. I got the SDAs as part of a lot, then fell in love with them. But when I first got them the only thing I had that would drive them without instantly becoming frying hot, or going into protect, was an old JVC JA Integrated amp. Someone once told me it was because of the "Darlington" outputs in these amps, myself, I have no idea. Well, for what it's worth I've found that NAD amps do an amazing job with these speakers. Even after I had them completely rebuilt-upgraded, and the tech who did the work told me they would now run a lot closer to 2ohms at most frequencies. My NAD 372, 375, and 326, {yes I like them} makes them sing beautifully. My re-capped SX950, that drives a set of Klipsch Fortes like there's no tomorrow, goes into protect within seconds. I'm not sure if NADs "Power Drive" makes any difference, by doing what Tube Radio is describing in the previous post, but it must help. Just my personal experience with 4ohm amp stress. Hope it helps someone. Forgive me for throwing in the "Modern" amps, they are just part of my experience with this subject..
 
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Its simple.... If the amp is rated for 8 ohms nominal load then you don't run a load with an impedance of 4 ohms or less. If the equipment you own is older and has speaker switches that will allow the usage of two sets of speakers at the same time you don't do it.

I live in the commercial/industrial world of electronics and just when I think I have seen everything stupid pops up again and surprises me. The same hold true in my world of 8 ohm, 4 ohm, 25 volt, and 70 volt speakers. There is an additional exception and that is stupid is carried one step further. Commercial/ industrial amplifier and speaker systems are usually 70 volt here in the states and can be 70 volt or 100 volt across the pond. School systems that is school intercom systems are usually 25 volt. I was talking about stupid rearing its head and the most common thing I see is some moron putting a 8 ohm speaker on a 70 volt line. Guess what the load is then? Its 8 ohm. The amplifier sees this as a virtual dead short and goes ballistic. To put this simply it overheats because the output stage cannot drive this load.

The same logic is true with simple 8 ohm and or 4 ohm systems. If they were designed lets say for a minimum impedance of 8 ohms your playing with fire by trying to run a 4 ohm load on it.

One last thing..... just because it says 8 ohms doesn't necessarily mean it doesn't dip even lower impedance wise when you check lower frequencies. Generally when low frequency is checked you will find it dips to about 4 ohms or possibly less. So how can they say its 8 ohms impedance? Simple.... the impedance on tag on the back of the speaker is a nominal reading. This is based on an AVERAGE of frequencies between say 20hz and 20K. Ever check the reading at high frequencies? You will find that it goes up in value.
 
I wonder if anyone makes a home or pro audio amplifier that actively varies the voltage rails in response to the load impedance for the optimum power output over a range of impedances like the JL Slash series of car audio amps do?

That would be nice to have as the amp could then deliver the same power output over a wide range of impedances and be better able to handle real low impedances.

Some do. My (now old) Onkyo AVR has three voltage rail levels. You can tell it that your speakers are 4 ohms and the rail voltage will drop from ~80 to ~60. If you don't drop it manually, and a temperature threshold is crossed it will drop the rail voltage to 60. If another temp threshold is passed it will drop to 40 volts. There are also 2 92mm fans that blow air onto the heatsink to drop temperatures before rail voltage gets dropped. It also weighs about 50lbs.

I have forced air moving through the cabinet which drops the reported idle heatsink temps from 60c to about 45c.

This receiver does a great job driving old Infinity speakers that are rated as 4 ohm and drop to less than that below 50hz.
 
That is similar, but the JL amps from what I understand are capable of constantly varying the B+ voltage to always provide the optimum power based on the speaker's impedance.

One benefit of that is one can connect a speaker of a certain impedance range (1.5-8 ohms for the JL amps) to the amp and still get the same amount of power which means for a 500 watt amp that same 500 watt spec is good for 1.5-8 ohm loads.
 
Impedance calculations - have I got them right?

I'm considering whether I can add a second set of speakers to a system and I've read through this long thread and picked up a lot of useful information about impedance and amplifier risks and would like to summarize what I think I now know about my choices in the hope that someone more knowledgeable can check I have it right before connecting things up.

I have a NAD 3020 amplifier currently attached to Eltax III speakers. The speakers state that they are 4-8 Ohm, 90W, 89dB sensitivity. I am considering adding a Pioneer sub-woofer which is rated at 4 Ohm, 100W 78dB sensitivity.

As I understand it, combining a 4 Ohm and an 8 Ohm speaker would result in a 2.66 Ohm load on the amplifier (using the parallel formula AxB/A+B). The NAD manual does not state a impedance rating, but it notes that "the 3020 will easily deliver over 58 watts/channel into 4 ohms or over 72 watts/channel into a low 2-ohm impedance, with no distortion due to triggering of protection circuitry" which seems to imply it can cope with a 2 Ohm load.

So my conclusion is that this combination would be a possibility - albeit that the 4 Ohm sub-woofer could be receiving more power than the existing speakers and might be too loud.

I have one alternative, a Pioneer A-207R amplifier, rated at 35W at 8 Ohm or 45W at 4 Ohm. This amp has two speakers outputs but, as I understand it, these will also operate in parallel meaning the combined impedance calculation is the same as above. This seems to be supported by a comment in the manual which states that "When speaker systems are connected to only SPEAKERS A or SPEAKERS B terminals, such speakers should have rated
impedance in the range of 4–16 W. When speaker are connected to both A and B terminals, they should have a rated impedance in the range of 8 –32 W."

So my conclusion is that this amp would not be suitable for driving this combination of speakers.

Do I have this right?

Moved this question to a new thread to make more visible and prompt a response.
 
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I'd like to hear more about amps that can handle 4 ohms. I have a set of Kenwood L-09ms. Haven't read through every page just yet and skipped to the end as I see this conversation is pretty old.

Also got a pair of 07-Ms and a pair of Dynaco Mark IIIs. How do you think they handle 4 ohms?
 
So what is the consensus on flagship amps from the Japanese when you use 4ohm nominal speakers? For example, the Yamaha as3000, the marantz pm11s3, the onkyo m-5000r, the denon pma-a100 etc. They all have similar power ratings and claim to double down into 4ohms. Will using 4ohm nominal speakers adversely effect sound quality vs. 8 ohm nominal? Just generally, not specifically, I know there are many variables. Thanks!
 
So what is the consensus on flagship amps from the Japanese when you use 4ohm nominal speakers? For example, the Yamaha as3000, the marantz pm11s3, the onkyo m-5000r, the denon pma-a100 etc. They all have similar power ratings and claim to double down into 4ohms. Will using 4ohm nominal speakers adversely effect sound quality vs. 8 ohm nominal? Just generally, not specifically, I know there are many variables. Thanks!

I have Onkyo A-9070 integrated (M-5000r's little brother with quite similar specs), and it was not able to drive nominal 4-Ohm Infinity RS-II's, going into protection even at moderate volume levels. Now, Emotiva SA-250 (clean 400 wpc at 4 Ohm) has no difficulty driving these speakers. And even at low volume levels the bass in particular is improved with Emotiva.

Now, your Onkyo setup is impressive! Those dual 5000r's must look great. Pics please :)

I have been seriously considering P-3000r pre-amp, but it is a bit pricey for my taste, and there does not seem to be used ones on the market. How do you like it? I am using A-9070 as a pre-amp at the moment and like how it sounds.
 
Thanks! The p3000r is probably the best value for money item ive got. It's very versatile, I makes quite a difference in sound quality over using the onkyo 9050 pre outs. I am especially interested in the nature of amp behavior because when running the dual m-5000rs in btl mode (only rated into 6 ohms running into my klipsch palladium p-17b's, (4 ohm nominal)the sound gets worse than with a single m-5000r running them. Alternatively, the sound notably improves running the btl setup on my revel f-208's (8ohm nominal). Also after reading this im paranoid about damaging my amps haha. Photos forthcoming, im on my phone right now!l
 
HankNova, if the amp is capable of driving 4 ohm loads it will either say so on the rear panel or in the owners manual.
 
HankNova, if the amp is capable of driving 4 ohm loads it will either say so on the rear panel or in the owners manual.

Not necessary.

A recent debate regarding Nakamichi PA-7 bears that out. Only lists specs for 8 ohms but clearly is capable of safe operation below that.
 
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