Why 4 Ohm Loads Stress Your Amplifier

[ANACRUSIS]

After a lil more reading ...

ron-c wrote:

One solution is the autoformer which will match the 2.7 to 2, 4, or 8 Ohms but the drawback is cost, weight and size.

autoformers, any transformer, will have relatively poorer frequency response at the lowest frequencies of the audible band. For the subsonic stuff, for us theater mavens, BADDD. And yes good ones are $$$$$$$.

I believe that is the reason stages in amplifiers are now directly coupled instead of using those pricey (if they sound okay and have good frequency response, meaning flat without a lot of notches of mountains) autoformers or *isolation transformers.

Funny, I've been looking for some of them for my set of car amplifiers still new in the box with some Boston Pro 6.4.3's and a trio of 10.4LFs. Accumatch they were called, think manufactured by Veritas.

All in all, good thread with a host of info that I'll have to go over more.

Another thing, got rid of the SX-1250--a fine receiver it is, compared to Yammie separates, no contest period. Not wanting to offend SX folks out there, but owned, tested, an SX-1250 and host of Yammie stuff. The tuner is definitely up to snuff, cosmetics pretty, and the unit is simply chock full of USEFUL features. (I'm a hobbyist and mathematian that can now boast of having the correct equipment for the job of benchmarking the equipment, RF & Power.)

Give me a T-7, M-4, and C-4 (European Black Models), that combination will eat the SX-1250 for lunch. Why? 75 pounds of poor dampening, poor low impedence driveability, and a poor equalization section.

The M-2 and SX-1250 represent the state of manufacture for that period, both with Toshiba output resistors, and huge toroidal transformer. However the amplifier topology/design/engineering of the Yammie stuff superior, IMHO.

M-2 cents.

 

[soundweasel]

Otay -- I'm all confuzzed! I just noticed that my recently acquired NAD 20 speakers are 4 Ohm. Does that mean I can't use them with any of the receivers I own -- Marantz 2245, Pioneer SX-780, Concept 11.0? I'm rather fond of all of them and would rather not toast any of them just to use a pair of speaker I got for $40. I'd toss the NADs before I'd damage one of my receivers. How can I hook them up with causing problems? If 4 Ohm speakers pose such a hazard to "most" equipment, why make the buggers at all? So much to learn.

 

[soundweasel]

OK -- I just checked the back of the SX-780. It says: "A, B -- 4 Ohm or higher/speaker, A+B -- 8 Ohm or higher/speaker". What does this mean? Does this mean that A and B are wired to handle any speaker 4 Ohm and above, just not simultaneously? How do I simply hook up one set of 4 Ohm speakers to run by themselves? Does this mean that I can run a single pair of 4 Ohm speakers off of either the A or B hookups, but not two pairs of 4 Ohm speakers on both A and B simultaneously because it will cause damage? Can I run a pair of 4 Ohm speakers on A and a pair of 8 Ohm speakers on B simultaneously?

The Marantz makes no indication that anything but 8 Ohm speakers can be used. And the Concept clearly states that no speaker of less than 8 Ohms can be used.

Looks like I can use the the Pioneer, but I'm just not sure in what configuration?!

Please help. Thanks.

 

[wualta]

What does this mean? Does this mean that A and B are wired to handle any speaker 4 Ohm and above, just not simultaneously?

Exactly that. It also means that the A+B position puts the speakers in parallel, so that if you hook up two sets of 4-ohmers and flick the speaker swich to A+B, you'll throw a 2-ohm load across the amp and probably trip the protection circuits, so don't do that.

One set of 4-ohmers should be just fine. Just be aware that all "4-ohm" speakers are not created equal and "difficult" speakers may trip the amp's protection circuits and still be "4-ohm" speakers.

How can this be?

Using a single impedance figure to predict what will happen with a specific amp and a specific speaker is like trying to predict how far a baseball will travel given only the earned run average of the pitcher. A speaker's impedance rating is only a (ahem) ballpark figure. More to the point, it's only ONE number.

Can I run a pair of 4 Ohm speakers on A and a pair of 8 Ohm speakers on B simultaneously?

I wouldn't. The resulting impedance would be less than 4 ohms, and the amp's label is specifically warning you against such a thing.

The Marantz makes no indication that anything but 8 Ohm speakers can be used. And the Concept clearly states that no speaker of less than 8 Ohms can be used.

I'm a little surprised, but I wouldn't argue with 'em. The Marantz's instruction manual may have info on driving speakers less than 8 ohms, but until you have further information, use the Pioneer.

 

[soundweasel]

wualta, thank you for the reply and clarification/confirmation!

I thought I knew what I up doing until I read the entire discussion thread, but then I started to "glaze over" and began to doubt what I thought the notations on my receivers meant. Whew, at least I can use my 4 Ohm NADs on one of the three receivers. Frankly, I was surprised the Concept states nothing below 8 Ohms. The Marantz has no notations regarding speaker impedance so I'll need to get my hands on a manual. Thanks for your help. Regards.

 

[gener8tr]

Okay, now that I have a headache from trying to figure this out, please answer me this...

Is my Pioneer SX-980 being hurt by running two sets of Advents (8 OHMS each) rather than just one?

If so, should I only have speakers A running while listening quietly at home by myself and limit A/B to only when friends are over and I'm showing off a bit?

Same question for my Sansui AU-317.

Thanks for the help!

 

[caygees]

When multiple speaker systems are selected, some receivers put them in series. A large Kenwood sitting on the shelf here puts speakers A, and B in parallel, when speakers C are also selected they're put in series with B. Most of the newer amps/receivers put multiple speaker systems in series, too cheap to put dual output transistors in the output stages so 4 ohm loads are out of the question.
Too see a big difference in output levels between some newer amps and older ones, sweep the audio spectrum at full output, just before clipping, older systems have larger transformers and filter caps so they see a much lower drop in clean power. Never buy a power amp that's been built after 82 in my opinion, not if you want to run less than 8 ohm loads.

 

[Tube Radio]

On my sansui 4000 receiver I did a test when I replaced the output transistors and I get a power output something like around 40 watts with 8 ohms and about 80 watts on 4 ohms. left it with the 4 ohm load for awhile didn't hurt it.

 

[ToTo Man]

So, is there a speaker switch you can by which lets your amp see 8ohms no matter what your speakers' impedances are (e.g. is there a switch that lets you connect a pair of 4ohm speakers along with a pair of 8ohm speakers, etc)?

 

[240 Volts]

So, is there a speaker switch you can by which lets your amp see 8ohms no matter what your speakers' impedances are (e.g. is there a switch that lets you connect a pair of 4ohm speakers along with a pair of 8ohm speakers, etc)?

From other AK'ers posts I believe this is the case, but remember "ye cann'a change the laws of physics captain" to quote Scotty.
All that these speaker switches do is to add extra resistor networks between the amplifier and speakers - so the load as seen by the amplifier never goes too low.

The downside is that power is lost/wasted in the resistors and the amplifier's damping factor is shot to hell. This may or may not adversely effect the sound depending on the characteristics of the speakers and on the personal taste of the listener.

- Richard B.

 

[EchoWars]

Most '4 ohm - 8 ohm' switches simply drop the supply voltage by switching to a different output tap on the transformer. I've never seen an amp or receiver that adds resistance (it would be a big, expensive resistor, and it's just not practical for a long list of reasons).

OK...I'll get out of Richard's thread... :tongue:

 

[240 Volts]

Most '4 ohm - 8 ohm' switches simply drop the supply voltage by switching to a different output tap on the transformer. I've never seen an amp or receiver that adds resistance (it would be a big, expensive resistor, and it's just not practical for a long list of reasons).

OK...I'll get out of Richard's thread... :tongue:

EW, you're always welcome in my threads - it helps keep me focused :thmbsp:

Agree entirely with your comment about the "4 ohm - 8 ohm" switches on amplifiers simply selecting a different tap on the transformer, but I interpreted ToTo Man's question as relating to a seperately bought add-on speaker switching box with an "impedance matching" feature.

- Richard B.

 

[Yamaha B-2]

Nice work 240. :thmbsp: If EW likes it, then so do I. And, as stated, it is current that it the killer. Current = heat. The enemy of solid-state devices and life of electrolytic caps.

I run fairly efficient speakers and also have a muffin fan above the heatsinks on my B-2 amps and CA-2010 integrated amp. Great, inexpensive insurance policy. Bought on eBay for $15. Comes with a built-in wallwart and has little rubber feet so sits very nicely on top of the vents for the heatsinks. Can't hear it run when the music plays unless at the lowest levels. Amps all run much cooler.

 

[ToTo Man]

I've got a fan above my CR-2020 as insurance too :thmbsp:. Not sure if it makes a 4ohm load any safer but it certainly does help circulate a bit of air inside to keep the voltage regulators a little cooler.....

 

[240 Volts]

..... The data at the start of the thread is fine, but the comparison is not -= you end up cpmparing apples and oranges. What needs to be done is a heat generated vs power generated graph for 4 and 8 ohms, with a limit drawn in for a typical limit on maximum power dissipation and maximum current - just pick an amp for an example.

Comparison graph for heat generated vs output power for 4 and 8 ohm loads has finally been added to the original post (#1) :thmbsp: The actual numbers on the graph are for a typical 160 - 200 watt amp, but the relative shapes of the curves are valid for all Class AB amplifiers. Thanks for the suggestion ilimzn.

- Richard B.

 

[Paul C]

The graphs are for sine waves. This is continuous power, but fortunately, real music is nowhere near continuous. The graphs certainly show a worst case scenario. When listening to music the capability to produce that sort of power on peaks is certainly necessary, but that is for a small percentage of the time. Most amps have plenty of cooling capability for listening to music even at high levels below clipping.

I have PA equipment with heatsinks nowhere near as massive as many home stereo amps--but they are externally mounted. I run these amps into 2 ohms for hours and they get just barely warm to the touch. I have had more heating from sunshine on the black aluminum at outdoor festivals.

While the charts are scary, and they do show a difference between 4 ohm and 8 ohm operation, you will seldom stress an AB amp. What will make more difference is how much cooling air is allowed to circulate around the amp. Is there sufficient space under the amp to allow airflow into the bottom? Is there space above to allow air out? Is there a buildup of dust coating the heatsink fins? Blow the dust out with canned air every so often. Get those old Stereo Review magazines off the amp! A fan certainly can't hurt.

 

[Yamaha B-2]

Paul C - That's a great pic! I played the alto/tenor/baritone, but never the bass. They are quite rare. Looks like fun. :thmbsp:

240 - Thanks. More good work. :yes:

 

[Paul C]

The cool thing about bass, even if you play a few clams, fumble a few passages, people are just so thrilled to see the huge thing played that they will overlook a less than perfect performance. In other words, even if your playing sucks, you are still cool.

 

[Yamaha B-2]

No question!

 

[subdermis2000]

Cr-1040

Anyone ever try on the CR-1040 A+B setting two 4 ohm pairs or a 4+8 pair combo?