Why 4 Ohm Loads Stress Your Amplifier

[richamor]

I've spent the last 2 hours reading all this. Veeeery interesting, waaay over my head.
You guys rock!

My brain hurts:drool:

 

[SweetDoug]

Adding a fan to most any electronic device will increase it's life. I've noticed alot of newer equipment that have small fans. I have a dvd recorder that has a fan at the back and a Wii has a fan. Adding a good quality PC fan to an amp would be easy. Find a source of 12v or so within the amp and wire it up. A fan doesnt take much current so you can make it part of the whole system quite easily.

I've recently switched to a pair of Energy 22's which are 4 ohms and my amp creates more heat so I am faced with rigging up a fan or two inside the amp, and that should prove to be fairly easy and cheap since I have alot of PC fans lying around.

As for a good quiet fan, try a SilentCat 120mm. It's a very quiet fan and moves a good volume of air. I put one in my PC and it cost about $8. The only question left is....blowing in, or out? I'm thinking it should be mounted on the inside of the cover and blowing outwards. That'll suck the hot air out. Ideally it would be set up like a PC where there is an intake from the outside where the fan sucks air in and blows the hot air out of the case.

I like the idea of adding a fan to equipment. I know it might be a real newb of a question, but I'm a little shy to go poking around inside an amp and wiring up a fan. Any 12 v PC fan from a computer will do? Tough to find a 12 v source without cooking myself, or the amp?

Doug

 

[HiFiCanada]

Adding cooling definately helps, and I think lack of cooling is the biggest contributor to why some manufacturers don't spec 4 ohm or lower loads. I know that on my Phase Linear, the manual is very clear that you MUST use cooling fans on the heatsinks if you hope to achieve rated output through 4 or 2 ohms without toasting the transistors.

If you could cryogenically cool your transistors, you'd be amazed at how much power they could dissipate. Like the 266mhz freezer cooled 486 computers that people throw together sometimes.

Thanks for the overview, I'm still absorbing all the math, but it seems to make sense, and definately correlates to my experience.

peace,
sam

I added cooling to my two Harmon Kardon 665PM from day one, that was now 26 years ago, and they are still going on strong as ever, no problems. I notice that the Harmon Kardon 665PM got hot anyways, just by being on, with out any music going throught them. So I got two old computor battery backup fans, 110 volts at that time that I placed in a spot that moved enought air to keep them cool. I was surprised that it does not take that much of a fan to cool these amps, as long as there is air movement.

 

[Tube Radio]

They have water cooling systems for computers so why not water cooling systems for output transistors in vintage power amplifiers

 

[Nat]

There are liquid cooling systems for amplifiers -- I've had several -- Kyocera, Luxman, and maybe an Akai or some other unexpected manufacturer. But generally the expense is significantly more than the advantage. I think the problem with cooling in large computers is simply the amount of circuitry, all of which produces heat (and that heat slows down the computational speed, which is the real problem), whereas the output transistors in amplifiers are main issue, and their overheating is an occaisional thing. The same money spent on a complicated cooling system might better be spent on higher power tansistors that would handle the extra heat.

 

[ibdplummer]

Ok , I an new to this , so don't yell at me ! I have a question . I have a 12" pioneer 4 ohm wired in series with a 6" 6ohm mid and a horn tweeter that is I assume 8 ohms . per side "2 channel" what is my ohm load on this setup ? Pioneer receiver , 8 ohm , Kenwood amp 8 ohm . What am I going to fry , if anything ? "Sound's pretty good " !..... O.K. , you can start yelling !

 

[grandville45]

Ok so I am guessing after reading all these pages etc. My Project/One 1500 was tested as putting out 300 WPC into 4 ohms is pretty respectable then? I am running 4 speakers and using the A+B switch is then 4 ohms correct? No wonder the Sm150's pound like they do!

 

[DaWoofer]

Long post with great info. I don't notice any heat difference in my 1980 running 4 or 8 ohm loads, and of course only one pair at a time. I run a fan most of the time now though running 4 ohm loads and it stays just warm to the touch. Can't say that about my big Kenny.

 

[eeprof]

There is a basic engineering principle here that many of my students had trouble with. In order for maximum power transfer to occur the internal impedence of the power source (amplifier) must equal to the impedence of the load (speaker). When this occurs then the power dissipated by EACH is the same. If the load Z is lower then the internal source Z then then the source (amplifier) will dissipate the most power in the form of heat of course. In fact, slightly over 3 BTU's per watt dissipated.
eeprof.

 

[restorer-john]

there is a basic principle here which it seems some professors clearly don't get here too, maximum power transfer to this load in this discussion will only occur if the source impedance is essentially zero. The problem (stress) occurs because the source is not an ideal voltage source, it's impedance is not zero and the devices cannot sustain either the current required or the dissipation demanded by the VI. Maximum power transfer by definition means it is transferred- not shared equally or bounced back and forth in an undamped 1:1 situation.

 

[Ken Boyd]

I have to admit I didn't go the very begining of the this thread so maybe its been already posted. Just because a speaker is rated for 4 ohm's doesn't mean that at many times the impedence doesn't go much lower. I have been told that some of the speakers I use sometimes drops down to below 2 ohms which requires the amp to supply much more current. Some amps are better than others at this. As far as fans it won't really help much if the amp can't deleiver the needed current when speakers drop to low impedences, it will go into clipping and then problems can occur. Many people here know much more then me and I am just telling it like I was told in a simplistic way. But heat build up does happen over a period of time and when you have lots of equipment all being in a rack the heat from multiple units can build up. I use Mid Atlantic amp racks which have options for fans which cool the entire rack. I set my fans to come on when I power up my amp racks main power strip, and on the top they have multiple openings for the fans, I usually run a few fans right on the top to pull the air out and the racks have slots on the bottoms and blank spaces to allow the cool air to be pulled in. Enclosed is a pic of one of my amp racks, it is a six foot Mid Atlantic Rack, and they come with many options from castors, rear lockable doors, and lockable front doors, either preferatored, solid, or glass, either smoked or clear. Lots of different options, I highly recommend them if you don't mind the industrial look. This amp rack is of my Phase Linear equipment and is still not complete yet so that is why it has some openings in the front. Since I am using the series II's that have the rack handles I can't use a front door as the handles stick out to far.

 

[Tube Radio]

Just remove the rack handles

 

[acdc7369]

It's a good thing my amp is designed to run at both 8 and 4 ohms. Because i do both

 

[waynechou]

Consider your bow shot. Your generous analysis has huge errors...sorry.
An ideal amplifier, like the Levinson ML 2 (25w RMS), will deliver twice the current if the impedance is halved, and twice again if halved again. So this amp is 25W into 8 ohm, 50W into 4, 100W into 2, 200W into 1....etc. The ML2 is also a nice example because it is class A. The same current flows across the outputs with or without a load. This enables the amp to drive short circuits with no damage.
Maximum amplifier heat is when running at 1/2 rated power. Run a 100w amp at 100w and it will warm up, but run it at 50 and HOT!. In the 100W example, the load is what gets hot! I won't explain the math of why the amp doesn't heat, except to point out that the specified test for amp power requires operation at 1/3 rated power until things heat up to max, and then the measurements are made.
You have to do the math in peak AC terms, not DC or RMS. The picture changes.
Secondary breakdown, not excessive peak current (and not peak voltage) are what shorts outputs. Secondary breakdown is often called the Safe Operating Area of a device. Designers use SOA analysis because almost all loads are reactive. The European DIN standard speaker load is reactive, not resistive, and contributes to the "lower power" that Euro stuff has, even though it plays louder than rated!
The limit for most amps are fuses. A fuse is purely a current device, opened by heating, which is an RMS function. If an amplifier can deliver 5amps RMS through a 4 amp fuse, the fuse will open! At 4 amps we might get 50 watts peak at 8 ohms, but at 4 ohms we don't get twice that, because the fuse opens.

I hope this plain English "long story made short" helps to clarify things.

WOW!! No wonder I can't seem to get my points across. It's really basically a lack of knowledge of electronic principles. But in spite of this, I commend anyone who tries to understand. First, you are correct that class A is limited by its inherent bias current, whether a load is attached or not. As the signal to the load is increased, it will deliver what was in the source bias to the load to the extent that it has, up to the value of zero. Thereafter it would present an obvious wave front of not being able to deliver the rest. Halving the load impedance, would not double the output, unless the amplifier was going into class B (I have no idea about how the referenced amplifier works) but theoretically would only be able to deliver half the power in class A (not double) if the load resistance dropped by a half. And probably, the chances that an amplifier that was designed for class A operation would not be damaged by a short-circuit, either. You might say you contradited yourself by stating doubling the power, by halving the load.

Another blog that I came across was the discussion of the reason of how power transfer is achieved. Concurring with "eeprof" (who seems to a professor of EE), maximum power transfer occurs when the source output devices are matched to the load. In a tube amplifier, this is accomplished by the output transformer. In the case of the transistor type, there is not only no need to do so, but it might also present heating/destructive problems if we tried to match the output transistor impedance optimally (powerwise) if we tried to do the same thing. If we tried to match the transistor ouput stage with the load, we might be able to find more power, but at some consequential cost, like destruction.

The effective output impedance of either, whether achieved with negative feedback or not, is a completely different issue over that of maximizing power transfer. Output impedance, as part of the circuit that shares where the signal will be shared, determines how the amplifer output's impedance, wire, and the speaker load will be shared. If the output impedance of the amplifier can be made to be zero, then all the signal waveforms are delivered unattenuated to the remaining wire and speaker circuits.

Wayne Chou

 

[rjohn]

If the amp is rated for 8 ohm's min, when using a and b you must use 16 ohm speakers. Some amps/receivers series (for the lack of a better term) the out puts. You can tell by switching your selector to a+b with speakers only connected to a. If you lose audio they are seriesed. If you do not, parallel.
If rated for 4 ohm on a parallel a+b must be 8 ohm. =4 ohms.
This has been said before.
Rob

I was really excited to find the above simple way to check if speakers are in series or parallel with my amp. The only problem is I have a Sansui 881 and it has jacks for three speaker systems. With one set of speakers hooked to the "A" system it plays on "A" but not "A+B". So I thought, OK, my amp hooks them up in series. But then I tried "A+C" and the speakers once again played even though there are no speakers hooked up to "B" or "C". Now I'm confused again. Could it be that "A" and "B" are in series while "C" is in parallel? Any ideas?
Thanks

 

[gp4Jesus]

nominal vs minimum impedence

Which speakers for your mains/surrounds/centers? Remember that nominal Ohm rating is not always so helpful. All speakers I know of vary, it's a range not a simple static measurement. (never seen a totally flat impedence curve either.)

Using only one center channel and boosting its volume instead of using 2 has another benefit. Less smearing of the 3-D image.

Great point about imaging. Lots of great tech & experience in this thread. Big power supplies don't guarrentee "safe" into low "nominal" loads. Band limiting won't neccesarily help either.

2 war stories w/no casualties:

1. Belles 1-30a peak!; twice the power into 4 ohms-160 WPC; no 2 ohm rating. Huge transformer, "Foster Lager size" PS caps (66Kuf total). Runs cool full range into "lazy" 8 ohms at defening levels.

2. Same amp-warm to hot doing stereo sub duty, below 110hz. "Nominal 4 ohms" 3.2 ACTUAL minimum impedence. Atop a milk crate helped cooling a lot. My $.02
Cheers tony

 

[Nikko75]

There are a lot of funny claims on the web claiming bigger it better, and that more power is safer. It's not. More power is just that, more power. More power equals higher dissipation. While putting a bigger transformer on an amplifier may allow it to double it's output each time a load is halved doesn't make it any more resilient. There is less chance of clipping and sending high order harmonics to the speakers, but it doesn't mean the amp will operate safer. Whether or not an amplifier will overheat, oscillate or become a conductive open circuit is completely dependent on the cumulative design, not how much current the transformer can supply.

 

[Tube Radio]

If the amp says it can drive 4 ohm speakers than there is no issue whatsoever driving 4 ohm loads.

 

[gp4Jesus]

I believe I read (here) that my NAD 2400 THX power amp is a rare exception in that it's not expensive and can handle loads down to even 2 ohms.

I also just read here in this thread:

"If the amp is rated for 8 ohm's min, when using a and b you must use 16 ohm speakers. Some amps/receivers series (for the lack of a better term) the out puts. You can tell by switching your selector to a+b with speakers only connected to a. If you lose audio they are seriesed. If you do not, parallel.
If rated for 4 ohm on a parallel a+b must be 8 ohm. =4 ohms."


I have yet to determine if my NAD runs series or parallel--I'll go home and perform the "test" by disconnecting the "B" side of speaker cables and then select "A" and see if I still have sound.

If I understand correctly, if I in fact have a series connection, then it doesn't matter about mixing/matching speaker ohms?

I really need to find out because I am running a large 4 ohm tower set And a small unknown ohm rating set across the room--I always thought this freakish NAD could handle it, but this morning for the first time in over 10 years the red Overload light came on and stayed on. The unit performed fine and after a few seconds the light went out, but I wonder if this is a warning sign of bad things to come?
This has been said before.

FWIW in the early '80s NAD rated the lowly 3020 to 2 ohms. One test I read, measured it's 2 ohm dynamic output @ 105 WPC!!

The buddy who turned me on to NAD's 3020 saw his flicker the 4 ohm Pyramid Met 7's LED! This takes 100 Watts!

I see NAD has a fan base on other forums, HT included, for sonics, features, & reliability

So, if NAD claims 2 ohm stability for your unit, while caution carries the day, enjoy!!!
Cheers tony

 

[gp4Jesus]

Adding a fan to most any electronic device will increase it's life. The only question left is....blowing in, or out? I'm thinking it should be mounted on the inside of the cover and blowing outwards. That'll suck the hot air out. Ideally it would be set up like a PC where there is an intake from the outside where the fan sucks air in and blows the hot air out of the case.

A well-place exhaust fan would enhance the convection cooling the engineer should have designed into the case.

I plan to close off the back of those rack openings & install AC fans.
Cheers tony