Why 4 Ohm Loads Stress Your Amplifier

Discussion in 'Solid State' started by 240 Volts, Feb 14, 2006.

  1. nonobaddog

    nonobaddog Member

    Messages:
    60
    Actually 3.7/2 = 1.85 so it is a little closer, but concern is definitely appropriate.
     
  2. nonobaddog

    nonobaddog Member

    Messages:
    60
    For what its worth, it seems to me that the DC resistance is frequently lower than the nominal impedance of a speaker system. This is just an observation so nothing to bank on.
    The impedance is like the DC resistance plus reactance of any inductance in the circuit so maybe it makes sense that the impedance would usually be higher but don't take my word for it.
     
  3. Bigboi33

    Bigboi33 Active Member

    Messages:
    110
    A quick question for a Pioneer Spec 2.
    Currently I am using this to feed a pair of Acoustat Electrostats that can dip down to 2ohms in the upper frequencies. It's been fairly hot out here lately and I've noticed the amp getting warm at fairly low-medium volume levels.
    Am I damaging or running the amp to hard with these speakers?
    I don't normally listen to loud. But I also do not want to damage the amp.
    Anyone with a Spec 2 and experience driving difficult speakers?
    I have a QSC USA Pro Amp that I can borrow. It is definitely spec'd down to 2ohms. But will it sound as good as the Spec?
    There is also a Carver TFM25 available to me, I've read that this is an amp that goes great with Electrostats.
     
    Last edited: Jun 29, 2016
  4. botrytis

    botrytis Trying not to be a Small Speaker Hoarder Subscriber

    Is it rebuilt? If not - I would stop using it.
     
  5. Bigboi33

    Bigboi33 Active Member

    Messages:
    110
    Yes, it has been gone through. It has been recapped and also voltages adjusted.
    My biggest fear is overworking the amp. I don't see any specs showing the Spec 2 was designed to drive anything below 4ohms.
     
  6. Tube Radio

    Tube Radio Super Member

    Messages:
    3,377
    Then I personally wouldn't recommend driving anything under 4 ohms with it.
     
  7. Cope

    Cope New Member

    Messages:
    21
    Location:
    Houston, TX
    My NAD 7155 is switchable between 4 and 8 ohms. I never play at loud levels. Assuming one pair of efficient speakers, would it matter if I ran 4 or 8 ohms, assuming the speakers matched the receiver setting?
     
  8. Nat

    Nat AK Subscriber Subscriber

    Messages:
    12,297
    Not an issue.
     
  9. onplane

    onplane What! No Wake???

    Messages:
    2,158
    Location:
    Rochester, NY
    The NAD amp I had dropped the rail voltage when you switched to 4 ohms. My guess is the reason for this is to protect the output devices so they won't over heat. If you are driving efficient 8 ohm speakers and set the NAD to 4 ohms, you are limiting your "head room". The greater the "head room" the less chance of clipping.

    If you have "efficient" 4 ohm speakers (I've never seen any, but they may exist.) and set the NAD to 8 ohms you risk heat problems if children or grand kids crank up the volume.

    Hope this helps...
    Jerry
     
  10. Alan0354

    Alan0354 Super Member

    Messages:
    2,090
    Location:
    Bay Area
    I know this is a very old post. I just read the first post. The first image of the amp is not even showing. I just comment on the first two graph and high lighted the two sentence in blue:

    This seems to refer to some really cheap amps. If they claim 225W capability into 8ohm, they better have output stage capable to drive a lot more than 7.5A. A good 200W power amp should have at least 3 to 4 pairs of output transistors. 7.5A is very very low for even 2 pairs of transistors. My Acurus is an OK amp, it is 200W, it has 4 output pairs. It should have no problem driving 20A. Good power amp usually spec double the power of 8ohm load if driving 4ohm. That's where the external heat sink comes into play.

    That's why when I buy an amp, first thing I look at is whether they have external heatsink. External heatsink does not imply high quality amp. But without heatsink, I question how much can the amp really drive. That's why some of those cheap SS amps have 4ohm and 8ohm switch where they even lower the rail voltage when switch to 4ohm to allow them to have a good marketing spec and still pass UL or CE certification.

    Of cause, driving a 4ohm stress the amp more when you crank it up. But a good power amp should be able to handle 4ohm speaker.....at least should design to handle that. I test my amp with 2ohm load!!! I don't think those amps that has no external heatsink can sustain continue max power specified, not even close. they can't even sustain the idle bias current needed to be a good amp.
     
  11. Tube Radio

    Tube Radio Super Member

    Messages:
    3,377
    depends on the maximum amplifier power.

    A 20 watt amp for instance may be able to drive a 4 ohm load and not need an external heatsink and may do fine relying on the chassis as the heatsink. some higher power amps such as Carver do use internal heatsinks and can handle 4 ohm loads without a problem.
     
  12. Alan0354

    Alan0354 Super Member

    Messages:
    2,090
    Location:
    Bay Area
    True, but you are very limited. OP was giving example of a 225W amp. To me, it is ok to have low power amp, but marketing like to say high wattage. The worst is the internal heatsink really limit the idle power dissipation. There is established theory on optimal biasing current of output stage. That can be limited by the size of the heat sink already. I am fighting this issue with my design because I made the mistake buying a chassis with external heat sink that can only dissipate about 65W of heat. I have to resort to lower the power of the amp in order to get high bias. I think that's what companies advertising "high bias" amp. YOu cannot get high quality amp without the optimal biasing.
     
  13. Tube Radio

    Tube Radio Super Member

    Messages:
    3,377
    If internal heatsinks were that bad, no one would be using them on higher powered amps period.
     
  14. Alan0354

    Alan0354 Super Member

    Messages:
    2,090
    Location:
    Bay Area
    Check on how expensive are the chassis with external heatsink. Chassis is the single most expensive item for the power amp, then comes the transformers. Manufacturers want to spec as high power as possible to make it sounds good. Most people will not buy an amp for any reasonable price if it specified 40W!!! Most people think 200W is always better than 40W.

    Go check the heatsink specification, you'll quickly see there's no free lunch. If you have internal heatsink, it is hard to get a heatsink that can dissipate over 20W. Cheap amp maunfacturers count on music is at much lower average power except occasionally high amplitude transient. That's why they have switch to lower the rail voltage for 4ohm speaker to avoid burning the amp.

    It is well established that optimal bias require quite a bit of power dissipation even without input signal. You just cannot optimize the sound with low bias. That's when manufacturers try to sell "high biased" amplifiers!!!

    IN page 2 of this thread, I was talking with Mondiafan about heatsink, you can take a look.
    http://audiokarma.org/forums/index....esign-forum-other-than-diyaudio.749061/page-2
     
    Last edited: Jan 15, 2017
  15. Tube Radio

    Tube Radio Super Member

    Messages:
    3,377
    What about the Carver TFM-35 amp that can do 380 WPC @4 ohms while using internal heatsinks?
     
  16. cube-tube

    cube-tube Well-Known Member

    Messages:
    883
    Location:
    Lakeland, FL
    4 ohm loads stressing you amp?... Really? Sure, maybe cheap SS receivers from the 70's and the cheap plastic ones from the early to mid 80's.

    Heck, the little 25 watt Technics SA-5170 I grew up using was even capable of running stable at 4 ohms. I abused the heck out of that thing only because I didn't know what I was doing at the time building speakers (I was 8 or 9 years old then). I built tall thin enclosures out of 1/2" plywood with a 4" x 10" and 3.5" speakers out of my brother's old 1983 Oldsmobile Omega. I was running those speakers in parallel with a single electrolytic cap on the 3.5" driver to take out the bass. So those things were running at 2 ohms, or near it. Still, never a problem with that receiver and it's still running today in our daughters system. The worst that ever happened to it was that the dial lights on it would dim really low when I would play it somewhat loud.

    Most all decent quality amplifiers from the 80's on up to present are all quite comfortable running at 4 ohms, even at moderately loud volume levels. As long as they have ample ventilation and are of a good design, you have nothing to worry about.


    The TFM-35 is quite capable of easily coasting along all day at 4 ohms, as most amps are these days. Years ago, I ran a pair of Magnepan's at very loud levels all the time with my TFM-35x, and not once had a single issue. Even with an old S.A.E. A205 amp as well.

    I'm running an even older M-500t (modded to MkII status) now on a pair of Martin Logan's, also rated at 4 ohms with a dip down to 1.5 ohms, although that 1.5 ohm dip is at 20 kHz, so very little power draw. Even so, running this amp at 4 ohms all day long has no effect on it whatsoever, and it doesn't particularly like low impedance loads itself.
     
  17. Funky54

    Funky54 Well-Known Member

    Messages:
    861
    Crap.. all this smart stuff just tells me I need a 1000 wpc at 8 ohm amp. :)
     
  18. KKnight

    KKnight Active Member

    Messages:
    324
    +1 I was going to say the same things. Bottom line is 4 ohms with a rebuilt/recapped vintage receiver/amp is OK at low/moderate volumes. Always been a rule-of-thumb.
     
  19. Tube Radio

    Tube Radio Super Member

    Messages:
    3,377
    Most likely the power transformer was barely adequate for the job.

    If the dial lamps still dim at all with a proper speaker load, a power transformer of the same voltages with a higher current rating will cure that.

    With what the person you quoted said if the manufacturer spec'd an amp to run at a certain power output into a certain load then it can handle the load regardless of whether the heatsink is internal or external.

    Also an amp with an internal heatsink or using the chassis as the heatsink can handle the proper bias current, because if it couldn't the amp manufacturer would have used a better heatsink.

    That only applies to quality amplifiers though.

    Heck I had a 25WPC Fisher 202 Futura Series receiver that uses the chassis as the heatsink in my teens that regularly drove two pairs of 8 ohm Fisher speakers for a four ohm load to loud levels and the amp did just fine.
     
    Last edited: Apr 10, 2017
  20. Alan0354

    Alan0354 Super Member

    Messages:
    2,090
    Location:
    Bay Area
    That's the whole problem of the cheap amp, they are NOT designed to handle the rated power. They bank on music has very high crest factor that even if you play to distortion, the average power is still very low.

    Also the problem with a lot of those cheap amp use very low bias current to avoid heating up. Good amps usually have much higher bias current to minimize crossover distortion. Also, good amp usually use transformer, and capacitors that is enough to give good performance. Use enough transistors to avoid over heating when pushed.

    Whether those cheap amps are good enough are totally subjective and relative. Some people think they are good enough, then they save money. But there are people that want more.
     
    Last edited: Apr 10, 2017

Share This Page