Why 4 Ohm Loads Stress Your Amplifier

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

  1. nonobaddog

    nonobaddog Member

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    4000 RPM in 1st gear - look stupid
    4000 RPM in 5th gear - get ticket
    The engine doesn't really care.
     
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  2. Pete B

    Pete B AK Member Subscriber

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  3. Bodyblue

    Bodyblue AK Subscriber Subscriber

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    If your amp or receiver is rated for a 4ohm load AND your amp or receiver is in good condition, then one will be fine. Should you take an old and dirty unit with no service history and run it flat out with 4 ohm speakers? Probably not, but there is too much hysteria that 4 ohm loads are a death knell for stuff.
     
  4. FidgetMaster

    FidgetMaster Active Member

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    That's why some integrated a small fan, they do get Hot, notice some of the sticky pad/adhesive used up against the heat-sinks kinda bubbled up and deformed and stuff. Just harder on the Transistors that driver is wanting more power. It's the same like in valve stuff it can be hard on the output xformer and the valves both.
     
  5. Tube Radio

    Tube Radio Super Member

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    Some amps such as the Carver TFM series will go into protect mode if they get too hot.
     
  6. roadrunnerx

    roadrunnerx AK Subscriber Subscriber

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    I am not a smart man... think Forrest Gump.
    Have used cooling fans for years. Tech taught me that heat was the enemy.
    Many companies did not want to warranty 4 ohm loads.
    The Luxman R-117 rated at 160 at 8 ohms but can handle 2 ohm peaks at 700..... cooling.
    Natural radiation cooling is limited. Reminds me of my firefighting days, radiation is a b****. Try a fuel or wood house fire.
    Side note: How many people stack something on top of amp or receiver?? I always look at this when buying.
     

     

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  7. McGowdog

    McGowdog Active Member

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    Very late to the party unfortunately.

    The topic interests me much.

    I used to not care about what I was doing to the amps, the speakers, my ears, the neighbors ears, the neighbors dogs, the cops, etc.

    I've gotten old, somehow, and do care about all those things, thusly.

    I did stupid stuff like put speakers in parallel to knock 8 ohms down to 4 in order to "double" the power from the amps because although p=iv, it also equals i^2/r and when you cut the resistance in half, the current goes up thusly and is in this case, squared.

    Long story short, ears bled and quality sound did not materialize.

    My brother noticed me doing this and I got a serious browbeating over this to say the least.

    So, I immediately stopped doing that, set my speakers up correctly, and learned a bit about staging, imagining, etc.

    Strange thing is, I have less need for loud and more need for, well how do you describe nice audio? A brain massage, I guess, a rise in consciousness for sure. Oh, and it's louder anyway if that's what I need.

    Then I went down the rabbit hole of upgrading cabling, innerconnects, sources, etc. Some m4a 24 bit 192k music is nice too if you have a source/dac that can play it. The oppo 103 (with or without the d) and the 105 are nice for that purpose. My oppo 95 plays some high rez, but not all.

    Anywho, back to speaker loads. Those are nice equations but you were saying something about how speakers vary throughout the frequency range.

    Would like to learn more about that. Meanwhile, I'll do some lab work using the equip I got and these ears of mine.

    Oh, I will now read through the years of comments.

    My first question wrt the topic based on my own bad experience is this, why 4 ohms, why add cooling? Just stop. Stick with 8 ohms and call it good. If you don't have good sound at 8 ohms and two speakers, assuming you have two working ears attached to a working brain, something's wrong. Start with the software (brain) and work back.
     
    Last edited: Mar 26, 2018
  8. wyn palmer

    wyn palmer Member

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    As the owner of a pair of Martin Logan Montis I find the above comment curious. The Montis fall to well under an ohm at 20kHz, and the impedance is almost purely capacitive. I drive them from the 4 ohm tap of a Rogue M180 tube amp- which has some negative feedback but looks quite inductive at 20kHz, and they sound glorious. Sure, it requires very short, very low impedance interconnect wire, but I wouldn't swap them for any other speaker. Technology is what technology is, sometimes the limitations of the physics dictate what the impedance is so why limit yourself to 8 ohms?
     
  9. tarior

    tarior Dirty pool, old man? Subscriber

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    Your amp is probably delivering very little power to the speakers at 20KHz. Not much information up there.
    You could probably darn near drive any amplifier into a dead short at that frequency with music program material. Not so much at 40Hz.
     
  10. wyn palmer

    wyn palmer Member

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    That's true- but the point is that you don't always have the choice as to what impedance you drive at what frequencies.. The Montis are particularly troublesome- not only are they capacitive at high frequencies, but at frequencies below the crossover (c. 340Hz) the impedance becomes purely resistive and increases to the few kohm range so in essence the load varies by almost four orders of magnitude versus frequency! Many amps just cannot drive them properly, or for that matter, safely. For example the Rogue tube amps have explicit (but unexplained) warnings about driving open circuit loads. Does a few kohm count as an open circuit? Also, response testing of the system can have interesting outcomes when you sweep to 20kHz- arcing of the panels is quite surprising when it occurs.
    Remember that for electrostatics the amp output current and voltage are almost in quadrature at all frequencies above crossover which means that simple VI calculations concerning the dissipation in the output stage are not entirely valid.
     
  11. tarior

    tarior Dirty pool, old man? Subscriber

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    Power factor calculations. Fun!
    I learned them years ago, and then promptly forgot them.:biggrin:
     

     

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  12. McGowdog

    McGowdog Active Member

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    Don't know about these electrostatic speakers, ribbon type speakers, magapans, etc. Would imagine they are designed to not damage amps.

    If they are anything like the magnapans, don't they have a built in amp for the lower end or something like that?

    I'm curious to learn more about how vintage as well as new tube amps handle 4 ohm loads vs new and old solid state.

    Much to learn about this.
     
  13. tarior

    tarior Dirty pool, old man? Subscriber

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    IMO, a speaker that is difficult to drive to the point of damaging most amplifiers for trying, is a poor design.
     
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  14. McGowdog

    McGowdog Active Member

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    I'm not going to spend a tenth or a fifth of my gross income on a set of speakers but hey that's just me. The most I've spent to date is twofiddy.
     
  15. wyn palmer

    wyn palmer Member

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    In my opinion, if a particular approach to the design of a loudspeaker results in particular benefits (which hybrid electrostatics undoubtedly do) then compliance with the physical needs of that design is the problem of the amplifier designer and not of the speaker designer. Amplifiers can be made to survive pretty well any load. They may not perform well, but they should survive, and if they don't sound acceptable then don't mate them with the speakers.
    As it turns out, despite the disclaimer, the Rogue M180s pair well with the Montis- assuming some intelligence is used- and indeed ML electrostatics and Rogue M180s have been displayed together in Audio shows over the last several years.
    Besides I've never actually had a believable explanation given to me concerning what catastrophic event will occur if the amp outputs are open circuited, and I confess to having accidentally done so on more than one occasion without any detectable consequences.
     
  16. tarior

    tarior Dirty pool, old man? Subscriber

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    I will certainly concede that there is no discounting synergy between an amplifier and a pair of speakers, in fact I insist that it is a real thing. However, with some of these designs, I almost get the impression that the loudspeaker designer went out of his way to build an "amp killer". Some kind of exclusivity thing, I guess.
    I've never owned any true electrostats, but I've heard quite a few, and they never really did much for me. But, and a big but, is that I wasn't the one who set them up and chose the amplification for them, and whatnot. The sound of the cylindrical wavefront is a neat effect, I will admit.
     

     

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  17. Bill Ferris

    Bill Ferris Super Member

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    FWIW. Ref. : amplifiers being able to drive no load("air", no load connections) I, in servicing tube & SS amps for over 42 years, I will and have tested(signal driven) every SS amp, before and after servicing with no load first, with zero concern, or any problems doing so..

    This checkout procedure is well documented in the service manuals of every manufacture of direct coupled SS amplifiers that I own(Crown, QSC, Phase Linear, BGW, etc.), and if working/testing on a amp of that type design, I follow anyway, even it not outlined in the SM, or if no service manual is available...

    In fact, to do so, can prevent further destruction, damage, if there are serious problems with the output stage.
    Then I proceed to dummy load connections, if the amp passes no load checkout, starting with 16 ohms and proceeding to lower value load of 8 & 4, or 2 ohms, if rated for, to determine the amps heath/performance specifications, while being driven under load..

    Now, I would never, ever drive any tube amp, much less power it up without a load connected, as I have had to replace quite a few output transformers/output tube sockets on customers tube amps over the years, who failed to have a speaker hooked up to their tube guitar, bass, or stereo amps. and they kept turning up the volume control while driving the amp, while waiting for sound to come out of a not connected speaker cabinet !! An expensive lesson, the customers did not forget again..

    Anyway, just my $.02`s worth of experience folks, FWIW.
    Carry on..
    Kind regards, OKB
     
  18. wyn palmer

    wyn palmer Member

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    Thanks for the response.
    I certainly do believe you, but as I said, I've never seen a non hand-waving explanation as to why it's bad to operate a tube amp without a load.
    The principals at Rogue Audio have unquestioned integrity so I certainly believe what they say also- I would just like to know why.
    Let me describe a set of circumstances.
    1.The Rogues are used to play organ music on the Montis with sections of essentially only sub 300Hz music.The speakers present a load of 3kohms or there about under those circumstances. The organ is played very loudly and at frequencies which use large voltage excursions of the amp.
    2. The system is tested for frequency response- the sweep is in two parts, one being from 1kHz to 10 Hz and is fairly lengthy in duration.
    3. The room is tested repeatedly for resonances at sub 250Hz frequencies.
    All of these actions apply high voltage sub 300Hz frequencies for lengthy periods of time, and although the panel is still connected there is no signal present at frequencies that would excite it and it is effectively open circuit.
    The Rogues do not blow up, or even seem to suffer any damage. Why is this?
    This is not a hypothetical situation in my home. All of these circumstances have happened many times, and yet the output stages haven't even changed bias current. No failed transformers, no wrecked KT120s.
    Transformers and output tubes do just fail- output tubes all too often. In the case of the failed amps that you repaired why was the diagnosis that the cause was an open circuit? What is your diagnosis as to the actual failure process/mechanism?
    What impedance doesn't count as an open circuit- for example if I was to place a permanent 300 ohm 5W resistor across the 4ohm output terminals of each of the monoamps would that be sufficient? Is 3k sufficient ? How about 1Mohm?
    I've run LTSPICE simulations on Pentode and Triode output tube amps (using models for KT120s and KT88s) with no load and seen nothing that would cause me to believe that the output components might fail. Maybe I'm looking for the wrong thing...
    All the best,
    Wyn Palmer
     
  19. Bill Ferris

    Bill Ferris Super Member

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    Ok Wyn, I will try to answer you in the best way I can from real world experience.
    Some examples : First was an observation of the aftermath of a tube amp connected to it`s proper load driven to max unclipped power on a test bench, by a previous employee that I was hired to replace..
    This story of the first event, was told to me by my new boss in 1978.
    The tech was running up a Fender guitar amp after repair to confirm it`s performance, when he some how unplugged the load resistor(dummy load while preforming the test. @ 1,000hz, he had his Simpson 260 analog meter connected to the amps output transformer at the time when the event happened..
    Well Sir. when the load was removed the output transformer`s primary voltage soared as it probably went in to oscillation as well

    The output transformer`s primary human hair diameter coated formvar(the varnish that is commonly used to coat bare wire for coil wire) became stressed with the sudden application of a very high voltage, and shorted it out from the spike of much higher than it was rated for insulation thus trashing the output transformer, plus the Simpson meter sustained serious damage as well !!
    The reason that I learned of this event, was the prior tech left the Simpson 260 meter when he quit, and my new boss offered to give it to me, thus telling me the story of the no load output transformer story, and the pretty smoked meter guts, and it looked like lightning struck the rotary selector switch and a few precision resistors in which I eventually rebuilt the meter..

    Another personal experience concerning this situation concerns a smart dummy load made by Sencore PA 81 that is not tube amp friendly, unless modified..
    It was designed for SS amps..
    If it even thinks whatever amp is connected to has DC, or a very low frequency component feeding it, it promptly disconnects the selected load resistor from the amp, with a LED indicator & clunk of the heavy duty relay disengaging the load!!

    Great, if your testing a SS amp, but a real significant emotional event if your testing a tube amp of any type under power, as the output transformer instead of making it`s normal sine wave singing that can be heard on most output transformers while running the amp in to a load resistor, makes instead, a screeching sound, that I don`t want to hear, as I know the high voltage feeding it is soaring to very dangerous output transformer primary damaging levels..

    There are more stories, but typing with one finger, because of my partial paralysis to explain what I know in real world experience is tiresome, and more than I feel like doing..

    Your tube amps, do/experiment as you please, I will respect my conservative methods/reasons, as they have served my customers & me very successfully for over 4 decades.

    Take care,
    Kind regards, OKB
     
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  20. wyn palmer

    wyn palmer Member

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    Thanks, so essentially what is happening is that when the tube amp is under heavy load and the load is removed the transient output current that has already been established has to go somewhere due to the winding inductances and/or the energy gets stored in whatever capacitances exist in
    the output stage to store it. That can cause a great increase in output stage voltage that, amongst other things, can break down the transformer winding insulation.
    That can only happen if the output tubes are not capable of conducting the extra current at the time of the open circuit occurring, as otherwise the current would flow through them. So, a permanent output load resistor- as low as possible- would help.
    It seems that the reason that I don't see a problem in simulation is that I'm not testing the circuit under the right conditions (so what is new?). It needs to be under circumstances where there is no conductive path through the tubes for the additional current, i.e. where one or both tubes are turned off, transiently, and the load is removed.
    Thanks again for the information. Much appreciated.
     

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