Why 4 Ohm Loads Stress Your Amplifier

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

  1. Tube Radio

    Tube Radio Super Member

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    Cheap amps yes.

    Even some quality amplifiers have a barely adequate power supply such as the Lafayette LA-375 amp I have. It has excellent sound quality and build quality, but the power supply would sag a few volts at full output power at 20Hz. An external power transformer of more amp capacity solved that little issue.
     
  2. Alan0354

    Alan0354 Super Member

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    There is no guaranty an expensive amp is designed correctly.
     
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  3. Tube Radio

    Tube Radio Super Member

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    True, but an expensive amp stands a greater chance of being designed correctly.
     
  4. danielhaden

    danielhaden New Member

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    Why it sounds sour with a 4 ohm load is because it wasn't prepared for it. What could make a good speaker is opposing what might make a good amp. There seems to be a disagreement, and the language after that point is hard to translate. What I can manage to translate is that linearity, current capacity and parallel output devices are good things. I can claim that you should go ahead and make the best speaker; however, after you have done so, then please don't claim that every amplifier is perfectly fine. It is sturdy amplifiers that drive the goodly speakers towards the expected performance.
     
  5. Alan0354

    Alan0354 Super Member

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    Robust amp with big heatsink and multiple output device fares a lot better driving 4ohm or lower impedance load. Bigger power transformer is important also, it's all about how much money put into the amp.

    1) Multiple output device is needed because of beta droop when transistor driving high current. Beta of most big transistor starts to drop when current over 3 to 5A. So when you drive a 4ohm over 20Vpeak, you go beyond 5A. The cheap amps try to use less output transistors, so when current requirement over the beta droop region, distortion increases. Also, put more demand on the driver transistor and cascade back to the voltage amplify section and create more distortion.

    2) You need big heatsink to dissipate the extra load of 4ohm speaker. If you look at amps with internal heatsink, they are small and usually max out at about 20W to 25W max dissipation.

    3) You need robust transformer and big filter caps to smooth out the rail voltages and prevent from drooping too much.

    These all cost money, there is no other way around it. A lot of cheap amps cannot afford to do any of these, they either count on you don't turn it up too loud, putting a switch to even lower the rail voltage if you use 4ohm speaker and tell you to switch to 4ohm for 4ohm speaker. This is more the CYA thing to pass CE or UL test.

    SS amp does not care what load you drive, it will amplify the input and give the same output voltage regardless with the load impedance is. The lower the load impedance, the more current it needs to drive to the same voltage and dissipate more power. Of cause if you have an amp with internal heatsink, less pairs of output transistor, you stress the amp driving 4ohm more if you crank it up, you stress the amp more.

    Class D amp is a different animal, it does not need external heatsink to be able to drive low impedance load. But that's a different ballgame. Class D amp literally modulate the input signal like AM or FM radio transmission, then at the output stage, it got demodulate back to audio signal. It definitely cannot be better than straight analog signal being amplified. There's a good reason people go digital in radio transmission, that the quality of sound is better because digital signal don't degrade from modulation and demodulation to transmit in air.
     
    Last edited: Apr 30, 2017
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  6. Hufsredo

    Hufsredo New Member

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    Well, I guess I need to return this P.O.S. amp, seeing that it has internal heatsinks.

    Yamaha mx 1000
     

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  7. Alan0354

    Alan0354 Super Member

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    Just basic engineering knowledge if one really study and understand.

    Maybe change to bigger transistors!!!:rflmao:
     
    Last edited: May 1, 2017
  8. Tube Radio

    Tube Radio Super Member

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    It's all about getting rid of heat.
     
  9. Alan0354

    Alan0354 Super Member

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    This afternoon was the first time I tested my amp at +/-40V, I was actually surprised how hot it got.

    Here are a few pictures of the heatsink I bought on ebay that measures 13" X 6" X 1 3/4". By all measure, it's huge. I am testing my own amp with 9 pairs of output transistor. I only use +/-40V rail and high bias of 0.8A just now. I had it on for 15mins with no air flow, just sitting in the room as shown. It got so hot I cannot hold on with my hands, not even close, maybe 1 sec max. From calculation, it's 0.8A X 2 X 40V= 64W of heat dissipation. Granded it is not a very thick heatsink but that size and got that hot with 64W. This gives a very good perceptive on power dissipation of those heatsink, gives a realistic perceptive of what heatsink can or cannot do.

    Test setup.jpg This is the setup I test the heatsink


    Ebay heatsink1.jpg This is the back view of the heatsink with my OPS pcb.


    Ebay heatsink2.jpg This is the front view of the heatsink


    You can see, the back of the heatsink is a little thin, but still it is 0.2 inches thick, it's by no means thin compare to a lot of other heat sinks. You'll be lucky to get a heat sink half the size of this inside a typical chassis particular with this kind of thickness. Also there is no air flow inside the chassis. I doubt any internal heatsink can dissipate over 25W. That is not comforting for an 80W or higher amp.

    Also, I use 9 pairs of output transistors in TO-264 package to avoid beta droop, more surface to lower the thermal resistance from transistor to heatsink. I only run +/-40V and only 75W into 8ohm or 150W into 4ohm. I don't care about how many watts, I only care about the quality of the watts.



    Of cause, TO-264 is smaller than the MT-200, but that will do!!!!:rflmao:

    I first bought a smaller chassis with heatsink similar size to this, a little smaller, but 2" fins instead of 1 3/4" and 3/8" thick back. So the power dissipation is about the same. The more I calculate, the more uncomfortable I got, so I spent $350 buying a Krell clone 50W class A chassis. I always question whether I was wasting money. The result today make me feel a lot better, that it's a right move. Save the smaller chassis for a lower bias amp, maybe do lower voltage, make it a bridge monobloc that get back the power to use heatsink of both sides for a single channel.


    Chassis1.jpg The one on the left is the new Krell clone, the right one is the old smaller chassis.
     
    Last edited: May 1, 2017
  10. grapplesaw

    grapplesaw New Member

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    This is age old discussion.
    My input is use the right equipment with your speakers of choice. Driving 8 ohms 4ohms or even 2 ohm loads without matching the amp will be impossible to solve after the fact with mods tithe amp. A class AB amp can be an example that does give wiggle room to control heat. Changing bias voltage and having lots of head room will all but rid you of your heat issues. Adding more transistors does share the load and spread the heat over more surface if configured corecly and only if you have chosen the correct performance transistors.

    I resently build a 18 output 120+ 120- volt rail amp which has 565 watts at 8ohms full complimentary configuration. I find the additional transistors help lower THD+N and runs relitivly cool. When driving a low ohm load at max power then all the heat issue come to play. My suggestion For those playing with very low ohmage loads is have have lots of redundant power available and be creative try new ideas such as water cool the sucker.
     
    Last edited: May 2, 2017
  11. Alan0354

    Alan0354 Super Member

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    Low bias is only one thing, when you crank it up, you still have issue with heatsink if it is too small.

    Yes, more transistor lower thermal resistance, BUT if they all mount on an inadequate heatsink, it won't help. Like my amp, 9 pair of transistors does not help me as the heatsink is the gating factor.

    This goes true the other way around, if you have a huge heatsink, but only one pair of transistor, then the transistor might cook even though the heatsink is still reasonably cool. The thermal resistance between the transistor and heatsink is too high if you have only one transistor and the transistor gets a lot hotter than the heatsink.

    AND there is a special case of some really cheap amp that use one pair of transistor on small heat sink. Changing transistor is not going to do any good if the person only stop and read up a little.
     
    Last edited: May 2, 2017
  12. Tube Radio

    Tube Radio Super Member

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    Also some poorly designed amplifiers use nothing to compensate for the heat being produced by the output transistors.

    Most good amplifiers will have some sort of device mounted to the heatsink to vary the bias based on temperature.

    When I built an amp that could put out 60Vrms (could be more, but transformer isn't rated over 50Vrms) into a 5K load (built to drive 1920s speakers which were high impedance and required lots of volts) from an input of 100mVrms using two transistors I found out a lot about how transistor operating points shift when heated unless something is present such as a diode mounted to the heatsink which alters the bias voltage based on temperature to keep the operating point the same.
     
  13. Hufsredo

    Hufsredo New Member

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    Exactly, if air is not moving over the heatsink it will eventually saturate. Internal or external, which is better MUTE point. What matters is air flow, be it convection or fan driven. I.E. grille below heatsink and grill above heatsink with Vertical fins.

    Your cooling fins need to be vertical, not horizontal.
     
  14. Alan0354

    Alan0354 Super Member

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    Those are just ebay special, hard to find such a big heatsink to fit my board. It's going to be better than any of the internal small heatsink no matter what. These heatsink is only for bringing up the boards, even the boards are not going to be in the real amp, the real amp is going to have the real heavy duty heatsink in the picture and brand new boards.
     
  15. Alan0354

    Alan0354 Super Member

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    Thermal compensation is another totally different story. I use inverted 3EF where the first EF is PNP driving the NPN driver, then drive the NPN power transistor( the other side is reversed with NPN-PNP-PNP). I have the first two transistors mounted on a separate heatsink as shown in the picture for temperature tracking. The Vb of the PNP and NPN cancel out. The bias spreader transistor is mounted right on top of one of the big transistor. I tested, the tracking is within 10% from stone cold to operating temperature. Another advantage of this inverted 3EF is I don't have 3Vbe drop, the output swing closer to the rails which is important when using low rail voltage.
     
  16. Ds2000

    Ds2000 All About every cool stereo component. Subscriber

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    Sometimes I think folks can get hung up on one aspect of a design, sort of unintentionally negating successful designs of the past.
    E.g. IMG_4139.JPG Can drive 4 ohms all day at rated output.
    2. IMG_4078.JPG Can drive 2 ohms all day at rated output.
    All in normal temp environs of course.
    Point is, there isn't just one way here.
     
  17. audiodummy

    audiodummy Active Member

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    101
    This sort of reminds me of two things...
    1. I've seen bookshelf speaker systems that rate their outputs at 6 ohms... I wonder if that's just a cop out to be half way between 4 and 8, or are they just being evil?
    2. I was looking at a LM4871 AB 3W single chip amplifier in a "phone amplifier". It was meant to be a 1-chip 3W bridge solution into a 3 ohm speaker. According to the datasheet, because of its power supply limits, there's no way for it to supply 3W unless the speaker has an impedance of 3 ohms. I was kind of surprised a SOIC8 AB amp could dump 3W to a speaker, though it did warn the IC needs good track widths for heat sinking (no additional heat sinking needed however)... Unfortunately I don't think this is something that can be applied to hacking existing amps to use lower impedance speakers.
     
  18. woodj

    woodj Super Member

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    2,807
    The heat sink must get hot enough to cause sufficient convection currents to cause the heat sink to dissipate the heat from the area of the single transistor.
     
  19. woodj

    woodj Super Member

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    That is probably the result after the impedance of all the drivers and the impedance/capacitance crossovers have been designed and combined.
     
  20. SVI2004A

    SVI2004A Synchro Bias

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    What is the 1st amp... 2nd is M-510
     

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