Talking about LM3886

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Joined 2015
Talking about LM3886
This article is just an opinion ,it should not be taken into acount seriously.Satisfied Gainclone users must even not grant any attention to this writing.
On page 3 of it's datasheet under absolute maximum ratings you read ,power dissipation =125W at 25°C Tc with Tj150°C and θJC=1°C/W.But on page 17 thermal protection is provided which shuts down at 165°C.This means maximum power dissipation of 140W is possible. To achieve such high power the design team created a breakthrough in semiconductor technology of 250°C Tj output transistors,highest available to my knowledge is Motorola's 200°C perforated emitter.(If someone knows better ,please inform).Inorder it functions safely at 250°C SPIKe protection is invented so that such a circuit if mounted on a decent heatsink providing Tc of 60°C it must provide an output power of at least 150W! Sorry I am mistaken, only 68W can be obtained from this amp.But why? Where did I go wrong?The answer is found on page 14 figure 37, it shows minimum Tc 90°C for only 50W Pd.In another words this amp is stuffed with so high tech innovations just to operate at boiling temperatures and if required it can go safely to 150°C with Pd of 15W before thermal shutdown ,(isn't it nice ?). I don't know any audio application that requires imperatively 90°C operating temperature,why is this amp so? The first row of this table betrays its origin, on the contrary what stated on page 3,operating ratings Ta max 85°C,to be a consumer grade component ,the table shows Ta 110°C nothing else but a servo amplifier for military/aerospace applications.It can be used as a ballistic missile guidance servo amp or in supersonic aircraft where not only high temperature operating is required but also continuous function at lower power but higher temperatures at reentry in the atmosphere or engine failure.If this does not convince you,admit that something sounds wrong.What really sounds wrong is when it is used with a high efficiency loudspeaker or headphones upon 8 ohms load,at low power the sound lacks bass as some diys did complain, on this, as well as on other websites .Whereas with gain of 20, anything below 3khz is worthless (according to me),on the other hand above 3khz it is irreproachable,which is unusual for a class B amp.If I heat the heatsink with a powerful hair drier, the hotter it becomes less it sounds wrong until at temperature of about 125°C above which it doesn't get any better, according to my ears (worn ones).Now ,I woudn't bother anyone with such doubtful experience if Gaincard amp did not use a tiny enclosure as heatsink, to allow the IC function at high temperature at high power.But what about at low power?Instead of using hair drier ,a heater resistor along with a thermo-switch mounted on the heatsink will do.A 50W high temp. resistor of 100 ohms powered by 48VAC in series with KSD301(nc)130°C thermo switch does the job.Heatsink to be retailored ( see note below).On figure 37 the table shows 20W Pd at 126°C ,figures 38,39 translate Pd into output power and supply voltage for 4 and 8 ohms.They show available output of 35W in both cases,with required gain of 16 and 24,this explains why this amp is compensated for average gain of 20.
Explanation: this is just a speculative explanation without any technical value.
This is a monolithic circuit ,all the componants are on the same die.When a transient power is developed, the output transistors gets hot in a short time, as the thermal resistance doubles, creats on the die a heat wave,which heats low power transistors, sequentially enhancing their betas,passing higher currents sequentially, this provoks feedback.The layout designer has done all to avoide positive feedback ,to have only negative thermal feedback. Everytime a transient occurs,as a beat or a consonant,instead of providing appropriate output voltage and balance the differential input via feedback,the thermal negative feedback balances the input internally, swallowing the transients from the output.As the opamp is an integrator,gain varies inversely with frequency,the negative thermal feedback pollutes transients which are bellow certain frequency.By heating the die, as beta of transistors vary less at high temperatures , in consequence ,the negative thermal feedback ratio decreases .An example of beta variation is given on figure 13 ,input bias current vs case temperature.This is the inverse function of input transistor's beta,and it shows how it gets stabilized at 125°C, where all thermal feedbacks are unhealed,which was not the case with the precedent version LM3875,from which it inherited the table of figure 37. Most probably all other low level transistors are similarly modified so that no any thermal feedback is provoked above 125°C.
All this "trallala"is to make this audio amp which is (according to me) a JLH simple class A converted to class B by adding a PNP transistor along with some diodes at the output stage ,to performe as good as it would have been with descreet components,or may be I am just hallucinating .
Note for heatsink
Convection (hot air going upwards) becomes important cooling function at 125°C.Chimney becomes a better option than radiating heatsink.All you need is a square section tube installed as a vacuum tube upon the chassis, with a hole venting the interior of the enclosure.I did not tried this yet ,I'll give the dimensions of the tube if.

its not only heat that limits output.
so it did not go wrong anywhere.
its a fine product and does its job well.
there are limits set by other than heat only , that limit maximum voltage (supply and therefore output) and maximum current it can handle. it ca not produce mroe than the product of those.
i took the effort to read it, not sure if everything got into my head.
allso heating an amp ic rarely made it sound better to me.
i prefer them as cold as possible within reasonable level.
if it gets up to boiling point then the highs become grainy and i don't like that.

anyways the OP says warming up the ic will make it sound far better.
near boilng point of water.
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