Power amp IC vs Power amp Transistors

[yustech]

hello..I'm trying to understand this.
lets say we have 2 amps and its gonna be like a shoot out or sound comparison.
both are identical solid sate type except for the power amp section.

amp A has LM3886t(found out about this when repairing Vox AD30VT yesterday)

amp B has maybe 2 BD909 or combination of 2 PNP+NPN power transistors that will produce the same watts as amp A.

Thank you all.


one more
what is equivalent to BD909(saw in laney hc30r schematic)

[teemuk]

...And besides the output devices mentioned, what else is there? Power amp is more than its output devices.

You can download the datasheet of LM3886 and check out the equivalent schematic. An IC like this is typically nothing but an ordinary amp circuit integrated inside a chip.

This integration has some benefits though: For one, you can make the circuit quite a lot more complex without it requiring a lot of board space or hundreds of discrete components. One can see how the circuit shown in equivalent schematic diagram has quite a lot of linearization going on while its still basically just a differential input amplifier followed by a single-ended voltage gain stage and a current buffer. Not shown in the schematic are the various protection circuits also implemented to the chip. Many discrete guitar amps lack them or only implement very simplified versions of them because otherwise these features would add too much complexity and cost (increased board space, component count, amount of design time etc.).

Second benefit is the close thermal coupling, which means the IC automatically tracks its temperature and maintains a steady bias. In discrete amplifiers you typically need to track the temperature of the output devices and feed this information to the biasing circuit, which then acts accordingly.

There are drawbacks too. Fabricating PNP power transistors to ICs is quite difficult so these chips often use the same circuit that solved the problem half a decade ago for discrete circuits: The Sziklai pair and quasi-complementary output. You can find it from LM3886 as well, and as you can see, the output devices are of same polarity. This generates distortion since the NPN Darlington pair works somewhat differently than PNP Sziklai pair.

Second problem is that fabricating resistors to IC is also somewhat difficult (and therefore designers wish to reduce the amount of them) and that capacitors fabricated inside can be only of very small value. If higher values of capacitance are required they must be introduced externally.

The area of the chip's case also limits the heat dissipation. This typically means that a somewhat tiny chip such as LM3886 cannot cool itself as effectively as a large number of discrete power transistors. Of course these things strongly depend on heatsink arrangement too.

Final drawback is that the chip's architecture is closed and one cannot modify or scope the circuit inside it. You are stuck with the circuit it holds inside and that's it. Want to, for example, disable one of the protection circuits or troubleshoot what inside the chip burned up when the chip stopped working and then fix it. No can do. The only modifications to whole power amp circuit you can do are those introduced to the external components required by the chips. If the chip goes obsolete (what it will surely end up being in a matter of few years) their price skyrockets and fixing any circuits with such chips becomes economically questionable.

[Jack1962]

LoL-what Temmuk said, also , I have found that the IC power amps are not as durable as the old power transistor outputs.

[yustech]

I guess for beginner like me i asked to much hehe.

Thanks teemuk and i also do agree with jack1962.

although i'd skip reading about that sziklai and that quasi thing,i do now understand why most under 20w guitar amps comes with those TDA IC,the design itself is done by the parts manufacturer.so less R&Ding for the guitar amp maker.i guess initial production time,skill labour and cost are very important.

Power amp ic does make replacement easy.

I also have to agree on your last paragraph.

I used to worked at Sharp service house and most of their hi fi uses those STKxxxx ic and it is expensive to replace.I accidently burned a few and its not good when the boss knew about it.On my work records it would went.hehe

Later i worked at a company servicing marantz,sansui and creek home audio stuff.most of them uses those 2sa,2sb,2sc,2sd and other 3 legged trannys,none wre using STK or similar IC's.Replacing them was rare case,even my NAD lasted for 14 years if it wasn't for the faulty transformer.

now i'm in the musical stuff repairs and i'm seeing both IC and Tranny fitted equip. coming in for repairs.Thats why question suddenly pop in mind.

Thanks for the elaborate answers,this is really a great forum.