Output stage consideration

Write by Andrea Ciuffoli. This is a first realise so will be corect and increase. In this following notes I would like to give You some elements to take the decision on what output stage use in yoour amplifier design. Before to read this I suggest you to read what is write inside the article on Distortion in power amplifiers - 4. the power stages from ELECTRONICS WORLD. At this time I don't analize Output stage that have internal loop inside as CFP, STASIS, ecc ... . The more simple and used in very few design is the darlington with 3 stages (it means 3 device on signal flow). This is considered old design but produce very high level sound quality, infact Mr. Bartolomeo Aloia have used this in many amplifiers as the ST200. This ST200 amplifier declare 100W on 8 ohm but drive loudspeakers until 2 ohm without problem and can give on output impulse until 1Kwatt ! and the distortion of the output stage without feedback is less 0.1%. The components that he have used in ST200 are siemens BD139/BD140, motorola MJ15028/MJ15029 and motorola 4 x MJ15022, 4 x MJ15023. In the circuit there is 3 power supply Vcc1,Vcc2,Vcc3 but normally only one for all this stages is OK. The resitences values are: R29,R30 = 15Kohm 1W R33,R34 = 2.2Kohm 6W R35,R36,R37,R38,R34,R40,R41,R42 = 0.33ohm 3W about power suply voltage +50V/-50V and about termal stability a normal Vbe moltiplier as been used. The Bartolomeo Aloia about this Vbe moltiplier have done some test and he have found that using normal diodes instead of this, to get termal stability, the sound is more good. This type is used also in new Hybrid amplifier that he have design with the new Japan components, look about this also this comparation. If we want increase speed performance and reduce open loop distortion, for example to compensate the no linear output caracteristic of transistors, we can put an PNP that driver a its complementary NPN. I have see, the fisrt time this solution in the circuit of Audio Research D-400. In the D-400 to prevent the limitation of this design, as explain very well in the article Bulding better buffer from ELECTROINICS WORLD, the Audio Research have put on emitter of first stage a dinamic current generators with high feedback error correction. In fact we can see that the output signal is token and used to increase the current of this generators. Now the slew rate is very high and the distortion very low ! But this design increase the numbers of components and the possibility to corrupt the sound quality. The Pioneer some years later, after some years lost to design amplifier with dinamic class A, have presented the model A-09 with a new original output stage design. The A-09 is also the only one commercial amplifier, produced by an electronics consumer productor, where nono global feedback is used. This means that Pioneer engenear have found something to increase performances of output stage (look this misure). Another positive of this design is that if you put all active components(transistors) on the same heatsink the effect of temperature are automaticaly compensate, so you don't need sensor transistor as the darlington. I have simulated this design and with normal transistors as TIP41/TIP42 I have get this good results: with +18V/-18v the output swing is +17/=17v Slew rate: ton 650V/usec and toff 2000V/usec Rout = 85mohm THD with sin output swing +15v/-15volt 0.079% 10KHz 4ohm 0.098% 1KHz 4ohm 0.057% 100Hz 4ohm 0.088% 10KHz 8ohm 0.050% 1KHz 8ohm 0.064% 100Hz 8ohm and the bias point should be ottimize (value of resistences: 59, 40, 2K2 ohm). But if you are searching a good design with lower components and more easy with high performance You can use MOSFET and feedback as used in HAFLER amplifiers. This type of design is show in the page 5 of the article Simulated attack on slew rates. In this design, that is the same of the HAFLER amplifiers, the power stage hava a voltage gain because the MOSFET are not used in follower configuration. Here with a low power supply voltage drive stage has been possible to get incredible performaces. The other vantage of this configuration is that there is no switch off of output devices so could be called "virtual class A". This "virtual class A" is used to generate a very good quality sound also by other company as Musical Fidality in many amplifiers as the A-370. And the result is will continue ... 1