I want to build an amplifier. I got a circuit diagram from a website, Amplificador Monofónico de 250 Vatios Cuasicomplementario.

I want to know the class of this amplifier.

Amplifier schematic

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    \$\begingroup\$ Have a look at Amplifier Classes and see if you can figure it out yourself. \$\endgroup\$ – Transistor Apr 26 '19 at 18:27
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    \$\begingroup\$ With 75 V rails and a 4 ohm speaker you're looking at ~1400 watts. You would need many more output transistors for that, those 4 would cook themselves fairly quickly. \$\endgroup\$ – Colin Apr 26 '19 at 19:01
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    \$\begingroup\$ @Transistor Interestingly a very poor description of Class AB in your link. Although they state there is a Class A current flowing, they show a clipped output. Much better description here: en.wikipedia.org/wiki/Power_amplifier_classes and IMO the best description ever of Class B is Lindsey Hood: keith-snook.info/wireless-world-magazine/Wireless-World-1971/… ...read some of the classic Wireless World audio designs on the site. \$\endgroup\$ – Jack Creasey Apr 26 '19 at 19:12
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    \$\begingroup\$ @Colin Remember that you are quoting PEAK power. The amplifier is likely no more than 100-150W RMS and probably limited by the power supply. semicon.sanken-ele.co.jp/sk_content/2sc3858_ds_en.pdf \$\endgroup\$ – Jack Creasey Apr 26 '19 at 19:17
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    \$\begingroup\$ @JackCreasey Agreed, but the amplifier has a gain of 40 dB, with line level audio it will clip heavily with those rails, and would be trying for a lot more than 150 W. \$\endgroup\$ – Colin Apr 26 '19 at 19:21

Stage 1: Differential PNP Cascode Pair, comparing input with output with a gain of 100. (Q1,2,3,4)

Stage 2: Differential NPN Cascode Pair (Q7,Q9,Q6) with compensation caps and an Active load (Q8) and crossover bias diodes. High gain.

Stage 3: Differential Emitter-Follower (Q10,11) with compensation.
Stage 4: Current-Sharing Differential Emitter-Follower Q12,13,Q14,15

When Vout= +/-Vdc/2 the transistors draw 50% power of load. Class A partial Class B with lots of negative feedback.

Mid 70's power design.

DC Stabilizer and crossover distortion has improved since then with better linearity.

  • \$\begingroup\$ Which stages are the class A or B? \$\endgroup\$ – Unknown123 Apr 27 '19 at 1:52
  • \$\begingroup\$ Though I understand little about the term, I thank you for helping. Just looking for a DIY power amp project building with friend. the amp should be able to extend the power if needed. \$\endgroup\$ – praHoc Apr 27 '19 at 1:57
  • \$\begingroup\$ Stage 4 will be class AB but when there is offset Voltage from Stage 1 input impedance , small signals would be in Class A, since the open loop gain is over 1k the large signal will reduce crossover with negative feedback with +/- polar signals buffered by upper/lower circuits in stage 4 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 27 '19 at 1:59
  • \$\begingroup\$ Make sure the heatsink is as good or better than the one in your PC for CPU. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 27 '19 at 2:01
  • \$\begingroup\$ @SunnyskyguyEE75 Stage 3 is actually the Class AB pair and NOT a differential Emitter follower pair at all. Stage 4 are simply current amplifiers and NOT Emitter followers. \$\endgroup\$ – Jack Creasey Apr 27 '19 at 6:22

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