Can this 3 transistor classA amp 50w@4ohm with 1.3A bias still be called classA? - Page 1
Forum section: electronics / projects

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: November 03, 2024, 03:27:17 pm
content: I have designed this amp and posted several years ago on diyaudio website as CCPP. I simplified and enhanced it to be 25w@8ohm.It has a fabulous sound, more tube than my SET 2A3C.I will give it to public domain by letting the PCB to be purchased directly from PCBWAY, but I don't know what kind of amp to advertise.On simulator I tried it with 4ohms and to my surprise it could go to +/-20Vp before saturation, that is 50w rms.The upper PMosfet doesn't cutoff but the lower Nmosfet does when the load current exceeds the bias 1.3A.There is positive current feedback that gives the output stage very high transconductance which results relatively low distortion with second harmonic dominant.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: November 03, 2024, 10:15:59 pm
content: Amplifier classes are really straightforward. Class A conducts all the time. Class AB turns on and off with some overlap. Class B turns on and off with no overlap.You have a heavily-biased Class AB.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 03, 2024, 10:41:30 pm
content: It would have been nice if the OP had bothered to label the output of this circuit. But yeah, it looks pretty much like a classic push-pull (= class B) amp with some biasing to make it class AB. Which should mean less crossover distortion, right? It looks like it does pretty well in that aspect. (Not sure exactly what those waveforms are, though, as they aren't labeled.)Surprised that you mistook it for class A, which as pointed out has 360° conduction (through the entire wave cycle) rather than the 180° of class B.

author: johansen
author_rank: Super Contributor
timestamp: November 04, 2024, 01:24:37 am
content: Quote from: Kokoriantz on November 03, 2024, 03:27:17 pmand you can do all that in software without the voodoo.you do realize you cannot have second harmonic amplification without distortion.. right?

author: coromonadalix
author_rank: Super Contributor
timestamp: November 04, 2024, 01:36:22 am
content: as soon you have a push pull look a like its not a pure class a

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: November 04, 2024, 01:44:59 am
content: Thank you for giving attention to this circuit.The output is the 4 ohm resistor with a voltmeter tracing green on the scope 20Vp.The amp is a CFP ClassA single ended with a CCS but the load is grounded via the current weighing resistor. If the CFP commands more current to the load, the CCS will decrease its current as much to keep the 0.5 ohm resistor constant current, in turn, the decreased current wil make the CFP pour more current to the load and have a positive current feedback which makes the CFP work at constant current and the CCS modulated. Until the load current exceeds that of the CCS, which will shut down, here, the CFP will increase its current to drive the load single ended. By this, the CFP is always conducting whose current is shown in red.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 04, 2024, 02:02:46 am
content: this is a class ahttp://diyaudioprojects.com/Solid/ZCA/ZCA.htmand sorry but you should be using more standard terms then ccs cfp etc ...and do show sine wave when it cross over positive and negative, but simulation eh ... all talk ...

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: November 04, 2024, 03:01:10 am
content: In electronics,CCS means constant current source.CFP means,complimentary feedback pair or Szikai.This amp originally built with a transistor to control the CCS and had floating ground.https://www.diyaudio.com/community/threads/new-class-a-constant-current-push-pull-amplifier-with-single-smps-and-hybrid-preamp.332060/.What do you mean by zero crossing, the current of the driving output is always positive as shown with maximum about 5A.

author: ArdWar
author_rank: Frequent Contributor
timestamp: November 04, 2024, 04:18:53 am
content: Maybe you indeed can call this class A, in a very roundabout way. The current sink is active but not exactly input controlled.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: November 04, 2024, 04:44:33 am
content: I am thinking of adding the terms, single ended, to be understood that classA is at command and denote the CCS as , negative Z load.

author: wasedadoc
author_rank: Super Contributor
timestamp: November 04, 2024, 12:16:56 pm
content: Quote from: Kokoriantz on November 04, 2024, 03:01:10 amcomplementary. (Not complimentary.)

author: mtwieg
author_rank: Frequent Contributor
timestamp: November 04, 2024, 02:04:35 pm
content: It's not class B, as all transistors are are conducting even with zero input signal.So it's either A or AB. As always, it's a matter how it behaves at its maximum drive level (all class A amp will become AB at a high enough drive level). If at the intended max output level both transistors are still conducting significant current, I'd go ahead and call it class A (for whatever it's worth).

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: November 05, 2024, 04:25:30 am
content: I will not advertise it as classA, not to have people deceived when discover its not 100% classA. For that I have 4 transistor version.https://www.eevblog.com/forum/projects/4-transistor-40w-class-a-0-0004-thd-amplifier/I need to replace the lm334 with an NTC.I will rather present as simplistic 3 transistor 50w retro sounding amp, then he will realize that it is also partially classA and will be interested still more.And now I must prepare 2 projects in parallel.Thank you for your replies, if engineers can't see the real function, how an armature DIY will. Probably I must add a youtube video to explain this simple but strange amp.

author: dietert1
author_rank: Super Contributor
timestamp: November 05, 2024, 06:53:41 am
content: The OP has some misconception about output power of his circuit. If the CCS bias current is 1.3 A, it will drive at most 1 A into the load at low distortion. That means about 0.7 Arms, which isn't 50 W into 4 Ohm, but 2 W. Just enough to drive headphones, while causing more than 120 W of continuous heat for two stereo channels. It doesn't make any sense.Regards, DieterEdit: Apparently the circuit has positive feedback (!) via the source resistor R2. So that isn't at all a CCS, but looks more like an oscillator than an amplifier. With a real speaker as load it may become unpredictable.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: November 05, 2024, 09:07:45 am
content: I wonder if the message is addressed to me indirectly.The stability is so robust, it doesn't need any Zobel or Thiel.Bellow is square wave with 2uF in// load.The distortion is 0.02% for 1w 8ohm to reach 0.4% at 25w and 1.7% 40w 4ohm, 2.2% at 59w, mainly 2nd harmonic.

author: dietert1
author_rank: Super Contributor
timestamp: November 05, 2024, 09:51:17 am
content: That 2 uF test load isn't the replacement circuit of a real speaker. You cannot use positive feedback for a linear amplifier. Build your circuit and try using it. A real speaker with a passive crossover inside will have a complex impedance.Anyway we can say that the class A output power of the circuit is about 2 W into 4 Ohms. The usual class AB audio amplifier with its 50 mA or so bias current has less class A power but much more negative feedback to reduce distortions. Better build amplifiers using LM3886 or TDA7293.Regards, Dieter

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: November 05, 2024, 10:56:17 am
content: See post#7. This amp is working even with a current controler transistor which makes the positive feedback ratio 0db, without, it is -2.5db.Any emitter follower is a positive current feedback with less than 0db.