Amplifier

Some operational amplifiers confuse me like this one. You are creative with this operational amplifier. I'll see how powerful it is, I think this is an AB amplifier.

It's around 4 volts with 4 ohms, so that makes it around 1 amp, 4Vx1A=4W, it's a 4 watt amplifier.

That should be 68 ohm, not 0.68. Anyhow, it's only got about 2.5mW going through it. A standard 1/4 watt resistor can handle 100 times that amount of power. With a 4 ohm speaker, you should be around 4 watts. That'll drop down to a little more than 2.5W with an 8 ohm speaker.

By now it's work in very unbalanced class A means both transistors operate with operating point so chosed to bias transistors in steady state that output voltage is at middle of power supply and they never are modulated to less than around 10%-20% near cuttoff so current is present all the time

This is somewhat very strange amp. Its looks like someone complexing it just to make different topology as main aim. This upper pair of trannies is a constant current sourcing supply. Yes its cool upper pair sourcing constant current and below darlington act as current divider and ..... in what this strange thingy can give me advantage compared to traditional way?? Means sound quality , power efficiency, simple yet clever and powerful design from those things its fail in simplicity becuse i must break brain "guess what author's has in mind doing that " and this is not a feature of clever design. Clever one is when u catch u head and shout "why never catch this idea earlier so simple so obvious so ingenious"

For the 0.68 ohm resistor im sure thats the correct value and as to the topology itself I too find it quite strange cuz it looks like class B amp but behaves as a class A but its not a class AB. So is this a nice design because recently im fired up on amps and wanna build something useful. The last amp I build behaves well is stable so far but it doesnt offer the advantages that come with a class A amp (hi fidelity good distortionless sound) .

Isnt using constant current source better than using a passive resistor that would dissipate hell-of-a-lot of power to produce a quarter of that power as a useful sound. The darlington class A amplifier is easily distinguished and is as basic as they come. The first stage is also typical for these kind of circuits. So I just answered my question myself. Its just pieced together from other circuits its not unique per se but it should offer the advantages of three different circuits simultaneously.

Yeah, I missed the .68, I saw the 68 and thought you wrote .68. That thing has over 27W plowing through it. And the general rule of thumb is to use twice of what you think you'll need, in case something gets shorted, etc. So you'll be looking for at least a 50W (probably 1%) 0.68 ohm resistor. That's VERY specific, and won't be easy to get your hands on.

Cant I just wind a thick piece of wire with a 0.68 ohm resistance? 50W resistor would be huge and costly. ;/

That's basically what a 25W resistor is, wound wire. I don't know how big they make them. Personally, I would try to find another way to go about it, rather than dumping 27W on a resistor.

Like what? This resistor is essential nevertheless its value.

Beware of wiring resistor by some thin copper wire or any other typical alluma or steel. Those have so high temp resistance variation than unregulate whole amp. Those resistors are wired from constantal resistive wire whose have almost zero temp coeff. It's best material known. Wiring is made bifilar to zero out inductance and made on case with smallest dieelectric const to minimize capacitance. Constantan has also high thermal resistance to melt off if a short circuit or heavy overheat occurs a ceramic case can take so much watts up to red heat almost and whole resistor survive and not change params.

This resistor is essential for current source to workon. Not all time it will dump power. Most of the time a transistor will take it on chest. This resistor is a refference voltage for current source and from his ohmage a constant current ampere value will be determined. At rated one amp if this resistor if have for example 1 ohm a one watt will be dissipated only on him and trannies take rest . Dissipation will oscillate between current source transistor and lower swinging darlington pair in synchrony with modulating signal. This is somewhat stupid idea cuz resistive current sources are always most dissipative method of powering. Fact those give uber linear environment for lower transistor pair and theoreticlly limit distortions cuz less nonlinearities shows up when current is always constant. But this trannie itself introduce new ones cuz any transitor no mater how well maintain always produce more dist. Than without him if it is in direct power loop. More it take on his poor body role of collector resistance plus additional loses from regulation. So concentrated heat generator at your service dissipating more when power output is limiting by volume pot. So at silent operation it will act a room heater. At full power it will dump power from herself to speaker cuz it will no longer need to limit it. But who use full volume all the time. At powering by traditional way if values are perfectly chosen and matched a voltage source power supply will produce the same result as from current source with only difference that power will be wated not on resistor but transistor. Small difference will be at lower output wattage but it will be so small and if we take zero additional thermal noises from two trannies less this is not good option this whole current source at all. A true pro designer will instead a resistive currentsource introduce a well designed non-saturable voltage dependent reactor instead colector resistance ths solution give constant current source, perfect linearity even at different frequencies, lack of semiconductor noises and nonlinear distortions and most valuable!!!! LossLess non dissipative resitancne in form of inductive reactance so overall power used will be limited by losses generated from current source and resistor. One disadantage is zero resistance for DC. Choke are usable down to 20 or even 15 hz presenting all time full curent limiting feature but during zero signall it dump bias voltage from herself to transistor so at this special moments to not dissipate half of power on our trannie ( normally half is at collector resistance half at trannie at iddle) a some ppl use current limit addon to if signall is zero for longer than 5 -6 second cut off output transistor and back it to power as signall appear. Such chokes can be made from ordinary speaker transformer cores and as u have full window for winding cuz u not need to wound secondary thicker wire can be used to boost power handling around four times possible. If it is a transformer made to be powered by pulsing DC none more will u need . If it is for AC a half mm air gap made by remove by any siutable means this half mm from center leg at contact point when of two core halves conjoin. Typical inductance at 20v max on choke at one amp no more than 50mH so it is tiny ammount and even with air gap and thick wire a 100 or 150 turns will u need no more .

I just wound up the transiator. The teacher who gave mw tge wire to wound it told me it was one of the best cuz it can be soldered (unlike others). I dont think it needs a bifilar winding because through that exact resistor only DC flows and I dont think it matters whether I put an inductive resistor or normal.

Yes theoretically it's no matters here especilly that ammount of this inductance is not very impresive and only hf circuits take such in considerations. I only afraid that current source. If it should operate reasonable at such currents it should be given very high supply voltage instead it will behave like constant current only on scheme. A high gain transistors will help but not darlingtons cuz those introduce too high dropout at saturstion limits by this availabe voltage during swing. A mosfet current mirror will should be better what u think bout this?? Anyway it wil be hottest place so proper distance from other components and heatsinks required.

Ive decided im not going to build this. Theoretically it will be a furnace with no input and is quite unstable in simulations. I mean it doesnt work properly even if 1 part is different.

Yeah especially that it wil, be concentrated source of heat. In normal operation even if a watt r two is lost more those loses are distributed between transistor and power resistor or second trannie so they are not so concentrated

Someone lso notice this amp and post it informig about some problems lol