OPamp + Darlington Will it work?

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yes that will work, in fact there was a wellknown italian design "Puccini" based on the same principle. It even has some tricks to maximise output power, by shifting the opamps supplies with the signal.

To be honest, if you do not like "chip amp" sound, then you probably won't like this circuit either. They are very similar.
 
Looks like a lot of fun, definitely worth building. I don't believe in 'chip amp' sound and would think you have quite a bit of scope to change the 'sound' of this design through component choices, some tweaks and changes, different op-amps etc.

Not sure why you need D1 as part of the Vbe multiplier unless you are trying to get a better match in the temperature compensation somehow.

Are you really using batteries or is that just a symbol for the schematic - because if this is a mains derived power supply with rectifier and filter caps etc. I'd be nervous about the PSRR of this design, the base junctions of the output transistors are very closely coupled to the power rails. Me thinks you should add resistor + capacitor filters of a decent size to both power rails to isolate the 'front end' of this amp.

The op-amp will want dedicated power supply decoupling capacitors.

I'd add a capacitor in series with R4, something around 100uF. This is to ensure that the inverting input of the opamp is able to 'see' the dc voltage at the amplifier output - and correct for dc offset. As part of a low dc offset I'd add a 22k resistor at the non-inverting input to match R3 when using bipolar input op-amps.

Lastly, I might consider a high quality capacitor (e.g. 0.47uF) in line with the input signal to provide a measure of safety from any dc on the source signal.
 
If you included the output stage in the feedback loop it would reduce the output stage distortions. Though it would be a little more complicated than just changing the take off point to the output, the blocking caps would cause havoc with the output DC offset. But I dont see much of a problem removing them and attaching the OA out directly to the base of Q1. And I also dont see the reason for D1, which is a hassle because it should be mounted on or near the darlingtons along with Q3 for temp. stability.
 
Again, without any gnf the PSRR will be relatively low. The regulators will help with the OpAmp if done well but the bias of the output devices feeds directly off the rails - perhaps these bias points should be moved so that they connect to the power rails after the RC filters.

You'll find it handy to have a probe point or connector for measuring the voltage between the emitters of the output devices - to check on the bias current.
 
Again, without any gnf the PSRR will be relatively low. The regulators will help with the OpAmp if done well but the bias of the output devices feeds directly off the rails - perhaps these bias points should be moved so that they connect to the power rails after the RC filters.

You'll find it handy to have a probe point or connector for measuring the voltage between the emitters of the output devices - to check on the bias current.



Does replacing resistors R3 R10( from last schematic ) by current sources solve the problem?
 
It would certainly improve the PSRR but why not simply include the bias inside the RC filtered part of the power supply ?

You could also bootstrap these resistors from the output, this would also improved PSRR and provide a more constant current through the bias string. You just replace R3 with two resistors and tie a capacitor from their mid point to the output.
 
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