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Do you know any linear sources providing offset on their output? Most of them is AC coupled. BTW, offset produced by the output stage is of comparable level and there is no feedback loop in this amp to compensate it. I bypassed those big blue ERO and it showed that they weren't that excellent. When insisting on AC coupling, add a quality bipolar cap parallel to the film cap.
Any output source has DC offset, unless it has capacitor in series that block DC component.
Feedback loop is not here because that involves more components and problems with ringing and oscillating if feedback loop is not made in proper way. Furthermore, if there was feedback loop, OPAMP rolling would be dangerous because LM xx, BB xx and AD xx are different speed and behave in different way so, simple said, every OPAMP needs specially tuned feedback loop for best performance.
Output source that is made directly from OPAMP output (without buffer) involves default DC offset that is stated in tech specs plus DC offset on input (multiplied by gain). Naked out from OPAMP might be excellent choice but has two bad things. First is that you can not down DC offset with output capacitor because OPAMP does not like capacitive load (usually) and when it does, it needs additional compensation and feedback loop (more components). Second is that output resistance falls to low level (usually 50K ohm) and we all know that OPAMP in tech charts specifies minimum load when it gives specified performance (some need 1kOhm some 10 kOhm, etc..). But generally, the bigger load the better performance. In Lovely Cube (Black Cube Linear, Matrix M stage) output resistance (what buffer sees) is 10 Ohm which is high and can be lower to 2.2 Ohm with changes in resistance (1.5 kOhm resistor to 2.5 kOhm) but it will make amplifier prone to oscillation and OPAMP rolling will not be so easy. Current through BC 550/560 transistors will be lower and ratio between BC/BD transistor current will be higher - reason for oscillation - needs better noise control and source and tightly matched transistors.
The most important thing is what kind of load sees OPAMP now (with bufFer). it sees almost pure resistive load (buffer) and its (buffer) resistance is close to 160 kOhm. 160 kOhm is enough for every OPAMP to behave within specs. Only reason what people see difference between diamond buffer and naked OPAMP out - is in load resistance. A pure technical reason without voodoo magic.
After I researched few things and make few Lovely Cube amplifiers I fount out something what makes me laugh. And that is - biasing OPAMP in A class. If we want to bias something in A class we need DC current through buffer, OPAMP or output stage or middle stage. Whatever... Our case - BJT transistor, that will always be in ON state and in area where BJT transistor behaves like amplifier (most important thing!). If we want to bias OPAMP in A class it is almost like we drive voltage amplifier (preamp) in A class. If OPAMP must drive 160 kOhm load then how much it needs to be driven in A class? Well, it needs only 40-80 uA. Improvement that people see when they connect 5 kOhm resistor between OPAMP out and negative rail is just opposite (downgrade) because it involves two things: noise from supply source directly to OPAMP out, and low resistance (OPAMP now must handle higher current where 98% represents pure heating and buffer becomes useless).
When high resistance buffer is in output stage (represents current source), biasing OPAMP in A class with DC current more than 200 uA is total nonsense.
