Hi Rupopulles,
Welcome and thanks for joining our community. I would certainly try to the join diyaudio if you haven’t already.
Here’re a few quick notes after a quick glance at the circuit here:
Add shunt resistors around 100K from your input gates to ground. Without defining the input impedance, the gates will float up and you’ll have problems. 100K is typical for this. No need to use a JFET for Q26. I’d just replace this with a BJT junction based configuration such as Q12/Q10. For these small signal transistors you can use something that is general purpose such as 2n5551/2n5401. You may have temptations to use high beta devices but I’d suggest not using those parts due to the much higher risk of parasitic oscillations and run away.
I’m curious why you’ve decided to float the output up instead of using a bipolar supply and aiming to get around 0VDC on the outputs. My guess is that you wanted to eliminate a supply. You just better hope that both sides stay well behaved in DC and don’t start to slip on you or your speaker will start to possibly see some significant offsets.
The output stage is where I start getting concerned a bit. The configuration is similar to a diamond output where the current running down the driver stage is reflected on the output. You need emitter resistors to define this current since you are essentially using the junction drops to get to 0 voltage on the output. You can start at Q14/Q15 and follow the junctions on each side of the complementary circuit. You’ll see that overall, the ideal DC operating point is 0V from the base of Q14/Q15.
One issue that won’t show up in simulations is thermal runaway on the outputs. I have found these configurations to be challenging to thermally stabilize and one way you can help mitigate this is to also add emitter resistors to Q14/Q15. My guess is that there will be some trial and error here to make sure the output stage is stable.
Finally, you don’t have any negative feedback in the design. Negative feedback is incredibly useful for power amplifiers. Feedback lowers output impedance, dramatically lowers distortion and helps with DC stability. I’m guessing this was a design decision but thought I’d mention that for power amplifiers, feedback is an incredible tool to optimize performance.
My suggestion is to add a negative power rail, add emitter resistors, and look into DC servos using a JFET opamp to servo the outputs in reference to ground. This will help with DC drift on the output especially since you aren’t using NFB.
That was a quick overlook on the circuit and didn’t look into gain/other specifics but I’m impressed coming from someone in their second year of EE. Good work.
If you’d like to talk further, PM me.
Best of luck and happy listening
-Darren
