My DIY Amplifier has a small sizzling sound

[dkfan9]

Make sure to disable the AC source at the input. In the sim you attached it's still on. Doing that, I get more reasonable seeming ULGF at 1.1MHz with 67deg PM and 18dB GM at 5.5MHz with DC servo removed, and similar with DC servo included (but with the same peaked shape).

 

[Bonsai]

I'm sure Bonsai will have more valuable insight on this, but to me your 1st plot looks more or less 'normal,
and the PM value (39 degrees) is too low. PM should be at 45 or more.
GM on the other hand is unusally high (35).
From my experience, amps with simulated PM below 45 degrees, are very likely to oscillate, perhaps not all the time, but in certain conditions.
Grounding issues, loops, and long/convoluted PCB traces can make it even more likely.
I would aim for closer to 60 dgrees in this case (this topology definitely can achieve this).

Phase margin and gain margin are usually extracted at the amplifier output before the output coupling inductor network. Without it, it doesn’t take a feedback amplifier much capacitive load to prompt it into oscillation, or peaky response behaviour.

However, with even a small inductor (0.5 to 1uH), very effective isolation of the load is provided. I regularly test my designs with capacitive loads of up to 2.2uF without any problems. Is 45 degrees of PM enough. Normally I design for 60 degrees, use an output L of 0.5-1uH and TMC compensation, but with a suitable output L, PM’s of 45 degree are also ok. You could not do this without an output L. Its important BTW to ensure you have good gain margin as well. With TMC, figures of 10-20 dB are normal. 😊

https://hifisonix.com/wp-content/uploads/2014/11/Output-L_1.pdf

 

[dkfan9]

Bonsai brings up a good point, Sevy. If you name the actual output node (after the L) "out", you need to change the node name in the loop gain formula you're plotting. For instance, in LTSpice demo circuit "LoopGain2" they name that node "x" and the LG formula used there uses "x" rather than "out". The probe and calculation is intended to measure the gain from the probe's positive end to its negative end, so the nodes you measure should be the nodes on either end of it. I used that method to get the GM/PM numbers I posted earlier.

 

[Grauwacke]

Bias current too low?
I fitted R21 modules into a silent amp (see https://www.diyaudio.com/community/threads/an-arguably-better-replacement-for-the-resistor-in-a-crc-power-supply-r21-ps-module.376003/page-10).
After switching the amp on, I was shocked by a sizzling noise accompanied by slight whistling.
What went wrong?
By mistake I had set the output voltage of the R21 module more than 10V below its input voltage. Accordingly, the bias current of the output transistors had dropped to a few mA. Readjusting it to the original level was the cure: Complete silence again.