The amp shown is nowhere near "professional grade". Professional means it does the right thing in unusual circumstances - naughty inputs, naughty outputs, naug

The amp shown is nowhere near "professional grade". Professional means it does the right thing in unusual circumstances - naughty inputs, naughty outputs, naughty mains power, running at full power on the grass under a pile of coats and scarves, pumping through old and fraying speaker leads lying on the ground, ... The design is missing all sorts of safety features apart from the most basic (prevent electrocution from broken mains wires touching the chassis). A 200VA transformer with a 350W amplifier is just asking for a transfomer meltdown (or fire, if the transformer doesn't have a functioning thermal fuse), unless there is a limiter in place to limit the output to 120W or less. (Fuses are also recommended, but in the professional environment you don't want them to blow, ever. Hence, a limiter is mandatory.) The bridge rectifier is undersized. In traditional mains-frequency rectification such as shown, the diodes only conduct for about 10% - 20% of the time, so the peak current in them is can be more than 10 times the average. At 36V and 10A output average, the rectifier current peaks are in the neighbourhood of 100A. A BR35 (35A 1000V) will provide a reasonable working life. At up to 36W dissipation, you can probably just bolt it to the chassis with some thermal compound. The design is also missing speaker protection features and surge/spike protection/EMI prevention, and RF intereference filters on the inputs. The design is also missing convenience features one expects of a professional amp, such as "power good" and "fault" lamps/circuitry. And handles. It's nice that there are good inexpensive amplification modules now. But as with software, the difference between a toy example and professional grade is thought, time and money spent on reliability, safety, and usability.