Top of the PCB. On the bottom right you can spot the transformer that steps down the voltage from mains to a more manageable voltage (probably 24v). Above the transformer we spot two heatsinked chips and to the left of the transformer we spot another heatsinked component. It is safe to assume that they are voltage regulators, one of which is adjustable (probably a variant of the LM137) that is controlled by the knob for amplitude.

The reason for you seeing so much wires everywhere is because the boards are all two layer, which means that there are only two layers (top and bottom) that copper tracks can go on. Thanks to advances in PCB manufacturing technology, we can go up to 26 layers, which results in less wires hanging around like that.

The yellow wire going from the big metal thing on the bottom right (Mains plug) is the grounding wire. If you ever poke around with equipment and leave that wire disconnected, it is VERY dangerous. It is there for your protection, and believe me, if they didn't have a good reason for that wire then they wouldn't put it in there, as it does increase the price of assembly significantly.