The man gets shocked because his hand is touching the positive terminal, and his legs are touching the ground, which is connected to the negative terminal. Since the positive terminal is at a high potential relative to the ground, he gets a shock. If the load is removed, he still gets a shock.
The bird doesn't because it's only touching the positive terminal, so there is no potential difference across it's body.
Remember voltage is always between two points. You can't just say "this point is at 5V", you have to say "this point is at 5V relative to this point" (although people often say the former because a common reference point, like circuit ground is assumed)
Regarding the energy being consumed part, as long as the voltage source is strong enough (i.e. has a low internal resistance) an assuming it has "infinite" capacity to keep supplying power (e.g. a mains source as opposed to a battery or capacitor) then it will supply current to many loads connected in parallel to it, without it's voltage dropping appreciably.
If it is a weak source, then it's voltage will drop if a low resistance load is connected across it - in this scenario it would be safe(r) for the man to touch it (depending on how much the voltage drops - this sort of stuff is all covered by Ohm's law, Norton and Thevenin)
Regarding edit
In an ideal situation where we assume the wires have zero resistance, then the resistance of the bird does not matter. You are right that the resistance of the bird could matter in the real world though.
We can take a look at this with a SPICE simulation. We will use your circuit above, and assume the resistance of the wire between the birds legs is 1mΩ. By varying the resistance of the bird we can see what would be needed to cause an appreciable current to flow through it:
Circuit:
Simulation, seeping the birds resistance from 1Ω to 1kΩ:
You can see the current through the load R1 (green trace in the bottom graph) is around 2A, and hardly changes at all.
For the bird, even with a resistance of only 1Ω, it only has 2mA flowing through it. At 1kΩ, it has 2μA flowing through it (note the logarithmic graph used on the X axis due to the large variation)
In reality, unless the bird is soaking wet, and the cable it's standing on is very thin, then the above values are a long way from typical. Typical bird resistance will probably be > 1MΩ, and the resistance between it's legs probably less than a few μΩ.
Hopefully this makes some sense - it all agrees with Ohm's law, and once you get the hang of it it will seem quite natural. It might be good idea to grab something like LTSpice (the simulator used in the above examples and have a play around with it to see what happens when you change various things. Or build a circuit on a breadboard and measure with a multimeter. Also, have a good book that covers this stuff handy (see "Art of Electronics", "Practical Electronics for Inventors", etc)
It was the cable after all.
It has a loose connection. Unless manipulated, it connects and sound gets through. That is why I didn't notice initally. Even then, I ruled it out as the cause for the hum as it worked most of the time (of course I got a replacement anyway).
I measured the resistance of the cable:
- left channel: 0.9 Ohm
- right channel: 0.9 Ohm
- ground: 3-200+ Ohm fluctuating, occasionally disconnects completely.
I went a bit CSI on that cable. The shield is twisted together and soldered to the jack by a few strands. It broke of there. The assembly is molded in rubber, the strands stayed in place and still made a lousy connection.
Sorry for the red-herring hunt.
Now that this is settled, what's with those 88 white and black buttons? They aren't labeled. Which one does "Flight of the Bumblebee"?
Best Answer
This is an important detail. The transistor tab is usually connected to the Collector pin, and your standard insulator will add about 15pF capacitance between the tab and the heat sink, so this is about knowing whether this capacitance can cause problems or not.
If the heat sink is grounded, then the collector will have 15pF to ground capacitance in series with the connection inductance. Then the heat sink will also capacitively couple to whatever traces or signals are around.
If your amp uses an emitter-follower output stage, then both power transistors collectors are connected to power supplies (or ground if it is a single supply amp). Since power supplies are decoupled to ground, there is already a low impedance HF path between collectors and ground, therefore grounding the heat sink should make no difference. However, if the heat sink is external, or part of the enclosure, and the enclosure is already grounded at another point, grounding the heat sink on the amp pcb will introduce a ground loop.
If your amp uses a common emitter output stage, then the collectors are connected to the output, and the capacitance will be between output and heat sink. Some amps omit the insulator for better thermal resistance and have the heat sink connected to the output voltage, but this requires it to be inside an enclosure and not accessible. Anyway, if you ground it, then you have to consider if the extra 15pF to ground could make your amp unstable. For an audio amp, this is unlikely.
If the output voltage appears fuzzy on the scope, your amp could have stability issues. You can try single shot mode and see if you observe oscillations or just noise. If the amp does have stability issues, they are probably not related to case-to-heatsink capacitance, the problem would come from somewhere else, like layout or decoupling.