Here's how to interface a piezo earpiece with a crystal radio.
Parallel-tuned, series-fed radio:
The earpiece is interfaced through a 33 kΩ shunt resistor. The resistor provides a path for the
detector current. The voltage drop across the resistor drives the earpiece.
Series-tuned, shunt-fed radio:
The earpiece is interfaced through a 2.5 mH RF Choke to prevent its capacitance from short-circuiting the detector. The voltage drop across the detector drives the earpiece.
The main purpose of mains-supply-earthing
(third pin) in the shack is to blow the fuse in the event of a live-chassis
Protection against static build-up / lightning strike is another matter
As a SWL in the mid-1970s, the antenna I used
with a Philips transistor portable receiver was a long wire at a height of
about 50 feet. One summer afternoon, the receiver front end (AF117) blew right
after a loud crackle of static.
Then again, my first tube homebrew CW rig had a 3-pin mains supply plug. The
antenna was a straight dipole 50 feet high. Operating on a summer afternoon, I
received a jolt through my Junker CW Key and survived to hear the crash of
thunder from a nearby lightning strike. I immediately yanked the twin-line
feeder and threw it on the floor (upper floor of my 2 storey house). After a
few moments I was surprised to see the arc from the banana plugs to the cement
Those days, in our sparsely populated area, my antenna was way above other
surrounding structures. During thunderstorms the static build-up on my 2m ¼ λ
ground plane antenna would cause a whine in my 2m receiver as it dissipated
through the front end coil to ground.
Conditions are totally different now, with my
2 storey house surrounded by high rises, cell phone towers and other structures
with lightning arrestors. Problems of static build-up and lightning are a
distant memory. I have not heard that static whine for years, even though I
still use that ¼ λ ground plane antenna.
And my shack earth is just mains-supply-earth (third pin)!
Of course, if you live in the countryside, the lightning arrestor at the feeder
entry-point, with a separate earth bonded to the mains-supply-earth, is a must.
While on the subject of bonding – a friend’s shack and attached bath had a
separate safety earth, not bonded to the mains-supply-earth. His 2m antenna was
mounted on a metal mast which was also separately earthed.
For reasons not
known, the shack earth was open when the water heater developed a ground fault. The path of the fault current was through the
floating earth wire, 2m rig, coax feeder braid, antenna mast and finally to
ground, resulting in a coax cable fire. A parallel path also caused considerable
damage to a Drake AC4 power supply. Fortunately he had the presence of mind to pull
the main breaker before he doused the fire.