Audio Engineering

Components 135

jack socket. This size of jack plug is almost always found with the headphones provided
for portable radios and cassette players. Now you know the size we’re talking about.
These aren’t the huge jack plugs used with electric guitars. The jack plug has a screw-on
barrel, often plastic but sometimes metal. Once you remove the cover, and if it’s a mono
plug, you’ll see two connections. There’s a short connection to the center pin and a longer
connection that goes to the ground terminal. You can recognize a mono jack plug by the
single insulator strip near the end of the jack plug tip. The stereo jack plug has two such
insulator strips. Jack sockets come in the normally closed and normally open types. In
the normally closed type of jack socket, there are two contacts that are in mechanical and
electrical contact, that is, it’s normally closed. The action of inserting the jack plug causes
the two contacts to mechanically spring apart, so the electrical connection is broken.
When you remove the jack plug, the electrical connection is made again. The normally
open type of jack socket has two close-by terminals that are not electrically connected
to each other. When a jack plug is inserted, these two contacts are mechanically brought
together and as long as the jack plug remains inserted, the electrical connection is
maintained between the two terminals. Figure 4.12 shows the differences for one
particular type of popular socket. For the basic application, such as connecting a speaker
to an amplifi er output, it makes no difference which type is used. But the normally closed
type of socket has a special use; it is used where an amplifi er is connected normally to
an internal speaker, and when an external speaker is plugged in, the internal speaker is
disconnected by the action of this jack socket. Portable radios have the same arrangement,
whereby plugging in the external headphones disconnects the internal speaker. This

Normally closed socket

1/8′′ jack socket

Plug Out: Pins B and C are shorted

Plug In: Pin A grounded, Pin C to

signal

Normally open socket

1/8′′ jack socket

Plug Out: Pins B and C are open circuit

Plug In: Pins A and B grounded, Pin C to

signal

Signal

Ground

Jack plug

A

B

C

Side view

Jack socket

Signal

Ground

Jack plug

A

B

C

Side view

Jack socket

Figure 4.12 : Jack socket conventions.