Quick Answer / Summary: Double jacks are either two isolated sockets in one housing or two sockets commoned to a single circuit. Check continuity before wiring. Spacing is normally 19 mm centre to centre, matching dual banana plugs. They suit speaker pairs, supply outputs and any panel where a matched pair must always be adjacent.
A double 4 mm jack puts two sockets in one body, usually at the 19 mm spacing that matches binding posts. It saves panel space and guarantees spacing, and it introduces a shared-mounting decision that single sockets avoid.
Isolated or Commoned
The distinction is invisible from outside and decisive electrically. An isolated double jack provides two independent circuits, which is what a stereo pair or a plus/minus supply output needs. A commoned double jack ties both sockets to one conductor, which suits distribution but shorts a pair if used as a stereo output.
Testing Before Wiring
Check continuity between the two sockets. Near-zero means commoned; open circuit means isolated. Doing this before connecting anything prevents the short that destroys an amplifier channel.
Colour Coding
Dual jacks are often supplied in red and black pairs. The colour marks the intended polarity, not the circuit independence, so it cannot tell you whether the sockets are commoned.
Spacing and Panel Layout
Nineteen millimetres is the spacing that dual banana plugs are moulded to, and a double jack built to the same figure guarantees a plug pair will seat without strain. Where a double jack has a different spacing, a moulded dual plug will not fit and single plugs must be used.
Panel Space Savings
One double jack replaces two singles and one drilling operation, and it removes the chance of drilling the pair at the wrong spacing. That is its main practical advantage in home-built panels.
Adjacent Circuit Clearance
Where several double jacks sit in a row, check clearance between bodies as well as between bores. Bodies that touch can create an unintended path if either has damaged insulation.
Replacing Two Singles Safely
When swapping two single sockets for one double, transfer the wiring one conductor at a time. Doing both at once is how a plus and minus get exchanged, and the error is invisible once the panel is closed.
Double Jack Check
| Property | Isolated | Commoned |
|---|---|---|
| Continuity between sockets | Open | Near zero |
| Suitable for stereo pair | Yes | No, shorts the pair |
| Suitable for distribution | Yes | Yes |
| Spacing | 19 mm typical | 19 mm typical |
| Colour meaning | Polarity | Polarity only |
Practical Applications
Building a speaker output panel, mounting a plus/minus supply pair, replacing two single sockets with one part, and checking a salvaged double jack before use.
Frequently Asked Questions
What is a double 4 mm jack?
Two 4 mm sockets in one housing, usually at 19 mm spacing. The sockets may be isolated from each other or commoned to one circuit.
How do I know if a double jack is isolated?
Check continuity between the two sockets. Open circuit means isolated; near-zero means they share a conductor.
Why does 19 mm spacing matter?
It matches the spacing of moulded dual banana plugs and of most binding post pairs, so a plug pair seats without strain on the sockets.