Quick answer: Banana jacks are 4 mm spring plugs that fit binding posts and banana sockets. Standard dual banana spacing is 3/4 inch (19 mm). Check the post diameter, recess depth and spacing before buying, and choose a screw or solder termination that suits your cable gauge.
Banana Jack Speaker Cable: Sizes, Spacing and Wiring
A banana jack is the spring-loaded cylinder that pushes into a binding post and stays there by friction. It is the fastest way to connect a speaker cable, and the easiest connection to get wrong. The plug is usually 4 mm, but the jack on the speaker may be 4 mm, 5 mm or a recessed binding post that needs a slim body. Dual plugs add a fixed spacing that must match the terminal pair. Measure before buying, because a plug that fits one amplifier will not necessarily fit another.
Jack, Plug and Binding Post
The terms are used loosely. The plug on the cable is a banana plug; the socket on the equipment is a banana jack. A binding post is the threaded terminal that accepts a banana plug in its end, a spade under its nut or bare wire through a hole. Most home amplifiers use a five-way binding post, which accepts all three. The plug’s spring leaves compress inside the 4 mm socket, so contact depends on the spring tension and on the socket diameter being close to 4 mm.
4 mm and 5 mm Sockets
Most consumer and prosumer equipment uses a 4 mm socket. Some older or industrial gear uses 5 mm, which needs a different plug or an adapter. A 4 mm plug in a 5 mm socket makes poor contact and falls out; a 5 mm plug will not enter a 4 mm socket.
Recessed Binding Posts
A recessed post surrounds the socket with plastic, which limits the plug body diameter. A slim metal plug or a right-angle plug fits; a bulky plastic body may not. Measure the recess depth as well as the diameter.
Single and Dual Banana Plugs
A dual banana plug holds two plugs at a fixed spacing, usually 3/4 inch or 19 mm, which matches the terminal pair on many amplifiers and speakers. It installs in one movement and keeps polarity consistent. The disadvantage is that it only fits equipment with exactly that spacing. Some amplifiers use 1 inch spacing, and some use separate posts that are further apart. Measure the centre-to-centre distance before ordering a dual plug.
Standard 3/4 Inch Spacing
The 19 mm spacing is common on lab equipment and older hi-fi. It is not universal. A dual plug that does not align will not seat, and forcing it can bend the binding posts or crack the plastic panel.
Stacking and Side Entry
Stacking banana plugs allow a second cable to plug into the back of the first, which suits bi-wiring. Side-entry plugs turn the cable 90 degrees, which helps behind a shallow amplifier. Both add length and leverage, so support the cable.
Termination Methods
Banana plugs attach by screw clamp, solder or crimp. A screw clamp is reusable and needs no tools beyond a small screwdriver, but the grub screw can loosen over time. Solder gives the lowest resistance and a permanent joint, but it needs a hot iron and a steady hand. Crimp is fast and reliable in production, but it requires the correct crimp tool and a plug designed for crimping. Whichever method is used, the joint must be mechanically sound before it is electrical.
Screw Clamp
Strip 10 mm, twist the strands, insert them into the barrel and tighten the grub screw firmly. Check for stray strands outside the barrel. Re-tighten after a month, because copper settles and the screw can lose tension.
Solder and Crimp
Solder needs a clean, tinned conductor and a plug with a solder cup. Crimp needs the matching die and a plug with a defined crimp barrel. Do not crimp with pliers; the joint will be unreliable and may pull out under load.
Polarity, Fit and Testing
Mark the positive conductor before fitting the plugs. Red is conventional for positive, black for negative, but the marking on the cable is what matters. After assembly, check continuity from the plug tip to the far end and confirm there is no short between the two plugs. Insert each plug into its socket and feel for a firm, springy fit. A plug that spins freely or falls out under its own weight is too small or worn.
Polarity and Channel Balance
A reversed polarity at one speaker collapses the bass and moves the centre image. Check both cables against a test tone before final dressing. A dual plug helps because it can only be inserted one way if the spacing is correct.
Fit Check
A good banana plug needs a light push and stays put. If it slides in with no resistance, the spring leaves are compressed or the socket is oversized. Replace the plug rather than bending the leaves outward, which can damage the socket.
Banana plug types and fit
| Type | Diameter | Spacing | Best use | Caution |
|---|---|---|---|---|
| Single banana | 4 mm | N/A | Separate binding posts | Check recess depth |
| Dual banana | 4 mm | 19 mm | Lab and hi-fi pairs | Fixed spacing only |
| Stacking banana | 4 mm | N/A | Bi-wiring | Adds leverage |
| Right angle | 4 mm | N/A | Shallow cabinet | Support cable |
| 5 mm banana | 5 mm | N/A | Older industrial gear | Will not fit 4 mm |
Real-World Application Scenarios
A desk system uses dual banana plugs at 19 mm spacing to connect a small amplifier to bookshelf speakers, so both channels go in with one push. A vintage receiver with recessed posts needs slim single plugs because the plastic body of a dual plug will not fit. A test bench uses stacking plugs to feed two instruments from one amplifier output without adding a second pair of terminals.
Related Guides on This Site
For connector types and terminal fit, read the speaker cable types guide and the speaker wire gauge guide. Polarity practice is covered in speaker cable polarity, and pre-made options in speaker cable bulk vs premade.
Frequently Asked Questions (FAQ)
Q: Are banana plugs all the same size?
A: Most are 4 mm, but 5 mm versions exist and recessed sockets vary in depth. Dual plugs also have a fixed spacing, usually 19 mm. Measure the socket and the terminal spacing before buying.
Q: What is the standard banana plug spacing?
A: 3/4 inch or 19 mm centre to centre. It is common on lab equipment and older hi-fi, but not universal. Some amplifiers use 1 inch spacing or separate posts that are further apart.
Q: Can I use banana plugs with spring clips?
A: No. Spring clips accept bare wire or a pin, not a 4 mm plug. Use a pin terminal or bare wire for spring clips and reserve banana plugs for binding posts and banana sockets.
Q: Do banana plugs affect sound quality?
A: A clean, tight plug has negligible resistance and no audible effect. A loose or corroded plug adds resistance and can cause dropouts. The fit matters far more than the plating.
Q: How do I stop banana plugs coming loose?
A: Use the correct diameter, check the spring tension, and support the cable so it does not hang on the plug. Dual plugs stay aligned better than separate singles. Replace plugs that no longer grip.