A guide to using turret tags for Loudspeaker crossovers.
When laying out a crossover it's
always nice to achieve that professional look to the end result. Of
course point to point or hard wiring as it's called is a technique often
used by diyers, but the layout often leaves something to be desired
and fixing of the components to the inner speaker cabinet can also be a bit of a problem.
Tufnol board / sheet is an insulating material made from paper and glue. It is produced in a number of grades and thicknesses, some of which are extensively used in the electrical industry. It can be drilled, filled and cut and has a good stiffness for component mounting.
Turret tags, Fig (1), are small solderable components which, when board mounted, allow the use of interconnecting wires and fitting of components between them.
To achieve a satisfactory layout it is useful to plan your component layout on a sheet of paper the same size as your board. Re-arrange the components to achieve a satisfactory mechanical and electrical layout. Remember wires cannot cross unless you intend to use the board underside as well. So.....
(1) Draw your board outline on a sheet of paper and cut to size.
(2) Layout your components on the paper and arrange / re-arrange them to a satisfactory position.
(3) Mark the positions on the paper, a small cross or circle, where you want to insert the pins. Note: for components with thick wires you should allow enough space to adequately bend / form the connection to the pin, (10 to 15)mm.
(4) Stick the paper onto the board to exactly cover the Tufnol board. Sellotape is good because it's transparent. It's also a good idea at this point to mark four corner holes for later mounting your board. M3 or M4 holes should be sufficient.
(5) Tufnol is relatively hard and small drills skid / slip over it's free surface so with a small hammer and centre punch mark the drilling positions.
(6) Leave the paper in place and drill the holes. Use a piece of scrap MDF or flat wood as a backing piece under the Board. The underside hole edges will otherwise break-away when the drill goes through. Known as break-out, this is not good, as we need a 'good, clean' through hole for proper turret fitting.
Note: (The use of the correct drill size is important for accurate tag fitting. 2.05mm is the specified size and can be obtained from QTA. 2.1mm is usable but less satisfactory).
(7) Remove the paper and re-use on the second board, assuming you are making two. If unusable draw a second outline and repeat the above procedures.
(8) Mark with a piece of tape which board side is to be the component side as it's easily confused.
(9) More easily done with fingers than a drill stand, rotate a small drill (4 to 6)mm between fingers and make a small countersink on each component side hole. Deburr, but not countersink the underside holes, by the same process.
(10) The tags are supplied with an insertion aid / pin, Fig (2).
On a scrap of MDF (150 to 200)mm square, drill a blind hole, (6.4mm or 1/4
"), i.e. one that does not go right through and press the pin into the hole with the
recess uppermost. Alternatively, clamp the pin in a vice with the pin recess uppermost.
If you do this you may need to prevent it from twisting.
(11) Insert the turret tag into the pin recess, Fig (3), with the turrets facing down.
(12) Keeping the Tufnol board level and with the component side
facing downwards press the board onto the Turret base. The board hole
will be a tight fit and you may need to persevere a little if it's hard
(13) Using a small hammer and centre punch, peen the turret base over, Fig (4).
Keep the assembly level and strike the punch once. Do not strike the
punch too hard as the turret may split. A little practice may be needed
to acquire the correct force.
(14) Repeat the process for all tags, Fig (5).
(15) If you misplace or damage a tag they can be drilled from the underside, (non-component side), and extracted with pliers. However, that particular hole can no longer be used.