A Pair of Wharfedale Super 3s

In early March, 2022, a member of AudioKarma posted a query regarding two visually identical Super 3 tweeters with slightly different magnetic strengths and voice coils. The field strength on the labels were stated as 14,500 lines and 13,000 lines. The voice coil in the stronger magnet was a little longer than that in the weaker magnet.

In a search for the resistivity of aluminum wire of 0.0056", this site was found. OMNICALCULATOR

It has a calculator for wire of any diameter and several materials. The resistivities are calculated at 20^o C(68^o F) The resistivity will increase as temperature rises, so in a room of 77^o F, the resistance of 36AWG copper wire will increase from 411W/1000 feet at 68^oF to 423W/1000 feet at 77^oF. Copper was used here as an example because my wire tables use copper wire at 77^oF

The bare aluminum wire of these voice coils measures 0.0056", which is 35BWG and AWG. This makes sense as in the fifties, Wharfedale would have used BWG.

The difference in resistance vs these two temperatures regarding these voice coils, if copper, would be about 0.2W. Since aluminum⁴ is about 40% higher in resistance than copper⁵, the difference in resistances would be about 0.3W

Using the number of turns on each layer and calculating the length of each layer, we get 209" for the longer coil and 176" for the shorter one. Using the above mentioned calculator, at 20^oC, the longer coil calculates at 8.596W and measures as 8.32W. A slightly higher measured reading was expected but this is close enough. The shorter coil calculates at 7.453W and measures as 7.63W.

The difference is due in small part by temperature and in larger part in determining the turns. The circumferences of each layer were determined as described in figs 5 and 6. The only way to accurately determine the length of the coil wire is to unravel it and measure it.

PHOTO 1 Click image for larger piccy

Both of these coils are from Super 3 units of which the cones were unsalvageable. The one on the right is open circuited at one of the connections at the cone. This one goes back to the early 80's when I was unaware of not being able to solder aluminum with ordinary solder and flux. It worked for a while only due to the mechanical contact of the aluminum wire to the braid. The ribbed cone is a Waldom replacement.

The winding of the coil on the right may seem to have come loose but not so. They were actually wound that way, not intentionally.

It is unknown which of these coils came from which of the two magnet assemblies. The poster's observation was that the longer one came from a 14500 lines magnet.

The wire diameter was measured as 0.005" (0.127mm), which is 36AWG¹ or 35BWG³ This equates to 0.000019635 in^2 or 25 circular mils

The purpose of checking the calibration or error factor of the meters will become evident when we attempt to find the resistance of 0.008" (0.203mm) diameter aluminum wire. This will be used to check the determination of the length of wire in the coils.

This meter is a Fluke 8050A dating to 1979. There are several here, some are hangar queens, used for parts.

The calibration of this meter along with a few others was verified several years ago with the help of an electrical engineer friend and done over the phone. Unbelievable as it may seem, they all came in within 0..3% accuracy. Original spec for resistance was 0.1%.

At the time of this work, the ohmmeter was again checked using a 10W resistor of 0.1% accuracy. The meter accuracy was 0.3%, measuring 9.97W. This, of course, includes the 0.1% tolerance of the resistor.

This spec is about the same as the much newer Agilent U-1241 shown next. Fluke does make bench meters of higher accuracy ranging from $1100 to $2000 but the cost just can't be justified for the purpose of checking loudspeakers or tape recorders.

PHOTO 2 A

PHOTO 2 B

FIGURE 1

FIGURE 3

1w1m VIO=J1; GREY=J2; BLK=K; RED=P1; GRN=P2

While it may be difficult to differentiate these curves, the point is to show the similarity of their sensitivities and responses, especially above 3khz.. The K unit has 13,000 lines and the P units have 14,500 lines. The J units have no labels but are original as are the other three with exception of the annuli and centering discs.

FIGURE 5

This represents the voice coil. The red area is the coil form and the green area is the coil winding.

The inside diameter is 1.022". The form thickness is 0.005" and the thickness including the coil winding is 0.021". This puts the two layers at 0.016"

The wire diameter including the insulation which looks like some sort of thread, possibly silk or reasonable facsimile thereof is 0.009", the wire itself being 0.008" which is 32AWG¹ and 32SWG² or 33BWG³.

FIGURE 6

This is looking at the coil as shown in photos 1 and 4

This diagram is for those who may wind their own coils Admittedly, it is of little value if one uses the same gauge wire as originally used. There are two simple ways to wind a coil. This does not exclude the somewhat complex winding of some coils and transformers. They are perfect lay on the left and square lay on the right. The perfect lay is the more widely used. The second layer will usually have one less turn than the first, the third will have one more than the second and equal to the first, the fourth will have one less than the third ands so on. For a given number of layers in square lay, the thickness will increase by the diameter of the wire for each layer but for perfect lay, it is different.

The top diagram in fig 2 is perfect lay. The wire radius is r. the overall thickness of two layers is the red line, UQST. P, Q and R are the centers of the wires. Therefore, QPR is an equilateral triangle bisected by the red line. UQ and ST are the wire radii but QS is less that the sum of the two.

By the theorem of Pythagoras, (PQ)² - (PS)² = (QS)² or (2r)² - r² = (QS)² This reduces to 3r² = (QS)² Taking sq. roots we get 1.732r=QS to which we add UQ and ST giving 3.732r for the height of a two layer coil. Since the wire diameter including the coating is 0.008", the radius is 0.004". BY multiplying 0.004 by 3.732 we get 0.014928" for the coil thickness. Add to this the 0.005" of the coil form or bobbin, we get 0.019928" which is very close to the measured thickness of the Super 3 coil. It actually measured 0.019" to 0.021" at different points around the coil but keep in mind the non enamel coating on the wire is somewhat compressible and there's the assumption that this covering, like single coating enamel, is 0.001"

PHOTOS 5L and 5R

Then foam is sandwiched between the two washers on the left. The tool is inserted into the chuck of a drill press and turns at less then 300 rpm. A blade is gently pressed against the rotating foam alongside the smaller washer, thus cutting a nice circle. The smaller washer is 1.012" in diameter, about 0.010" smaller than the inside diameter of the voice coil form. Once cut, the foam is a little larger than the washer and when sandwiched, it just touches the inside of the coil, maintaining concentricity with minimum friction. See red curve in fig 4

Photo 5R shows the other end . This nut is the one that comes off; the other is locked. A 1/4-20 hex head bolt could be used if it is threaded to the head.

PHOTO 6

The left disc was cut using the tool shown ion photos 5A & 5B

The bottom disc is one of the two phenolic spacers used above and below the foam making a sandwich. This keeps the foam a little above the pole piece to allow equal friction to the coil in both directions.

The disc ion the right was cut by hand with scissors. Its uneven roundness accounts for the jaggedness in the green and black impedance curves in fig 4 above

I can't believe I still have this since before graduating from high school in 1957.

3 BWG Birmingham Wire Gauge, an old British system that was used world wide.

It is unknown to me if this system is still used. In 1972 I contacted Wharfedale in England for replacement cones for the W15FS woofers. .Specs were also requested and the tech gave me the wire gauge in BWG 33, which is 0.008" bare. The diameter as measured on a W15FS is 0.009 with the enamel coating.

Using aluminum reduces the coil mass by 30%. While that may not seem like a great reduction in mass considering the overall light mass of the coil, there is acceleration and momentum to be considered. The more mass, the more reluctant will be it's tendency to move and the more reluctant it is to stop. This improves transient response. Many tweeters now use an edgewise wound ribbon coil and one layer. This places more wire in then field with less mass.

The g forces acting on a tweeter diaphragm as it oscillates at well over ten thousand times a second can be as high as 9g. One must look upon the tweeter with awe as aircraft have been known to come apart when subjected to such forces.

This meter is but a few years old.; It's ohmmeter specs are shown.
PHOTO 3A	PHOTO 3B	FIGURE 2

click for larger image
PHOTO 4A	PHOTO 4B	PHOTO 4C