Wharfedale Super 12 RS/DD

 

 

 

The repair and data acquisition of a Super 12 RS/DD purchased in August, 2017 "AS IS" with, possibly unbeknownst to the seller as having an open voice coil.  It was open at the junction on the cone of the coil wire end and the braided wire leading to a terminal.  The repair was tedious as cleaning and extending the aluminum wire had to be done carefully so as not to break it, thus making it shorter.  The lead coming from the cone apex to the junction is only an inch and covered with black adhesive which, fortunately, yielded to the application of pure acetone.

Aluminum solder and flux from a kit purchased at a heating & AC distributor worked well enough, the main drawback being that the solder was about 1/8th inch in diameter and became a thick paste with a 25 watt soldering iron.  A solder gun was used , very carefully, as the melting point of that solder is around 800 degrees; 20 gauge solder melts around 350 degrees.  Another inhibitor was the heavy gauge of the aluminum solder wire which acted like a heat sink, thus requiring more time applying the solder to the aluminum wire in close proximity to the cone.

In short, it was a P.I.T.B. (Pain In The Butt)

 

 

 

 

 

The Voice Coil

The coating is blue which surprised me as all I've seen was white, what seems to be a silk thread wound around the bare wire.  The label on the magnet is indicative of a later date of manufacture, the cone is marked 6/63.  While this is my first RS/DD, previous 12 inchers I have, namely the Co-Axial 12 and the Super 12 FS/AL have silk covered aluminum wire.

There are two layers comprising 48 turns each (if I counted correctly).  The total length of the wire in the coil is about 47 feet.  The inside diameter of the form is 1.754".  The wire gauge is probably BSW, British Standard Wire.  The coil wire measures 0.0074", bare wire diameter.  This is 36 BSW (0.0076").  The closest AWG (American Wire Gauge) is 32 (0.008").  Next is AWG33 (0.0071")

The DCR of this coil is 10.9 ohms.

 

 

 

Cone Eyelets

The wire on the right is the one that originally was broken; the left had to be clipped close to the braided wire to prevent breakage closer to the coil.  Aluminum wire this thin is extremely delicate and won't tolerate bending like copper will.

The braid was carefully removed from the eyelets with the 25 watt iron to avoid burning the cone.  Tension was applied to braid with long tweezers and when the solder softened, out they came.  The holes were then drilled clean with a 3/32" bit using a Milwaukee 2401-20 compact drill and a 5/64" drill bit.  This unit can hold a gentle and steady speed of 1 revolution every 2 seconds.  It was much easier than using the bit by hand.  The braid was then rounded with a pair of crimping pliers to slide into the holes easily.

Two 1" lengths of bare copper wire obtained from a piece of zip cord were formed with a small loop at one end into which was inserted the aluminum wire.  The copper wire was taped to the cone with the loop elevated from the cone about 1/4".  Once soldered, the junction was verified with an ohmmeter.

The aluminum wire was glued to the cone just beyond the solder junction.  After this set, the terminal strip with the braids was re-attached to the speaker and braids were inserted from the other side. The copper wires were wound around its ends and soldered.  Again. verified with an ohmmeter across each solder junction and then across the whole coil after the second wires was soldered.  The braids and remaining copper wires were then glued.

This process took the better part of an hour.

 

 

The Terminal Strip and Braids

The free ends have been rounded.

The solder joints were also checked with an ohmmeter.  By the way, the ohmmeter used was a Fluke 8050A. I have several and three have been checked with laser trimmed precision resistors and the DC voltage scale checked with MAXIM precision voltage reference chips wired in series to 40VDC with an accuracy of 0.0015V.  The AC scale is within spec, 0.03%, checked with a recently calibrated Tektronix DM501A multimeter which, believe it or not, isn't quite as accurate as the Fluke.  When the DM501A was repaired and calibrated by Norway Labs at a cost of $300, I was informed that the unit was calibrated slightly better than spec.  Using this, AC and DC current could easily be checked with the precision resistors.  Both the Flukes and Tektronix are accurate +/-0.1% on resistance.

Quite a testimonial to these meters as they date to 1980.

 

 

Waiting for the RTV to set.   I left it overnight.

 

 

 

The Assembled RS/DD

 

 

 

 

FIG. 1

Z  vs  F

Black:  without the whizzer cone/dust cap combination

Red:  with the whizzer cone and cap

The high impedance may be due to the high compliance which also affects fs, a low 23hz.  This is attributed to the spider losing its stiffness after so many years and also the possibility of the RTV used in the annulus isn't as stiff as the original synthetic rubber or latex.  The cone is marked 33 next to the date.  A new spider may remedy that as well as increase the power handling.

The added stiffness with the cap installed can be attributed to increased air pressure in the volume between the cap and spider, thus reducing cone motion and back EMF.

 

 

 

FIG. 2

 

These three curves were run at 1w, 1/2w and 1/4w, all gated and at 1 meter distance, on axis.  The reference for 1w is at 1 khz, where the impedance is 14.1 W  Therefore, at 1w, 1/2w & 1/4w, the applied voltages would be 14.1, 3.74 & 1.87, respectively.  If the impedance were assumed to be 15 W, these voltages would be 3.87, 2.74 & 1.94, respectively.  Now, with these voltages applied, the power transferred into a 14.1 W load would now be 1.06w, 0.53w & 0.27w, respectively.  Not much of a difference in this case.

In short, claiming that this speaker will go to 20k is somewhat of a stretch.  What isn't stated is that at 20 khz, it's about 15 to 20 dB down.

It can be argued that if the RS/DD was all that good above 15 khz, there'd be no need for the Super 3, although in all fairness, the Super 3 will have the advantage of lower voice coil inductance and lower cone mass.

 

 

 

 

 

Below is a conglomeration of response curves (Fig 3) run from various points near the cone and within the volume extending about 2 inches from the apex.  One was at the center of the dust cap about 1/8" away, another at the circumference of the cap, another at the edge of the whizzer and looking between the whizzer and the cone, another about 2 inches from the apex and 1/8" from the cone surface, one from 1/2" above the dust cap center and another about an inch above the cap and one about 2" from the cap center.  The black curve is taken at 1w-1m gated.

None of the near field responses exhibit the rise in the octave between 500hz and 5khz.  This is most likely due to such things as the sum of cavity resonances and cone breakup, the latter happening anywhere on the cone dependant on frequency and cone material variations.  Chances are, age made that worse.  The near field responses would only see the erratic breakup at one point, hence the radical variations in the curves.  I very rigid cone should shoe little to no such variations as the pressure that close to the apex is quite uniform.  That pressure would drop to ambient pressure at the outer edge of the cone.  The air mass in front of the cone has a high impedance relative to the room at the outer edge of the cone, where the impedance (pressure) drops quickly. 

On cone breakup, this usually happens at frequencies above a few hundred hertz as can be seen in the plots.  In short, near field seems good below about 400hz.  I would expect breakup to be coupled to the amplitude which, if high enough would cause breakup at lower frequencies.  Here's a photo of that phenomenon.

 

 

FIG. 3   The Conglomeration

 

 

 

In the next graph, FIG 4, we have the same black curve, 1w 1m gated and the average of the above curves 83 thru 89 (pink), not including the black one, 75.  This pink curve is much smoother than the black one due to its taking only a few samples close to the cone, a place not usually occupied by one listening to music.  The next piccy after this one is cool.

FIG. 4

 

 

 

In this set, we have the above curves, 75 and 90 (the thin ones) and the sum of these two,  the green one.  This curve bears a much better resemblance to the one below right in the Wharfedale spec sheet.  The slight differences lie in one main thing, the speaker in the lower chart is about 57 years younger than the one I used and as it does with people, time takes its toll.  The drop in the low end is more realistic than the near field low frequency responses as near field is a compromise to anechoic or free space.  In a room, the response below about 300hz is anything but flat; it can exhibit variations as high as +/- 10dB.  Taming these can become a nightmare.  Sound absorbing materials work at frequencies a little above 125hz; bass traps aren't any help as they trap the bass as sensed in the location of the trap, not in the listening location.  A diffuser, such as the Schroeder diffuser/quadratic residue diffuser can do a much better job but at frequencies below 400hz, they become very large, very fast and simply put, are not practical in an average house room.  They simply won't fit.

A parametric equalizer would be a more practical alternative if it is used to attenuate the peaks and NOT boost the dips and the mic is placed at the listening position.  In many cases, obtaining a relatively smooth response in the listening creates an acoustical nightmare just about everywhere else.

FIG. 5

 

 

 

 

FIG. 6

On and 30 Degrees Off Axis

Both curves measured at 1/4w 1m, the blue being 30 degrees off axis.  

The green curve here is the same as  the green one in  FIG. 2.

The effect of beaming can be seen around 1 khz where the level is 3 dB down.  Supposedly, beaming starts a frequency whose wavelength is equal to the speaker diameter.  Others say it's about an octave higher.  The latter may be referring to perceptible change in SPL.  Based on that, a 3dB drop around 1 khz can be seen.  Given that this 12 inch speaker has an overall diameter of 12.25", the 3 dB drop would be around 1111hz.  An octave higher, 2khz shows a drop of 10dB which would be quite noticeable but not necessarily liked or disliked.

Bass Box Bro states that beaming starts when the wavelength equals the speaker circumference, here being 38.5", corresponding to a frequency of 354hz.  In the graph here, a difference of 1 dB can be seen at about 375hz, corroborating the statement made by Bass Box Pro.

All the above may seem academic and for the most part, it is.  Calculations can show that it begins around 354 in this case but can it be heard?  Like many things in audio, it can become very subjective.

What was noticed is the subtle difference above 10khz which may be the effect of the whizzer cone, sometimes referred to as a high frequency dispersion unit.  

It is resemblant of a horn or wave guide.

 

 

 

The following is the result of entering the Theile-Small data obtained in LMS into Bass Box Pro, v.6.

 

The Theile-Smalls

VENTED

This speaker and probably a new one would work well in either a vented or closed box of 1 cubic foot internal.  

A vented enclosure of 2.5 cubic feet will allow a little lower bass extension with the aid of amplifier bass boost if desired and such an enclosure can be designed to be  close to golden section dimensions.  It would also be more aesthetically appealing as well as floor standing or on a stand if preferred.

A larger vented enclosure approaches the point of diminishing returns, acoustically speaking.  Aesthetically speaking may be different.

 

For a comparison on responses, see the bottom set of graphs.

 

The Theile-Smalls

CLOSED

The closed box would give a better transient response in the bass. It would be easier to construct due to the lack of a vent.  The front would have to be at minimum, 13 inches square inside to accommodate a front loaded speaker. This leaves an internal depth of 10.25".  While this would work, it doesn't seem to be aesthetically appealing.  Add to that the square, which will support a 1047hz wavelength as well as its harmonics.  The speaker already has a rise of about 5dB between 1500 and 4500hz.  Most likely, the internal padding will absorb this but why take the chance.

These designs are supposedly optimum but when I saw the Qtc for the closed box, I was curious.  So, I changed that to the accepted "ideal" of 0.7071 and the box volume was reduced to 0.557 cubic feet.  This would have little to no visual appeal unless one were seriously in need of space.  Well, I.M.H.O. as subjectivity is always lurking around.

A larger box of around 2 cubic feet would defeat the purpose of the closed box damping.  The Qtc drops to 0.415 and may produce a sloppy bass although I have not tried it.

 

 

These graphs pertain to the above alignments

 

Shows the expected amplitude response for the power input specified

 

 

Maximum loudness capable within Xmaz or the thermal limit, whichever comes first.

In all probability, Xmax will limit the output

 

 

Shows the maximum power the speaker can absorb before Xmax or the thermal limit is reached.

Xmax will probably be reached first

 

 

Shows diaphragm excursion at the specified input power.  The darker part of the line(s) indicate Xmax will be exceeded

 

 

Shows the relationship/phase in degrees between the electrical input signal and acoustical output signal, pressure wave/sound

 

 

Expresses the phase response in milliseconds.  Ideally, both phase and group delay would be straight lines.

Either of these last two, if excessive or sudden can have adverse effects on transient response and video sound tracks , i.e. lip syncing

An interesting story about phase and time delay.  It was probably in the 40's in a movie studio when playing back the sound track of a movie involving tap dancing.  A double click of the dancers shoes was noticed.  It was traced to the speaker system, most likely an Altec A7 or similar.  By moving the treble horn so its voice coil was in vertical alignment with that of the woofer, the double tab was eliminated.

 

 

The Bottom Graphs

RED is 1 cubic foot; YELLOW is 2.5 cubic feet, both vented

 

 

The larger cabinet will extend the bass a little but at the price of a steeper slope which could be corrected by a slight bass boost, carefully.

 

The larger cabinet will initially result in lower bass acoustical output which, as previously, can be compensated for by a slight bass boost.

 

However, that larger cabinet provides less damping on the speaker, thus reducing its appetite for electrical power.

 

The cone displacement will reach Xmax sooner than with the smaller cabinet.

 

 

 

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