Super 8 FS/AL

May 24, 2017

 

Well, they were FS, foam surround but now they're Super 8 CSP/AL, cloth surround, purple.  Now they have purple power.

I had 7 but now there are only 6, as one of the units on this page found its way to an appreciative home to match one that was sent to me for repair.  Several months ago I repaired a pair of Super 3 tweeters for this same person.  After running tests on them, I discovered they were close to identical also.

Today, a close to matched pair is easy due to more modern manufacturing methodologies but randomly finding two 50+ year old speakers with such sonic similarities seems rare.  

Of course, it may be just a co-incidence.

 

 

 

 

 

   

This photo just shows the date on the cone of the one recently repaired, March 1958.

Damm, I was 16 then and remember when I saw and heard a half-brother's Wharfedale 3-way system he built.  It was such an improvement over the portable record players to which I was used to listening that I became addicted to loudspeakers.  It can be seen and is described HERE and HERE.

I still have it and still use it with the original Rek-O-Kut N-33H Rondine turntable and the EICO HF-60 preamp and HF-30 mono power amp. Just can't shake away that nostalgia.

 

 

 

 

 

 

A photo taken while the glue set.  Note the phenolic spider.

 

 

A close-up of the aluminum voice coil.  The wire is covered with what I believe to be silk.

 

 

 

 

The green curve (with the higher peak) is that of the twin I had and the black curve is the one recently repaired, JAS1.  The difference is possibly due to a stiffer spider.  Considering that these Super 8's are intended for use above 800 hz, maybe even 400 hz, the 20 hz difference in Fs is of no consequence.

Another possible cause of the difference in Fs could be the magnet strength as they will lose strength over time, especially if played a lot and at high levels.  Temperature can also have a negative effect on magnet strength.  While this may be true for Alnico, I'm not certain if ceramic magnets are affected the same way.

All it takes to erase a magnet is a strong enough AC electric field.  While the voice coil has such a field when played, that field is very weak compared to the magnet but then there's time and 60 years is a lot of it.

To give an idea of how small the voice coil field is compared to that of the magnet, consider pro musicians who feed hundreds of watts into speakers and I know of none that suffered from weak magnets.  However, when such pro speakers are sent back to the manufacturer for cone replacement, the magnets are usually re-charged.

 

 

 

The blue is the JAS1 recently repaired.  Both responses are measured at 1w1m gated signal and in a sealed box of 1 ft^3 which has no effect above 300 hz.  It was used in lieu of an IEC baffle which would have to measure  about 4.5 ft by 5.5 ft for an 8" speaker.  The idea of cutting a 6" hole in a sheet of plywood to check a couple of speakers didn't seem appealing. Both devices will eliminate doublet action below about 1700 hz, which causes cancellation when the back wave meets the front wave radiated by the diaphragm; they are out of phase by 180 degrees.

The cause of the positive and negative peaks at 4500 hz and 5000 hz are as yet unknown.  Despite that aberration, they sound extremely similar when fed with pink and white noise.

The peak at 2700 hz and the dip at 3500 hz are possibly due to cone cavity resonance and/or transverse waves along the cone.  these effects can be eliminated by more exotic cone materials and/or sandwich cones.  Another method is coating the back of the cone with a rubber like material, which I did on a pair of 5 inch units and with remarkable success.

 

 

 

 

 

The graphs below are spectrum analyses of each speaker, arranged in pairs for easy comparison. The upper being that of my speaker, the EB1 and the lower being that of the recently repaired unit, JAS1.

In all graphs, the longest trace on the left is the fundamental with the next major traces being the harmonics.  The stuff at the very bottom is most likely noise from the instrumentation and is of no consequence as it's about 60 to 70 dB below the fundamental.

The reason for placing these graphs here is to show visually their very similar sound to the ear.  This is probably no big deal as I'm sure that all so called high end speakers will exhibit similar or even better properties but these little fellas are 60 years old which, to me, is quite a testimonial.

Using a Tektronix AA501A distortion analyzer and a Behringer ECM-8000 microphone driven by an ART Studio MP tube amplifier, the following harmonic distortion figures were obtained.  Actually, it's THD plus noise THD+N.

Freq (khz)          THD+N in %

                           EB1        JAS1

                                                                                            1                0.59         0.69

                                                                                            2                1.07         1.08

                                                                                            3                2.52         1.86

                                                                                            4                1.13         1.72

The speaker was driven with 1/2 watt producing about 92 dB in the room at 1 khz.  The SPL at 5" from the dust cap was measured at 107 dB.  While I don't know the maximum SPL tolerated by the mic and the spec sheet makes no mention of that, a test with a scope showed no clipping of the waveform when the mic was driven to about 117 dB.  This was done in close proximity to a 5 inch speaker at 1 khz, the SPL also being  measured with an SPL meter with a max limit of 126 dB and an accuracy of better than 2%.

 

 

 

 

Fundamental: 1k

EB1

JAS1

 

 

Fundamental: 2k

EB1

JAS1

 

 

Fundamental: 3k

EB1

JAS1

 

 

Fundamental: 4k

EB1

JAS1

 

 

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