Scientific Fidelity Tesla loudspeaker Vifa on the Tesla Tweeter
Editor: It is not Vifa's assignment to comment on tests of loudspeakers [that use Vifa drive-units], as Vifa cannot take on the responsibility of the use and the sound of its speakers in the final product. As is known, we are involved in the design of neither cabinets nor crossovers.
Meanwhile, it is Vifa's assignment to deliver faultless, high-quality products which fulfill the high acoustical demands our customers are making today.
Mike Maloney [of Scientific Fidelity] was never promised a letter from Vifa wherein Vifa acknowledged that the domes were defective. [See April 1992, Vol.15 No.4, pp.236 & 305.—Ed.] On the other hand, we promised to look into the problem. After this, we received a butterfly containing a coil and cone, which probably was from one of the "defective" boxes.
To begin with, it is impossible for us to evaluate a drive-unit which has been separated and exposed to various tests, and which is not returned in full. (We didn't receive the magnet system.)
The returned butterfly was examined, and there was no error on the dome. This result has also been given to Scientific Fidelity.
In the meantime, the fact has been established that the magnetic oil ["ferrofluid"] cannot dissolve the glue used in the domes. This is a result of a long cooperation with our supplier of magnetic oil.
It is of great importance to note the following fact: All domes which use magnetic oil will change if they are stored for a long time, because of the following phenomena: 1) the colloid stability; 2) aggregation of the particles.
1) This might cause a small part of the oil to become nonmagnetic and run out of the air gap. This will not have any influence on the acoustics, as only a very small part of the total amount of oil runs out.
2) The magnetic oil gets lumpy, which in some cases can dampen the resonance frequency up to 3dB, depending on the viscosity of the oil at the time the dome was filled. This is a reversible process; the dome can regain its normal frequency response either by playing it or by heating it up.
We have analyzed the crossover [circuit] in one of the tested Tesla samples and found that the filter is loading the D25AG-35-06 tweeter far beyond—up to four times—what we allow in our data sheet. The D25AG-35-06 is overloaded, therefore, and will distort at low listening levels. (Fig.1 shows the IEC 268-1 amplitude weighting; fig.2 the Scientific Fidelity crossover and weighted amplitude; fig.3 the Vifa standard crossover proposal and weighted amplitude.)
Fig.1 IEC 268-1 amplitude weighting.
Fig.2 Tweeter weighted amplitude vs frequency, Scientific Fidelity crossover (dB/V rel. 1mm/V +40dB).
Fig.3 Tweeter weighted amplitude vs frequency, standard Vifa crossover (dB/V rel. 1mm/V +40dB).
It would have been better had Scientific Fidelity and Vifa worked together on this problem, instead of [Scientific Fidelity] implicating Vifa in a discussion which we were not aware of and had no opportunity of influencing.—Birger Jorgensen, Vifa