I bought Fostex t90a supertweeters for my Altec 604E 1.5 years ago.
Since most people use these tweeters with a 1st order filter, so I used with a 1st order filter. Moreover, it was technically simple. I had quality Audio Note Cu 1uf copacitors and autotransformers that make it easy to adjust the volume of super tweeters.
The effect of the supertweeters was very positive. More detailed, refined sound, more air and 3D soundstage.
But over time, I began to come to the conclusion, that the high frequencies are still a little lacking, and if I increase the volume of the super tweeters, the upper mids become too bright.
Then I remembered what my friend Sergey ( an experience DIY speaker builder) said that for supertweeters it is advisable to use filters of the 3rd or 4th order. (For example, the Harbath Hl5 has a supertweeter connected by the 3rd order.)
I thought that such a filter would allow me to raise the high friquencies without climbing to the upper mids.
I contacted Sergey for advices and began to build a filter. I used 2 pairs of capacitors that I had: 1uf Audio Note Cu and MultiCap RTX.
The coils were calculated by the online calculator Bessel filter of the 4th order. I bought cheap coils at madisoundspeakerstore.com. First, I set the filter to 12K Hertz, but then I got a small dip in the frequency response in the region of 10-12K. So I decided to rebuilt the filter to 10K hertz.
The 4th order filter greatly weakens the output of super-tweeters, so I had to abandon the autotransformer and connect directly through the Dell 0.5 Ohm resistor, as Sergey advised.
It is too early to draw final conclusions. The parts have not passed break in and I do not listen to it for so long enough. In addition, Sergey advised to shunt the 1uf capacitors with small polyester capacitors.
But the impressions are as follows:
The upper middle is less bright, but cleaner, smoother, more detailed. Vocals, saxophone sound clear, without distortion typical for a 1st order filter (when the tweeter and supertweeter are played in parallel in this frequency range). The chorus sounds smoother and cleaner without mud or squeals.
The focus of the instruments is sharper, the soundstage is more transparent.
There are many more upper high frequencies. A lot more details of cymbals, brushes and so on. Flutes, violins have more overtones. More air and better presence effect.
