Monday, September 28, 2009

Horn Loaded Tweeters: Ideal for Automotive Applications?

Given that the AN214 IC based audio amplifier lives in the 12-volt world of car audio, are those Pioneer horn-loaded tweeters that are often paired with it really good for automotive use?

By: Vanessa Uy

Maybe it was those “old” circa 1993 Lexus adverts that convinced me that there are still those who believe that it is still possible to engineer high-end sound for the acoustically harsh environment of a car’s indoor cabin. Especially that particular advert where Classical guitarist Manuel Barrueco performed and recorded a suite by the 19th Century Spanish composer Isaac AlbĂ©niz in the cabin of the “supposedly” recording-studio-quiet 1993 Lexus LS 400 driving at 55 mph. Though it probably work well, given that more recently Lexus had been working with Mark Levinson for a car audio system with a sound quality that is commensurate with the luxury of their famed “entry-level” sports cars.

From the point of view of acoustic science, horn-loaded tweeters are better suited for automotive applications than direct radiators (i.e. ordinary cone speakers) because horn-loaded tweeters have better radiation pattern and smoother off-axis response than their coned counterparts. Horn-loaded tweeters eliminate the problem of upper-frequency dips in a typical car interior. Given that there are a number of horn configurations, which type does suit well in getting as close to a high-end high fidelity sound in an acoustically non-ideal environment of your typical car interior?

Horn tweeters come in three main configurations; one is the round horn, the square horn, and the rectangular horn. Although the Pioneer sourced tweeters that are often used in AN214 IC audio amplifier based systems are typically square and rectangular horn-loaded tweeters. These Pioneer sourced tweeters vary in impedance from 8 ohms to 16 ohms and the type used during the 1980s seldom generate significant acoustic output above 8KHz. The higher impedance types - 16-ohm versions with Alnico magnets - are my preferred choice (if you can still find one since they are virtually extinct) since they provide an easier impedance load for the partnering amplifier. Allowing one to use an AN214 IC audio amplifier configured with reduced negative feedback for better musicality. Even though Pioneer had been recently producing ribbon tweeters that reached out to 120,000 Hz, they are still to gain widespread usage with an AN214 IC-based Pioneer car stereo from the 1970s.

In the audio world, there exists two camps on which type of horn – round or rectangular – provide the optimum sound quality and they’re respective point of views are well defended when it comes to which one sounds best. Audio Note manager Herb Reichert belongs to the camp professing that round horns are the ideal horn shape because – according to him – round horns overcome some of the sonic limitations of traditional horn-loaded speakers. According to Reichert, the more a horn tends toward the rectangular shape of traditional horns, the more it will produce sound with a "honky" or cupped-hands coloration. Public-address (PA system) horns are rectangular because acoustic engineers want to spray the whole auditorium with sound. High-fidelity horns should be round so that they can launch a perfect hemispherical wave front – where the acoustic energy is in phase and of equal amplitude at any point on the acoustic wave-front. Any horn that is not round will sound more colored than an equivalent round horn says Reichert.

While Jean Bernard Gabet and Jean Phillip – designer of the Jadis Eurythmie II horn-loaded loudspeaker – belong to the rectangular horn believers. After examining existing horn-loaded loudspeaker technology, both of them concluded that a properly designed horn system cannot be round but rather must be rectangular in order not to sound too colored.

Given that most tweeters used with the AN214 are either the black-colored Pioneer PT-103F horn-loaded tweeter with a square-shaped horn aperture Alnico magnet equipped with an impedance rating of 8 ohms with a 15-watt power rating. And the much sought after gold-colored Pioneer PT-6 horn-loaded tweeter with a rectangular horn aperture Alnico magnet equipped 16 ohm with a 15 watt power rating and said to be the perfect soul-mate of a tweeter for the AN214 amp. Although there is the black-colored Pioneer PT-205 horn-loaded tweeter with an16 ohm impedance rating which nobody mentions using with the AN214 probably due to its rarity.

Even though most of these tweeters begin to roll-off at 8KHz, they seem to be the primary tweeter of choice for those MJ2955 PNP transistor-based transformer-coupled booster amp for the AN214. Maybe they can tolerate the high frequency parasitic oscillations of interstage and output transformers of such amps that are plagued with stray capacitance and stray inductance due to improper transformer winding.