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Fostex T 500A MK 2 Horn Super Tweeter
Article By Bill Gaw at www.enjoythemusic.com

As most of you know by now, I'm a horny type of guy. While my wife may dispute this, my audio system (actually my second partner) would have to agree as all seven speakers are of that persuasion. While the subs, woofers and mid range horns and drivers have been acceptable and stable, using Electrovoice 12 and 15 L drivers for the woofer horns, and TAD 2002 and 4002 drivers for the mid Edgar Tractrix horns, the tweeters have been a problem, for two reasons.

First, to get the best possible sound from these expensive drivers one needs to match the woofers to the mids and the mids to the tweeters in as many specifications as possible to minimize parts needed to run them. Especially with horns and single-ended or low wattage amps, passive crossovers and parts significantly detract from their sound, especially higher order crossovers. Thus, for the main speakers I use VacuumState RTP-5 preamps that were built with 24dB active crossovers as part of the preamp's regular circuitry and Behringer 24dB active crossovers as the preamps for the other speakers. Thus theoretically, no parts are in the circuit between the preamps and amp other than what would have to be there anyway. Only the tweeters have 1 capacitor on them for a 6dB crossover to the mid-horn.

Second, most horn tweeters tend to have frequency ranges that tail off just above 20kHz, such as the Fostex T-500 which this unit replaces, or they extend into the upper 30's but with a somewhat ragged response curve that adds some tizziness in the hearable high end such as the T 900. The lower end of their frequency range may go as low as 3kHz to match up well with the mid range drivers, but the drop-off has to be steep as they begin to have frequency anomalies not too far below that.

If one is using a single amplifier for both the mid and tweeter, one has to match the various driver's efficiencies as close as possible so one doesn't need a volume control in between to muck up their sound. Also, their impedances should be matched. Preferably both should be 16 Ohms, or if one is 16 and the other 8 Ohms impedance then have different efficiencies to match them up. Over the past several years I've gone through several different horn tweeters, and had settled on Fostex T-900A's as the best at about $380 each, but they didn't match up perfectly with the mid-horns, having a lower efficiency requiring the mid horn to be padded down. Also, while they were flat out to 38kHz they had a peak at 5kHz to 8kHz, which is right at the crossover point with the TAD drivers that had to be compensated for.

About three months ago, Fostex came out with the T 500A Mk II driver that seemed to fit perfectly in specs with the TAD driver. While they are 8-Ohm impedance compared to the TAD's 16 ohms, they are slightly less efficient, thus should be a close to perfect match. Its frequency response is flatter than the T-900A, reaching out to 30kHz with useable response out to 50kHz. I know what you're going to say; "Only your dog can hear out to that range." That may be true, especially with my 60 year old ears, but with SACD and DVD-A and even Dolby and DTS digital with 88 to 96 kHz sampling frequency, that information is there and can intermodulate with lower frequencies allowing us to hear its effects on the lower overtones. I think it's that intermodulation that gives, for instance, the shimmer and sheen of violins that is missing in CD.

Anyway, on an online discussion board I came across John Kalinowski, another horny guy who was purchasing several of these tweeters for himself. He graciously offered to get me two at a group rate and also suggested using Mundorf silver-gold caps and the Fostex R100TX transformers for any necessary padding. So the extra pair of T-900's was removed to my rear two speakers with the T-500's placed on the main left and right ones. Using the WinAudio MLS program from Dr. Jordan design ( more to follow possibly next month) the tweeters were found to match almost perfectly with the TAD drivers using just a 2.2 mic. Mundorf cap without the transformer. Thus the transformers were transferred to the side and center speakers and the rheostats previously used on the main speakers were transferred to the rear horns to pad down the tweeters, and with the MLS program, all speakers were matched as well as possible.

As this is not a full review of the product I won't use any fancy high-end gobblydegook to describe my findings. First, listening only to the two main speakers, the difference was subtle but important. Gone was peakiness in the mid-high frequency range. With DVD-Audio and SACD recordings, there was a beautiful sheen to the violins not heard previously on my system and only on those systems heard in the past that used the newer super-tweeters with response extending out beyond 50kHz. You have to hear one of these speakers to understand what I'm talking about. I'm sure my ears cannot pick out sound in that range, but there is a distinct difference.

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