Electro-Voice X825
A crossover network sold on a promise: add it to a two-way and get a three-way, for the price of a step.
Overview
The X825 is a crossover network, and what it was selling is the interesting part. Electro-Voice built its 1950s component business around what the company called the Building Block Plan. A listener started with a single coaxial loudspeaker and added pieces as the budget allowed, hearing the improvement at each step. The X825 was the piece you bought to move from two-way to three-way. It was the mid-range step.
The company's own advertising spelled out the recipe. In a 1956 campaign running in Audio, E-V told buyers to start with an SP12B coaxial full-range speaker, then step up with a T35B VHF driver, an AT37 level control and an X36 crossover for the top end. For added mid-range, the ad said to install a T10A HF driver with an 8HD horn, an AT37 level control and the X825 crossover. That pairing - T10A, 8HD, AT37, X825 - was the mid-range building block. The X825 split the signal so the compression driver could take over the middle of the band. High Fidelity (December 1956, p.11) ran the same "one economical step at a time" pitch, built around hearing the improvement before spending the money.
The key spec shows up in the kit literature rather than the ad copy: the X825's passband is 800 to 3,800 cps. That is a mid-range window, not a full-range crossover. It handed the T10A driver the presence region and left the extremes to whatever the rest of the system already covered. In the Building Block kits, the X825 came as part of the BB3 mid-range kit, sold with the T10A compression driver, the 8HD diffraction horn, an AT37 level control and a wiring harness.
Electro-Voice's marketing for these kits leaned hard on the diffraction horn and the level control, and the X825 rode along. The company claimed the E-V diffraction horn spread uniform energy across the listening room for better stereo coverage, and that the level control let the owner trim the presence range to taste and to the room. The X825 itself was the least glamorous part of the chain - a passive network with no controls and nothing to adjust. Its job was to be transparent and to cross where the T10A wanted to be crossed.
There is no independent measurement of the X825 on its own in the record. The period coverage is advertising - E-V's own claims, placed in the magazines, not a reviewer's bench test. It was sold as a system component, frequently bundled, so its value today is as the matching piece for a T10A/8HD/AT37 mid-range build, not as a standalone curiosity. Per the 1956 advertising, the whole mid-range kit cost $116.50 net for the driver, horn, control and crossover together. That makes the X825 one line item in a modest upgrade, not a flagship purchase.
The practical caution is the same one that applies to any passive crossover of this age: the capacitors and other parts inside are decades old, and a network that has drifted will not cross where it was designed to. The surviving documentation is the advertising and the kit literature, not a service manual, so the component values are not laid out in the material available here.
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