UREI 813B (1983)
A 195-pound studio monitor with a horn you're explicitly warned not to clean, built to be flush-mounted into the wall of a control room.
Overview
The UREI 813B is a Time Align studio monitor designed for critical listening in professional installations. The manual is blunt about what that means. It is a single coaxial loudspeaker plus one additional low-frequency driver, and UREI's own literature calls the result a "one-point sound source." The coaxial driver is the 801B, which carries a JBL 2425H in its high-frequency section. UREI claims that combination pushed the high-frequency envelope beyond 17.5 kHz. Series B, in UREI's telling, is the third generation of the company's monitors, and the manual credits advances in materials and fabrication with doubling both the sensitivity to input signals and the power handling capability over what came before.
The headline numbers are 40 Hz to 17.5 kHz at ±3 dB, an 8-ohm nominal impedance with a stated minimum above 4 ohms, and a sensitivity of 101 dB SPL per watt per meter. That sensitivity is the big one. At roughly 100 dB for a single watt, an 813B will reach alarming levels on amplifier power that would barely wake most domestic speakers. The power rating is 150 watts across 40 Hz to 20 kHz with pink noise. The manual also states it the other way: 23 volts RMS for one hour with pink noise band-limited from 40 Hz to 20 kHz.
That power figure is why UREI recommends an amplifier rated at 400 watts per channel into 4-ohm loads. The manual is candid about the reason: the extra headroom is there to avoid clipping short-duration signal peaks when reproducing contemporary music at high levels. The recommended partner is UREI's own Model 6500 Dual Power Amplifier, rated at 450 W per channel into 4 ohms. It also carries the company's Conductor Compensation circuit, which UREI says eliminates signal deterioration originating in the cable between amplifier and speaker, so the system sees the zero output impedance it was designed around.
This is a big box. The 813B weighs 89 kg (195 lb), with a shipping weight of 98 kg, and the enclosure displaces 326 liters (11.5 cubic feet). Modern Recording (February 1986, p.41) listed the cabinet at 36 x 31 x 23 inches with a flat black paint finish and a black cloth grille screen. It described a 15-inch cone bass driver, a 15-inch cone mid driver, and a 1.75-inch horn for the high end. The same listing put the price at $2,096.00, against $2,196.00 for the larger 815B, which adds a second 15-inch bass driver and 40 more pounds.
The time-domain work is what separates the 813B from a generic big box. Time Align is a trademark of E.M. Long Associates, and UREI's claim is that the design achieves accurate time-domain response, which the company frames in terms of reduced listener fatigue at high sound pressure levels. The coaxial layout is central to that: a true one-point source, with the patented high-frequency horn using a diffraction buffer for acoustic impedance matching and "shadow slots" that UREI says eliminate the midrange shadowing common to conventional coaxial horns. The magnet structures across the 800-series are ceramic, chosen so sensitivity does not degrade over time even under continuous use at high monitoring levels.
Then there is the detail that catches first-time owners. The blue foam at the mouth of the horn and the padding inside it are not packing material. The manual states this in capitals: they are installed for acoustical reasons and must not be removed. A pair that gets "cleaned up" by someone well-meaning is a pair that has been damaged.
The 813B ships with the Model 840 crossover network and control panel. The panel is mounted on the enclosure baffle with three adjustments: High Frequency Trim, High Frequency Drive, and Midrange. Separate fuses for the high and low frequency drivers sit beneath the controls. On the left side, under a removable panel, are three incandescent lamps that indicate excessive current peaks near overload. They also provide constant current power limiting to protect the system. The manual warns that running levels so high that one or more lamps stay lit continuously will compress the signal's dynamic range and will probably cost the operator some hearing.
The recommended setup is unusual by domestic standards. UREI designed the 813B for flat response with the front panel controls fully clockwise. It advises leaving them there and handling room correction with an electronic equalizer, specifically the UREI Model 539 1/3-octave Room Equalizer. Smoothest low-frequency performance comes from mounting the cabinet into a hard wall with the front panel flush with the wall surface, away from adjacent boundaries. The 813B is supplied in mirror-image pairs, and UREI recommends mounting the control boxes on the lower outside edge of each cabinet, away from the center of the room. This is particularly important with the accessory grille cover fitted, to avoid high-frequency diffraction at the mixer's position. For stereo imaging, the manual gives a spacing of approximately 0.9 to 1.1 times the distance from the mixer's position to the wall, with both cabinets angled so the listening position sits on the axis of both blue horns.
Room placement changes the low end dramatically, and UREI documents it. Free-space (4 pi steradian) response is one curve, half-space (2 pi) with the cabinet flush in a large wall is another, and quarter-space (pi) placement at the intersection of two boundaries is a third. Electronic equalization can compensate to a degree, but the manual draws a hard line: no more than 6 dB of boost below 80 Hz, and none at all below 30 Hz, to avoid exceeding the system's displacement limits. A high-pass filter is recommended to roll off below 30 Hz. UREI points to the Model 539 for that, or the Model 501 Subsonic Processor, a two-channel filter with a 30 Hz corner frequency in the FLAT position, installed ahead of the power amplifier inputs.
Wiring gets its own guidance. Heavy gauge stranded wire of at least 16 AWG (2 mm) should connect speaker and amplifier, with heavier gauge for runs beyond roughly 7.5 metres (25 feet). Owners using transformer-coupled amplifiers with selectable impedances are told to use the 4-ohm tap.
The manual is also upfront about failure modes. Amplifiers with output transistor protection circuits can be fooled by reactive loads into "chatter" near clipping. And despite the 813B's protective fuses and lamps, heavy clipping can still take out the high-frequency diaphragm of the coaxial driver. The manual notes that with such an amplifier, it is possible to blow the HF driver on clipped low-frequency signals. The network carries a warning too: driving the system at high levels with a transducer disconnected or with an open voice coil can develop voltages exceeding the capacitor ratings in the network, and the warranty is void if the network is operated without a proper load.
Maintenance is close to nonexistent by design. The manual states plainly that no preventive maintenance is required, or even possible. Lamps and fuses live in the control box behind the front panel's colored-lens cover plate. Operating the 813B without that plate installed affects the tuning of the enclosure. Lamp and fuse types are specific: type 1680 lamps and 7A 3AG fuses for the 800W low-frequency driver and the 801B low-frequency section, and an 11 4 1 lamp with a 1.5A 3AG fuse for the 801B high-frequency section. Substituting the wrong types risks damage to drivers and network that the warranty will not cover.
Both loudspeakers come out the front of the enclosure after removing the retaining bolts and disconnecting the wires. The manual supplies the color code: black and red to the 801B HF terminals, white and green to the 801B LF terminals, brown to the 800W's unmarked terminal, and yellow to the 800W's purple-dot terminal. Transducer service is explicitly restricted to qualified loudspeaker repair technicians, and field service of network components beyond lamps and fuses is not normally authorized. UREI directs owners to a dealer or the UREI Customer Service Department instead.
Anyone weighing an 813B today needs a wall, a serious amplifier, and a room that can take a 195-pound cabinet per side. In return it gives efficiency and headroom that most home systems cannot approach. Clipped signals, mismatched loads and the wrong fuse all lead to driver damage. The blue foam stays in the horn.
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