UREI Model 6500 (1981)
Two fully independent mono blocks in one 7-inch rack chassis, each with its own transformer and fan, and a feedback trick meant to make the wire disappear.
Overview
The Model 6500 is the biggest amp in UREI's early-Eighties power amplifier line. The load rating tells the story: 275 Watts RMS per channel into 8 ohms, and 600 Watts per channel into 2 ohms. UREI's own 1981 literature notes that the 8-ohm figure is measured at 0.05% THD across a bandwidth greater than 20 Hz to 20 kHz. That is a full-bandwidth distortion number, not the single-frequency rating that flatters lesser amps. For a studio running big soffit-mounted monitors, the 2-ohm capability is the whole point. The amp can take parallel or low-impedance loads without folding up.
The 6500 differs from its stablemates - the 6350 at 255 watts into 8 ohms, the 6250 at 150, and the compact 6150 at 75 - in its architecture. UREI built it as two totally independent plug-in channels, each removable from the front panel, each with its own power supply and its own continuously variable cooling fan. It is a service-friendly design. A failed channel slides out and gets swapped, and the whole amplifier doesn't have to come out of the rack and onto a bench. The two halves also share nothing electrically, which is handy in a control room where one channel might be asked to do very different work from the other.
The headline feature is Conductor Compensation. UREI's advertising billed it as a circuit that corrects for wire losses and transducer-related load anomalies, delivering accurate waveforms all the way to the speaker's terminals. In plain terms, the amplifier is designed to account for what the cable and the load do to the signal, rather than pretending the run from rack to monitor is a straight wire. It reads like marketing until you notice the company built it into its Series "A" monitor line as well. The 811A, 813A, and 815A were all fitted with a BNC connector specifically to accommodate the Conductor Compensation feature of the 6500.
The package is standard 19-inch rack mount at 7 inches high. That makes it the tallest of the four amplifiers in the line, which is the price of two independent supplies and two fans. Contemporary show coverage placed the 6500 alongside UREI's time-align studio monitors at its introduction. Recording-Engineer (April 1981, p.27) carried the launch material describing the amplifier as extending UREI quality from its low-level signal processing through to the Time-Aligned monitors, and DB-Magazine (May 1981, p.35) ran the same announcement. The amplifier's own documentation is dated 1981, and the Series "A" monitor literature from the same year confirms the two product lines were designed to work together.
The record has gaps. There is no independent bench test of the 6500 in the available period material. The magazine appearances are UREI's launch announcement reproduced in the trade press, not a third-party measurement session. The 275-watt and 600-watt figures are the manufacturer's own ratings, and collectors should treat them as such. The trade coverage does establish that UREI positioned the 6500 as the flagship of a four-model amplifier family, with the smaller 6350, 6250, and 6150 filling out the range for less demanding installations.
Owning one today has trade-offs. The front-panel plug-in channels and independent supplies make diagnosis straightforward, and a single failure does not sideline the whole unit. The continuously variable fans beat the on/off blowers that make some period amplifiers unusable in a quiet room. On the other side, a 7-inch, two-supply, 600-watt-into-2-ohms amplifier is heavy, hot and current-hungry. The Conductor Compensation scheme is only fully realized when the amp is paired with the matching UREI monitors and their BNC inputs. Run it into ordinary speakers and you are using a fraction of what it was built to do.
That is the essential character of the Model 6500: not a general-purpose hi-fi amp dressed up in rack ears, but the power stage of a complete UREI monitoring system, engineered around the assumption that it would spend its life driving Time-Aligned monitors through Conductor Compensation. Judged on those terms, the 275-watt full-bandwidth rating and the 2-ohm stability are not bragging rights — they are the minimum required to make the rest of the system behave.
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