Universal Audio UREI 560 (1970s)
Four narrow notch filters, a built-in mic preamp, and a single job: kill feedback before it howls.
Overview
The UREI 560 is not a tone-shaping equalizer in the usual sense. It's a feedback suppressor, built for sound reinforcement. The manual says the Model 560 "was designed specifically for use in sound reinforcement systems," and the whole design follows from that. It has four active notch filters, each adjustable in depth from 0 to 20 dB and tunable from 60 Hz to 6000 Hz. Mic preamplification is built in, so the box can sit at either mic or line level in the signal chain.
The premise is simple. In a typical single-microphone reinforcement system, gain before howlback can be significantly increased and intelligibility improved by centering a narrow band-reject filter on three or four of the most prominent system resonances. The 560 gives you exactly four of those filters in a compact chassis. The documented tuning procedure is almost mechanical. Bring the system gain up slowly until feedback appears, tune the first notch until it stops, push the gain up again, catch the next resonance with the second filter, and repeat. The manual claims a typical result of 12 dB or more in additional gain before feedback, with no undesirable changes in fidelity.
That's a claim that either works in the room or it doesn't. The documentation makes clear that UREI built this as a practical install tool, not a studio toy. The manual notes the 560 is "not a cure-all for serious sound system deficiencies" - it won't make inferior components behave like superior ones, and it won't fix bad acoustics or a badly engineered installation. It's a scalpel for high-Q resonances, not a room correction system.
The 560 turns up in period documentation. A test in Audio (September 1978) describes using the unit to notch out a 1 kHz tone on playback, with all suppressor filters set at 1 kHz. Collectors and users today still call them "feedback reducers" and list them as vintage units.
Key Features
Four Independently Adjustable Notch Filters
The core of the 560 is its four active notch filters. For each filter, a black outer knob selects frequency and a red center knob sets notch depth from 0 to 20 dB. A frequency multiplier toggle splits the tuning range into two bands - 60 Hz to 600 Hz on the low side and 600 Hz to 6000 Hz on the high side - so the same four filters can chase a low-frequency room mode or a honking midrange resonance depending on the multiplier setting. The filters are state-variable by design, and the manual puts the bandwidth at approximately 1/6 octave at 5 dB attenuation (3 dB points). That narrowness is the point. A wide filter would color the program material, while a narrow one can be dropped on a resonance and leave everything around it alone.
The manual's tuning guidance is worth repeating because it's the difference between the 560 working and the 560 frustrating its owner. Start with all depth controls fully counterclockwise and the multiplier switches at X1. Bring the gain up slowly, stabilize the first feedback tone, then turn the depth control clockwise and tune the frequency knob until the feedback disappears. Switch to X10 if the frequency is above the low band. Reduce attenuation for finer tuning once you're close. Then repeat for the second, third, and fourth resonances, retuning earlier filters as needed if previously treated frequencies start howling again. The manual suggests adjusting gain at least 3 dB below the threshold of feedback to avoid ringing — and reminds the reader that 3 dB of gain is twice the previously available power. That's the payoff, and it's a real one in a lecture hall or a church where every extra dB of clean gain matters.
Built-In Microphone Preamplification
The 560 carries its own mic preamp, which lets it sit at either microphone or line level. The mic input bridges low-impedance (150-250 ohm) microphones and is transformer isolated. The line input is a 20 k ohm bridging, floating differential input that accepts balanced or unbalanced sources. Gain runs from 20 to 50 dB on the microphone input, adjustable with a screwdriver control at the rear of the chassis, while the line input is unity. The manual describes the 560 as "basically a unity gain device." The preamp exists to bring a mic signal up to the level the filters want to see, not to add character.
Here the folklore starts. A Gearspace user claims the mic pres in the back are "exactly the same as the famed Urei 1109's." That's a single forum post, unverified by any manufacturer documentation, so treat it as an enthusiast's claim. The 1109 equivalence is unconfirmed. What the manual does confirm is that the preamp is transformer isolated and conservatively specified, which fits UREI's house approach but doesn't say which circuit it shares.
Transformer-Isolated Inputs and Outputs
The 560's connectivity is workmanlike and built for install work. Microphone input and output use XLR-3 connectors. Line-level input and output use T.R.S. phone jacks. The line output is transformer isolated and intended to be bridged by a 600 ohm or higher load. The manual notes that if the line output is feeding a high-impedance circuit, a 560 to 620 ohm resistor should be shunted across the output terminals to assure proper loading. It can be installed either on the T.R.S. jack inside the unit or at the input of the following equipment. The mic-level output sits 50 dB below line level and is also transformer isolated, which makes it easy to drop the 560 into an existing chain.
Maximum output is quoted at 7 V R.M.S. (20 V peak-to-peak) into a high-impedance load, or 6.15 V R.M.S. (+18 dBm) into 600 ohms, with distortion under 0.5% at maximum output from 30 Hz to 20 kHz. Frequency response with the filters off is ±0.5 dB from 20 Hz to 20 kHz.
Overload Indicator and Protection Notes
The front-panel pilot light doubles as an input overload indicator. It's normally on when the 560 is powered, and it flashes intermittently when an overload condition occurs.
The manual also includes a protection-circuit suggestion for the tuning procedure: a pair of germanium diodes ahead of the power amplifier to clip excessive levels while the user is deliberately provoking feedback to find resonances. UREI notes the circuit is not guaranteed to prevent damage, since power amplifiers may be set too high and signal levels at the protection circuit may be too low to make the diodes conduct before dangerous levels are reached.
Compact Chassis and Optional Rack Mounting
The 560 measures 2.5" x 8.5" x 10" and ships with a built-in power supply, so there's no wall wart and no external box to lose. UREI offered optional rack adaptors for permanent installation. The SR-60 mounts a single 560 in 3.5" of standard 19" rack space with 9.5" of depth required behind the panel, while the DR-60 mounts two units side by side in the same vertical space. Power requirements are 120/230 VAC strappable at 50-60 Hz, drawing 7 watts maximum.
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