The Clandestine Origins of the UREI 1A

Special guest writer for this month’s "Analog Obsession" is classic UA and UREI repair guru extraordinaire, Mr. David Kulka! David’s Studio Electronics is UA’s first-string repair service for all things vintage. His work is truly a cut above the competition, and his knowledge and resources have helped us out of many jams. David wanted our users to know about a piece of UA’s heritage in the government-contract business that was heretofore unknown to all of us, even Bill Jr. …

A Rare Model
Servicing all manner of UA and UREI gear since the '70s, I've encountered many unusual pieces and unique repair situations. I've refurbished a pair of LA-3As that had been in a fire, performed emergency repairs on an 1178 for L.A.'s Pirate Radio while the unit was live and on air, and restored one-owner gems with great pedigrees (like Malaco's UA 175-B, which was used on "Groove Me" and "Mr. Big Stuff," and Jim Messina's Pultecs). There was the time I was tweaking an LA-2A up at Indigo Ranch, and noticed that a quiet gentleman had appeared behind me, intently watching my every move: Bob Dylan!

This summer, another kind of project came up. The famous name was UREI and the gig involved a Limiter model I had never encountered before--the very rare 1A.

In the early months of 2008, I'd seen a few of these pop up on eBay. One or two clients wrote me about them, asking whether they were genuine UREI products. From the first fuzzy pictures I saw, the 1A resembled an 1176LN, but sported a natural aluminum faceplate and a different control set. Then in June, to my pleasure, two of them (serial numbers 046 and 049) landed in our shop. Now I had a chance to pop the covers and see what these oddball units were all about.

The 1A was designed to add just the right amount of limiting to your propaganda broadcasts, with simple "set and forget" front panel controls.

Government Contract
It turns out that the 1As were a custom run of limiters that UREI built for the U.S. government, commissioned by the United States Information Agency (USIA). Founded in 1953, USIA was America's propaganda service, overseeing the worldwide, multilingual operations of Voice of America (VOA) along with Radio Marti, which blasted Spanish programming to Cuba from directional powerhouse transmitters in Florida. But as VOA scaled down services in recent years and decommissioned some of the older gear, some 1As found their way into the marketplace.

All AM and shortwave stations need high-quality limiters, and the VOA was no exception. Due to interference from other stations, "jamming" from unfriendly governments, noise from lightning and power lines, and numerous other problems, these stations have at most around 20 dB of dynamic range. At lower levels audio is buried in background noise, and if levels are too high the transmitter will over-modulate, causing distortion and possible damage. To prevent overload and maintain good intelligibility and punch through the background noise, good limiting is essential.

There was also a UREI 1B. Between one and two hundred of these were built, with some going to WIC Broadcasting in Canada. The 1B seems very close to the 1A, but the input switch is 20dB/0dB; perhaps there are other minor differences.

Topology/Controls

The 1A was designed to add just the right amount of limiting to your propaganda broadcasts, with simple "set and forget" front panel controls. It's a FET-based limiter, like the 1176LN, but there are some important differences. For starters, like the LA-5 (which was designed for sound reinforcement) the 1A's compression ratio is a brick-wall-ish 20:1. Attack time is fast (about 100 microseconds), as is release time (55 milliseconds, about the same as an 1176LN at its fastest setting). 

The balanced input signal passes through a low pass filter and EQ network that gives a 9dB boost at 4.3 kHz, but then begins to steeply roll off at 5 kHz. The boost portion roughly corresponds to AM radio's NRSC pre-emphasis curve. (In the old days of tube radios and speakers that rolled off at around 7k, the NRSC boost added needed treble and, combined with speaker roll off, formed a simple noise reduction system. We bypassed this portion of the circuit in our units; other 1A owners will want to do the same.)

With the Meter switch in VU mode, input level is displayed. Oddly, there's no way to see output level on this unit. The 10dB/0dB switch simply adds 10 dB of input gain in the "10" position; the Fine Output control adds a few dB of range to the output circuit. The 1A has one more very unconventional circuit element that owners will probably want to disable. Hardwired to the output amplifier is a circuit comprising four rectifier diodes and a 10-volt zener. This circuit forms a fail safe brick wall limiter: At about 6 dB over 0 VU, the diodes just say "no more" and clip the audio level. The distortion heard would not be pretty, but the transmitter would be protected. If you don't have a transmitter, you should probably snip out the five diodes.

Using the 1A Today
With the filter and brick-wall diodes removed, the 1A can be a pretty handy limiter. Its dynamics are much like the fabled radio compressors of yore: When properly set up, it's limiting action can be similar to a CBS Volumax, an Inovonics 201 or maybe a Gates Sta-Level. If you have a 1A and want a 12:1 or 8:1 compression ratio, this can be accomplished with some minor circuit changes.

You can see in the photo of the front that someone (not us) has modified the front panel of this particular unit. Attack and Release controls have been added to the right of Fine Out. Adding Attack and Release adjusts is a nice idea, though we'd have done it more elegantly.

Text by David Kulka, Studio Electronics Inc.
Images by Richard Horn, Negative Altitude Photography

— David Kulka