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Found in the Attic: Wilkinson GCA-1 AGC Amp

Posted by Tom Vernon on Oct 5, 2015 1:01:00 PM

Wilkinson GCA-1 AGC amplifierThe 1970s was the decade when peak limiters and AGC amps were transformed from equipment that kept a station's modulation within FCC limits into creative tools to tailor a unique on-air sound. With the right audio processor and careful tweaking, you could also beat out the competition in loudness wars – or not. CBS Labs Audimax/Volumax combinations, Gates Solid Statesman limiters and later, Optimod 8000s were common. However, the equipment market was flooded with various AGCs, compressors, squashers, clippers and limiters that promised to make your signal stand out. Some delivered on their claims, others, not so much. Lurking in the deep recesses of the attic is a Wilkinson GCA-1 AGC amplifier. Time to dust it off and take a look.

This processor design dates to 1970. Along with its companion peak limiter, the LA2-CS, it was sold by Wilkinson for a number of years, and in 1978 by TTC when they bought out the company. A total of around 300 GCA-1s were manufactured.

It was one of the first AGC amps to use a true RMS detector. Many contemporary processors used a simple diode detectors in various forms – bridge, full wave, voltage doubler, or capacitor-diode pump. All of these circuits were peak detectors, meaning they responded to the peak, and not the average, or RMS program level.

One of the biggest problems with peak detectors in AGC amps is that they are more responsive to asymmetrical waveforms, which are predominately speech, and less responsive to symmetrical signals, which are the norm in pre-recorded music. The end result is that music always sounds louder than speech. Sometimes AGC amp circuits had work-arounds for this problem, usually involving slowing down the attack time. The reasoning was that the AGC wouldn't respond to peaks of short duration. In practice, this didn't work very well.

True RMS detectors worked reasonably well at solving these problems, and were a cut above conventional A_View_Insidediode detectors. The problem was that they produced an unnatural sound in the process. These unnatural artifacts of course, sounded worse the harder the processor was pushed. With the slower attack times of the GCA-1, it was not too noticeable.

Gain control in the GCA-1 was provided by a voltage-variable attenuator with a Raytheon CK1116 Raysistor as the control element. The Raysistor was just an incandescent lamp and photocell in a TO-5 can. It had a very long life, as with all lamps however, it would eventually burn out. Replacement required a complete recalibration of the processor, as the characteristics of individual Raysistors varied considerably. The savvy engineer kept a few spares in the parts bin, as the GCA-1 was dead in the water without them.

The Raysistor was a slow-responding device by design, and it was the limiting factor in attack time. Recovery time could be adjusted from .25 to 2 seconds by changing a resistor-capacitor combination.

In the early 1970s, integrated circuit technology was in its infancy, and like most broadcast gear of the time, the GCA-1 was designed around discrete components. The circuits included a regulated 25-volt supply, a wide band, low distortion amp with about 60 dB gain, an electronically-variable attenuator, level discriminator and sense amplifier, and true RMS detector.

Specifications included distortion of 0.1% or less, response of 50 Hz to 20 Khz +/-0.25 dB, a compression ratio of 16 to 1 and a gain control range of +15 to -20 dB from normal gain setting.

The GCA-1 was designed by Bill Johnson, who is best remembered as the tech support guy from Wilkinson (and before that, ITA) who always went the extra mile for customers. Johnson also designed the Wilkinson FME-10 exciter, which in the early 70s, was the gold standard for exciters, having unparalleled noise specs.

Wilkinson Electronics was founded the late 1960s by Guffy Wilkinson, a former ITA salesman. The company bought the ITA transmitter line, and manufactured updated designs featuring solid state drivers and other refinements. They eventually sold a full line of AM and FM transmitters and audio products. Guffy passed away in 1979, and the company began to have financial difficulties. Wilkinson Electronics was sold to TTC in Arvada, Colorado in 1981, which in tBackurn was sold to Larcan in 1993. 

This GCA-1, serial number 004, was purchased in 1971 by a classical music station, where it was in the program chain for a number of years before moving to backup status. It was retired in the early 1980s. When set up on the bench and put through initial test setup procedures some 30 years later, it was still meets original specs.

The GCA-1 had a somewhat undeserved reputation for not sounding good or not working properly. Often this was because commission-hungry salesmen and dealers sold them to stations with Top-40 or AOR formats, for which they were not well suited. However, when properly installed and set up at stations with easy listening or classical formats, it often turned out to be just what the doctor ordered.

Topics: Vintage Technology