Adcom GFP-750 preamplifier
To break through our complacency, Adcom would have to produce an outright unlistenable turkey—or a product that raised the bar so high that any audio manufacturer would get a hernia just thinking about raising it again.
Well, brace yourself, Bridget—Adcom's GFP-750 is a solid-state preamplifier/passive line controller that will demolish your expectations.
At least...do no harm (footnote 1)
The GFP-750 looks like an Adcom component. It ain't fancy—just a solid black box with four rotary knobs and a discreet brass rectangle sporting three levers. I do mean solid: This 15-lb preamp has a rugged, no-nonsense feel. Two of the four sturdy knobs control electronic switches that choose the source, and set the outputs to stereo, reverse, or mono. The other two, motor-driven for remote control, adjust volume and balance. The three switches toggle between passive and active output, power on and off, and engage a processor bypass—a handy way to integrate multichannel capabilities.
The rear panel accommodates one pair of balanced XLR outputs and one balanced XLR input pair. It also features four additional RCA inputs, a processor loop on RCAs, two pairs of RCA outputs, and remote sensor and repeater jacks.
The GFP-750 is a Nelson Pass design—essentially a variation on the Pass Labs Aleph P and Pass's DIY project, The Son of the Bride of Zen (footnote 2). The circuit couldn't be simpler. Says Pass, "It's a differential pair—the end. A single gain stage, balanced input, balanced output, no feedback....We took a pair of MOSFETs, and the inputs go to the gates of the MOSFETs. The MOSFET sources are tied together and biased with a current source, with the signal taken off the drains. [The circuit] can run unbalanced on either side, although the performance is best when it's run balanced."
Other than glue logic and the transistors used to drive the relays, the 12 active devices—four gain blocks, each with three devices—are HEXFETs from International Rectifier. The output stage is intended to function as a pure voltage source, making the GFP-750 relatively immune to cable differences or low amplifier impedances. As we've come to expect from Adcom, the preamp sports a honkin' big toroidal transformer, with multiple secondary windings for each channel. A large heatsink on the left-hand side carries the power supply regulator chips.
Both signal and control circuitry is carried on one large double-sided printed circuit board, though these are physically separated. The parts quality is superb throughout, and care has been taken to keep signal paths as short as possible. The input switching relays, for example, are all adjacent to the rear-panel sockets. Did somebody mention the proverbial brick outhouse?
In passive mode, the signal sees only input switching and the attenuator.
Footnote: 1 All headers by Hippocrates.
Footnote 2: See www.passlabs.com for more information on these products. While there, you can also access patent 5,376,899, which covers the GFP-750's circuit.