Adcom GFP-750

GFP-750

The GFP-750 Preamplifier

The GFP-750 is part of a series of ultra-high performance separate components from Adcom. It shares this position with our exciting GFA-5802 power amp and GCD-750 CD player. All three of these components were conceived under Adcom's long standing philosophy of providing exceptional performance for the money. But our goal was to go beyond delivering exceptional performance and value, to the "next level"; by delivering the sonics and effortless power, along with the finesse and sophistication of units costing two to three times as much. The fact that we exceeded our own expectations is a testament to our engineers' diligence and their unswerving dedication to these goals.

The preamplifier's role

From a design standpoint the proverbial audio component "acting like a piece of wire, but with gain" is always (or should always be) the goal. But approaching that goal is, in reality, an incredibly difficult task. With the GFP-750 we believe we've come significantly closer to this ideal than anyone near this price category has ever come before.

The preamplifier is "control central" for any system. It provides source selection, adds gain when necessary, lets you adjusts levels, balance, and other parameters, and sends the signal off to the system's "engine room", the power amp. As the switching and control center, the preamp must be extremely quiet and distortion free. Because the signals it receives and then sends out to the power amp will be multiplied hundreds of times before you hear them. So if the preamp changes the signal in any way, it'll be magnified big time when you finally hear it! Fundamentally then, a great preamp must allow control flexibility and source selection, while adding or taking away nothing from the original source.

Choose active or passive control

The GFP-750 can be operated either as a completely "passive" (no gain) unit, with absolutely minimal sonic impact, or as a sophisticated active (utilizing active components and providing gain) preamplifier. In passive mode the GFP-750 is strictly a "line level input switcher", with its high quality four gang volume attenuator that lets you adjust the volume. While its active circuitry is both unique and superb (details later), you always have the option of using the GFP-750 in this ultimate passive mode, for control with maximum fidelity.

Adcom's passive approach

Any "line level" source that you have (CD player, tuner, DVD player {audio output}, tape deck, etc.) can be directed through the GFP-750 in the passive mode. The only effect the GFP-750 will have on the sound is volume adjustment, using the built-in precision four gang volume attenuator. In this mode the GFP-750 adds no noise, no distortion, in fact, it changes nothing. So, as far as your ears are concerned, it's sonically invisible. And that's our goal, isn't it?

But what if you need the active capabilities of the 750? Do you sacrifice fidelity for convenience and added control? Read on and see why you don't.

Adcom's active approach

The engineering design specifications for the GFP-750 were simple:

Minimize circuitry (the fewer parts and gain stages, the less chance there is for sonic degradation).
Reduce or eliminate negative feedback.
Incorporate true differential balanced audio signal paths for the best sound quality.
Use high quality MosFet transistors throughout.
Power the internal circuits of the GFP-750 with a power supply that can keep up with any conceivable signal demand.
Incorporate an easy way to add home theater processing while maintaining the best performance.

Adcom's creative engineering solutions

The GFP-750 has only one active gain stage. It's a pure Class A design, long regarded as the best sounding type of amplifier circuit. The active devices in this gain stage are exclusively reference grade HexFet transistors (an advanced hybrid MosFet device) that offer superb, dynamic sound with a smoothness and openness heretofore considered unobtainable in solid state devices. Countless hours of intense listening have proven to us that well designed amplifier stages using MosFet devices are the smoothest and most natural sounding. So you'll find these more expensive MosFets and HexFets throughout Adcom's line, because we won't compromise performance to save money on parts.
The GFP-750's active circuits have extremely wide frequency bandwidth for uncolored sound, yet they've been designed to be impervious to RF (Radio Frequency) interference, as an added assurance that noise won't be an issue.
The GFP-750 features both balanced XLR (for the CD) and unbalanced gold plated RCA input circuits. Using the balanced XLRs assures the quietest signal transfer between components. Balanced XLR circuits are also included for output connection to an appropriately equipped power amplifier, like the GFA-5802.
The GFP-750's output stage acts as pure current source (as opposed to a voltage source). That means it's effectively isolated from the power amplifiers input impedance and the impedance of the cables used between them. (High quality cables are important, but should have less of a sonic impact on the GFP-750 than some other preamp designs.)
The GFP-750 incorporates a power supply that would more typically suit a receiver or integrated amplifier! It includes a large toroidal (donut shaped) transformer which offers less internal resistance to current flow and lower induced hum than traditional transformer designs. This transformer has multiple output taps, one for each channel. Individual voltage regulators are mounted to special aluminum heat sinks for maximum stability and to allow high current draw during dynamic passages. Selected high capacity storage capacitors are mounted close to the individual circuit they service to supply clean, filtered power to the active components.
The GFP-750's component parts are carefully selected reference grade devices, for maximum sound quality and minimum noise.
The GFP-750 is direct coupled, eliminating capacitors entirely, where possible, to reduce their sonic impact.
Input switching is accomplished with hermetically sealed high quality relays. This eliminates the noise and other problems associated with alternate switching methods.

The Home Theater connection

A specially located and buffered (isolated) external processor loop has been included in the GFP-750. It allows easy connection of an external surround sound processor with no sonic impact. When this loop is employed, the GFP-750's volume control is bypassed allowing the processor to adjust all channel output levels. In this way, you can create a Home Theater while maintaining the GFP-750's (and the processor's) sound quality. It's quite an elegant solution to the problem of expanding an ultra-high performance system without affecting the sound.

Other important touches

The GFP-750 comes with a slim, elegant remote control that lets you select inputs, adjust volume and balance, mute the output, and select the external processor loop.
The front panel includes red LEDs which indicate Passive mode, Power on, and Processor loop engaged. There's also a convenient front panel control for Stereo, Stereo Reverse, and Mono.
On the rear panel all input/output jacks are gold plated for maximum conductivity and to eliminate corrosion. There's an ON/Off "vacation" switch to completely shut down the GFP-750 when it won't be used for a long time.
Infra-Red "repeater" jacks are included to allow sending IR signals along to other remote controlled units, using IR flashers available from your Adcom dealer. You can also connect a remote room sensor to the back of the GFP-750.
The GFP-750's physical construction has been chosen to eliminate any concerns about external vibration coloring the sound. This design also helps shield the internal circuits from outside electrical noise. The GFP-750 is built using an extruded aluminum front panel with a heavy gauge steel chassis and top. The entire unit is finished in an attractive and durable powder coated black finish.

The GFP-750, like its Adcom stable mates, offers true audiophile high end sonic performance at a surprisingly affordable price. Its simple, clean control layout, combined with full remote capability, Home Theater expandability, and State-of the Art sound will certainly satisfy the most critical and quality conscious listener.