Power Amplifiers

ADCOM GFA-555se Stereo Power Amplifier


Design and Setup

The design is little changed from the original, this is indeed a special version of the original and that's as it should be. Wouldn't be a proper tribute otherwise and besides, why mess with success? The features list in the GFA's owner's manual lists a few features that I didn't include in the specifications list above. Namely, precision matched devices used throughout the signal path, custom toroidal power transformer and independent power supplies for each channel. Those last two items tell us that while this amp is not a dual mono design it is nearly so. I'm glad they didn't go so far as to call it a dual mono design. Some manufactures might have but I'd like to reserve that term for boxes that share only a power cord between the two channels. I'll admit though that the crosstalk between channels that share only the initial mains transformer should be down in the noise. More important to me is the fact that the rectification and regulation of the power supply is independent in the two channels. The practice of spreading out the power supply regulation is one that I've come to look for in the best hifi. One item that I did list in the specifications: 60,000 µF. That is a lot of capacitance at this price point.

One of the upgrades over the original GFA-555 is the addition of balanced inputs. Selection between the single ended and balanced inputs is via switches (one per channel), on the rear panel. An additional switch is provided to select bridged operation – neatly turning the 555se into a monoblock. This yields more than twice the power per channel (600W into 8 Ohms), not to mention the separate mains transformer mentioned in the paragraph above. Why more than twice the power? When in bridged mode each of the channels 'helps' the other to see less impedance. A push/pull type of operation that means that each channel sees half the impedance it would see if operating alone. This imposes some speaker limitations while in bridged mode. The owner's manual has a thorough discussion on the subject. In short you should not connect the amp to a speaker with any significant impedance dip under 4 ohms. There is no such warning when not in bridged mode but there is an asterisk alongside the power ratings for 4 Ohm loads and Bridged Operation into 8 Ohm loads. At the bottom of the page the asterisk decodes to 'With supplemental cooling system'. Meaning a fan I suppose. There are pretty hefty heat sinks on the back of the unit but I never noticed them getting beyond mildly warm. I was able to keep a hand on them indefinitely.