Adcom GFA-555 MKII Solid State Vintage Amplifier

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This is the legendary Adcom GFA-555II complete with its original owners manual.

This example was clearly well cared for and is in wonderful condition.

Sure there are are plenty of 555's on the market, but most were passed around from audiophile to audiophile and got beat up in the process and over time.

The GFA-555 was quite the amp in the 1990's and one of the top selling amps of all time. It is super powerful, yet it comes in a fairly compact and lightweight package making it easy for a single person to move around and put in place. 

While the Adcom company we knew and loved is no longer in business, prices on the top models seem to be going up as audiophiles on a budget rediscover the brand. They are simple in design and use common parts so servicing them, if the need arrises, should be an easy task performed by just about any technician. 

We tested this amp on our test bench to its rated full power and it worked effortlessly without any signs of strain or noise.  Sounded excellent in our listening area tests too.

Click below to add our recommended matching cables from Kimber Kable, all brand new as SkyFi is an official Kimber dealer.

Kimber Kable - RCA Interconnects

Kimber Kable - Speaker Cables

Inputs

100k Ω.

Two gold-plated, Teflon-insulted brass RCA jacks.

They are close enough together to make connection a breeze.

Gain

27 dB.

Input Sensitivity

1.75V for 200 W into 8 Ω.

130 mV for 1 W into 8 Ω.

Rated Power Output

200 watts per channel into 8 Ω from 20 - 20,000 Hz at less than 0.04% THD, both channels driven.

325 watts per channel into 4 Ω from 20 - 20,000 Hz at less than 0.04% THD, both channels driven (requires fan option to do this for any length of time).

600 watts into 8 Ω from 20 - 20,000 Hz at less than 0.04% THD, bridged mono (requires fan option to do this for any length of time).

850 watts into 4 Ω from 20 - 20,000 Hz at less than 0.09% THD, bridged mono (requires fan option to do this for any length of time).

Dynamic Headroom

2.5 dB at 4 Ω.

Power Bandwidth

1.7 - 100,000 Hz at - 3 dB.

Frequency Response

10 - 20,000 Hz +0, -0.25 dB at 1 W RMS into 8 Ω.

Damping Factor

Not less than 800 from 20 - 20,000 Hz into 8 Ω.

Noise

Not less than -110dB down from 200 watts into 8 Ω, A-weighted.

Power Input

120 VAC 50-60 Hz.

72 VA idle.

1,500 VA maximum.

675 VA at 200 W into 8 Ω.

I measure 37.5 watts at idle, cold.

60 watts at idle, warm (0.73A).

92 Watts (1.09A) when delivering 1 watt per channel into 8 Ω.

845 watts when delivering 265 watts per channel into 8 Ω.

It draws zero power when turned off because it has a real power switch. It's not like modern equipment that never really turns off and consumes power and remains a fire hazard any time its plugged in. The GFA-555 II is completely disconnected when switched off.

Size

Chassis: 6-3/4 x 17 x 12-3/16 inches (172 x 432 x 310 mm) HWD.

Overall: 7-1/4 x 17 x 12-3/16 inches (185 x 432 x 310 mm) HWD.

Weight

35 pounds (16 kg).

39 pounds (18 kg), packed.

Quality

Made in USA.

The SkyFi Testing Process for Solid State Amplifiers:
We start with a visual inspection of all internal components to make sure there are no signs of stress or aging. Capacitors are checked for telltale sings of bulging or leaking, resistors are checked for signs of overheating or cracking, and transistors are checked for signs of stress or damage.

We then power up the unit carefully and run a simple 1k sine wave while monitoring the output on an oscilloscope for signs of distortion or noise. Finally, we run the amplifier to its full rated power into the appropriate resistive load provided by our Sencore amplifier tester. If there is an issue, it will usually be noticeable at this point. If everything reads clean, we will often test for THD (Total Harmonic Distortion) measured by our Keithley distortion meter, and compare it against the original manufacturers specifications. The amplifier then remains powered on to "burn in" on our test bench for a few hours to make sure there are no heat related issues. If there are balanced inputs available they will be also be tested.

Final testing involves pushing actual music thorough the amplifier. We've learned over time that some issues are only noticeable to a trained ear while listening to familiar source material. Our test bench has reference vintage KEF speakers that we're very familiar with which quickly reveal any discrepancies. Some amplifiers will then move into our listening room where they are tested with our in-house reference speakers of choice.