Adcom GFA-555 MKI Stereo Amplifier - 200W x 2 - Serviced

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General:
This is the legendary Adcom GFA-555 MKI complete with its original owners manual.

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

Sure there 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.

SkyFi Cosmetic Notes:
• Light blemishes on top cover and top of faceplate.

SkyFi Service Notes:
• Electrolytic capacitors replaced on driver board.
• Differential input pairs replaced with matched pairs for optimal DC offset.
• Bias set to service manual specification.
• Both channels test to approx. 210W before clipping.

Brand Background:
ADCOM started in the 70s. Our first offering wasn’t amplifiers but phono cartridges. They were no ordinary cartridges; however, they were extraordinarily musical "moving coil" designs that extracted a lot of information from a record groove. These products were so successful that other companies asked us to begin making cartridges for them, too! And so, we became a manufacturer!

Early Electronics... ADCOM's first electronic product, the groundbreaking GFA-1 power amplifier, appeared in December 1979. It was substantially more powerful than most of its competition at 200 watts per channel and was one of the first "high current" designs available. . Again, it was an affordable and standout performer. Critical response was phenomenal and the GFA-1 became the "hot ticket" amplifier of its day, as did the successor GFA-1A.

Ownership:
Single Owner

Connections:
RCA Inputs Jacks, 5 Way Binding Post for Speaker Output. Captive power cable.

General Sound:
Smooth, uncolored, undistorted, natural and clean

Cosmetic Condition:
8/10 = Very Good. Excellent front faceplate, one minor flaw on chassis side or top. See our detailed rating description here.

Working Condition:
Working perfectly and tested in our lab and listening room.

Included:
Unit, manual, and power cord.

Packing:
Will be packed using our highly developed in-house process and custom packing materials.Specs:
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 MK I is completely disconnected when switched off.

Dimensions:
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 lbs.

Recommended Cables:

Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - Speaker Cables - Better

Kimber Summit Series Monocle XL Speaker Cables (PAIR) - Best

Kimber Summit Series BiFocal XL Bi-Wire Speaker Cables (PAIR) - Best If Applicable 

We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect the PCBs for discoloration from resistors or transistors that may have been running hot. On vintage units we often spot check select capacitors for value and ESR.

If the amplifier passes visual inspection, we move on to a controlled power on sequence using a Sencore safety analyzer to monitor current draw in real time. Once the amplifier is determined to be safe to operate, we connect it to full AC mains for function and power testing. We connect the speaker outputs of the amplifier to a Sencore PA81 Power Analyzer which acts as a dummy load, DC offset monitor, and oscilloscope interface. We start with a low level 1KHz test signal at the amplifier’s input and slowly increase its amplitude while monitoring the output on an oscilloscope for signs of noise, clipping, distortion, or improper channel balance. We continue increasing the signal level until the amplifier reaches clipping. At this point we take an output power measurement and compare it to the spec sheet of the amplifier to verify proper performance. If the device under test has both balanced and single ended inputs they are both tested at this time. We finish off the bench evaluation with a 1KHz square wave check and a 20Hz to 20KHz sine sweep to assess the amplifier’s frequency response characteristics. This battery of tests will usually reveal if the amplifier has any issues that need further attention.

Before the device leaves the bench, we perform a listening test with actual music using a variety of preferred test tracks. Our benches are outfitted with familiar monitor speakers which help us identify inconsistencies that will not always show up on our test gear. The main things that we are listening for are hum or noise with no signal present, proper center image, clicks, pops, or any other obvious undesirable audio characteristics.

If the unit passes all of these tests it is moved to our long term testing rig where we simulate real word operating conditions for 6-8 hours. This allows us to monitor the unit for signs of thermal runaway or intermittent issues that only crop up when the unit has fully come up to temperature.