ADCOM GFA-555 II
ADCOM GFA-555 II. enlarge.
Rear, ADCOM GFA-555 II. enlarge. This free website's biggest source of support is when you use these links, especially this link directly to them at eBay, where they sell for about $350 (see How to Win at eBay). It helps me keep reviewing these oldies when you get yours through these links, thanks! Ken. You also can buy today's ADCOM GFA-555SE, which appears very similar, for more money. (The GFA-555ms is a smaller model.)
The ADCOM GFA-555 II is among the world's greatest audio power amplifiers of all time. It was designed by the legendary Nelson Pass (interview, chat room), whose amplifiers today sell new for five figure price tags. Considering that you can get a used GFA-555 II like this one for about $350, I doubt there's a better bargain for high-end amplification. Its performance is exemplary; I haven't heard any other amplifier surpass it today.
While only rated for 200 watts per channel into 8 Ω, this 20-year old veteran easily put out 260 watts from each channel simultaneously at only 0.007% THD into 8 Ω. If a Japanese brand was marketing this, they'd rate it as 350W per channel into 6 Ω at 0.7% THD, or call it a "700 W amplifier."
ADCOM actually rates it as able to drive 2 Ω loads, and rates it for 850 watts at 0.09% THD into 4 ohms in bridged mono! I'm sure the Japanese could inflate this to over a kilowatt.
Measured with tone bursts, it easily put out an undistorted 370 watts per channel into 8 Ω.
This amplifier makes as much power as you can get from a linear audio amplifier from standard 120 VAC 15 Amp wall socket, and it runs cool and has less noise than any other power amplifier I've measured! It also makes no transformer buzz; it needs no fan and runs silently, which should be a requirement for any high-fidelity amplifier. It also has no thumps or whistles when turned on or off.
I have this lab report first, since I always run old gear through the lab to be sure it's stable and not going to blow up good speakers. More serious auditions are still to come, but from what little I've heard, it's clean, quiet, neutral and effortlessly powerful.
The clip indicators are dim at best. They start to light at 0.3% THD, they're halfway on at 1% THD and on fully at 3% THD, but if I designed this amplifier, they'd be a heck of a lot brighter and stay lit for several seconds regardless of how short the clip.
Because idle power drain is so low and the power supply uses such enormous capacitors, the GFA-555 II plays for a long time after turned off. The power LED stays on for 15 seconds if not playing anything.
Unlike most amplifiers with crappy mono bridging switches, the GFA-555 II uses a nice toggle switch that still works great after 20 years. Other amps that use slide or pushbutton switches have usually gotten dirty and gone flaky, while the GFA-555 II still sounds like new.
The GFA-555 II is state-of-the-art. It's funny to say, but the GFA-555 II much better than any of ADCOM's preamps. You're best driving this amp from a real preamp, or directly from the variable output of a CD player, skipping a preamp entirely. ADCOM's preamps are noisier than this amplifier deserves.
I have not tested today's ADCOM GFA-555SE, which appears very similar. The ADCOM GFA-555ms is a smaller model. I'm a cheapskate, so I prefer the real thing for a fraction of the price of today's recreations, but for most people, buying new is much easier.
Pages 1 and 2, ADCOM GFA-555 II Ad, October 1991. bigger.
Page 3, ADCOM GFA-555 II Ad, October 1991. bigger.
ADCOM GFA-555 II.
Two gold-plated, Teflon-insulted brass RCA jacks.
They are close enough together to make connection a breeze.
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).
2.5 dB at 4 Ω.
1.7 - 100,000 Hz at - 3 dB.
10 - 20,000 Hz +0, -0.25 dB at 1 W RMS into 8 Ω.
Not less than 800 from 20 - 20,000 Hz into 8 Ω.
Not less than -110dB down from 200 watts into 8 Ω, A-weighted.
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.
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.
35 pounds (16 kg).
39 pounds (18 kg), packed.
Made in USA.
All made with the R&S UPL.
Measured at 1 kHz, 1 watt output into an 8 Ω load:
This calculates out to 23.1486x gain.
122.2 mV in is 1W out into 8 Ω.
1.222 V is 100 W out into 8 Ω.
1.728V is 200 W out into 8 Ω.
ADCOM GFA-555 II Frequency Response, ±5 dB scale.
ADCOM GFA-555 II Frequency Response, ±1 dB scale.
ADCOM GFA-555 II Frequency Response, ±0.2 dB scale.
ADCOM GFA-555 II Frequency Response to 110 kHz, ±5 dB scale.
ADCOM GFA-555 II infrasonic Frequency Response.
Frequency response is as flat as amplifiers get.
ADCOM claims the ability to amplify DC, but as we can see, clearly it can't.
Maximum Output at 1 kHz into an 8 Ω load: 260W
ADCOM GFA-555 II THD at power limits at 1 kHz.
It puts out a clean, continuous 265 watts per channel into 8 ohms.
It puts out a clean 370.5 watts per channel into 8 ohms on tone bursts: a 10 ms burst of 1 kHz (10 cycles) at 1 per second: 77V Pk = 54.447V RMS = 370.5W RMS burst power at 2.3600 V RMS input (120.1V AC power input).
0.002% THD at 200 W.
0.0022% THD at 216 W (1.8V in)
0.0028% THD at 241 W (1.9V in, 808W from wall at 117.5V)
0.014% THD at 253 W (1.95 V in, 117.5V from wall)
0.057% THD at 256.5 W (1.96V in)
0.1% THD at 257.2 W (1.963 V in, 117.5V from wall)
0.11% THD at 258W (1.965V in)
0.18% THD at 259W (1.97V in)
0.68% THD at 265W (2.0V in, 845W from wall)
Unloaded, I measure 57V RMS at 0.1% THD (120.5 V RMS power input).
ADCOM GFA-555 II THD at 1 kHz.
ADCOM GFA-555 II THD at 1 Watt.
ADCOM GFA-555 II Distortion Components at 1 Watt.
19+20 kHz Difference-Frequency Distortion
per DIN IEC 268-3 or 118:
ADCOM GFA-555 II DFD at 1 watt RMS.
ADCOM GFA-555 II DFD at 100 Watts RMS.
Damping Factors and Output Impedances
All the fancy math below confirms is that this amplifier has the same output source impedance as 6 inches (15cm) of 16 AWG speaker wire, in other words, any further reduction is insignificant.
Damping Factors at 8Ω
Method and raw data
Voltage drop when applying an 8 Ω load:
R source = (R load/attenuation ratio) - R load.
Z source = (Z load/attenuation ratio) - Z load.)
DF = Zload/Zsource = 1/(1-(10^(dB drop/20)))
ADCOM GFA-555 II Output noise spectrum, fed from 5 Ω source impedance.
Unweighted noise, AC coupled:
The GFA-555 II is about 5 dB quieter than the GFA-545 II, and most importantly, is completely free from the power supply hum that plagues the GFA-545 II and GFA-535 II if you listen at desktop distances.
Measured into 8 Ω when fed from a 5 Ω source impedance:
Driving a B&W CDM2 Loudspeaker
Frequency Response driving B&W CDM2 speaker (in yellow), ±0.2 dB scale.
The ultra-low output impedance and high damping factor of the GFA-555 II means that its frequency response is unaffected by speaker loads. In this trace, the green trace is the left channel driving an 8 Ω reference resistor, and the yellow trace is the right channel driving the B&W loudspeaker.
THD driving a B&W CDM2 speaker at 100 mW (in yellow). Note expanded scale down to 0.0002% THD.
This graph shows the GFA-555 II driving an 8 Ω reference resistor on the left channel in green, and the yellow trace is the right channel driving the B&W loudspeaker.
Driving a real load, the distortion actually dropped in the critical midrange! This is because this speaker's impedance rises far above 8 Ω in that range, asking for less power. Ignores the lower frequencies, as I showed above looking at the 1 W THD, the channels don't match, and at only 100 mW here, noise is confusing the lower-frequency readings.
I tested this at 100 mW to save my ears; even at 100 mW sine waves are painful.
While I don't advocate anyone needing this sort of power, the ADCOM GFA-555 II amplifier is so clean, quiet, well-built and well-mannered that it's a steal at only about $350. You could easily spend over $25,000 for a new amplifier today that might not sound as good or handle as well as does this GFA-555 II.
If you've found my efforts in documenting this classic equipment helpful, this free website's biggest source of support is when you use these links, especially this link directly to them at eBay, where they sell for about $350 (see How to Win at eBay). It helps me keep reviewing these oldies when you get yours through these links, thanks! Ken.
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