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ADCOM GFA-555 II
200 WPC Power Amplifier (1990s)

© 2011 KenRockwell.com. All rights reserved.

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ADCOM GFA-555 II

ADCOM GFA-555 II. enlarge.

 

ADCOM GFA-555 II

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.)

December 2011   ADCOM Reviews   Audio Reviews   All Reviews

 

Introduction         top

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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.

 

Specifications         top

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Name

ADCOM GFA-555 II.

 

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.

 

Price, USA

December 2011: about $350 used. (see How to Win at eBay).

 

Measurements         top

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All made with the R&S UPL.

 

Gain

Measured at 1 kHz, 1 watt output into an 8 Ω load:

Left
Right
27.289 dB
27.292 dB

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.

 

Frequency Response

ADCOM GFA-555 II

ADCOM GFA-555 II Frequency Response, ±5 dB scale.

 

ADCOM GFA-555 II

ADCOM GFA-555 II Frequency Response, ±1 dB scale.

 

ADCOM GFA-555 II

ADCOM GFA-555 II Frequency Response, ±0.2 dB scale.

Looking at the sonic signature in this +-0.2 dB trace, it matches the GFA-545 II and GFA-535 II, which is warm, clear, transparent and detailed.

 

ADCOM GFA-555 II

ADCOM GFA-555 II Frequency Response to 110 kHz, ±5 dB scale.

 

ADCOM GFA-555 II

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

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).

I measure:

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).

 

THD

ADCOM GFA-555 II

ADCOM GFA-555 II THD at 1 kHz.

 

ADCOM GFA-555 II

ADCOM GFA-555 II THD at 1 Watt.

 

ADCOM GFA-555 II

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

ADCOM GFA-555 II DFD at 1 watt RMS.

 

ADCOM GFA-555 II

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

 
Left
Right
50 Hz
1,551
1,930
1 kHz
1,848
1,930
20 kHz
1,127
1,400

 

Output Impedances

 
Left
Right
50 Hz
5.2 mΩ
4.1 mΩ
1 kHz
4.3 mΩ
4.1 mΩ
20 kHz
7.1 mΩ
5.7 mΩ

 

Method and raw data

Voltage drop when applying an 8 Ω load:

 
Left
Right
50 Hz
-0.0056 dB
-0.0045 dB
1 kHz
-0.0047 dB
-0.0045 dB
20 kHz
-0.0077 dB
-0.0062 dB

R source = (R load/attenuation ratio) - R load.

 

Noise

Adcom

ADCOM GFA-555 II Output noise spectrum, fed from 5 Ω source impedance.

 

A-weighted noise:

 
Left
Right
Input fed from 5 Ω source impedance
-95.0 dBV
-92.0 dBV
No input connection
-75 dBV
-75 dBV

 

Unweighted noise, AC coupled:

 
Left
Right
Input fed from 5 Ω source impedance
-91.6 dBV
-87.5 dBV
No input connection
-73.6 dBV
-74.2 dBV

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.

 

DC offset

Measured into 8 Ω when fed from a 5 Ω source impedance:

Left
Right
-1.1 mV
-1.5 mV

 

Driving a B&W CDM2 Loudspeaker

ADCOM GFA-555 II

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.

 

ADCOM GFA-555 II

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.

 

Recommendations         top

Intro   Specs   Measurements    Recommendations   More   

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.

 

More Information         top

Intro   Specs   Measurements    Recommendations   More   

ADCOM GFA-555 II Service Manual.

Nelson Pass' Do-It-Yourself pages .

Pass Labs.

 

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Mr. & Mrs. Ken Rockwell, Ryan and Katie.

 

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