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100 WPC Power Amplifier (1991-1994)

© 2011 KenRockwell.com. All rights reserved.

Intro   Specs   Performance   Compared   Usage   Recommendations

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ADCOM GFA-545 II. enlarge.



Rear, ADCOM GFA-545 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 $600 (see How to Win at eBay). It helps me keep reviewing these oldies when you get yours through these links, thanks! Ken.


December 2011   ADCOM Reviews   Audio Reviews   All Reviews

Adcom CGA-545 II Review

Right now, all I have is this lab report. Audition is still to come.

Need harmonics

555 pro: front fuses


General Obsetvations

32 Watts idle, cold.

40 Watts idle, warm.


dim power LED

Distortion LEDs dim, light at 1% THD (barely lit at 0.5%, full at 3%)



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

27.152 dB
27.164 dB

This calculates out to exactly 22.8x gain.

124 mV in is 1W out into 8 Ω.

1.24 V is 100 W out into 8 Ω.


Frequency Response


ADCOM GFA-545 II Frequency Response to 20 kHz, ±5 dB scale.



ADCOM GFA-545 II Frequency Response to 20 kHz, ±1 dB scale.



ADCOM GFA-545 II Frequency Response to 20 kHz, ±0.2 dB scale.



ADCOM GFA-545 II Frequency Response to 110 kHz, ±5 dB scale.



ADCOM GFA-545 II infrasonic Frequency Response.




Adcom GFA-545 II Output noise spectrum, fed from 5 Ω source impedance.

A-weighted noise:

Input fed from 5 Ω source impedance
-90.5 dBV
-90.5 dBV
No input connection
-75 dBV
-65 dBV


Unweighted noise:

Input fed from 5 Ω source impedance, unweighted
-84 dBV
-82.5 dBV


DC offset

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

+160 µV
-240 µV


Damping Factors and Output Impedances

Output Impedances

50 Hz
2.58 mΩ
2.63 mΩ
1 kHz
2.67 mΩ
2.76 mΩ
20 kHz
3.68 mΩ
3.96 mΩ


Damping Factors at 8Ω

50 Hz
1 kHz
20 kHz


Method and raw data

Voltage drop when applying an 8 Ω load:

50 Hz
-0.0028 dB
-0.00285 dB
1 kHz
-0.0029 dB
-0.003 dB
20 kHz
-0.004 dB
-0.0043 dB

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


Maximum Output at 1 kHz into 8 Ω load

128W at 121.1 V AC power input at 0.1% THD.

134W at 121.1 V AC power input at 1% THD (also at lights on).

10 ms burst of 1 kHz (10 cycles) at 1 per second: 52.1V Pk = 36.9V RMS = 170W Pk at 122.6V AC power input.




ADCOM GFA-545 II THD at 1 Watt.



ADCOM GFA-545 II THD at 1 kHz.



ADCOM GFA-545 II THD at power limits at 1 kHz.



ADCOM GFA-545 II SMPTE Intermodultion Distortion

SMPTE (60Hz+7kHz @ 4:1) Intermodualtion Distotrion vs. input voltage.

10 mW (12mV in or 283mV out): 0.0022%

100 mW (39mV in or 894mV out): 0.0035%

1W (124mV in or 2.83V out): 0.011%

10W (392mV in or 8.94 V out) 0.013%

93W (1.2 V in or 27.3V out): 0.22%.


19+20 kHz DFD (per DIN IEC 268-3 or 118)


ADCOM GFA-545 II DFD at 1 watt RMS.



ADCOM GFA-545 II DFD at 62.5 watts RMS.


Driving a pair of B&W Matrix 805


ADCOM GFA-545 II Frequency Response driving B&W Matrix 805, ±0.2 dB scale.



ADCOM GFA-545 II THD at 100 mW driving B&W Matrix 805. Note expanded scale down to 0.0002% THD.






Introduction         top

Intro   Specs   Performance   Compared   Usage   Recommendations


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The ADCOM GFA-545 II is a high-voltage amplifier and bias source for Stax' electrostatic headphones.

The SRM-T1 has two Professional (580 V) and a third Standard (230 V) output, so any Stax earspeaker ever made since 1959, from the original SR-1 to today's SR-009, works perfectly. (Stax calls its headphones "earspeakers.")

The SRM-T1 is an FET-input, tube-output, pure class-A, DC-coupled amplifier. It has no capacitors or transformers in the audio path. It's housed in a completely non-magnetic case to ensure no inadvertent magnetic coupling happens. It uses two 6FQ7/6CG7 vacuum tubes in its output stage.

It has enough output to deafen us with any of Stax' headphones, without any distortion.

It is fed from any device with 1/4" RCA jacks, or even directly from iPods and iPads with any 99¢ 3.5mm to RCA cord. If you do your iPod transfers correctly, this and a set of SR-Lambda Professionals sounds indescribably incredible driven directly from an iPod, if you'll just listen.

This SRM-T1 was Stax' first tube-output amplifier. It was replaced in 1993 by the SRM-T1S which debuted with the first SR-Omega. The SRM-T1S is the same as ths SRM-T1, except for a switchable input selector.


Specifications         top

Intro   Specs   Performance   Compared   Usage   Recommendations




Unlike the SRM-1/MK-2, I've only seen "SRM-T1" punctuated one way.



There is one loop-through line-level input.

Loop-through means they are wired together; you can insert this in between any two other components, and even if it's turned-off, the signal goes through.

100 mV, 50 k Ω.



60 dB (1,000x).

In other words, a 100 mV input can produce a 100 V output at maximum gain.


Frequency Response

DC-20 kHz ± 1dB at 100 V RMS driving an SR-Λ Signature 1.

Appreciate how no other amplifier rates its performance actually driving a real, reactive load as Stax has rated this amplifier.



0.02% at 100 V RMS driving an SR-Λ Signature 1.

Again, this is real performance driving a real, reactive load.



The three headphone audio (plate) outputs are connected in parallel.

Bias voltages: 230 V DC (Normal) and 580 V DC (Pro).

Maximum output: 300 V RMS at 1 kHz.


Size (actual measured)

195 x 87.5 x 346 millimeters (7.68 x 3.44 x 13.62 inches) WHD, rectangular case only, excluding tube humps, feet, knobs and connectors.

195 x 103 x 377 millimeters (7.68 x 4.06 x 14.84 inches) WHD, including tube humps, feet, knobs and connectors. Leave more room for the plugs when installing.

Stax specifies 195 × 103 × 375 millimeters (7.68 x 4.06 x 14.76 inches).



3.9 kg (8.6 pounds).


Line Voltage and Power

Voltage: Rear-panel selectable 100 V, 117 V, 220 V or 240 V.

Frequency: 50 - 60 Hz.

Power Consumption: Rated 45 Watts. With the main amplifier off (red pilot light) but the pre-heater left on (green pilot light only), it's rated 12W.



2011: about $600 used at this link to them at eBay. (see How to Win at eBay).


ADCOM GFA-545 II. enlarge.



Rear, ADCOM GFA-545 II. enlarge.


Performance         top

Intro   Specs   Performance   Compared   Usage   Recommendations



See also Electrostatic Headphones: How They Work, and Why They Sound Better.


It sounds smooth, perfect and detailed, exactly as I had dreamed. I thought those people at SAPPHIRE's tube table were making all this up, until I heard this for myself.

The ADCOM GFA-545 II sounds absolutely awesome. Music sounds as music should; nothing more, nothing less.

With any of the Stax Lambda Pro or ancient Stax New SR-3, it just sounds great. I plugged it in, and it just grabbed me to shut up and listen to the music, not the gear, for hours and hours and hours. Bravo!

Seriously, even the 1970's Stax New SR-3 with this amplifier sounds so much better than the Ultrasone Edition 8 or Sennheiser HD800 that it's amazing.

With the Stax Lambda Pro, it's audiophile death: once an audiophile gives a listen to this setup, he'll be converted into a music lover and be cured of ever wanting any more equipment. He'll finally be able to stop researching about all this on his computer and start actually enjoying music for a change!


Channel Balance

The potentiometer is ganged and clutched. This means balance is set by holding the sleeve and turning the top of the LEVEL knob.

The potentiometer on this sample tracked extremely well. Only below about 70 or 80 dB attenuation was there even the slightest imbalance between channels. One slight adjustment of the two knobs, and the darn thing is now perfect all the way smoothly down into silence. This is extraordinary performance.



It sounds great, without any noise or distortion even at deafening levels.

The gain structure is perfect: most of the time, I have the gain control set to the 12 o'clock position.

Left and right channels are friction coupled in one knob. To shift the channel balance, just hold both and turn each part of the knob in different directions.



As far as I can hear, it's silent, so long as you have it installed correctly to avoid hum or buzz.



I never heard any.



Everything should be this simple: two power switches, and one gain (volume) control.

The gain control feels great: it's slick, smooth and undamped. It flicks up and down with ease, and the big knob makes changing levels a joy.

It's easy to find and grab the knob and turn it from any angle, even in the pitch dark, for fast and precise adjustment. I love real volume control knobs; you can take digital up/down buttons and shaft encoders and put them in the doo-dah (garbage) can.

Stax uses a premium semi-sealed Alps 50 k Ω potentiometer, and it's as smooth as the glassy feel that Penny + Giles used to advertise for their professional audio controls.

The power LEDs flicker with the 60 Hz power, not driven by DC.



WARNING: DO NOT OPEN unless you are a licensed technician. Deadly voltages can be present even after being unplugged.

Output stage: two 6FQ7/6CG7 vacuum tubes.

ADCOM GFA-545 II, interior

ADCOM GFA-545 II, Interior. enlarge.


ADCOM GFA-545 II, interior

ADCOM GFA-545 II, Interior. enlarge.

The SRM-T1 is very well made.

The wiring is neat and clean, and they use fancy super oxygen-free copper wire with giant crystals that I wouldn't know where to buy (I build my own gear with military teflon-insulated silver wire).

Pure DC coupled: no transformers and no capacitors in the audio path.

It runs pure class-A, with FETs in the first amplification stage, and vacuum tubes in the output stage.

It's got plenty of power supply capacitance; when turned off, it keeps playing for quite a while.



As a pure class-A amplifier, and as a tube amplifier, it runs hot.

This amplifier is supposed to get hot, even sitting idle.

Imagine a 45W bulb in a small box like this, and that's exactly as hot as this is supposed to get.



The SRM-T1 is built internally more like a piece of military or industrial hardware than almost anything from what passes for professional audio gear today.

The big volume control knob just glides. In fact, the Alps potentiometer is so smooth and the knob so big that you can spin it and keeps going by inertia.

The amp is built on a heavy inner aluminum chassis with nonmagnetic top and bottom covers, and a solid aluminum front panel.

Stax deliberately made everything nonmagnetic to eliminate unintended audio interactions.

The nonmetallic metal covers are held in place by machine screws screwed into solid metal inserts. Stax never got lazy and tried to use self-tapping bung screws like we lazy Americans; they took this very seriously.


Compared         top

Intro   Specs   Performance   Compared   Usage   Recommendations

With the Stax Lambda Pro, compared directly with the Stax SRM1-MK2, I was amazed that it really sounded as those folks at SAPPHIRE's tube table keep telling us: it's ever so slightly smoother, with less high treble. Since the Stax Lambda Pro tend to be on the bright side, I really like the subtle difference I hear with this SRM-T1 compared to the all solid-state SRM1-MK2.

I'm not doing this double-blind, so it could be my ears playing tricks on me as so often happens in audio, but I really do prefer this over the Stax SRM1-MK2 and Stax SRA-12S for use with the Stax Lambda Pro.

Likewise, this SRM-T1 sounded just a little bit better even with the ancient Stax New SR-3.

Darn. Now I want a tubed iPod. I thought those tube people were crazy, except for mics and mic preamps.

Oddly, probably because its bigger and has even more ventilation and a perforated circuit board, even though it draw more power, it feels like it runs a little cooler than the Stax SRM1-MK2.

I don't know that this is an issue of tubes versus bipolar transistors in the output section, or simply that this SRM-T1 sounds like it has a few less dB of gain at 16 kHz compared to the SRM1-MK2. This couple of dB make all the difference between smooth and too bright; Stax headphones make even the slightest imbalances obvious.


Usage         top

Intro   Specs   Performance   Compared   Usage   Recommendations



This amplifier is a class-A tube power amplifier, so it gets warm.

Don't cover any of the top or bottom convective cooling holes.

Never put anything on top of this amplifier. Not only are you likely to damage the amplifier, you may burn whatever you put on top.

Proper airflow is critical. No fan is used for noise reasons. Air flows naturally with convection from bottom to top, so leave all the slots and holes free.



Watch for High Voltage when plugging and unplugging headphones: even when off, it's easy to touch the connector pins and potentially (hee hee) get a shock.

Never, ever open this thing. Lethal voltages are present, even if unplugged. The capacitors can store enough energy to kill a man for quite some time.


Recommendations         top

Intro   Specs   Performance   Compared   Usage   Recommendations

The only place to find these limited-production amplifiers is here at eBay, where they usually sell for about $600 if you know How to Win at eBay.

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


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