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Adcom GFA 5200 Measurements

orchardaudio

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A local customer of mine brought over their Adcom GFA 5200. I took some measurements of it and posting up for the community.

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Amplifier's Manual:

All the measurements below are with the amplifier operating in stereo mode with an 8-ohm load on each channel. Only RCA inputs are available on this unit.

Bench mode
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Signal-to-Noise Ration (SNR), no weighting
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Distortion (THD+N) vs Measured level at 1kHz (Starkrimson Ultra show for comparison)
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Distortion (THD+N) vs Frequency at 10W output (Starkrimson Ultra show for comparison)

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Intermodulation (IMD) Distoriton vs power level (starkrimson ultra show for comparison)
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Wideband FFT with no input
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Multitone
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Frequency Response

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Attachments

  • gfa5200.pdf
    592 KB · Views: 299
Page 1 BIAS ALIGNMENT is important and is a essential procedure.
 

Attachments

  • ZZZZ - hfe_adcom_gfa-5200_service.pdf
    33.2 KB · Views: 196
That looks like something that would be done at the factory.
It's like an engine adjustment. It's routine stuff to perform. The instructions are bare minimum in span. If not comfortable then at the least connect a meter if you have one and check for any irregularities. It's a worthy device to justify checking the idle offset and bias. It would be interesting to see a oscilloscope connected to the power supply rails while cranking out the watts. To check for rail voltage stability and ripple.
 
What year did those come out? If it hasn't had any servicing done or electrolytics replaced then that is pretty impressive.
 
What year did those come out? If it hasn't had any servicing done or electrolytics replaced then that is pretty impressive.
I searched all the <.pdf> and found no dates. There is use of a font within a font's underline. I don't really remember that. Perhaps that can be used to date it or maybe finding a review date could date this bad boy. It's a Adcom 50 watter and they are known to last so why not give it a fighting chance.
ZZZZZZ - adcom - Screenshot 2022-08-01 152850.png
 
That looks like something that would be done at the factory.
Naw, setting bias and DC offset on these guys is easy peasy if your comfortable with your hands inside a powered amp.
All you need is a 8ohm dummy load and multimeter.
Just be careful, there are dangerous voltages loose inside there. ;)
I'm far from a tech but have gone thru all 6 of my Adcoms to keep them in top tune.
There's a thread here where a couple of the guys gave me some guidance.
 
What year did those come out? If it hasn't had any servicing done or electrolytics replaced then that is pretty impressive.
IIRC the GFA-5x5 models began in 1990, and the GFA-5x5 II's began in 1995.
Following that guideline the GFA-5x00 models began their model run right around 2000
 
IIRC the GFA-5x5 models began in 1990, and the GFA-5x5 II's began in 1995.
Following that guideline the GFA-5x00 models began their model run right around 2000
@orchardaudio has the amplifier, he can pop the top and check the component date codes and report back.

Obviously bias and offset should be checked prior to any serious testing on a 27 year old amplifier. Regardless, it performed very well and considering you can buy those wonderful little Adcoms in the US for $250-$350, they are an absolute bargain.
 
DC offset is 12mV on one channel and 11mV on the other, this is with the inputs shorted.

Sorry, I don't have much time to do much of anything else.
 
Last edited:
DC offset is 12mV on one channel and 11mV on the other.

Sorry, I don't have much time to do much of anything else.
Not horrid but a quick fix with an insulated screwdriver and a tiny tweak of pots R039 and R139 to zero out.
@orchardaudio has the amplifier, he can pop the top and check the component date codes and report back.

Obviously bias and offset should be checked prior to any serious testing on a 27 year old amplifier. Regardless, it performed very well and considering you can buy those wonderful little Adcoms in the US for $250-$350, they are an absolute bargain.
Adcom had the habit of printing the year right on the circuit board in the earlier models, not sure on the 5x00's
 
Did you know that "Adcom" is an acronym for "Additional Compensation"?

I have three of these 5200 amps: one early model with common PS (date sticker says "4-95") and two later ones with separate front end PS (both "3-97").

If you rub one just so, you can make it have 20W in class A:
 
@orchardaudio -- The 750 kHz'ish spur is about 300 mVrms (checking units)?
 
Found schematic. Most service manuals seem to not include it.

Didn’t realize Adcom did FET amps.


I have two old 535II’s (bipolar designs) and the big problems with those were…

- Filthy pcb’s (took ages to clean)
- Badly mismatched gains on diffpair input transistors (one was someone hacking it)

…which when fixed sounded great. Wish I had means to measure them.

I did not find any problems with caps, only other devices with a problem were rusty front panel micro LEDs.

On the 5200, I would check the power transistor source resistors measure OK.
 
Didn’t realize Adcom did FET amps.
All the GFA-5x5 II and earlier amps were bipolar. GFA-5x00 and later were FET
 
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