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Adcom GFA-5500

highend64

highend64

AK Subscriber
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I am about to purchase an Adcom GFA-5500 to replace a BK ST2140(which I dont have anymore). Currently I am without an amp but that is ok since I have been really busy and do have time to listen to my system. I will be getting it sometime this week. One of my questions is would it be a good idea to service the amp and see if it within specs before trying it out? Also, I know this amp was a Nelson Pass design. Does this model run Class A at up to so many watts? It would be powering a pair of Vandersteen 2ci speakers. Thanks
 
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I finally got the amp and so far it works and sound great. Do I need to worry about rebiasing or dc offset?
 
always check with a multimeter that its not putting out DC voltage. I would go though the service manual adjustment, if anything just to look around inside. nice amp.
 
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highend64 said:
I finally got the amp and so far it works and sound great. Do I need to worry about rebiasing or dc offset?
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Download the manual from Hi Fi Engine if you don't have one. I purchased my GFA-5500 new and a few years back I noticed one heat sink was a little warmer than the other. Checked the bias and one channel was a little off.

As a matter of caution I always take any used amplifier / receiver I get and 'open up the hood' and check for proper voltages before using.
 
TomD62 said:
Download the manual from Hi Fi Engine if you don't have one. I purchased my GFA-5500 new and a few years back I noticed one heat sink was a little warmer than the other. Checked the bias and one channel was a little off.

As a matter of caution I always take any used amplifier / receiver I get and 'open up the hood' and check for proper voltages before using.
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Thanks! I do have a multimeter handy but I feel uncomfortable messing inside the amplifier.
 
painttoad said:
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wrong hobby for you;)
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Actually his listening to music is a fine hobby for him. It's the messing around inside the 'no user serviceable parts inside' stickers that is a subset of the music listeners, especially here on the vintage and cheap ak where we want the good stuff, cheap and can't pay to get it fixed so have forced ourselves to dive in with the voltmeter and poke around. All this is fine until the probe slips, small spark no sound and one learns about minigrabbers.

enjoy the music highend 64, we got plenty of folks needing help with gear that got 'probed', me included. But I have minigrabbers now.
 
Blue Shadow said:
Actually his listening to music is a fine hobby for him. It's the messing around inside the 'no user serviceable parts inside' stickers that is a subset of the music listeners, especially here on the vintage and cheap ak where we want the good stuff, cheap and can't pay to get it fixed so have forced ourselves to dive in with the voltmeter and poke around. All this is fine until the probe slips, small spark no sound and one learns about minigrabbers.

enjoy the music highend 64, we got plenty of folks needing help with gear that got 'probed', me included. But I have minigrabbers now.
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sure, listening is fine but, buying unknown used and asking if he should service the amp and make sure it's up to spec while being uncomfortable messing around inside the amp means that he is overstepping the listening part of the hobby and moving into the next phase.

minigrabbers are a must have.
 
He didn't say he was going in, but it did read like that. Not that we can see if it is within specs without test equipment, implying a tech visit. But checking DC offset from the outside would be one thing he could do. Bias and all internal adjustments could be left for the pros of friends.
 
Sorry to bring the old thread back but I am planning on getting the Adcom GFA 5500 serviced for a check up to see if it is within specifications. I need to see if there is any reputable company that will service the amp in the Central California area? Does anyone know anybody in the area? Thanks
 
highend64 said:
I am about to purchase an Adcom GFA-5500 to replace a BK ST2140(which I dont have anymore). Currently I am without an amp but that is ok since I have been really busy and do have time to listen to my system. I will be getting it sometime this week. One of my questions is would it be a good idea to service the amp and see if it within specs before trying it out? Also, I know this amp was a Nelson Pass design. Does this model run Class A at up to so many watts? It would be powering a pair of Vandersteen 2ci speakers. Thanks
Click to expand...
From the Adcom service manual. It also states to make sure the amplifier is at room temperature before proceeding with adjustments.

INTRODUCTION
This service manual is intended to assist trained and qualified technical personnel in verifying the
performance of, adjusting, and repairing the ADCOM GFA-5500 amplifier. The procedures described here are
not intended for persons unfamiliar with the appropriate safety and test procedures.
TEST PROCEDURES
• All tests are performed with a 115V, low-distortion (less than 2% THD), AC-power source, 8-ohm
resistive load (except slew rate), and a signal source of not more than 600 ohms.
• An 80kHz low-pass filter is employed during THD distortion measurements.
• Signal-to-noise measurements are “A” weighted.
• Damping factor is measured by comparing the 1 watt output voltage with and without an 8 ohm load.
• Slew rate is measured with an inductive load, and is derived with a dual-time-based oscilloscope reading
the slope of a full power 5kHz square wave. DO NOT OPERATE THE AMPLIFIER AT FULL-POWER
SINE WAVE ABOVE 22kHz OR FULL-POWER SQUARE WAVE ABOVE 5kHz.
CORRECT BIAS ADJUSTMENT IS CRITICAL TO THE PERFORMANCE OF THIS AMPLIFIER.
MAXIMUM OUTPUT POWER, MINIMUM THD AND HEAT DISSIPATION ARE AFFECTED BY THE BIAS
SETTING AND MUST BE CORRECT TO MAINTAIN THE SONIC QUALITY AND LONGEVITY OF THE
AMPLIFIER.
BIAS ALIGNMENT
Prior to performing BIAS ALIGNMENT turn unit on and allow to idle for approximately 5 MINUTES before
attempting adjustments.
1. Connect millivoltmeter across emitter resistor R52 for left channel (R51 for right channel)
2. Adjust bias pot P2 until meter reads 50mV +/- 5mV
DC OFFSET ADJUSTMENT
1. Connect millivoltmeter across speaker output
2. Adjust DC offset pot P1 until meter reads 0mV +/- 10mV
 
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