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post #1 of 73 Old 03-25-2016, 07:43 AM - Thread Starter
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Adcom GFA-5500 troubleshooting

November 2019 Thread Update:

See posts 55, 62, and 63 for updated listening impressions. I executed on the upgrades offered by John Hillig at Musical Designs/Musical Concepts in the form of the LX Elite Mod with Mini-Platinum upgrade.

The results are just stunning.


I'm going to refer to the amp, in its current, highly modified state, as the GFA-"5500 MCEP".




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************************************************** **************
I just acquired a used Adcom GFA-5500 two channel amplifier. it is the non-IEC power cord version. I took the liberty of checking bias and DC offset; the bias was quite low at under 15mV on each channel (service manual says 50mV +/-5mV). I adjusted both and it's stabilized at just over 49mV on each channel.

Next, I adjusted DC offset. Right channel was fine at .7mV. Left was way off at 19mV. Now they're both hovering around 1mV after temp has stabilized and some current run through the amp.

My issue: bass, while much improved from my first listen and now very tight and well damped, seems underwhelming.

Using my ir thermometer, there is a running difference between heat sink temps; roughly 117 for the left channel and 110 degrees F for the right channel across the face of the heat sink.

My question: Is this an indicator that I need to start looking at cap replacement? If so, which ones; the smaller low capacity caps on top (there are 8) or the four primary beer-can sized filter caps underneath them?

I checked the fuses and they are all functional (look normal and had continuity according to my fluke meter), but I'm going to replace them anyway as they are a hodge podge of mismatched brands. I know I know....blame the OCD.

Any input is appreciated. If I need service, I'm in the Orlando, FL area and am willing to drive quite a ways to avoid absurd shipping costs. Recs for nearby businesses are welcome (or far away ones with reasonable shipping costs).

Thanks in advance, gents.



Oh, and as a note, I'm coming from an Arcam AVR 300 with a healthy amp section itself. Using it as a pre amp now.
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"If I had asked people what they wanted, they would have said 'faster horses'." -H. Ford | Krell Showcase 7.1/Anthem Statement D1 pre/pros | Musical Concepts' LX Elite Mini-Platinum Mod Adcom MOSFET GFA-5500 | Acurus/Mondial A200x3 | Bluesound Node 2/Apple TV 4K | Toshiba SD-9200 CD/DVD-A | Vizio 55" LED/LCD | L+R: Chane MTM Prototype | Chane A2.4 center | custom-finished Chane A5rx-c surrounds | Member: NATIA, LEVA, & AES.

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post #2 of 73 Old 03-25-2016, 07:53 AM
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I had my old GFA-5500 serviced here (see below). Excellent quick turnaround. Its a flat rate.


http://www.ebcelectronics.com/Adcom-...r-Service.html


My previous Adcom's were GFA-5500, GFA-5503 and GFA-2535L. They performed and sounded well but have since been sold and replaced.
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post #3 of 73 Old 03-25-2016, 10:36 AM
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If one channel is getting warmer than the other, it sounds like the BIAS is off, i bet both channels need to be re-biases probably need some service caps, resistors.. Maybe some contact cleaning such as the ones that go to the LARGE psu caps..
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post #4 of 73 Old 03-25-2016, 12:11 PM
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Originally Posted by dashpuppy View Post
If one channel is getting warmer than the other, it sounds like the BIAS is off, i bet both channels need to be re-biases probably need some service caps, resistors.. Maybe some contact cleaning such as the ones that go to the LARGE psu caps..

In general, the manufacturers of most power amps tend to set the idle current for the amplifier pretty low to be super-safe; too much so IMO.

As an example, one 125 watt power amp I had for a long time had the bias set at the factory so that each DC power supply was supplying around 45 milliamps to the amplifier with no input signal.

This minimized Class A operation and maximized Class B operation.

I adjusted the amplifier bias control for 70 milliamperes of idle current and found that the sound quality of the amplifier improved considerably, and the amplifier ran with heat sinks only slightly warm; not really hot at all. This improved the power transistors's linearity near the crossover point in the output stage and increased the percentage of the time that the amplifier ran Class A, lowering overall distortion.

In most amplifiers there are fuses in the DC power supply lines, making it easy to remove the fuse and substitute a current meter while making the bias adjustment.

The amount of idle current will be different for amplifiers, depending on their power rating, but in many cases you will find that increasing the bias setting for a higher idle (no-signal) current will improve sound quality. Just be sure that the change does not cause the amp to run too hot.
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post #5 of 73 Old 03-25-2016, 12:38 PM - Thread Starter
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Quote:
Originally Posted by dashpuppy View Post
If one channel is getting warmer than the other, it sounds like the BIAS is off, i bet both channels need to be re-biases probably need some service caps, resistors.. Maybe some contact cleaning such as the ones that go to the LARGE psu caps..
Bias has been adjusted and is virtually identical between channels.

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post #6 of 73 Old 03-25-2016, 01:15 PM - Thread Starter
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Quote:
Originally Posted by commsysman View Post
In general, the manufacturers of most power amps tend to set the idle current for the amplifier pretty low to be super-safe; too much so IMO.

As an example, one 125 watt power amp I had for a long time had the bias set at the factory so that each DC power supply was supplying around 45 milliamps to the amplifier with no input signal.

This minimized Class A operation and maximized Class B operation.

I adjusted the amplifier bias control for 70 milliamperes of idle current and found that the sound quality of the amplifier improved considerably, and the amplifier ran with heat sinks only slightly warm; not really hot at all. This improved the power transistors's linearity near the crossover point in the output stage and increased the percentage of the time that the amplifier ran Class A, lowering overall distortion.

In most amplifiers there are fuses in the DC power supply lines, making it easy to remove the fuse and substitute a current meter while making the bias adjustment.

The amount of idle current will be different for amplifiers, depending on their power rating, but in many cases you will find that increasing the bias setting for a higher idle (no-signal) current will improve sound quality. Just be sure that the change does not cause the amp to run too hot.
This amp is supposed to be biased pretty heavily into class-A operation for the first 10 or 15 watts according to posts by Nelson Pass over on DIYaudio. According to the service manual, 50mV of bias current is the factory spec. Wouldn't increasing the bias cause it to run hotter at idle and not cooler? Also, honest question: what is too hot for this application?

"If I had asked people what they wanted, they would have said 'faster horses'." -H. Ford | Krell Showcase 7.1/Anthem Statement D1 pre/pros | Musical Concepts' LX Elite Mini-Platinum Mod Adcom MOSFET GFA-5500 | Acurus/Mondial A200x3 | Bluesound Node 2/Apple TV 4K | Toshiba SD-9200 CD/DVD-A | Vizio 55" LED/LCD | L+R: Chane MTM Prototype | Chane A2.4 center | custom-finished Chane A5rx-c surrounds | Member: NATIA, LEVA, & AES.
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post #7 of 73 Old 03-25-2016, 03:48 PM
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Quote:
Originally Posted by commsysman View Post
In general, the manufacturers of most power amps tend to set the idle current for the amplifier pretty low to be super-safe; too much so IMO.

As an example, one 125 watt power amp I had for a long time had the bias set at the factory so that each DC power supply was supplying around 45 milliamps to the amplifier with no input signal.

This minimized Class A operation and maximized Class B operation.

I adjusted the amplifier bias control for 70 milliamperes of idle current and found that the sound quality of the amplifier improved considerably, and the amplifier ran with heat sinks only slightly warm; not really hot at all. This improved the power transistors's linearity near the crossover point in the output stage and increased the percentage of the time that the amplifier ran Class A, lowering overall distortion.

In most amplifiers there are fuses in the DC power supply lines, making it easy to remove the fuse and substitute a current meter while making the bias adjustment.

The amount of idle current will be different for amplifiers, depending on their power rating, but in many cases you will find that increasing the bias setting for a higher idle (no-signal) current will improve sound quality. Just be sure that the change does not cause the amp to run too hot.
Some, not all though.
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post #8 of 73 Old 03-25-2016, 10:31 PM - Thread Starter
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Quote:
Originally Posted by commsysman View Post
In general, the manufacturers of most power amps tend to set the idle current for the amplifier pretty low to be super-safe; too much so IMO.

This minimized Class A operation and maximized Class B operation.

I adjusted the amplifier bias control for 70 milliamperes of idle current and found that the sound quality of the amplifier improved considerably, and the amplifier ran with heat sinks only slightly warm; not really hot at all. This improved the power transistors's linearity near the crossover point in the output stage and increased the percentage of the time that the amplifier ran Class A, lowering overall distortion.
Okay, this worked. I adjusted bias such that it is stable at roughly 72mV for both channels.

I gained about 6 degrees for the running temp of both heat sinks and the chassis increased about 4 degrees. So I placed a low speed 120mm fan on top of the grate and the chassis is now running cooler than with bias set at factory 50mV. With the fan, heat sink temp dropped to within 3 degrees of temp at 50mV. Fan has an inline resistor and is inaudible from more than a few inches.

Bass fullness and punch HAVE ARRIVED. I'm sure I'm due for cap replacement, but this buys me some time. And it sounds excellent while doing it.

Thank you, commysysman!


"If I had asked people what they wanted, they would have said 'faster horses'." -H. Ford | Krell Showcase 7.1/Anthem Statement D1 pre/pros | Musical Concepts' LX Elite Mini-Platinum Mod Adcom MOSFET GFA-5500 | Acurus/Mondial A200x3 | Bluesound Node 2/Apple TV 4K | Toshiba SD-9200 CD/DVD-A | Vizio 55" LED/LCD | L+R: Chane MTM Prototype | Chane A2.4 center | custom-finished Chane A5rx-c surrounds | Member: NATIA, LEVA, & AES.

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post #9 of 73 Old 03-26-2016, 09:00 AM
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Okay, this worked. I adjusted bias such that it is stable at roughly 72mV for both channels.

I gained about 6 degrees for the running temp of both heat sinks and the chassis increased about 4 degrees. So I placed a low speed 120mm fan on top of the grate and the chassis is now running cooler than with bias set at factory 50mV. With the fan, heat sink temp dropped to within 3 degrees of temp at 50mV. Fan has an inline resistor and is inaudible from more than a few inches.

Bass fullness and punch HAVE ARRIVED. I'm sure I'm due for cap replacement, but this buys me some time. And it sounds excellent while doing it.

Thank you, commysysman!

I'm glad that worked out for you. In general, the rule of thumb is that if you can press the palm your hand against the hottest point on the heat sinks and leave it there for a period of time, it is not running too hot.
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post #10 of 73 Old 03-26-2016, 09:04 AM
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Originally Posted by BufordTJustice View Post
This amp is supposed to be biased pretty heavily into class-A operation for the first 10 or 15 watts according to posts by Nelson Pass over on DIYaudio. According to the service manual, 50mV of bias current is the factory spec. Wouldn't increasing the bias cause it to run hotter at idle and not cooler? Also, honest question: what is too hot for this application?

If they already have it biased fairly high, maybe it should be left there.

The rule of thumb is that you should be able to place the palm of your hand on the hottest spot on the amplifier and leave it there comfortably for a while.

If it is too hot for that, you may need to back off a bit on the bias.
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post #11 of 73 Old 03-26-2016, 06:51 PM - Thread Starter
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If they already have it biased fairly high, maybe it should be left there.

The rule of thumb is that you should be able to place the palm of your hand on the hottest spot on the amplifier and leave it there comfortably for a while.

If it is too hot for that, you may need to back off a bit on the bias.
I can leave my hand on any part of the heat sink or amp indefinitely even without the fan present. I should be good.

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post #12 of 73 Old 03-28-2016, 06:45 AM
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Hmm....


I picked up one of these last year and had to adjust bias/DC offset as well. I set everything to factory spec based on the service manual as mentioned already. I have been running it untouched since then. My speakers are Ascend Sierra-2's crossed at 80Hz through my Rythmik sub so the amp is getting frequencies which are rolled off below 80Hz at 12db/octave. The sub is doing the heavy lifting for low bass. Would re-biasing to 70+mv be beneficial in this case?
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Hmm....


I picked up one of these last year and had to adjust bias/DC offset as well. I set everything to factory spec based on the service manual as mentioned already. I have been running it untouched since then. My speakers are Ascend Sierra-2's crossed at 80Hz through my Rythmik sub so the amp is getting frequencies which are rolled off below 80Hz at 12db/octave. The sub is doing the heavy lifting for low bass. Would re-biasing to 70+mv be beneficial in this case?
nope..
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post #14 of 73 Old 03-28-2016, 09:39 AM - Thread Starter
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Hmm....


I picked up one of these last year and had to adjust bias/DC offset as well. I set everything to factory spec based on the service manual as mentioned already. I have been running it untouched since then. My speakers are Ascend Sierra-2's crossed at 80Hz through my Rythmik sub so the amp is getting frequencies which are rolled off below 80Hz at 12db/octave. The sub is doing the heavy lifting for low bass. Would re-biasing to 70+mv be beneficial in this case?
Case by case basis.

First, consider that, although I have technical knowledge, I'm not Nelson Pass or John Curl.

Second, consider that I'm using a 120mm cooling fan on the top grate that moves at least 20 CFM (this 800 RPM fan with an additional in-line resistor added: http://www.newegg.com/Product/Produc...82E16835185004).

Third, consider that I keep my house no warmer than 73 degrees F, I don't smoke, and I have no pets or kids.

Fourth, I'm taking a gamble and I know it. This could possibly fry my amp (though I highly doubt this to be the case). Caveat emptor, as it were.


I've had several very high volume 2.0 listening sessions (Led Zep's Celebration Day in 48k/24bit FLAC...the entire album....among several others) and the heat sink and chassis temps are stable even without the fan in place. With the fan, the running temps of the internal boards, caps, transformer, and case are slightly cooler than before increasing to 70mV. The heat sinks are running slightly warmer by a few degrees.

I'm very pleased with the audible improvements. I found that, on my amp, adjusting bias after amp has been on and idle for at least 30 minutes to about 68mV results in a final bias reading of right-at 72mV or slightly higher. Understand that it's likely going to vary slightly after you put some current (and heat) into the amp, post adjustment. I have checked after 3 movies and at least 6 hours of music in 2.0 and mine remains stable at 72mV. But, and I can't emphasize this enough, this is outside of the design parameters of the amp from the factory and you're assuming all risk. At a minimum, I rec using a Noctua ultra low noise/speed 120mm or larger fan situated directly on top of the amp. I position mine toward the rear, centered over the smaller caps.

A quality and silent cooling solution would be the following:

http://www.amazon.com/Premium-Quality-Cooling-NF-A14-ULN/dp/B00AA89BN4/ref=sr_1_1?ie=UTF8&qid=1459175267&sr=8-1&keywords=noctua+nf-a14+uln


and

http://www.amazon.com/Coolerguys-100-240v-Molex-Power-Adapter/dp/B000MGG6SC/ref=pd_sim_147_30?ie=UTF8&dpID=41Wg4Jh1iQL&dpSrc=s ims&preST=_AC_UL160_SR134%2C160_&refRID=1MYSESQRYM EEQZR9ZM4H

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post #15 of 73 Old 03-28-2016, 11:10 AM
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Yeah, I learned enough to be dangerous! One heat sink was not warming up when I first got it, so began the quest for knowledge. After speaking with an actual person working at Adcom and describing the condition, he said that bias needed adjustment and stressed how critical it was. After calling several repair shops he had recommended and not being pleased with the people I spoke with, nor the cost involved to ship it both ways on top of the service charge, I found the service manual online and borrowed a multimeter.


I wonder if it being way out of adjustment had anything to do with the previous owner trading it in for a Parasound Halo A21?
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Yeah, I learned enough to be dangerous! One heat sink was not warming up when I first got it, so began the quest for knowledge. After speaking with an actual person working at Adcom and describing the condition, he said that bias needed adjustment and stressed how critical it was. After calling several repair shops he had recommended and not being pleased with the people I spoke with, nor the cost involved to ship it both ways on top of the service charge, I found the service manual online and borrowed a multimeter.


I wonder if it being way out of adjustment had anything to do with the previous owner trading it in for a Parasound Halo A21?
It's certainly possible the trade could have been a result of that. I've heard the A21 and it is a fine amplifier, but for most common loudspeakers with easy loads, I doubt much of a difference should be audible between the two.

Honestly, if you're happy with the sound you're getting, I'd leave it alone or make VERY small adjustments.

My amp sat unused for at least 3 years according to the seller. I suspect that, coupled with its original date of manufacture, may have had something to do with my issue. Regardless, I monitor with my IR thermometer during every listening session.

"If I had asked people what they wanted, they would have said 'faster horses'." -H. Ford | Krell Showcase 7.1/Anthem Statement D1 pre/pros | Musical Concepts' LX Elite Mini-Platinum Mod Adcom MOSFET GFA-5500 | Acurus/Mondial A200x3 | Bluesound Node 2/Apple TV 4K | Toshiba SD-9200 CD/DVD-A | Vizio 55" LED/LCD | L+R: Chane MTM Prototype | Chane A2.4 center | custom-finished Chane A5rx-c surrounds | Member: NATIA, LEVA, & AES.
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post #17 of 73 Old 03-29-2016, 08:51 AM
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It's certainly possible the trade could have been a result of that. I've heard the A21 and it is a fine amplifier, but for most common loudspeakers with easy loads, I doubt much of a difference should be audible between the two.

Honestly, if you're happy with the sound you're getting, I'd leave it alone or make VERY small adjustments.

My amp sat unused for at least 3 years according to the seller. I suspect that, coupled with its original date of manufacture, may have had something to do with my issue. Regardless, I monitor with my IR thermometer during every listening session.
I think I'm ok with the way it is, no need to run the boost higher. (Already thinking of doing that in my daily driver!)
You have me thinking of picking up a IR thermometer now, as if I need something else to spend money on.


I found a company in Colorado that will take the GFA-5500 and "improve" it by replacing the factory "spec'd-for-budgetary-constraints" components with superior parts. I think the price was upwards of $400 plus shipping. I think I'll wait until things start to fail and then go from there.
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post #18 of 73 Old 03-29-2016, 08:58 AM
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Just throwing a pic in the mix. This is when I had the hood open after blowing the dust & cobwebs out with my leaf blower in the garage! I then warmed it up and did the adjustments.
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post #19 of 73 Old 03-29-2016, 10:18 AM - Thread Starter
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I think I'm ok with the way it is, no need to run the boost higher. (Already thinking of doing that in my daily driver!)
You have me thinking of picking up a IR thermometer now, as if I need something else to spend money on.


I found a company in Colorado that will take the GFA-5500 and "improve" it by replacing the factory "spec'd-for-budgetary-constraints" components with superior parts. I think the price was upwards of $400 plus shipping. I think I'll wait until things start to fail and then go from there.
I found that service as well. Maybe once mine fails.

Yeah, check Amazon for IR thermometers. They're extremely handy.

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I found that service as well. Maybe once mine fails.

Yeah, check Amazon for IR thermometers. They're extremely handy.

I will have to check that out.


Not so sure about the amp soup-up. Differences among amps are slight as it is, so you would probably have to have an unmodified unit on hand to compare with the modified one in an A/B test to hear any changes.
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post #21 of 73 Old 03-29-2016, 12:31 PM
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Quote:
Originally Posted by HOTDIGITY View Post
I will have to check that out.


Not so sure about the amp soup-up. Differences among amps are slight as it is, so you would probably have to have an unmodified unit on hand to compare with the modified one in an A/B test to hear any changes.
Changing caps and other things will improve sound, remember it's a old amplifier
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post #22 of 73 Old 03-29-2016, 01:19 PM
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Originally Posted by dashpuppy View Post
Changing caps and other things will improve sound, remember it's a old amplifier
I suppose there's some logic to that, but how noticeable an improvement would there be?
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post #23 of 73 Old 03-31-2016, 06:16 AM
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Ok, so having my curiosity sparked by this thread I contacted a guy who performs service on Adcom equipment, such as amp repairs and rebuilds. After a few email exchanges he remarked that bias should not really drift with age. If it does it is usually due to the Zener diodes leaking or input Mosfets cooking which can cause a loss in bias. Distortion runs high with improper bias. So, the fact that we had to adjust bias could mean one or both of those conditions exist.


I'm on the fence. I can certainly keep running the amp as is and take my chances, or be proactive(paranoid!) and cough up the cash to ship the amp to him and have him do his thing, then pay for return shipping. Now I'm wrestling with doing this or staying on course to save my cash for something else I've been planning for over 1 year.


The guy does awesome looking work based on his webpage.
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post #24 of 73 Old 03-31-2016, 09:15 AM
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Quote:
Originally Posted by HOTDIGITY View Post
Ok, so having my curiosity sparked by this thread I contacted a guy who performs service on Adcom equipment, such as amp repairs and rebuilds. After a few email exchanges he remarked that bias should not really drift with age. If it does it is usually due to the Zener diodes leaking or input Mosfets cooking which can cause a loss in bias. Distortion runs high with improper bias. So, the fact that we had to adjust bias could mean one or both of those conditions exist.


I'm on the fence. I can certainly keep running the amp as is and take my chances, or be proactive(paranoid!) and cough up the cash to ship the amp to him and have him do his thing, then pay for return shipping. Now I'm wrestling with doing this or staying on course to save my cash for something else I've been planning for over 1 year.


The guy does awesome looking work based on his webpage.
Now that you have the bias set, check it again in one month to see if it's drifting. If it's stable, button it back up and enjoy. Replacing the electrolytic caps is a good idea on old amps, but it's not absolutely necessary.
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post #25 of 73 Old 03-31-2016, 09:42 AM
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I was thinking of checking again. It's been almost 1 year since I did the adjustment (Bought it last April).
I will need to borrow the meter again from my coworker. If it s drifted it may be time to bite the bullet.
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post #26 of 73 Old 04-01-2016, 09:28 AM - Thread Starter
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I agree that bias shouldn't drift much over time. However, one can get very different readings depending on how warm the amp is and how much current is pulled through it after adjustment and before verification readings are taken.

My bias has remained stable at 72.4. However, I'm going to back my bias down to about 60mV and see how it sounds. Heat generation is stable, but a little hotter than I'd like for anything outside a pure Class-A amp. Granted this is biased into class A for the first 10-15 watts according to Nelson Pass. So I'll run it at 60mV for a while and report back.

72.4mV is nearly 50% higher than the factory spec, and that got to me from a design standpoint.

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post #27 of 73 Old 04-01-2016, 10:11 AM
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yeah, sort of like higher resistance with temperature rise.
I should double check it anyway just to make sure it is set correctly once it is at operating temp.


Before I adjusted it initially one channel did not even get warm. I think the bias on that side measured below 10mv, but it's been a while and I'm going from memory.
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post #28 of 73 Old 04-01-2016, 03:04 PM
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I purchased the three-channel version, the then new GFA-5503, for our theater system about 1997. It is still in the system today and has always provided bullet-proof performance. After a loud action film the amp will routinely become very warm at the top of the case and on the heatsinks. One can still touch those areas but it is not comfortable to do so. This is normal.

Also note that the amp becomes sonically smoother after it sits powered up (idling) or played for about 45 to 60 minutes.
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Music room: Cary SLI-80 tube integrated amplifier, McIntosh MA6500 integrated amplifier, Quad 99 preamp, Quad 909 power amp, Acoustic Research AR9 loudspeakers, Yamaha CD-N500 CD player, Teac UD-503 DSD DAC, Phase Linear 8000II linear-tracking turntable.
Theater room: Panasonic 65S60 plasma television; Yamaha RX-A2020 (preamp section); Adcom GFA-5503 and GFA-5400 amplifiers; Polk LSi25, LSiC, and LSiF/X loudspeaker system; Velodyne FSR-18 servo-subwoofer.
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post #29 of 73 Old 04-03-2016, 08:43 PM
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Ok, I picked up a meter and IR thermometer today at Harbor Freight.
I let the the amp get nice and warm then opened it up. Bias was slightly higher on the left channel @ 60mv vs lower 50's for the right. Set both to the max tolerance of 55mv. I could only get DC offset to around 2 per channel but well within the tolerance of +\-10.
I listened to some Tidal for an hour and grabbed the thermometer. Not sure where on the heat sink to point it but I got a max of about 101 I think, varying slightly from left to right. I will recheck in a month or two to see where things sit.
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post #30 of 73 Old 04-15-2016, 11:17 AM - Thread Starter
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Out with the old. In with the new. Feedback network cap replacement.


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