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Yet another Adcom GFA-535
Yet another Adcom GFA-535
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Old 12th November 2020, 05:11 AM   #21
Phloodpants is offline Phloodpants
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Join Date: Feb 2001
Location: Madison, WI
Oh hey! Super late reply, but why not?

Here's a copy-pasta from my GFA-555 bias adjustment procedure on my website, edited to apply to GFA-535 MK1

Bias Adjustment:

Bias spec is 5mV across the emitter resistors.

I recommend checking all 4 emitter resistors on each side. Take an average, it doesn’t need to be exact. Connect the negative probe of your meter to the speaker output and then probe each output transistor's emitter.

The GFA-535's bias has a rather pronounced positive thermal coefficient. That is, when it gets hot, the bias goes up. This makes bias adjustment kind of fussy, as the bias will fluctuate with the temperature of the room, and air circulation.

What I do is let the amp sit on the bench, with no clutter around it, with the cover on. From stone cold, I power-on and adjust bias to about 3mV. This will climb as the amp warms up a bit. After 15 minutes or so, I’ll then re-adjust to 5mV. Then I put the cover back on and let it sit for about an hour. Check, adjust, check, adjust…. lather, rinse, and repeat as many times as needed.

What I’m looking for is that the bias hangs around the target, at room temperature with good ventilation.

Once adjusted properly, the heatsinks should feel just a little warm to the touch, about 100F. You can use an infrared thermometer to make sure both sides are approximately the same.
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Old 30th November 2020, 03:42 AM   #22
topomouse is offline topomouse  United States
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Join Date: Jun 2015
Default Another Gaf-535

See if this link Adcom GFA-535 Rejuvenation and Mods - DIY • Canuck Audio Mart Hifi and Audio Forum
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Old 30th November 2020, 03:44 AM   #23
topomouse is offline topomouse  United States
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Join Date: Jun 2015
Default Another Gaf-535

See if this link Adcom GFA-535 Rejuvenation and Mods - DIY • Canuck Audio Mart Hifi and Audio Forum
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Old 27th December 2020, 02:52 AM   #24
BlueRob is offline BlueRob  Mexico
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Join Date: Dec 2008
Quote:
Originally Posted by Phloodpants View Post
Oh hey! Super late reply, but why not?

Here's a copy-pasta from my GFA-555 bias adjustment procedure on my website, edited to apply to GFA-535 MK1

Bias Adjustment:

Bias spec is 5mV across the emitter resistors.

I recommend checking all 4 emitter resistors on each side. Take an average, it doesn’t need to be exact. Connect the negative probe of your meter to the speaker output and then probe each output transistor's emitter.

The GFA-535's bias has a rather pronounced positive thermal coefficient. That is, when it gets hot, the bias goes up. This makes bias adjustment kind of fussy, as the bias will fluctuate with the temperature of the room, and air circulation.

What I do is let the amp sit on the bench, with no clutter around it, with the cover on. From stone cold, I power-on and adjust bias to about 3mV. This will climb as the amp warms up a bit. After 15 minutes or so, I’ll then re-adjust to 5mV. Then I put the cover back on and let it sit for about an hour. Check, adjust, check, adjust…. lather, rinse, and repeat as many times as needed.

What I’m looking for is that the bias hangs around the target, at room temperature with good ventilation.

Once adjusted properly, the heatsinks should feel just a little warm to the touch, about 100F. You can use an infrared thermometer to make sure both sides are approximately the same.
Hey Chris. I am rejuvenating this thread LOL. After the mods you help me with I have changed speakers a couple of times. Being a pair of Klipsch Heresy my current ones.... Super efficient and the Adcom makes them sound the best from the amps I have in my lineup. With this I started to notice a pop and/or squeak sound at turn-off.... When the caps drain I guess. To make the story short I opened the 535 again and check Bias again... Way to high so re-adjusted to 5mv. After that I checked DC offsets and noticed it is higher than I would like R=47mv L=14mv.
On the other hand with this speakers the buzz is evident in the R side even after the mods.

The first question is how can I reduce the DC at the outputs? Any ideas?

Cheers and Merry Christmas.
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Old 27th December 2020, 02:53 AM   #25
BlueRob is offline BlueRob  Mexico
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Quote:
Originally Posted by topomouse View Post
Better late than never... Thanks! I'll check that link.
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Old 28th December 2020, 09:12 AM   #26
Phloodpants is offline Phloodpants
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Join Date: Feb 2001
Location: Madison, WI
It didn't squeak before and now it does?

The squeak itself is perfectly normal for an Adcom. It's just because there is no speaker relay to mute the output when you turn the amp off. As the capacitors drain towards 0V, the feedback collapses at some point, things go all wobbly and it oscillates a bit. It's totally normal. Some Adcoms do it, some don't. But they don't tend to develop, or lose, a squeak. It should behave the same throughout its life, whether it squeaks or not.

And 47mV is normal for OEM, but very high if you have hand-matched transistors, or matched pairs purchased from me, and the offset shouldn't change over time.

Make sure you're shorting the input when you measure.

Not sure what to tell you, but maybe start by swapping the input transistors from the other channel and see if it gets better.
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