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Adcom GFA-5002 increase ClassA bias on purpose
Adcom GFA-5002 increase ClassA bias on purpose
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Old 18th August 2019, 02:16 PM   #1
sckramer is offline sckramer  United States
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Default Adcom GFA-5002 increase ClassA bias on purpose

First off ***WARNING*** this is an experiment/for personal enjoyment and could be wrong and dumb, please don't do it.

I took this perfectly workable amp, and purposely upped the bias slowly by 1watt increments alternating left/right channel until I got to 90watts* (45watts a channel classA vs 6), up from 12watts out the wall factory.
Stopped increasing when the heatsinks hit 55degC idle and stable, heat was my limiter--> increase from fairly cold idle.

Why? Because this amp is an old backup, sounds kinda meh and I've been exposed to new amps and how they sound vs my classD.

Anyway this changed the sound, now has sweet dense tone, opened it way up, complex resolved transients and is fun to listen to. It does not sound like added distortion** or anything like that.
Is it just super warmed up, or more ClassA operation?! dunno YMMV


Could have dreamt it but, always thought about doing this because somewhere I swear Nelson Pass said why not crank the bias until the heatsinks are hot on his smaller Adcoms for more classA operation.

DC offset is still 14mV, 20mV -- that didn't really change. They both shift around +/- 3mV

Also I don't know the test point to properly bias it to factory, maybe someone has a snippet on how to do that on the 5002?

*by watching the power draw from the wall
**read you can check this by running a sine wave in and check the output sine for clipping, I am lazy and did not do this.

System: YggdrasilA2(UNISON/digionesig)-->Jenson 1:1 bal/se-->freya passive-->GFA5002-->Elac adante/debut 6.2 / MLdescent i x2

Thanks!!
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Last edited by sckramer; 21st October 2019 at 11:55 PM.
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Old 18th August 2019, 04:38 PM   #2
Fsatsil is offline Fsatsil  United States
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The GFA-5002 looks to be an updated home theater version of the GFA-535 with a bit less power but some added HT features. These little amps sound pretty good as long as you donít ask them to drive difficult speaker loads at insane volumes. Iíve attached the factory bias instructions as well as a picture of a GFA535 showing the test points. If your amp doesnít have test points you would connect your meter to one of the emitter resistors instead.
IMO there is little if anything besides heat to be gained by cranking up the bias on these Adcom amps with the BJT outputs.
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Old 19th August 2019, 06:11 PM   #3
Nelson Pass is offline Nelson Pass  United States
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Adcom GFA-5002 increase ClassA bias on purpose
Quote:
Originally Posted by Fsatsil View Post
IMO there is little if anything besides heat to be gained by cranking up the bias on these Adcom amps with the BJT outputs.
Of course it's worth the experiment. Historically I have seen examples of
BJT amplifiers that have higher measured thd at bias higher than "optimum",
but they often sounded better for it, even if it produces a bit of a hump in
the power vs distortion curves. Good example would be the Ayre power
amplifier.
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Old 19th August 2019, 06:18 PM   #4
sckramer is offline sckramer  United States
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Thanks!

Also looking around more do these Adcoms really use those JRC4558 opamps on the input stage (ugh)? Hoping maybe it's to support the auto-on audio sense circuit or drive the pre-out feature!
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Old 19th August 2019, 06:20 PM   #5
Nelson Pass is offline Nelson Pass  United States
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Adcom GFA-5002 increase ClassA bias on purpose
You will appreciate that companies do not inform me of additions and improvements
after they have paid for a design.

Probably think I'll send them another invoice.....

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Old 19th August 2019, 06:23 PM   #6
sckramer is offline sckramer  United States
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Quote:
Originally Posted by Nelson Pass View Post
Of course it's worth the experiment. Historically I have seen examples of
BJT amplifiers that have higher measured thd at bias higher than "optimum",
but they often sounded better for it, even if it produces a bit of a hump in
the power vs distortion curves. Good example would be the Ayre power
amplifier.
Interesting-- Thank You!
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Old 19th August 2019, 06:42 PM   #7
sckramer is offline sckramer  United States
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Quote:
Originally Posted by Nelson Pass View Post
You will appreciate that companies do not inform me of additions and improvements
after they have paid for a design.

Probably think I'll send them another invoice.....

Too Funny!!!

I just looked to see if Adcom is in business, and there are?!

...Like the 5802 vibe on this beast... but c'mon, off center with the gold plate?
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Last edited by sckramer; 19th August 2019 at 06:51 PM.
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Old 20th August 2019, 10:39 PM   #8
Fsatsil is offline Fsatsil  United States
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These old Adcom amps are IMO some of the best deals available on the used market right now. The big 200w+ amps like the Gfa-555 are still a bit pricey but the smaller 100w amps like the 545 and 5400 are often available for around $150 in excellent condition.
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Old 11th September 2019, 04:42 PM   #9
Phloodpants is offline Phloodpants
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No worries, that's just part of the protection circuit! Quite impressive, really. They are used to protect against DC offset on the input.

The preamp-out is simply tied in parallel to the input; no buffering.

The circuit is essentially identical to the old 535 MK1, though using a transistor-based biasing scheme instead of Nelson's thermistor-based setup. I don't really know if that's an improvement or not.

I like how they've moved the power supply caps onto the amplifier board itself! That tightens up the loops considerably. On the old 535, the star-ground is way back at the power supply board.

Quote:
Originally Posted by sckramer View Post
Thanks!

Also looking around more do these Adcoms really use those JRC4558 opamps on the input stage (ugh)? Hoping maybe it's to support the auto-on audio sense circuit or drive the pre-out feature!
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Old 12th September 2019, 02:11 AM   #10
dBel84 is offline dBel84  United States
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I own the Mosfet based 5200 and 5300 and have increased the bias on both amps to get the heatsinks around 40 degrees.

Neither sound near as good as the M2X or Sissy SIT but they really hold their own regardless. Easily the best second hand bargains on the market .

I saw a 5400 for $115 a week or so ago and it took great will power not to pick it up but I knew it would merely sit on my shelf and hoped that someone who would actually use it would pick it up.

..dB
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