Adcom GFA-555 questions...

rtate · 16th March 2009, 05:31 PM

I have just purchased a GFA-555 and have just read 15 pages of threads on this amp but still have a couple of questions.

1)Amp serial # is 6709 can I determine the build date from this?

2)offset is 38Mv and 70Mv, is this a problem?

3)bias current measured at the fuse blocks is about 48Ma
Is this ok?

4)the psu caps are 15,000uf 80v, the measured rail voltage is 80vdc with no by-pass caps present at the cans.
There is a black crust around the vent holes of the caps.

I want to replace the ps caps.How much can I safely increase the capacitance?

I read long debates on By-passing the ps caps and I would like to try it for myself and report, however I would like some advice on by-pass cap values from other experience...

I also think that I will replace the electroltyics on the driver board.
Are there any other replacements/improvements that I should consider?

Thanks

rtate · 16th March 2009, 08:15 PM

As an update I measured the mv across the emitter resistors and
one channel was 7.5mv and the other measured 5.2mv

The service manual calls for 16mV.

That is quite a long way out.
I'm alittle leary of making such a large adjustment.
The amp works fine now but possibly alot better with the correct bias adjustment???

carlmart · 16th March 2009, 11:35 PM

Quote:

Originally posted by rtate
As an update I measured the mv across the emitter resistors and
one channel was 7.5mv and the other measured 5.2mv

The service manual calls for 16mV.

That is quite a long way out.
I'm alittle leary of making such a large adjustment.
The amp works fine now but possibly alot better with the correct bias adjustment???

You may have some improvements by setting the bias levels to factory ones. But do not expect major changes in audio quality.

That may come from improving some of the passive parts on it, particularly the capacitors, and perhaps by using supply regulators like the ones designed by Kit Ryan.

rtate · 17th March 2009, 11:26 PM

I really would like to up the power supply caps but I'm not sure how far I can safely go...

Suggestions???

infinia · 17th March 2009, 11:39 PM

No don't add more bulk C, the power on surge is already too much on those. You'll be burnin up more stuff.
If you have to tweek just read the past threads.

carlmart · 18th March 2009, 12:25 AM

Quote:

Originally posted by infinia
If you have to tweek just read the past threads.

That would be very wise.

WithTarragon · 18th March 2009, 01:05 AM

RE: Bypass Caps on PS
I used 22 or 44uf on each cap (i forget which). It does tighten up the bass a bit. BTW, I have never been able to find a clear explanation for this effect.

RE: bias
The trick on the bias, as you probably already know, is that the amp must be fully warmed up. The service manual (Adcom will e-mail you a copy), outlines the procedure for this using a power resistor.

Good Luck.

rtate · 18th March 2009, 02:34 AM

Thanks WithTarragon!! That's the kind of info I need!!
With 22 or 44uf can i assume that they were electrolytic caps??

I have some Wima 10uf 100v caps that I was thinking of trying..

So what is the benefit of using higher values??

WithTarragon · 18th March 2009, 03:12 PM

There are different philosophies on this one. I would not bother using a smaller size. There are some designs which will use a very small cap for the bypass also. If you do this the amp may actually oscillate. It would need to be located very close to the output. I would not bother.

Incidentally, some of the later ones in the 555ii series already have bypass caps.

Yes, these are polarized electrolytic caps. If you don't know what you are doing, then have a friend discharge the big caps for you (with something other than a screwdriver). These store huge voltages and it is not a joking matter (I don't recall if there are bleeder resistors or not).

The easiest thing to do is to get some bipolar (non-polar) electrolytics used for speaker crossovers. There is no need to spend more than a couple of bucks per cap

Good Luck

rtate · 18th March 2009, 03:56 PM

My amp is not the MK-II version and although the service manual schematic says that there are .1uf 100v film caps across the large caps there are none present. There are 3.9k 2watt bleeder resistors present.

I think that the large caps have been changed at some point and the by-pass caps were left off.

16th March 2009, 05:31 PM	#1
rtate diyAudio Member Join Date: Jan 2002 Location: Burlington, Ontario, Canada	Adcom GFA-555 questions... I have just purchased a GFA-555 and have just read 15 pages of threads on this amp but still have a couple of questions. 1)Amp serial # is 6709 can I determine the build date from this? 2)offset is 38Mv and 70Mv, is this a problem? 3)bias current measured at the fuse blocks is about 48Ma Is this ok? 4)the psu caps are 15,000uf 80v, the measured rail voltage is 80vdc with no by-pass caps present at the cans. There is a black crust around the vent holes of the caps. I want to replace the ps caps.How much can I safely increase the capacitance? I read long debates on By-passing the ps caps and I would like to try it for myself and report, however I would like some advice on by-pass cap values from other experience... I also think that I will replace the electroltyics on the driver board. Are there any other replacements/improvements that I should consider? Thanks

17th March 2009, 11:39 PM	#5
infinia diyAudio Member Join Date: May 2005 Location: Californication	No don't add more bulk C, the power on surge is already too much on those. You'll be burnin up more stuff. If you have to tweek just read the past threads. __________________ like four million tons of hydrogen exploding on the sun like the whisper of the termites building castles in the dust

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16th March 2009, 08:15 PM	#2
rtate diyAudio Member Join Date: Jan 2002 Location: Burlington, Ontario, Canada	As an update I measured the mv across the emitter resistors and one channel was 7.5mv and the other measured 5.2mv The service manual calls for 16mV. That is quite a long way out. I'm alittle leary of making such a large adjustment. The amp works fine now but possibly alot better with the correct bias adjustment???

17th March 2009, 11:26 PM	#4
rtate diyAudio Member Join Date: Jan 2002 Location: Burlington, Ontario, Canada	I really would like to up the power supply caps but I'm not sure how far I can safely go... Suggestions???

18th March 2009, 01:05 AM	#7
WithTarragon diyAudio Member Join Date: Jun 2007	RE: Bypass Caps on PS I used 22 or 44uf on each cap (i forget which). It does tighten up the bass a bit. BTW, I have never been able to find a clear explanation for this effect. RE: bias The trick on the bias, as you probably already know, is that the amp must be fully warmed up. The service manual (Adcom will e-mail you a copy), outlines the procedure for this using a power resistor. Good Luck.

18th March 2009, 02:34 AM	#8
rtate diyAudio Member Join Date: Jan 2002 Location: Burlington, Ontario, Canada	Thanks WithTarragon!! That's the kind of info I need!! With 22 or 44uf can i assume that they were electrolytic caps?? I have some Wima 10uf 100v caps that I was thinking of trying.. So what is the benefit of using higher values??

18th March 2009, 03:12 PM	#9
WithTarragon diyAudio Member Join Date: Jun 2007	There are different philosophies on this one. I would not bother using a smaller size. There are some designs which will use a very small cap for the bypass also. If you do this the amp may actually oscillate. It would need to be located very close to the output. I would not bother. Incidentally, some of the later ones in the 555ii series already have bypass caps. Yes, these are polarized electrolytic caps. If you don't know what you are doing, then have a friend discharge the big caps for you (with something other than a screwdriver). These store huge voltages and it is not a joking matter (I don't recall if there are bleeder resistors or not). The easiest thing to do is to get some bipolar (non-polar) electrolytics used for speaker crossovers. There is no need to spend more than a couple of bucks per cap Good Luck

18th March 2009, 03:56 PM	#10
rtate diyAudio Member Join Date: Jan 2002 Location: Burlington, Ontario, Canada	My amp is not the MK-II version and although the service manual schematic says that there are .1uf 100v film caps across the large caps there are none present. There are 3.9k 2watt bleeder resistors present. I think that the large caps have been changed at some point and the by-pass caps were left off.

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