Adcom GFA-555 mk1 Bypass Cap Process Check


2012-06-05 3:57 am

I am very grateful to this forum and have learned a lot by reading through most of the past 555 threads (even the 65+ page monster). The one mod that I am struggling to find clarity on is adding bypass caps to the driver and output boards. I understand the purpose but need confirmation on the process for this amp in particular. I have tried to dissect grainy old images and have referenced the mk2 and Hoppesbrain's work and schematics to put together what I think is correct.

If the electrical makes sense, I have a question about the mechanical process. Do I just drill small holes to pass the leads? I've not been able to find a good image of the backside of a modded board. I could run long jumpers but the photos I've seen have been much cleaner and flush to the board.

This is my first attempt at working on an amp so your confirmation is valued!



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Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi mzisman4,
I'm not worried about the capacitors on the output boards, but I do have concerns about the capacitors on the driver boards. What I want you to do is carefully check the ground paths where your added capacitors connect. You will be injecting supply noise into those grounds. Note that I have not looked at the board carefully, but I am of the opinion that the driver boards do not require any further bypassing than what the factory has designed in already. Remember that where ever the grounds of those capacitors go, that is where the supply noise goes as well. So make certain that you really do want those noise currents running through there.

On the driver board, it works fine to tie all four capacitor's ground leads directly to the star-point in the middle of the board, and then out to the star-ground of the amp. I would recommend fattening up the ground wire to 16ga.

The best thing to do though, is to run a separate wire for the bypass grounds, straight to the amp's star-ground. That's how I do it on my replacement input boards. That way, there are no signal grounds running over the same wire as the bypass grounds.

For the output module caps, I re-purpose a bit of the extra PCB trace coming off the end of the output bus. I cut about an inch off the end and use this as a ground connection for the cap, and to mount the ground wire. Hopefully you can see how in these photos.




2012-06-05 3:57 am
Anatech, I appreciate the insight. It's helpful as I begin my dive (decline?) into the hobby!

Phloodpants, Thank you so much the info here and on your site. The way you use the bottom of the board is really clever. Do the smaller caps (.1uF like in your custom schematic?) help protect the polarized caps?

I think I understand how to wire these up. I'll revise my images and repost for one more review.
unknow pads?

Hi guys is my first time posting here on this section I'm making a clone PCB for a friend and I don't know where this pads are going connected I follow the schematic and I figure it out that this will go connected like this but I need to be sure first here on this section let me know if this is correct? by the way this is only for personal use no to make it and sale it I know I know


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You've just got them backwards. Referring to the left channel... The terminal you've marked as "To star ground" actually goes to the left channel speaker output. This feeds the zobel network, as well as the negative feedback. (It connects to the 22.1K feedback resistor.) The other terminal you have marked as "Left channel out" goes to star ground. As OEM, the ground actually goes to the binding post ground terminals, but you can connect it to the star too, it makes little difference.