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Yet another Adcom GFA-535
Yet another Adcom GFA-535
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Old 23rd March 2020, 07:09 PM   #11
BlueRob is offline BlueRob  Mexico
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Quote:
Originally Posted by Phloodpants View Post
@BlueRob Thanks for the compliment.

No, I'm not suggesting cutting those two bypass caps out. In the diagram, I drew two arrows that read "Bodge Wire" A wire is run from the bottom of R901 & R904, and a scraped-away portion of the PCB. This routes the return currents from the charging pulses of the bypass cap where it belongs: On the other side of the analog ground reference points.
Sorry I didn't explain myself correctly. When I said cutting the board I meant cutting or interruption of the track that goes to C905 and C908 as you marked. Going that way by cutting the track in the Xmarks will still leave the issue with C907 and C906 since these will not be benefited from the track cutting... At least that is what we forsee. And thus moving all bypass caps to the bottom of the boad directly to the main caps leads might be better.

Saludos!
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Old 24th March 2020, 12:03 AM   #12
Phloodpants is offline Phloodpants
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C906 and C907 already have their ground return currents correctly separated, on the other side of the star-ground point. (The center screw.)

The drain resistors also need to have their return currents moved to the other side of the star. The bodge wire takes care of that, and the bypass caps. Whatever you do, the two ground reference leads coming from the amp board should never have to share a path back to star-ground, with any bypass caps or drain resistors. The return currents of the bypass caps and drain resistors need to be routed to the "dirty side" which is in this example, anything above that center screw.
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Old 24th March 2020, 12:07 AM   #13
Phloodpants is offline Phloodpants
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The purpose of the track cutting isn't so much to shorten the paths, but to route the currents to the correct side of the star.

Yes, you can simply solder the bypass caps to the capacitor terminals on the bottom. This is slightly better, due to the shorter path, and therefore less inductance.

But you still have to re-route those drain resistor currents. R901 and R904.
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Old 25th March 2020, 06:04 PM   #14
BlueRob is offline BlueRob  Mexico
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Quote:
Originally Posted by Phloodpants View Post
The purpose of the track cutting isn't so much to shorten the paths, but to route the currents to the correct side of the star.

Yes, you can simply solder the bypass caps to the capacitor terminals on the bottom. This is slightly better, due to the shorter path, and therefore less inductance.

But you still have to re-route those drain resistor currents. R901 and R904.
Thanks again for the explanation! Will it be OK to do the bridge wire from the removed speaker terminal holes to the lead of the resistors? Since I will change those crapy speakers terminals for a decent pair of binding post I will have those connections points available for the bridge on each side. Please refere to the image. I want to make sure it's OK before proceeding.

Saludos!
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Old 26th March 2020, 05:36 AM   #15
Phloodpants is offline Phloodpants
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Nope, because then the return currents for those caps would now be polluting the return path for the speaker wire. Your binding post negative wires should go where the originals connected. Just scrape away some of the solder mask in the spot by the arrow.
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Old 28th March 2020, 04:08 AM   #16
BlueRob is offline BlueRob  Mexico
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Originally Posted by Phloodpants View Post
Nope, because then the return currents for those caps would now be polluting the return path for the speaker wire. Your binding post negative wires should go where the originals connected. Just scrape away some of the solder mask in the spot by the arrow.
Noted! I'm finished with the power supply board, cutting where indicated... I opted to use a Dremel and go through the whole board instead of just the copper track.
Bridge wires installed aled as indicated. Decided to move bypass caps under main caps and replaced bleed resistors with a more robust option but left them in their original location.

L and R boards all done also so now it's time for cables which brings me to the first question. Since I'm replacing RCA input connectors I will relocate them to the center of the chasis. For these input cables I'm planning to use a shielded two conductor wire. Obviously each conductor goes from the RCA +&- to one of the boards (1&E1)respectively . The mesh wire shield on the RCA side will be connected also to the negative and at the board end this mesh shield will not be connected at all, only the conductors. Is that the best way? Hope that crude explanation make sense LOL
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Last edited by BlueRob; 28th March 2020 at 04:11 AM.
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Old 29th March 2020, 06:11 AM   #17
Phloodpants is offline Phloodpants
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Cool, power supply board looks good!

Connect the RCA shields to the negative terminal on the amp boards. Their grounds will be properly routed, as long as you connect point E2 on the amp board, back to star ground at those large-ish holes on the supply board. You want the amp's signal ground reference to match the RCA shields.
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Old 4th April 2020, 07:54 PM   #18
BlueRob is offline BlueRob  Mexico
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Originally Posted by Phloodpants View Post
Cool, power supply board looks good!

Connect the RCA shields to the negative terminal on the amp boards. Their grounds will be properly routed, as long as you connect point E2 on the amp board, back to star ground at those large-ish holes on the supply board. You want the amp's signal ground reference to match the RCA shields.
Chris, first of all I want to thank you very much for sharing your knowledge, that was invaluable information. I completed the amp with the mods. First tested the Power Supply. TBH I was scared to hear caps popping, sparks flying and smoke raising but no, first power up after surgery without issues . Measured B+ B- voltages and looked OK at both boards. Then I installed the fuses on the R board and did an initial BIAS adjustment. Then the same on L board. Let the thing marinate and continued to adjust it when it stabilized more or less.

Listen Session and Initial Impressions:
Where to start....the thing is quiet, minimum background noise (hum) you have to put your ear in the speaker to listen to it. Tried my usual flavors (rock & Jazz) using my current and loved NAD3120 as preamp.
Well the ADCOM doesnt make me as happy as my NAD. There is a lack of bottom end, mids are not well defined and the top end is decent, overall the Adcom in my opinion is not on par with the NAD3120, maybe Adcom takes the lead only in channel separation but that is it. I will give it more burn-in time but I dont think it will live to my expectations. I have heard much praise about the NAD but being my first "serious" amp it is the first time I get to compare it.

Next steps in the ADCOM as per previous comments is to match those transistors. I will need to source or DIY the means to do so at some point or buy matched pairs. Also I did not changed the 331J caps since I did not have any decent cap of that exact value to do so, not sure if that cap will make a hell of a difference to improve much the SQ.

On the other hand my father has offered his F5 Pass project, eager to star with that one soon.

Saludos!

Rob.
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Last edited by BlueRob; 4th April 2020 at 08:10 PM.
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Old 4th April 2020, 09:06 PM   #19
Phloodpants is offline Phloodpants
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Nice!

The matched transistors make a fairly substantial improvement to the sound, though I don't think it would affect the bass much. I'm surprised you don't find the bass satisfying; most people find that it's excellent despite it's small size. I suppose it depends on the speakers too.
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Old 5th April 2020, 09:01 PM   #20
BlueRob is offline BlueRob  Mexico
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Quote:
Originally Posted by Phloodpants View Post
Nice!

The matched transistors make a fairly substantial improvement to the sound, though I don't think it would affect the bass much. I'm surprised you don't find the bass satisfying; most people find that it's excellent despite it's small size. I suppose it depends on the speakers too.
Will continue to listen and appreciate evaluating further

By the way, what is your methodology for Bias adjustment in this thing. The service manual is a bit... Shy in my opinion. How do you warm it up, what do you do with the input and output during warm up and during adjustment? Etc.

Cheers!
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