Adcom GFP 750 cap upgrade

Gerres26

Since everything has been crazy in the world as of late and everyone is stuck at home I decided to give my Adcom GFP 750 a little refresh since it’s going on 20 years old or so. I ordered the parts up a little while back but finally broke down and replaced everything on it last week.

The upgrades included cleaning connections and some internals with deoxit, replaced power rectifier diodes with new matching parts, changed all power supply electrolytics to Nichicons, changed all electrolytics in the signal path(input & output) to Elna Silmic II and also replaced the bypass caps with Clarity Cap ESA. Shout out to @vr3 for helping me source the correct values.

Anyway here are a few pics.





As you can see I had to place the Clarity Caps on the underside of the board due to placement issues. I had to go with ESA series instead of the new CSA series because the CSA’s are just barely too thick to fit.

It’s only been a week but man this thing sounds really nice. Just like when replacing crossover parts in a speaker, the results are quite an improvement. As many know, this is a Nelson Pass design and a very close cousin to the Aleph P preamp. Although it’s not super high end I felt it was plenty worthy of the upgraded parts and at least in this case they definitely payed off.

pitdogg2

Very nice.

GlennDog

Well done, Sir!

I’ve got one of those, too. Unfortunately, the face plate is beat up curtesy of the previous owner. Fortunately, that does not affect the SQ

May I ask, what was the total cost of the parts ?

Gerres26

GlennDog wrote: »

Well done, Sir!

I’ve got one of those, too. Unfortunately, the face plate is beat up curtesy of the previous owner. Fortunately, that does not affect the SQ

May I ask, what was the total cost of the parts ?

Let me check my email receipts and I’ll let you know. Funny, my faceplate isn’t in the greatest shape either, so I have some ideas on how to revamp it some but this virus thing has put that on hold for now unfortunately.

GlennDog

About a year ago I found a NOS face plate on eBay for $95… Seemed high to me. I think I offered him 65 he came back at 90 and I left it at that
If I had this unit reworked I’d probably reconsider dropping another hundie on it (for the face plate)

Gerres26

GlennDog wrote: »

About a year ago I found a NOS face plate on eBay for $95… Seemed high to me. I think I offered him 65 he came back at 90 and I left it at that
If I had this unit reworked I’d probably reconsider dropping another hundie on it (for the face plate)

Too funny, I offered probably the same guy $50 as a shot in the dark and no response lol. I have possible plans of stripping the black anodizing, sanding it down to give it a brushed aluminum look and then maybe having the letters engraved or lazed(lased)?into it. It’s on hold since all of the guys that run the R&D equipment at work aren’t allowed to be there due to the virus. I need them to make sure the scan I made of the faceplate can be transferred over after I strip it with no issues.

Looks like all of the electrolytics and diodes ran me about 40 bucks at Digikey and the Clarity Caps ran me about 65 from HifiCollective.

GlennDog

Wow that’s an extremely affordable refurb. Would that be considered a complete rebuild?

I guess what I’m asking is, were there upgrades that could have been done that you chose not to do?

Gerres26

Maybe there are a couple other things that could be done like changing the other film caps for a different brand but the consensus from my research is that they don’t go bad like the electrolytics and the ones that I didn’t replace with the Clarity Caps weren’t going to affect the performance of the pre.

I researched a few different sites about possible upgrades/refurb advice and what I have done were definitely the go to updates for it. Of course you could choose different brands of caps but he Silmic II s are very well respected. Check out HoppesBrain website, they do a bunch of upgrades to gear and you could read what he does to the GFP 750.

JayCee

I have a GFP-750 in the closet. The tech that updated my Parasound HCA-1500A offers a GFP-750 upgrade and firmly believes he can take what is considered a very good bang/buck pre and make it stellar. Several years ago I started researching and almost went down the same path as you w/DIY. I've moved on to new gear but it's definitely one of my top used purchases. Keep saying I'll offer it up on CP as I purchased it from a former CP member. Does me no good in the closet but.....I....can't.....let.....go....yet. I have a problem!

Any chance of convincing you to post the parts/values for future reference. I have, probably, the same links as you buried in my bookmarks but would take me a while to catch up the last 5 years.

Congrats on the work and outcome. Looks nice and I'm sure it sounds even better!

Gerres26

I’ll tell you, this has to be one of the biggest bang for my bucks I’ve gotten in this hobby. Once I get time tonight or tomorrow I’ll track down all of parts and values for you guys that are interested.

vcwatkins

Thanks for taking the time to share, @Gerres26 . Impressive and surprisingly affordable. Due to space constraints, I'm using one of these in place of the Joule. When our households are more settled, I will slot the Joule back in and update the Adcom based on your work here.

Thanks again,
Craig

pitdogg2

Those Elna Silmic II's are very sweet caps one of my favorites to use. The Nichicon FG, KZ, KA KT and Silmic's are pretty much all you'll ever need as far as electrolytics go in my book. When I did my Sony X33 ES CD player the silmic II's really made a difference to my ears.

https://forum.polkaudio.com/discussion/127903/can-i-join-the-club#latest

Gerres26

pitdogg2 wrote: »

Those Elna Silmic II's are very sweet caps one of my favorites to use. The Nichicon FG, KZ, KA KT and Silmic's are pretty much all you'll ever need as far as electrolytics go in my book. When I did my Sony X33 ES CD player the silmic II's really made a difference to my ears.

https://forum.polkaudio.com/discussion/127903/can-i-join-the-club#latest

I used those Nichicon FGs in a Forte F44 preamp that I recapped not long ago with really nice results as well. I was lucky that the Silmics fit in the Adcom(albeit barely) so I wanted to try them in this application and I have to say they’re excellent.

audioluvr

Great job! The GFP 750 is a great platform. I'm sure when it's fully broke in it'll be a keeper.

Gerres26

Alrighty, sorry for the delay in getting the parts info... been a busy few days here.

ClarityCap ESA series purchased from HifiCollective:
4 x 1uf 250vdc
4 x 2.2uf 250vdc

All other parts purchased from Digikey. Nichicon PW series caps were used in the Power supply and all others not in signal path and are as follows:
3 x 1000uf 100v
1 x 2200uf 50v
1 x 100uf 100v
1 x 100uf 25v
1 x 10uf 16v
1 x 1000uf 16v

7 zener diodes 9.1v 1w D041

All caps in signal path were Elna Silmic II and are as follows:
4 x 100uf 100v
6 x 10uf 50v
4 x 470uf 16v

Hopes this helps you guys.

pitdogg2

one of these days I need to get my diode learn on.

Gerres26

pitdogg2 wrote: »

one of these days I need to get my diode learn on.

I do as well, I know there are some interesting ones out there though. I just picked ones that matched for this application as I read a few stories of one or more of them burning up and taking out a mosfet with it. So I figured better to be safe than sorry.

JayCee

Thanks, @Gerres26, appreciate you taking the time to post the list. BOOKMARKED!

xschop

Gerres26 wrote: »

GlennDog wrote: »

Well done, Sir!

I’ve got one of those, too. Unfortunately, the face plate is beat up curtesy of the previous owner. Fortunately, that does not affect the SQ

May I ask, what was the total cost of the parts ?

Let me check my email receipts and I’ll let you know. Funny, my faceplate isn’t in the greatest shape either, so I have some ideas on how to revamp it some but this virus thing has put that on hold for now unfortunately.

Nice recap work. The faceplate looks like an easy candidate for the silverface-mod. Does it look like the lettering is laser-etched underneath the anodizing?

Gerres26

They look to be painted or silk screened on. But I haven’t tried scraping any letters off to be honest. If they were engraved underneath this project would be that much easier that’s for sure.

porkfried

@Gerres26 Nice work! I just ordered your BOM. Looking forward to hearing it.