replacing ADCOM GFP750 caps?

seliger · 17th July 2004, 04:19 AM

hi all,

having a little sucess recently in modding my cdp ,
am bout to mod my adcom gfp 750 pre......thinking of upgrading all the caps in the unit........currently its mainly JAMICON in the audio stage and PHILIPS in the power section.............any sugestions......... how much sonic improvment will i be able to get ........say if i change them all to blackgates or nichicon FG series..........

any comment???

thanx
jc

Nelson Pass · 18th July 2004, 07:30 PM

Why don't you try something and let us know?

seliger · 19th July 2004, 07:14 AM

ha ..... have just finished the operation while watching COPA america Brazil vs Mexico live.........!!!

the preamp is still alive........... used Nichicon FG instead of Blackgates....... coz i already have them....... replaced with exact same value.........first impression ......everything sounds positive.... the BASS is wonderfull!!!!!!!! it's so tight and with authority....... but the mid high seems a little bit harsh.........maybe it's not burn in yet........... will use a burn in disc for the next 2 or 3 days..........actual result will only be known after it's been burn in i guess......

wonder how the BGs will sound like.........

tim pattinson · 24th September 2004, 07:15 AM

Are you pleased with the results?

Interested to know...

Gregorio · 24th September 2004, 01:08 PM

What is can be modified besides the caps that might make an improvement in this preamp?

hbarki · 18th April 2005, 01:11 AM

How about upgrading the pot to stepped attenuator? Goldpoint makes some very nice ones, or you can build using ELMA switches.

commstech · 18th April 2005, 06:14 AM

Quote:

Originally posted by hbarki
How about upgrading the pot to stepped attenuator? Goldpoint makes some very nice ones, or you can build using ELMA switches.

I had used TKD stepped attenuator and i get clicking noises in between the steps.

markk02474 · 26th October 2006, 07:03 PM

I'm surprised to see no mods for this preamp, so I'll list some!

The major evil in the preamp are the DC-blocking capacitors that the inputs go through. These are 10uf electrolytics bypassed with a 1uf polyester. It doesn't matter if the preamp is in active or passive mode, they are still in the signal path! This explains why many reviewers didn't find any improvement by using passive mode. These parts are in the back, right corner and marked C163, C164, C171, and C172. Jumper wire replacement is my personal preference, but DC offset from sources could reduce the life of the volume control. The volume control appears to be on both the input and output.

In each channel there are four sets of electrolytic caps with mylar bypasses that appear to be in the signal path. Some Black Gate N electrolytics with polypropelene bypasses could be substituted. The layout is compact, so options are limited.

There are two bridge rectifiers made up of 1N4004's, one for the 12V getting regulated to 5V for control circuits, and one for the +82V analog supply that gets regulated down to 60V. Substitue your favorite diodes for each application - I'm not sure which ones people here think are best, and will be reading posts.

Power supply caps for the 60V rails are generic electrolytics with 1uf polyester bypasses. Could be better.

I like having remote control, so I don't plan on using a stepped attenuator for the volume control.

I'll report on the effects of the mods as I do them!

anatech · 26th October 2006, 07:33 PM

Hi markk02474,
Do they still use the switching IC's for signal switching? If so, they are the worst evil, followed by muting transistors, if used.

Your power supply caps are bypassed, so no gains worrying about caps there. The supplies are regulated, so don't get freaked out about the diodes. I find the 1N400X and 1N54XX series are just fine.

You may want to replace the coupling caps and bypass caps, but most improvements are very, very small with a switching IC in the signal path. Most of these tuner / preamps sound similar due to these signal switching IC's in my experience. I did do warranty service on Adcom, Carver and many others for years, that is where my experience is coming from.

-Chris

markk02474 · 26th October 2006, 07:46 PM

No Switching ICs or muting transistors, just relays in the signal path input and output. About 20 relays total in the unit!

In stock form, I think this preamps sounds good, and better than it ought to given the DC blocking caps!

17th July 2004, 04:19 AM	#1
seliger diyAudio Member Join Date: Jul 2004 Location: singapore	replacing ADCOM GFP750 caps? hi all, having a little sucess recently in modding my cdp , am bout to mod my adcom gfp 750 pre......thinking of upgrading all the caps in the unit........currently its mainly JAMICON in the audio stage and PHILIPS in the power section.............any sugestions......... how much sonic improvment will i be able to get ........say if i change them all to blackgates or nichicon FG series.......... any comment??? thanx jc

24th September 2004, 01:08 PM	#5
Gregorio diyAudio Member Join Date: Jun 2004 Location: Midwest	What else? What is can be modified besides the caps that might make an improvement in this preamp? __________________ Gregorio

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18th July 2004, 07:30 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	Why don't you try something and let us know?

19th July 2004, 07:14 AM	#3
seliger diyAudio Member Join Date: Jul 2004 Location: singapore	ha ..... have just finished the operation while watching COPA america Brazil vs Mexico live.........!!! the preamp is still alive........... used Nichicon FG instead of Blackgates....... coz i already have them....... replaced with exact same value.........first impression ......everything sounds positive.... the BASS is wonderfull!!!!!!!! it's so tight and with authority....... but the mid high seems a little bit harsh.........maybe it's not burn in yet........... will use a burn in disc for the next 2 or 3 days..........actual result will only be known after it's been burn in i guess...... wonder how the BGs will sound like.........

24th September 2004, 07:15 AM	#4
tim pattinson diyAudio Member Join Date: Feb 2004 Location: auckland	Are you pleased with the results? Interested to know...

18th April 2005, 01:11 AM	#6
hbarki diyAudio Member Join Date: Dec 2003 Location: NoCal	How about upgrading the pot to stepped attenuator? Goldpoint makes some very nice ones, or you can build using ELMA switches.

26th October 2006, 07:03 PM	#8
markk02474 diyAudio Member Join Date: Oct 2006	I'm surprised to see no mods for this preamp, so I'll list some! The major evil in the preamp are the DC-blocking capacitors that the inputs go through. These are 10uf electrolytics bypassed with a 1uf polyester. It doesn't matter if the preamp is in active or passive mode, they are still in the signal path! This explains why many reviewers didn't find any improvement by using passive mode. These parts are in the back, right corner and marked C163, C164, C171, and C172. Jumper wire replacement is my personal preference, but DC offset from sources could reduce the life of the volume control. The volume control appears to be on both the input and output. In each channel there are four sets of electrolytic caps with mylar bypasses that appear to be in the signal path. Some Black Gate N electrolytics with polypropelene bypasses could be substituted. The layout is compact, so options are limited. There are two bridge rectifiers made up of 1N4004's, one for the 12V getting regulated to 5V for control circuits, and one for the +82V analog supply that gets regulated down to 60V. Substitue your favorite diodes for each application - I'm not sure which ones people here think are best, and will be reading posts. Power supply caps for the 60V rails are generic electrolytics with 1uf polyester bypasses. Could be better. I like having remote control, so I don't plan on using a stepped attenuator for the volume control. I'll report on the effects of the mods as I do them!

26th October 2006, 07:33 PM	#9
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi markk02474, Do they still use the switching IC's for signal switching? If so, they are the worst evil, followed by muting transistors, if used. Your power supply caps are bypassed, so no gains worrying about caps there. The supplies are regulated, so don't get freaked out about the diodes. I find the 1N400X and 1N54XX series are just fine. You may want to replace the coupling caps and bypass caps, but most improvements are very, very small with a switching IC in the signal path. Most of these tuner / preamps sound similar due to these signal switching IC's in my experience. I did do warranty service on Adcom, Carver and many others for years, that is where my experience is coming from. -Chris

26th October 2006, 07:46 PM	#10
markk02474 diyAudio Member Join Date: Oct 2006	No Switching ICs or muting transistors, just relays in the signal path input and output. About 20 relays total in the unit! In stock form, I think this preamps sounds good, and better than it ought to given the DC blocking caps!

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