Adcom GFP-750 DIY mod finally finished. Warning.....NOT dial up friendly (loads of pics)

[Pricklely Peete]

Hi all,

I had this venerable Nelson Pass designed passive/active preamp sitting around gathering dust for the last year or so (was in use for years previous in the main system) and wanted to upgrade it a number of times over the years and never got around to it until a month ago.......ran into all kinds of trouble during the mod (most of it caused by my own inept bumblings with new parts I'm unfamiliar with). It's primary use now is as a source switcher and pre for the head fi system...allowing flexibility to use any type of headphone amp (balanced or single ended by using the passive mode as a buffer). The features of the preamp allow so much additional flexibility and control...the addition of it is a god send for a DIY tweaker junkie like myself. I can simply add DACs, amps etc at will now without having to tear the whole system down every time....the remote control feature is also a welcome addition...if those of you that are unfamiliar with the model...look it up at Adcom for all the details.

In any event...I borrowed some suggestions from an old thread at DIY Audio.com and used some of my own concepts to complete this massive overhaul (73 parts in all).

Here are some before and after pics....please note that the Vishay caps are installed incorrectly in these pics...that was one of the bumbles ...since corrected.

Stock trim...good design...shockingly bad caps in some key sections.....Jamicon and Teapo.....need I say more ? The shortcuts to meet the price point (1.5K US) are correctable...the jacks also need to swapped for decent ones (once I get all 10 pairs of them).

The parts list for the changes....

Fairchild UF diodes
Epcos metalized poly film and Vishay/BC component Axial film caps
Vishay 1% 1W metal film resistors
Nichicon electrolytic caps (PW and VK series)
Panasonic FC series electrolytics
Soft annealed 22gage solid silver jumpers in 4 spots (out and input DC blocking caps removed as per DIY Audio mod)
NOS PIO K40Y-9 .047uf 200V and 1.0 uf 160V Russian caps (bypass CDE, Nichicon, Vishay caps)

Here are the pics......(Vishay cap polarity has been corrected, pic shows incorrect installation)

Bypass caps shown are partial completion. The CDE's have an additional set of 1000uf 100V Nichicon PW's added on the underside to the main PSU filtering bank. Secondary PSU filtering was doubled from 2200uf to 4700uf.

Preliminary Sound Quality evaluation results

Certainly a vast improvement in extension, timbre, sound staging and dynamics across the board. Simply in another league over stock. I would say this preamp now lives up to it's Stereophool Class A rating it was given years ago but left me wondering why when I first bought the pre ( in stock trim obviously).

I shall add to this post's SQ impressions as the amp burns in.....I don't want to go into too much detail yet since it needs time to settle in fully. Used examples of this amp are well worth the money IMO simply because this mod can take it well beyond what it is in stock form. The cost of the mod is less than 60US.....certainly affordable by anyone's standard.

Those that own and want to modify a great SS preamp this is your bargain ticket to true Hi end SQ on the cheap. PM me for details if you wish...I would me more than happy to discuss the finer points.

That's it for now......time to enjoy some tunes

Peete.

 

[mrarroyo]

I have been trying to locate a GFP750 for a while. Of course I need it to be in good shape and at a reasonably low price, no luck.

I would pair it w/ my GFA555, meanwhile I have to do w/ my GFP550 both bought new back in circa 1986. Still sound very good.

 

[Pricklely Peete]

Quote:

Originally Posted by mrarroyo /img/forum/go_quote.gif I have been trying to locate a GFP750 for a while. Of course I need it to be in good shape and at a reasonably low price, no luck. I would pair it w/ my GFA555, meanwhile I have to do w/ my GFP550 both bought new back in circa 1986. Still sound very good.

I guess they are getting harder to source...I visit Audiogon quite a bit so if I come across one I'll let you know via PM.

Mine is in perfect working order...the remote is a little worse for wear although still fully functional. I would say the pre 9/10 condition with very minor cosmetic nicks here and there and the remote a 5/10.

That amp is nice (555). The GFP550 is no slouch but the 750 is really a big step up in SQ from it's little brother (as you most likely know).

Peete.

 

[wiatrob]

Just stumbled across this:

Adcom GFP-750 with matching Dolby GDD-1 Decoder

 

[mrarroyo]

Thank you guys, I have seen a few at about $850-$900 range. I guess I am bein unrealistic in hoping for $500.

 

[Pricklely Peete]

Quote:

Originally Posted by mrarroyo /img/forum/go_quote.gif Thank you guys, I have seen a few at about $850-$900 range. I guess I am bein unrealistic in hoping for $500.

The lowest I've seen them go for is around 735 -750US and that was a few years back..with demand and dwindling supply the price has risen since that time. At 800+ I would expect a near perfect example IMO including remote/manual and original packaging. Haggle is my advice

Peete.

 

[va3zrt]

Hi Peete,

Read the threads on your 750 mods...very interesting!...FYI, if anyone is interested and looking for one, I currently have a mint 750 for sale. I'm the original owner who bought it NOS from a dealewr in n/w toronto. If I don't end up selling, I would be very intereted in exploring the mod's you did a little further. I'm asking $775, but I'm a little flexible.

If anyone interested, my email is: bcthomas@sympatico.ca.

It is also posted on Canuck Audio Mart.

Thanks & season's Greetings!...

...Brad

 

[ptiJean]

Hello from France
I'm searching for a 230 voltage gfp 750, I wish to tweak it hard to hear how it can sing.
In case you see one, just let me know.
Would be to fit my Tannoy D500 speakers, with Sonic Frontiers Anthem AMP1 amp (pushpull EL34 with 6L6 =C= tubes).

 

[clarkie]

Hi Pete: I read your thread about the Adcom GFP750. I too have one and am thinking of tweaking it. I live in Cambridge ON. Is there any more info about the components etc that you used? Do you still have yours?

Regards Clarkie

 

[joefriend]

Hi Peete;
 
I have had a 750 for a several years and now have access to an expert guy who can do the mods for me.
 
Can you provide the link to the older threads in DIY Audio you refer to as well as a copy of the schematic and any other write up or supporting info to help me get your mod done?
 
Many Thanks!
 
My email is;  josephamico@comcast.net

 

[HiFiGuy528]

You make me want to bust out mine.  It's been in the closet in original box for many years.