Looking for schematic for Adcom GFP-555 (not GFP-555II)

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Hi all, first post here.

I've been looking around for a schematic & IC parts list for the circa 1986 Adcom GFP-555 original preamp (not the GFP-555II), with a view toward updating the opamps to the National Semi LME49710/LM4562 series of ultra low-noise, low distortion units. Want to make sure I'm ordering the right parts, etc. I'm only interested in updating the phono input, line input and output driver stages, not the headphone amp.

I see there was an article on the GFP-555 in the Sep 2006 issue of audioXpress (which I don't have). If this article is worth reading would appreciate guidance. Would cost over $40 to order the back issues, but I will do it if necessary.

Appreciate any guidance the forum can give me. I have searched and read all other posts related to Adcom GFP 555's but them seem to be written for the 555II, which I think was configured differently. I am the original owner of my unit, and use it every day in my main system. I need to pull it out (which is like heart surgery in my physical situation) to clean the main input selector switch, and I figure as long as I'm in there, might as well take care of other things as well.

Well, I never got a reply from Adcom, but I did manage to find the GFP-555 service manual privately. I want to report back to the Forum that it is definitely possible to use newer opamps. Here's what worked for me:

You need opamps that accept +/- 22V power supplies. Adcom provided zener-regulated +/- 21.5 volts through to all parts of the circuit except the headphone amp. My power supply caps, etc. are in fine shape, so I didn't mess with the power side.

National Semi makes their new series of ultra-low distortion opamps in high voltage models: singles are the LME49870MA (4 required), duals are LME49860NA (1 required). The singles are surface mounts only, so you need four carriers (Digi-Key part no A744-ND in gold) to mount them in the DIP sockets on the circuit board (thanks, Adcom, for making that part so easy!). The soldering was straightforward with a very fine tip temp controlled iron and ordinary 60/40 solder. It's helpful to smear a little flux paste on the carrier traces before heating. Be fast--don't want to cook the goodies! Clean well afterwards and make sure there are no solder bridges.

Impressions: phono stage is now very low noise and transients are crisper than the originals. Not much change in the other stages, but overall the performance is excellent. I did not A/B between L & R channels, but I think the gain is not much changed (it shouldn't vary much, given that these are operational amplifiers with fairly simple feedback networks).

I should also say that the Adcom build quality is quite impressive--excellent component qualitiy, use of bypass caps, ground plane circuit board, etc. I would have located the power transformer and A/C lines a bit farther away from the output section, but there is very little A/C noise anyway.

I would like to reduce the preamp's line out impedance from the current spec'd 470 ohms to something like 100 ohms, as I do drive some reasonably long single ended lines to power amps, and I am slightly concerned about loss of extreme highs. The Adcom circuit routes the line out signal through a series 475 ohm resistor, which I may replace with ~100 ohms at some point. I want to model this in LTSpice first, though.

Hi Doug(zilla), I have tried to find an Adcom gfa-555 ser/man for a few days now and haven't had any luck yet. Got to reading your post here and I see you managed to find one. Could I impose on you to send me an email scan of the schematic as I have a customers 555 with some power supply problems I can't seem to track down. Very low + & - 22 volt supplies, no obvious problems. Have not gotten any responce from a guy on ebay who advertises he has one so I'm getting desperate! Let me know if you can help, or where you got yours so I can see if they have another. Thanks in advance, Steve
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