Adcom GFP 555 Preamp upgrade

First let me say than you for inviting me into the forums, and secondly forgive me for all the mistakes I make as I learn to use the system

This is an ADCOM GFP 555- a Mk1, which a customer for whom I had upgraded a NAD 2200, asked to to work on.

The 555 is a straightforward design from the mid eighties and is generally well made with quality components and PCB. As with most units of that vintage the main areas for improvement are the power supply, IC's (if there are any) and capacitors.

There are four sections to the circuit.

The power supply- The original power supply caps were 4700uf, with the voltage regulation using transistors in a standard configuration. I replaced them all and effectively doubled the capacity of each. The very low value electrolytics were replaced with polyester film caps. The relay drive circuit cap values were left the same, but again low impedance types were used. The relay itself was replaced with a new sealed miniature type.

The main thing about the power supply is that it provides 21.5v for the rest of the circuit, excepts for a 15v (+ve/-ve) for the headphone amp. The 21.5v supply limits the choice of IC that can be used to replace the ADCOM 5E IC's originally fitted. I changed the zener in the regulation circuitry (D903) to 10v from 12v which brought the 21.5v to 18v, ensuring a wide choice of IC's could be used. The original IC's were all socketed, which made replacement simple.

The phono preamp is based around a pair of ADCOM 5E IC's , both of which were replaced with OP37 very low noise types. The decoupling caps were doubled in value and the electro caps in the feedback loop of the IC changed to Audio specific types, but of the same value (UKZ).

I did debate using OPA627 IC's but at nearly $40 each, the customer and I decided this was not worth the expense

Preamp section- This is based on another pair of ADCOM 5E IC's, acting as buffers, which again swapped with OP37's. The tone section uses a single JRC2041DD, which was replaced by an LM 49720. Some additional decoupling was added to the the power rails for this IC. More importantly there were two tantalum caps on the "Norm" output and these were replaced with polypropylene film.

Overall the unit measures very well and sounds clean and detailed, with a good sound stage and dynamics.

and then of course I could not resist trying a pair of OPA627 as the preamp buffers, and yes I do think the preamp sounds a little better, a little less bright perhaps, which was my overall sensation when comparing to my reference (upgraded- see the audioscience review) APT preamp. That said it was soemwhat dependant on the source material.

So I will now be having another chat with the customer....:)


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Welcome to The Haven Quirkaudio! Thanks for sharing and nice stack!

Any thoughts on the TC-7240 on top? I was real close to buying one but never did. Seems like a quality switcher and gets excellent reviews.

I'm a huge fan of 80s Adcom gear - such as my limit has been. I still use my GFA-555 occasionally. I bought it in 1986 with a GFT-1A and GFP-1A (got a great deal on the earlier gen units). The tuner is still here and functions well but I live in a signal killing valley and no proper antenna up on the 3rd story, so I don't use it much.

The pre was replaced in the late 90s with a GFP 710 and I like it. The 1A volume control failed and I couldn't find anyone local who could/would replace it. Too bad, I really liked it. I regret purging it to the side of the road years ago.

I still use my later Adcom gear: a GCD 700 and like it very much except for the clunky carousel noise when it rotates to the next CD. But I can live with that. I bought an ACE-515 so-called surge protector at some point for the my main system, it makes powering all my gear convenient with a single master on/off switch.

A GFS-3 speaker selector still gets use on one of my secondary systems.
That gold coating on the circuit board looks really nice, it is neat that they added such details to the insides of their products. They obviously knew that audiophiles are techy types who would appreciate such details. Those selector switches are very interesting looking also, I've never seen anything quite like those. Very nice work on the restoration/updating of a good looking preamp.