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Adcom GFP 750 pre amp upgrade Big Sky Audio

Recently I upgraded/modded my aging Adcom GFP 750 through Big Sky Audio. Very pleased with the results, so sharing here for whoever might be interested. You probably already know the GFP 750 was well regarded in its price range when it was produced, but that was 20 years ago. I’ve had mine for 5 years.

My system had a bit of smear, which I attributed to the 750 after removing it from the system and just listening DAC to amp with no pre. In that set up, there was much better Left Right differentiation, and details such as vocal harmonies, rhythm guitars and background keyboards I felt were more defined. I was not interested though in using no pre amp, as that created other weaknesses. So, the options were either to get something newer or update/mod the 750.

Since I was satisfied with every other aspect of the 750 - soundstage good, punchy bass and drums, smooth tone aside from the smear I mentioned earlier, I decided upgrading was the way to go. After a bit of research and phone calls I settled on Big Sky Audio.

Couldn’t be happier with the results. The detail, the definition that I wanted is there now, sounding like the DAC direct to amp, and like other systems I listen to occasionally. Listening to Gaucho, an album laced with vocal harmonies and intricate keyboard layers going on behind the vocals, I felt the system had a smoother, clearer and even airier presentation as compared to the pre mod 750. Left Right differentiation is much better, and lead vocals and soloists seem to be in a more stable and defined center. Listening to classical music, the upgrades are even more apparent.

[For specifics on the mods, please go to Big Sky Audio’s website. I did the mods that are described there]

There were no downsides that I could hear. I don’t feel that anything was weakened in an effort to get more detail. The tone was always good to my ears, and that’s unchanged except smoother.

So, my decidedly mid-fi system is 5-10% better. Very happy with the improvement.

(The rest of the system is: Adcom GFA 565SE; MQA enabled ProJect S2 DAC; Tyler Acoustics floorstanding speakers; two Velodyne subs, left and right; Tributary cables and interconnects; mostly streaming these days, via Tidal.)

philtangerine
213runnin
914 posts
 
Thanks for relating your experience, the 750 is truly one of the greats.  I’ve been tracking the 565SE with the goal of acquiring one by the end of the year.  If you don’t mind, could you describe it’s sound characteristics?

One owner I  read said it would be slightly forward sounding.  I found the 555SE to be slightly warm sounding and am curious whether Adcom has mixed it up a bit as one moves up the line.
philtangerine OP
29 posts
 
I'm having a hard time describing, because its just neutral to me. It just kind of sounds like whatever the recording is. Bass is tight and punchy. You can really feel drums on my system. I guess its warm or mid rangy. I wouldn't call it bright or hyper detailed. With the recent upgrade to the pre, my system sounds more like vinyl than it ever has. I guess forward sounding is fair. Definitely don't feel like I'm 15 rows back. I haven't heard any $900 (used) amps I thought were better overall, just different. This is a fuller sound than Rotel, much bigger soundstage than Cambridge Audio. Similar to Parasound I guess.  Krell is a brighter sound, to my ears.  It just seems to let the recording come through. Dire Straits sound loose and laid back on my system, but Motorhead sound like Motorheard, high octane.  I don't know how helpful that is...but as I say, its just kind of seems neutral to me.
213runnin
914 posts
 
What you’re describing sort of sounds like my 555MS.  It’s pretty neutral sounding,  warmer when I have a Denon AVR driving it vs a Schiit Freya+ .

Come to think of it, when I had a Parasound P5 driving the 555SE, that’s when I got the impression of warmth.  Anyway, thanks for your thoughts, they tell me I’m heading in the right direction.

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