adcom GFP565

[Andy28]

Does anyone know the identity of the op amps used in the Adcom GFP preamp 565? Adcom uses their own numbering system and I want to know whether I can substitute the resident op amps with the OPA627.

[EchoWars]

If the pinout is the same (check carefully), and the voltages are acceptable to the 627, then the answer is probably yes.

[jackinnj]

Originally posted by Andy28
Does anyone know the identity of the op amps used in the Adcom GFP preamp 565? Adcom uses their own numbering system and I want to know whether I can substitute the resident op amps with the OPA627.

they were Linear Technology LT1057s

The GFP-565 was subject to a very lengthy series of mod articles by Gary Galo in AudioXpress from November 2003 to February 2004 -- Galo had written the original review of the 565 for Stereophile in 1990. Another good reason to subscribe to AX.

FWIW -- I have a modded GFP-565 and love the thing. This is a great preamp for the Jung super-regulators.

[jackinnj]

The 5 pin TO-220 was an LT-1010 driver chip. The most recent modification used an AD825AR and AD811.

Here's a modification -- I've installed a beefier transformer and a HexFred diode board -- Galo used a separate box, but with a little squeezing it proved unnecessary:

[Herb S]

Interesting mods. Thanks for the pix. I never liked the external xfmr that Gallo used, it's nice to know it can fit in the original case. Did you use the Triad that Gallo suggested? Also what about the superreg, the Per Anders seems very expensive is it really better than the one used in the Gallo article? It seems the pc board used by Gallo is NLA. Any other suggestions?

[jackinnj]

The super-regulator boards are still available from Old Colony -- Old Colony Sound Lab -

According to the 1995 articles in TAA, it's best to use dual separate rectifier/caps in the power supply to supply each board. If you order the boards from Old Colony you get the articles, plus the follow up with the pre-regulator.

[jackinnj]

Turns out that mounting the open-frame transformer (the one recommended in the article) as shown above isn't a good idea -- it radiates energy all over the place -- I used my RF sniffer and was able to detect microvolts even 10 inches away. The original transformer, mounted as it was in its own little enclosure is a little better. Mounting the transformer in a separate container (I used an instrument case from a dead HP 465 amplifier) really knocks down the noise radiated by the transformer.

FWIW, I found that the HexFred diodes didn't cause any ringing, even given the quite large leakage inductance of the Triad transformer.

[jackinnj]

Here's what I described from the post of a few days ago, a comparison of a stock Adcom GFP-565 with one in which the power supply is removed, transformer changed out to the Triad unit recommended by Galo, and connected to the preamplifier by shielded cables using XLR's -- line noise is reduced by mucho dB:

[infinia]

Hi
Thanks for updating your findings, Im a big fan of Adcom gear and some older G. Galo SB project write ups.
Interesting that 60Hz is dominate I would think 120Hz,. In my way of thinking this means either meas ground loop issues or radiated line.

[jackinnj]

Yupp, there does seem to have been a ground loop -- when I reinstalled the stuff after the floods we had in early spring I mistakenly plugged the analyzer into a different socket.

And by the way, don't leave your battery powered tools on the floor if it's gonna rain!