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]

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:

An externally hosted image should be here but it was not working when we last tested it.

 

[Herb S]

Adcom GFP565

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:

An externally hosted image should be here but it was not working when we last tested it.

 

[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!

 

[jackinnj]

Here's the connection of the Jung Super-Regulator "sense" connections to the supply rail -- you can also see the wima mkp 1u's

An externally hosted image should be here but it was not working when we last tested it.

the sense connections are shielded and as close as possible to their load.

and here's the connection of the positive and negative output from the Jung regulators to the supply busses:

An externally hosted image should be here but it was not working when we last tested it.

 

[djordan]

ADVICE NEEDED FOR GFP-565 REPARIR or REPAIR / UPGRADE

Repair Only?
My very-qualified tech tells me I should get a full set of caps. Any recommendations on types and sources to buy from?

Repair / Upgrade?
I'm considering tracking down the AX articles and challenging myself to figure what's involved.
Could anyone advise on whether the parts required for this mod are still available? What would a triad transformer cost?

Cost-effective Repair / Upgrade?
What would be a good bang-for-the-buck upgrade / modification with easily available, modest-cost pricing?

Thanks in advance for all replies!

 

[jackinnj]

This is one of the finest "out of the box" mass market pre's ever built. The AX modification series simplified the signal routing, (eliminating the tone controls), replaced the wimpy power transformer and placing the raw power supply in an aux chassis. The buffer can be improved upon a bit with some of the newer devices.

The Jung-Didden Super-Reg boards don't seem available from AX as Circuit Cellar has taken over Old Colony -- WJ has the articles archived on his website -- one of the lowest noise, lowest impedance regulator designs ever.

 

[djordan]

If the regulator boards aren't readily available, this might be a little too ambitious for me to consider.
Could there be differences in SQ with different brands / type of Caps?
Any suggestions?

 

[djordan]

Are the op-amp / buffer chips plug and play

My trusted technician put in a new set of caps (sourced from Adcom)
and ALSO installed sockets for the Op-amp chips.

Could I just order replacement chips that would fit the 8 pin sockets?
Which one would you suggest?
These would go into sockets 201 and 202 correct?
Would I need to do any further electrical adjustments?

Thanks!
David

 

[jackinnj]

I believe that U201/202 are operated in Class-A, (as are the opamps in the phono stage) so you can't just "roll" opamps. You could upgrade to OPA627 if you were to remove the additional bias, but then it gets beyond a simple series of tweaks.

 

[superhdcd]

Can you explain what you mean when you say that in order to install the OPA627s that you must first remove the bias?

The schematic for the GFP-565 clearly shows biasing at IC202/IC201 using a CRD (as opposed to a resistor) followed by the IC204/IC203 buffers (LT1010). I mention the resistor because IC205/IC206 uses the same opamp (LT1056?) for the tone controls but with a simple resistor and no buffers.

I am also thinking of doing this exact same upgrade as the previous poster.

The OPA627 is a single op-amp, right? So in order to make it fit I would first need to place two OPA627AUs onto an adapter for a DIP socket. But since the circuit is already CRD biased I don't need to change anything.

Is this correct or am a totally confused about something?