ADCOM GFP 555II upgrade question

[starkeyg]

Hello,

Current project has been upgrading an old GFP555II preamp. I have already done considerable refinement of the power supply and was looking to upgrade the opamps used in it to something newer and hopefully better.

Some time ago was I told that it used LT1010 devices, but the pinout does not match. Also does not match the BUF634 device. The adacom part number I need info on the most is marked 6A.

As well as the 6A, if anyone know what the tape output drivers are, they are maked 7A, it would help as well.

From what I have been told, I believe the older 555 model may have used the LT1010's compounded with another LT opamp, but the 555II seems to use just a single device, not a compound one.

Device 6A is in a standard DIP8 package and has power going to pins 4 & 7. While device 7A is in a standard DIP8 package with pinout matching most opamps - BB2164, LM6172, ...

Any information or suggestions to research would be appreciated.

I have searched quite a bit and found nothing on the web so far.

Regards,
Greg
Certifiable Audio Nut

[TerryC]

I was able to glean the following from the Audio Xpress articles for the 555 and 565:

1A = LT1010
6A = LT1056
7A = LT1057
8A = LT1028C

The Audio Xpress articles are invaluable to DIY owners of these Adcom pre-amps. Various op-amp upgrade recommendations are made. The caveat is that the power supply rails have to be brought down. The stock op-amps allow for higher that typical supply rails, and Adcom chose to run them close to max.

[starkeyg]

Thank you for the feedback. Now can look for a suitable sub knowing the specs of what is in there.

Regards,
Greg

[EchoWars]

The opamp is used only as a servo. The best choice for the job is the Analog Devices AD820.

[mjraudio]

Originally posted by EchoWars
The opamp is used only as a servo. The best choice for the job is the Analog Devices AD820.

Not necessarily. While the AD820 is a good chip, considerations should also be taken regarding the power requirements of the op-amp IC. Walt Jung recommended a good quality OP97 op-amp for the GFA-565 power amplifier. I would imagine that if the pinout is the same for the GFA-555 and GFP series chips, this chip would be suitable.

From my understanding, I have concluded that OP07 is an older design while OP97 is a newer and more efficient design.

[EchoWars]

Power requirements? the 820 draws less than a milliamp quiescent, and about 200µA more than the OP97. In other specs, they trade desirability as a servo.

Either is certainly fine, but the 820 is the one I'm most familiar with. I may try the OP97 in the future after reading the specs though.

[mjraudio]

Originally posted by EchoWars
Power requirements? the 820 draws less than a milliamp quiescent, and about 200µA more than the OP97. In other specs, they trade desirability as a servo.

Either is certainly fine, but the 820 is the one I'm most familiar with. I may try the OP97 in the future after reading the specs though.

Adcom 2A IC for GFA 565

There have been other users who have reported success with the AD820, I was just going off Walt Jung's recommendation. The DC nulling circuit seems to be very sensitive to minor changes/adjustments. Right now on my GFA-565 that I fixed, he DC offset at the speaker terminals is about 2-5mV, but if i put a book on top of the chassis for 2 minutes, it rises significantly to around 70mV.

My other functioning amp gives out a constant 250mV sometimes raising to 270mV. I ordered to new Analog Devices OP97FS op-amp ICs but when I ordered them I didn't do the research and from just looking at them they are very small in comparison to the OP07 LT1012 originals (ADCOM 2A). Darn it.