Adcom gfa-585 dc servo opamp

[georgehifi]

Can any one help, I`m down here in Australia with no Adcom agent for spare parts, Adcom in their wisdom have taken off the original number and manufacturer and put on their own
ADCOM 2A 8933, number, does anyone know what the real number is for this i.c.
BTW if this was done in Australia Adcom would be taken to court, as we have a law down here which prohibits the coverting of part numbers if they are not designed and manufactered by yourself.

[DrG]

Assuming that it's a single op-amp and that DC servo is it's only function, an LF411 or OP97 should work fine...

[EchoWars]

I'm repairing a GFA-555II amp right now, and both servo opamps need replacing. Since the opamp is being used on a single 20V supply, I wanted to replace with a good opamp capable of rail-to-rail swings. In this thread someone suggested a Analog Devices AD822. The 822 is a double opamp, but the AD820 is a single version of the same, and after reading the datasheet it looks like an excellent replacement. I have several on order...

[georgehifi]

Thanks for replying guys, but i was told that you must use the one that Adcom used or else there will be trouble, has anyone done a subsitute succesfuly, especially with the gfa-585.

[djk]

Belligerence from the peoples' republic of Australia, I love it!

Its a custom part from Linear Technology.

http://www.diyvideo.com/forums/attac...p?postid=66400

But, as you can see from the schematic, any good single opamp will do (LF411, AD711, OPA604, etc).

You cannot use an old FET opamp like a LF351 or TL071 that has phase-reversal when driven hard.

[EchoWars]

Originally posted by georgehifi
Thanks for replying guys, but i was told that you must use the one that Adcom used or else there will be trouble.

Trouble for Adcom perhaps, because you didn't buy their part.

djk hit the nail on the head...buy a good quality single opamp and put it in.

[georgehifi]

Thanks again guys, DJK you said any good non fet input single opamp, i`ve got a couple of NE5534an hangging around, are they what you call good enough.
Cheers George

[Nelson Pass]

Originally posted by EchoWars
djk hit the nail on the head...buy a good quality single opamp and put it in.

Better yet, buy a crummy op amp and put it in.

[Fred Dieckmann]

"Since the opamp is being used on a single 20V supply" Are you sure? That doesn't sound likely for a servo.......

I believe what Mr. Pass is saying is that the choice of op amp for the servo is not that critical sonically.

The NE5534 is not suitable for a servo. It not stable for gains under 3 if uncompensated. Most servos are integrators and require unity gain stable op amps. This bias current for the NE5534 is 2000 nA worst case. Most servos I have seen use an input resistor of 1 Meg or more to be able to use a small film cap of 1 uF or smaller and still get a long time constant. 2000 nA times 1 megaohm is 2 volts, way to much error voltage for this circuit. Get a schematic or draw one for at least the servo circuit before rushing to replace the part at random. If it is a single op amp, a LF351 should be easy to find and most jfet input unity gain stable op amps should work fine. Worrying about phase reversal when overdriving the input is nonsense with an integrator type circuit where the inputs usually sit at around ground. Sorry for all the wacko advice you got from every one but Mr. Pass. If you really want to use a premium part for the servo, something like the OPA604AP would be hard to beat for the money. make sure there is not something else in the circuit that is causing the problem as it could be something besides the servo.


I believe you can get a service manual
which is usually worth the investment if you going to fix something.

[Elso Kwak]

Hi Fred,
On crusade again?
For what it is worth my limited experience with the servo IC:
I replaced the MC1458 by the OP07 in my Ampzilla II. This requires some surgery as the MC1458 is a dual and the OP07 a single opamp. It works on +/-13V supplies. Sonically the mod did not change a bit.
Later I completely removed the servo which made the designer of the beast very angry: OOO-GA, KING ZILLA ROYAL ORDER OF THE BANANA
http://www.ampzilla2000.com/bonge2.html

But I liked the sound of my amplifier much better without the servo!