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.
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.
Assuming that it's a single op-amp and that DC servo is it's only function, an LF411 or OP97 should work fine...
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...
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.
Belligerence from the peoples' republic of Australia, I love it!
Its a custom part from Linear Technology.
http://www.diyvideo.com/forums/attachment.php?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.
Its a custom part from Linear Technology.
http://www.diyvideo.com/forums/attachment.php?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.
Trouble for Adcom perhaps, because you didn't buy their part.georgehifi said:
djk hit the nail on the head...buy a good quality single opamp and put it in.
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
Cheers George
The usual bad advice....
"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.
"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.
Servo and Crusade
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!😎
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!😎
Fred...the 555II has a single 20V zener setting up voltage for the positive supply, the neg side goes to ground. Inverting input connected to the power amp output via a 4.7M resistor, integrated with a .1µf cap. Non-inverting input goes to ground, also integrated with the same 4.7M/.1µf setup. Output feeds base of first diff-pair transistor via a 1.5M resistor, and is it's only source of bias.
As for using 'cheap' opamps, I popped in the only single opamp I have here, while I wait for the AD820 to show up. Stuck in a LM741C. DC offset is sitting at about 2V. Lol!
So yeah, I think something a little better is called for. 😉
As for using 'cheap' opamps, I popped in the only single opamp I have here, while I wait for the AD820 to show up. Stuck in a LM741C. DC offset is sitting at about 2V. Lol!
So yeah, I think something a little better is called for. 😉
OK, OK. I always use the crappiest op amp so that nobody
will accuse me of putting it in the signal path.... 😉
will accuse me of putting it in the signal path.... 😉
Yes, you can! But only if you design the servo slow enough. 20 Hz at full power (or 15, 25 or whatever) shall not make the servo opamp clip. This gives the maximum speed of the servo. Usually you must choose slower or much slower speed.djk said:
The chump gets stumped
Looks like the AD820 will go about 0.2 below the negative rail for input common mode. I believe the dual version of this device is the AD823 for anyone curious. I was aware of any jfet input op amp that would do that. I learned something. It happens on this forum much more often than I let on......
Looks like the AD820 will go about 0.2 below the negative rail for input common mode. I believe the dual version of this device is the AD823 for anyone curious. I was aware of any jfet input op amp that would do that. I learned something. It happens on this forum much more often than I let on......
Yeh, Fred your right , put in the ne5534 and it pulsated the out put rapidly, i put a .0022uf comp cap across 1 and 8 and it slowed the pulse down to 1 pulse every 1 second, still not right.
Which one do i use guys that`s easily available at the corner shop is the lf351 the answer, the shop says the tlo71 is the same.
Cheers George
Which one do i use guys that`s easily available at the corner shop is the lf351 the answer, the shop says the tlo71 is the same.
Cheers George
No it won't. It must be a low power part, since the op amp rails come from the lm329 zener supply. Use an OP97 or an AD820, which work OK.
wj
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