Adcom gfa-585 dc servo opamp - Page 2

EchoWars · 24th September 2003, 07:03 PM

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.

Nelson Pass · 24th September 2003, 07:08 PM

OK, OK. I always use the crappiest op amp so that nobody
will accuse me of putting it in the signal path....

peranders · 24th September 2003, 08:16 PM

Quote:

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

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.

Fred Dieckmann · 24th September 2003, 08:56 PM

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......

georgehifi · 24th September 2003, 09:10 PM

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

WaltJ · 31st October 2003, 10:32 PM

Quote:

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

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

djk · 1st November 2003, 12:50 AM

I stand corrected.

24th September 2003, 08:56 PM	#14
Fred Dieckmann diyAudio Retiree Join Date: Oct 2002 Location: Spain or the pueblo of Los Angeles	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......

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24th September 2003, 07:03 PM	#11
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	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.

24th September 2003, 07:08 PM	#12
Nelson Pass The one and only Join Date: Mar 2001	OK, OK. I always use the crappiest op amp so that nobody will accuse me of putting it in the signal path....

24th September 2003, 09:10 PM	#15
georgehifi diyAudio Member Join Date: Sep 2003 Location: Manly Australia (Jewel of the Pacific)	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

1st November 2003, 12:50 AM	#17
djk diyAudio Member Join Date: Feb 2001 Location: USA	I stand corrected.