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Adcom gfa-585 dc servo opamp
Adcom gfa-585 dc servo opamp
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Old 24th September 2003, 08:03 PM   #11
EchoWars is offline EchoWars  United States
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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.
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Old 24th September 2003, 08:08 PM   #12
Nelson Pass is offline Nelson Pass  United States
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Adcom gfa-585 dc servo opamp
OK, OK. I always use the crappiest op amp so that nobody
will accuse me of putting it in the signal path....
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Old 24th September 2003, 09:16 PM   #13
peranders is offline peranders  Sweden
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Adcom gfa-585 dc servo opamp
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.
/Per-Anders (my first name) or P-A as my friends call me
Group buy: SSR03 Super Regulator Power Supply. Sign up HERE 53 pcb's in interest so far.
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Old 24th September 2003, 09:56 PM   #14
Fred Dieckmann is offline Fred Dieckmann  United States
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Red face 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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Old 24th September 2003, 10:10 PM   #15
georgehifi is offline georgehifi  Australia
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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
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Old 31st October 2003, 11:32 PM   #16
WaltJ is offline WaltJ  United States
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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.

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Old 1st November 2003, 01:50 AM   #17
djk is offline djk
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I stand corrected.
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