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Old 17th September 2003, 10:51 AM   #1
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Default Good DC-Precision OpAmp

I have an Adcom GFA-555II amplifier with a bOrked opamp in the input section. It is simply being used as a DC servo for the diff-input pair. The only ID on the chip says "Adcom 3A". Not too informative.

Examination shows that it is a standard pinout for a single channel 8-pin dip opamp. Neg is connected to ground, and positive is at 20V.

So, seems as if I could just pop in a reasonable replacement as long as it's DC characteristics are good.

Mostly, we all here discuss audio opamps. How about something for this application with good DC specs, pin-configured as a single-channel 8-pin dip? Any suggestions??

Thanks guys...
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Old 17th September 2003, 11:11 AM   #2
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OP07, OP97, OPA277 maybe?

You might check it further if the inputs are near ground and pin 4, then you must check for a type which has common range down to negative rail (pin 4).

Remember though that for audio almost any opamp will do. Offset of 1 mV isn't much in audio amps.
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Old 17th September 2003, 11:28 AM   #3
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OP27
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Old 17th September 2003, 11:34 AM   #4
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Echowars, why do you want to change the opamp since it's use only for DC? Your listening to DC
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Old 17th September 2003, 12:54 PM   #5
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Default DC Servo

A good DC precision op amp is the LT1012CN8. Very low bias currents also. Due to the very low bias currents you can use a larger value resistor and a smaller value capacitor for RC networks, this is usually cheaper and saves board space.
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Old 17th September 2003, 02:22 PM   #6
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Years go by and Erno Borbely keeps using the LF411 on his servos. Why not try that one?


Carlos
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Old 17th September 2003, 03:16 PM   #7
mlloyd1 is offline mlloyd1  United States
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Yep. Similarly, I've been using the old AD711 for years in this (as well as other) role. It works well as a servo device. I got a bunch cheap years ago.
mlloyd1

Quote:
Originally posted by carlmart
Years go by and Erno Borbely keeps using the LF411 on his servos. Why not try that one?
Carlos
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Old 17th September 2003, 03:39 PM   #8
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i have even seen the opa627 beein' used as servo opamp.......would it be superior to others in that position...?
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Old 17th September 2003, 05:24 PM   #9
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Red alert!!

It is not true that any old DC opamp will do! This opamp (if it is the servo) will have to handle audio freqs. The integrator function means that it will have to supply a charging current to the integrator cap from the signal it is fed.

At the output, especially at low freqs, you will find a replica from the signal, albeit low level. However, this low level signal is then injected directly in the amp. Any distortion in the servo output (AC) means amp distortion.

Please think this through, don't make it too easy on your self.
What happened to those people that claim to be able to hear a servo? For once, I can accept this claim.

Jan Didden

Edit: With almost all currently available opamps having very low offsets, paradoxically, DC precision probably is the least of your worries.
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Old 17th September 2003, 05:30 PM   #10
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Jan, I generally use a trick of putting a passive RC pole at the output of the integrator, with its frequency set a decade or so above the integrator time constant (that will put the pole at typically 50-60 Hz). That way, the effect of the high frequency characteristics of the servo opamp are minimized.
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