Good DC-Precision OpAmp

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.

OP27

Echowars, why do you want to change the opamp since it's use only for DC? Your listening to DC

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.
d.b.

Years go by and Erno Borbely keeps using the LF411 on his servos. Why not try that one?


Carlos

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

carlmart said:

Years go by and Erno Borbely keeps using the LF411 on his servos. Why not try that one?
Carlos

Click to expand...

i have even seen the opa627 beein' used as servo opamp.......would it be superior to others in that position...?

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.

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.

Yes, SY, that helps. You can also put this in front of the integrator to limit hf signal into the opamp. But these two methods can give problems with stability (low freq motorboating) and/or irregular freq response. After all, you're messing around with the feedback loop. I don't say it can't be done, but some care and experience is required. But my main point was that selecting the opamp solely based on DC performance is the wrong priority.

Jan Didden

Hi,
You could try Analog Devices AD822
We use it often and have so far been very satisfeid with the performance on AC and DC signals.
Not cheap though.

FWIW, I use an LF412 (dual 411) in my crossover as a servo; works just fine.