DC-Servo no-compromise opamp

[ClaveFremen]

Hi,

I want to swap a TL071 used in a DC-Servo circuit with a better perfoming opamp and I'm thinking about OPA627 or OPA827.

Someone has any opinion on it? Which is better betweeen the two?

Thanks in advance

[r_merola]

If the OP is applied in the correct way there is no inpact in sound performance.

Please, remember the servo is a low pass filter and fc must be low enought to do not impact in audio.

So, if the project is correct you do not need to change the OP in servo.

[Mooly]

I agree. The TL07x series are ideal and even untrimmed will hold the offset to usually under 1mv or so.

What area of performance are you trying to improve ?

[DF96]

I don't recall ever seeing a circuit which had no compromises. DC-servo is itself a compromise: you swap the problems of capacitors in one part of the circuit, for the problems of a servo and capacitors in another part of the circuit. You are swapping a simple passive component for an active solution - in other circumstances people try hard to go the other way!

[kevinkr]

Originally Posted by ClaveFremen

Hi,

I want to swap a TL071 used in a DC-Servo circuit with a better perfoming opamp and I'm thinking about OPA627 or OPA827.

Someone has any opinion on it? Which is better betweeen the two?

Thanks in advance

Neither! In addition to all the prior comments you want to choose an op-amp with very low DC errors for the DC servo. Choosing an op-amp with spectacular AC performance doesn't make a lot of sense for this application as it is used in an integrator with an HF cut-off well below the lowest audio frequency. (High open loop gain, and low input referred DC errors are what you want.)

I experimented extensively with DC servos in the distant and not too distant past and frankly preferred a really good film coupling cap to the servo.

[ClaveFremen]

Originally Posted by Mooly

What area of performance are you trying to improve ?

Sonic...

Originally Posted by kevinkr

Neither! In addition to all the prior comments you want to choose an op-amp with very low DC errors for the DC servo. Choosing an op-amp with spectacular AC performance doesn't make a lot of sense for this application as it is used in an integrator with an HF cut-off well below the lowest audio frequency. (High open loop gain, and low input referred DC errors are what you want.)

A DC-Servo, after all, is a filter and every analog filter it's not a brick-wall so also some audible low frequency could be affected.... isn't it?

Too me both seems far superior to TL071 (see attachment)

Which parameters should I consider?

Originally Posted by kevinkr

I experimented extensively with DC servos in the distant and not too distant past and frankly preferred a really good film coupling cap to the servo.

Sadly the cap the DC-Servo should avoid is 220uF...

[ThorstenL]

Hi,

Originally Posted by ClaveFremen

I want to swap a TL071 used in a DC-Servo circuit with a better perfoming opamp and I'm thinking about OPA627 or OPA827.

Someone has any opinion on it? Which is better betweeen the two?

On paper they appear broadly similar.

The real concern is however is if the Servo is implemented to minimise it's pull range to what is needed.

I recently came across a design which had a 100K feedback resistor and a 22k resistor from the servo into the feedback node. The Servo was a 072 running on 15V rails, the offset it actually corrected caused less than 0.1V to appear on the Servo Op-Amp output.

So the servo was injecting whatever it did into the feedback node with five times as much gain as the actual signal and it was doing so unnecessarily.

Using a 1M resistor to feed in the servo output caused much more voltage on the servo, but no more actual DC output from the circuit, meanwhile the noise, distortion etc. from the servo entering the signal path was reduced by 34dB!

I have yet to apply an OPA627 (I keep a substantial stock, but need to solder them to an SMD Adapter), but I doubt the effect from this would be as large as optimising the servo's pull-range, at least in this case.

Actually, my next step will be to seek a different point in the circuit to feed the Servo signal into which is inside the feedback loop and with a low amplification factor for AC, so any potential impact is further minimised.

Then we can apply a premium OPA, in which case the OPA627/827 is a very good choice, both due to the DC and AC precision.

Ciao T

[pdul]

What about trying a one point DCservo measuring and compensating in the same point at the output using AD712/711. It works well with linestage.

[ThorstenL]

Hi,

Originally Posted by pdul

What about trying a one point DCservo measuring and compensating in the same point at the output using AD712/711. It works well with linestage.

This can work okay, however two things to watch, if the injection node is very low impedance the pull range may not suffice and for some reason sims show often severe infrasonic peaks...

Ciao T

[ClaveFremen]

Thanks Thorsten

The circuit is already done and working, I was investigating for a friend's MyRef Evolution that has a DC Servo and he would like to change the opamp.