alternatives to LF411

[AndrewT]

Hi,
is the OPA228 a good alternative to the LF411 for use as a DC servo in a power amp?

or would the unity gain stable version OPA227 be a better choice?

Are there other cheap alternatives?

[jcx]

How about applying the 1st order "tests" of op amp substitution:

Fet or Bjt input?

Unity Gain Sable?

Similar GBW?

I believe you can quickly see that your proposed replacemnt fails all ways - specific circuits may work with different type op amps but in general you don't want to change these classifications when substituing

[sreten]

Hi,

Sorry but am I missing something ?
Selecting an op-amp for a DC-servo ?
Just about anything will do AFAIK.

/sreten.

[SY]

You want unity gain stability, low offset voltage, and low bias current. The LF411 is just about perfect.

[Samuel Jayaraj]

You also have the following choices: OP07, OP77 and OP90.

[AndrewT]

Hi,
so these are the important specs

unity gain stability, low offset voltage, and low bias current

for a DC servo?

In which case the OPA227 is better than OPA228. Except that it's 70% more expensive (£1.76 vs £1.07 each).

I note that 411 has a highish GBW product @15MHz for a unity gain. Is this relatively unimportant?

[Jan Dupont]

Just choose an opamp with low offset voltage and one that is stable at low gain, and it will work just fine

[peranders]

Not only at low gain, more like unity gain. That is important. Offset voltage is not important because no opamp will have more than 5-10 mV and the limit is < 50 mV (according to me).

One other criteria is also low distortion up to 1 kHz but this is fulfilled with just about any decent type.

My recommendation is, input bias current < 25 nA, SR < 10-15 V/us (= not too fast)

One more thing to think of is max output power at min frequency, this should not saturate the servo. Let's say the absolute max power is 100 W (8 ohms) and the choosen min frequency is 20 Hz, this will cause the servo to heavy action but it must work within it's limits otherwise you'll get some distortion due to a servo which clips.

[AndrewT]

Hi,

choosen min frequency is 20 Hz, this will cause the servo to heavy action but it must work within it's limits otherwise you'll get some distortion due to a servo which clips

if the servo rolls off at about 1Hz then how onerous is the above requirement?

What if 10Hz at full power were generated by the source (film sound effect)?

[peranders]

To understand this you must look what happens at the servo output. If you have 10 Hz 100 W/8ohms, how much signal goes through the servo?

To understand this better I'll recommend you to take a peek at my simulation files for the DC servo. They are somewhere in the link below.

http://www.diyaudio.com/forums/searc...der=descending

EDIT: here... http://www.diyaudio.com/forums/showt...728#post376728

If you simulate the servo you can directly see how the signal looks like around the servo.