alternatives to LF411

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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
 
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.
 
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)?
 
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/sear...62682389&sortby=lastpost&sortorder=descending

EDIT: here... :nod: http://www.diyaudio.com/forums/showthread.php?postid=376728#post376728

If you simulate the servo you can directly see how the signal looks like around the servo.
 
Notice that the servo has lower supply voltage than the main amp.

Set the input voltage so you'll get desired output voltage (power). Then make a frequency sweep and observe the servo output. If the servo goes clipping you must make it slower.

As mentioned you must deside max output voltage(power) at the lowest interesting frequency. 10 Hz at full power isn't particular important. 20-25 Hz is probably a better choice.
 
AndrewT said:

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

You may have dropped a decimal place somewhere - LF411 is "min GBW 3 MHz", typ ~ 4 MHz - other than offset V this is a early fet input "jelly bean" part

nA bias current could be bad if your servo uses MegaOhm input R to get long time constant with NP0 or small film cap

there are too many cheap FET and Cmos input op amps to count - looking for Vos trimmed to < 1mV will narrow your choices but then they may qualify as "precision" op amps and command a price premium
 
Hi Jcx,
I follow your reasoning.
That's why I started looking for an alternative, but without knowing the really important specs.
I can get 228 for the same price as 411.

The data sheet says a little bit of feedback capacitance corrects the stability for gain of +2 or -2, but omits data for +1 or -1.

The DC servo uses the opamp as integrator, leading me to suspect that the bigger feedback capacitance needed will sort the unity gain requirement.

Yes, there appears to be different data for 411, confusing.
 
the feedback integrator cap is a short circuit near the op amp's unity gain frequency - therefore it imposes a requirement for a unity gain stable op amp

too fast a subtitute means other supply bypass and output loading issues can become problems, probably a 2x faster device wouldn't be too risky - opa132/4 if you can get low Vos, on the lower side of GBW it is likely the inegrator doesn't really care so even <1 MHz low power devices could work

but what's the probelm with buying a lf411 - still being made and as a replacement you shouldn't even have worry about Rhos supply silliness
 
Andrew, you can manage with almost any opamp because you can use resistor values of 47-100 k + 1 uF to get sufficiently low speeds. This will have no effect of 25 nA input bias current.

Change my simulation files to the setup you have. Since it's almost DC it will be a very accurate simualtion. Have you noticed that if you decrease the adjustment range of the servo, it becomes also slower. The only real important thing is a 100% stable opamp at the gain of 1.

My values are perfect for me and AD8620 and OPA2134 will work splendid for me.
 
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