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Old 18th August 2006, 11:01 AM   #1
AndrewT is offline AndrewT  Scotland
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Default alternatives to LF411

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?
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Old 19th August 2006, 03:44 PM   #2
jcx is online now jcx  United States
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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
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Old 19th August 2006, 04:07 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

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

/sreten.
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Old 19th August 2006, 05:03 PM   #4
SY is offline SY  United States
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You want unity gain stability, low offset voltage, and low bias current. The LF411 is just about perfect.
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Old 19th August 2006, 05:11 PM   #5
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You also have the following choices: OP07, OP77 and OP90.
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Old 20th August 2006, 06:26 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
so these are the important specs
Quote:
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?
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Old 20th August 2006, 07:15 AM   #7
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Just choose an opamp with low offset voltage and one that is stable at low gain, and it will work just fine
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Old 20th August 2006, 07:28 AM   #8
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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.
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Old 20th August 2006, 08:58 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
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)?
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Old 20th August 2006, 09:22 AM   #10
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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.
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