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Dc servo circuit for AD815 pre
Dc servo circuit for AD815 pre
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Old 30th November 2020, 02:56 AM   #1
kp93300 is offline kp93300  Malaysia
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Default Dc servo circuit for AD815 pre

I have a constant dc offset of 40mv at the output of this pre. The op amp is TL 072. Tried several other jfet and bipolar op amp but dc offset remains the same
What's the problem ?
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Old 30th November 2020, 03:16 AM   #2
abraxalito is online now abraxalito  United Kingdom
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I would check the voltage at the output of the TL072 to see it hasn't saturated. My first guess is there might be not enough adjustment range for the servo to correct the output with 1Mohm output resistor.

With 15V supplies you might get 13uA maximum which corresponds to 8mV at the AD815's output.
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Old 30th November 2020, 03:26 AM   #3
kp93300 is offline kp93300  Malaysia
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Richard
I use +/- 15 v for both AD 815 and TL 072.
I will lower the value of R2 and report back.
Thank you
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Old 30th November 2020, 03:28 AM   #4
abraxalito is online now abraxalito  United Kingdom
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I'd suggest 100k for R2, it will give 80mV adjustment range.

The AD815 has a typical input bias current (at the non-inverting input) of 2uA (5uA max). If you go for a 47k input resistor that bias current gives almost 100mV offset at the input, assuming AC coupling of the source. Multiplied by the gain you have (roughly X4) that's almost 400mV at the output. This tends to suggest your input R can't be much higher than 10k which is rather a low value for a preamp input resistance. Or alternatively you need a lower than 100k for R2.
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Last edited by abraxalito; 30th November 2020 at 03:45 AM.
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Old 30th November 2020, 04:10 AM   #5
bansuri is offline bansuri  Italy
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put a 100k from input to gnd to accomodate the bias current.
Then check the offset without R2, it should not be more than 12mV. If it is higher the opamp is faulty.
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Old 30th November 2020, 04:14 AM   #6
abraxalito is online now abraxalito  United Kingdom
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Quote:
Originally Posted by bansuri View Post
put a 100k from input to gnd to accomodate the bias current.
With the typical 2uA bias current that'll give 200mV input offset.
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Old 30th November 2020, 04:53 AM   #7
bansuri is offline bansuri  Italy
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sorry I thought both were tl72, why do you use this line driver, it has more distortion and noise than a good audio opamp.
With the high bias current only low input resistors can be used
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Old 30th November 2020, 05:34 AM   #8
abraxalito is online now abraxalito  United Kingdom
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As we're doing DIY here it would be possible to measure the input offset and arrange a current source of the correct polarity to compensate for the bias current. Then a 100k input resistor would be practical - the input impedance is stated as 7M.
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Old 30th November 2020, 08:36 PM   #9
PRR is offline PRR  United States
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Dc servo circuit for AD815 pre
Quote:
Originally Posted by kp93300 View Post
....I will lower the value of R2 and report back.
Quote:
Originally Posted by abraxalito View Post
...check the voltage at the output of the TL072 to see it hasn't saturated.....
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Old 1st December 2020, 12:49 AM   #10
kp93300 is offline kp93300  Malaysia
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l change r R2 to 100k. Now l measure 4 mv . dc at the input of AD 815.
Richard, is 4mv acceptable for the TDA amp from you ?

Bansuri, l am still at the testing stage . It sounds pretty decent with TL 72 as dc servo op amp. What would you recommend , avoiding the expensive ?
Actually, l just like to build things as a hobby. many of my amp do not need pre from an engineering point of view .
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