Dc servo circuit for AD815 pre

kp93300 · 30th November 2020, 02:56 AM

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 ?

abraxalito · 30th November 2020, 03:16 AM

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.

kp93300 · 30th November 2020, 03:26 AM

Richard
I use +/- 15 v for both AD 815 and TL 072.
I will lower the value of R2 and report back.
Thank you

abraxalito · 03:45 AM

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.

bansuri · 30th November 2020, 04:10 AM

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.

abraxalito · 30th November 2020, 04:14 AM

Quote:

Originally Posted by bansuri

put a 100k from input to gnd to accomodate the bias current.

With the typical 2uA bias current that'll give 200mV input offset.

bansuri · 30th November 2020, 04:53 AM

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

abraxalito · 30th November 2020, 05:34 AM

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.

PRR · 30th November 2020, 08:36 PM

Quote:

Originally Posted by kp93300

....I will lower the value of R2 and report back.

Quote:

Originally Posted by abraxalito

...check the voltage at the output of the TL072 to see it hasn't saturated.....

kp93300 · 1st December 2020, 12:49 AM

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 .

30th November 2020, 03:16 AM	#2
abraxalito diyAudio Member Join Date: Sep 2007 Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...	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. __________________ I know everything you know about this times 1000 - Amir @ ASR

30th November 2020, 03:28 AM	#4
abraxalito diyAudio Member Join Date: Sep 2007 Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...	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. __________________ I know everything you know about this times 1000 - Amir @ ASR Last edited by abraxalito; 30th November 2020 at 03:45 AM.

30th November 2020, 05:34 AM	#8
abraxalito diyAudio Member Join Date: Sep 2007 Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...	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. __________________ I know everything you know about this times 1000 - Amir @ ASR

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30th November 2020, 03:26 AM	#3
kp93300 diyAudio Member Join Date: Mar 2007	Richard I use +/- 15 v for both AD 815 and TL 072. I will lower the value of R2 and report back. Thank you

30th November 2020, 04:10 AM	#5
bansuri diyAudio Member Join Date: Aug 2008 Location: Dolceacqua	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.

30th November 2020, 04:53 AM	#7
bansuri diyAudio Member Join Date: Aug 2008 Location: Dolceacqua	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

1st December 2020, 12:49 AM	#10
kp93300 diyAudio Member Join Date: Mar 2007	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 .