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Old 1st September 2004, 12:25 AM   #1
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Default Help: Opamp Question/Problem in Theta DAC

I just removed the original LF353N from my Theta Pro Basic 3 DAC and replaced with DIP adapter and OPA2604. The improvement is amazing, everything just become more 3D and realistic. However, when playing female vocals, at very very extreme high notes, I hear some noise; the noise sound like what you will get when you have input overload but very brief. When the high notes come down, the noise goes away. I replaced the OPA2604 with OP275, OPA2111 and the noise still exist. I played the CD in my friend's DAC and couldn't hear the noise so I know it's not the CD. Can anyone give me some idea what's could go wrong?

Here's what I get out of the circuit following the traces.
I/V -> Buffer -> some kind of filter which feeds back to I/V
AD841 -> LM6321 -> LM353 (Original)
AD841 -> LM6321 -> OPA2604

Thank you for your input.
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Old 1st September 2004, 02:16 PM   #2
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Just found out more info....

I/V -> Buffer -> DC Servo
AD841 -> LM6321 -> OPA2604

The I/V has a 5K + 470n in the feedback
Output of I/V feeds through a 1.5k resistor to LM6321
DC Servo consist of 47uf/47.5K looping output of LM6321 and input of AD841

Can anyone suggest a solution to eliminate the extreme high female vocal frequency noise?
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Old 6th September 2004, 11:37 AM   #3
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propably your 2-604 is too slow for the servo, thus the problem in the high freq.s. you could use sth like op27 instead or something in the range..
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i am currently using 60,000 uF of capacitance in my preamplifier..
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