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TL072: Audio preamplifier

Part Number: TL072
Other Parts Discussed in Thread: TL074H, TPA3116D2, TLV9054

Hi,

A problem in the attached circuit is the pop noise, that is heard in the speakers at the output of IC1, received at its input, perhaps from the incorrect design in AC. I think adding the elements shown in purple with a dashed line (capacitors 1.0uF and 22pF), could solve this problem. The noise appears only when the POT3, stereo potentiometer is turned or the IC7 and IC8 are digitally changed. When pins 1 and 8 of the digital pots are shortened, the noise disappears. I doubt that the change in impedance with decreasing / increasing internal resistance of IC7 and IC8 causes this noise.

I have another question: 

Whether the use of autozero OPA scheme(shown below on left edge of a sheet) could solve the problem?

Regards,

Tsvetan

Sheet2_SCH.pdf

  • Hi Tsevetan,

    there are several mistakes in your circuit:

    1. The wiper of POT3 is connected to a 3.3V DC voltage. This makes noise when moving the wiper. Insert 1µF AC coupling caps at R64 and R66.

    2. The digipots see a negative voltage at pin 8. This violates the absolute maximum ratings of digipots, as the voltage at pin 8 must be within the supply voltage.

    Kai

  • Hello Tsvetan,

    Kai's comments are valid. I would like to add the following comments.

    The TL072 common mode of 3.3V is not a good choice. the VCM should be 4V or more greeter than the VCC- pin.

    I marked the 3.3V planned operating point. See how this point is on the input offset error cliff.

    I do not know the supply voltage value. What is the op amp supply voltage? The data sheet suggest that it should be at least 10V.  The TL072 is not a rail to rail output so that will limit the maximum amplitude of the amplifier circuit.

    The soon to be available as sample TL074H is rail to rail output and minimum VCM and minimum VCC are more compatible with lower voltages. We also have other (available now) op amps that could be implemented.

  • Hi Kai and Ronald,

    At first I want to thank you for the your help and advices. I changed the scheme, but the following questions arose:


    - How to move the lower end of the digital pots to the appropriate voltage, without violating its maximum ratings? Is there another way to avoid the negative voltage on it? The maximum allowable input voltage of TPA3116D2 is 6.3 volts.

    - How can I add mute for each channel separately, avoiding the pop/click effect?

    Thanks,

    Tsvetan

    4237.Sheet2_SCH.pdf

  • Tsvetan,

    The op amp could be changed to TLV9054 or similar and it can run on same supply as the digital pots.

    Have you tried setting digital pot to minimum (full mute) or -20dB lower setting (semi mute). I'm not sure if this would pop or not.