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TL072: Audio preamplifier

Tsvetan Ginin
Intellectual 360 points
Part Number: TL072

Hi,

The attached circuit is designed for a power supply of 9 volts, and the power supply of the class D amplifier, TPA3251D2 is 26 volts. Can I use 26 volts without conversion (down to 9 volts) to power the preamp with some changes to the circuit? The microphone, that will be connected to the preamp has an impedance of 200 ohms. Is it matched to the input of the preamp? Can I replace POT1 with a digital potentiometer without significant changes in the circuit? Please also for a quick check of the circuit, because I am not familiar in depth with the audio circuitry.

Best regards,

Tsvetan

PREAMPLIFIER_SCH.pdf

  • 0
    Guru 140200 points

    Hi Tsvetan,

    R29 and R38 (3k3) look like pulling up the microphone signal to the supply voltage. For a dynamic 200R microphone this is wrong. They should not go to the supply voltage but to signal ground. Then they form a standard 3k3 load which is a proper load for a dynamic 200R microphone. Or shall an electret microphone be connected to the input and R29 and R38 is needed for the powering?

    IC5 needs a clean, stable and noise-free supply voltage, because the supply voltage is coupled into the signal chain via R30/R33 and R39/R40. There's a low pass filter and the 200R dynamic microphone is in parallel to R33, so that the supply voltage noise is not directly coupled into the signal chain. On the other hand, IC50 provides a very high gain. So I would still use a regulator (9V or so) for the analog circuit.

    POT1 can be exchanged by a digital pot. But keep in mind that it must be able to handle true AC signals.

    Kai

  • 0 in reply to kai klaas69
    Intellectual 360 points

    Thanks for the quick reply Kai!

    Thanks also for the explanations.
    Can I get a link to reference designs of amplifiers with digital volume control or AGC?

    Regards,

    Tsvetan

  • 0 in reply to Tsvetan Ginin
    Guru 140200 points

    Hi Tsvetan,

    you might want to have a look at these sites:

    Kai