Electronic – Using digital pot to control gain of hi-fi audio op amps

digital-logicoperational-amplifierpotentiometer

I'm looking to use a digital potentiometer (AD5206) to control the op-amps on the hi-fi audio XMOS board (schematic can be found at bottom of document). I'm wondering what would be the best way to go about implementing it:

  1. Keep the fixed gain at the opamp itself, then use the digital pot to 'attenuate' the input. So the digital pot would be placed at the input to vary the level of the output signal.

    Advantage – Allows me to fairly accurately control the upper limit avoiding clipping; any capacitance in the digital pot itself would be AC coupled to the opamp input anyways and not going to matter at the output.

    Disadvantage – ?

  2. Use the digital pot in the feedback loop controlling the gain of the opamp.

    Advantage – ?

    Disadvantage – Any capacitance caused by the digital pot is going to affect the high freq roll off; If the digital pot is a break-before-make type, it'll cause the loop to open up for a sub-fraction of a second causing the output to saturate and from which it'll likely need another fraction of a second to recover from; lower limit isn't easily controllable.

Best Answer

2) Use the digital pot in the feedback loop controlling the gain of the opamp.

it is generally recommended against this approach due to its altering of the circuit's feedback characteristics. and some opamps may not be stable in <1x gain.

I would take the first approach instead.

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