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.

  • \$\begingroup\$ The circuit you talk about covers 9 pages so copy and paste the section you want and embed as a picture in your question. \$\endgroup\$ – Andy aka Mar 6 '17 at 8:18
  • \$\begingroup\$ Whether you were to use it in the feedback loop to vary gain, or as an attenuator, adding zero crossing detection and only changing at zero would minimize audiable glitches. There are some digital pot like ICs such as Ti's PGA2310 which do this for you. \$\endgroup\$ – Colin Mar 6 '17 at 8:50

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.

  • \$\begingroup\$ You always have maximum noise gain with this approach. \$\endgroup\$ – Colin Mar 6 '17 at 11:46

If you wanna go really hifi you could try to use the digital pot to control the Voltage on the Ec-Pin of a dedicated VCA-chip like the THAT2181, like they are usually implemented in high quality mixing consoles and similar things. Using a dedicated VCA chip can be really give a big improvement depending on your demands (noise, offsets, drift, harmonic distorsion, ...)


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