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I'm trying get an audio signal from a high impedance source (guitar or mic) to an ADC.

Some casual research suggested that I should filter my signal before amplifying it.

My current mental design is: Source -> Buffer (optional depending on impedance) -> Butterworth filter -> Boost -> ADC.

  1. Is this the "correct" way to design this circuit?

  2. Does it make sense to throw in an RC filter (or a second active filter), e.g. before the ADC?

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    \$\begingroup\$ considering potential interference yes to above and possibly CM choke \$\endgroup\$
    – D.A.S.
    Commented Apr 2, 2018 at 21:05
  • \$\begingroup\$ Yes to both questions? @TonyStewart.EEsince'75 \$\endgroup\$
    – tiftik
    Commented Apr 2, 2018 at 21:27
  • \$\begingroup\$ It could be combined into one circuit that buffers, filters and boosts gain remotely and consider with large CM choke for improved balance. PS noise than is common mode is often hard to track down and eliminate \$\endgroup\$
    – D.A.S.
    Commented Apr 2, 2018 at 21:56
  • \$\begingroup\$ Transducers (mediates between some physical phenomenon and electrical signalling of some kind) often require some kind of specialized circuitry that maximizes some desired parameters. For example, a guitar uses variable reluctance and the magnitude of the signal is \$\propto\$ velocity. So, one may need to design a pre-amplifier (though many do not so) that provides the right frequency pre-emphasis as well as handling the details of the type of input well. So the first step would be to design a circuit tailored for the type of input device. Noise may drive choices in the design (JFET vs BJT..) \$\endgroup\$
    – jonk
    Commented Apr 2, 2018 at 23:32

2 Answers 2

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There isn't a single answer. It's clear that "Buffer" comes first, and "Sample" comes last. The choice of amplify/filter depends on exactly how much you need to amplify, and how much you need to filter.

If noise/hum/etc is greater than your signal, by all means filter first. If you don't, you'll lose headroom on your amplification stage. If the unwanted component is smaller than your desired signal, a lot depends on how tricky your filter needs to be. If you're just using a simple RC filter, then you might as well put it before the amplifier. If you're trying to use a sophisticated filter, you're generally better off giving yourself as much noise margin as possible by filtering after amplification.

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  1. Buffer (to take and save the signal from the guitar because it has an high IN impedance and lower OUT impedance, so the signal goes all on it.)
  2. Filter (to clean the signal)
  3. Amplify (to amplify the signal)
  4. Sample (to digitalize the signal)
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    \$\begingroup\$ Could you explain why? \$\endgroup\$
    – C_Elegans
    Commented Apr 2, 2018 at 21:36
  • \$\begingroup\$ @C_Elegans no problem \$\endgroup\$
    – Martino Pistis
    Commented Apr 2, 2018 at 21:39
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    \$\begingroup\$ Right at the guitar leads, have some RFI filtering, to keep cellphone energy away from the fast (rectifying) junctions of ICs. \$\endgroup\$
    – analogsystemsrf
    Commented Apr 3, 2018 at 3:10

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