RFI filter and DC blocking cap
In many books on audio theory, line inputs have RFI filter and
DC blocking capacitor, but a lot of practical circuit I found doesn't use them.
I'm planning to build a preamp (op-amp based) with some inputs
(CD, DVD, tuner, no tubes or phono source) and I'm asking if this
circuitry are essential;
and if they are, is it better to use a filter on every input or just
one after the input selector?
If they were essential then they would have to be present in all circuits. They are not essential, but they are very useful. You can omit the DC block if you can guarantee that all sources include a DC block themselves. Some sources use a DC servo instead, but I would not trust a servo as much as a cap.
RFI filters are useful unless you can guarantee that the equipment will never be used near a transmitter, including broadcast, mobile, taxis firms, Wi-Fi etc. etc. Putting them on each input can stop RF getting into the box. After the selector can help but the RF is already inside the screen by then.
'Practical circuits' may be designed by professionals who want to save money, or amateurs who simply want an item to work OK in their particular system. Neither can necessarily be regarded as best practice.
I recommend RF filtering at the input of every piece of equipment.
I do it in two stages.
22pF to 47pF across the input socket of analogue audio frequency inputs. Pretty obviously this does not apply to the input to a TV or FM radio.
appropriate RC at the input to the PCB/Amplifier. This is also where I fit the High Pass filtering (DC blocker).
If you intend to use the highest quality components and to cover all reasonable source types, then it is cheaper to locate the filtering at the input of the preamp rather than inside every source component you have.
Thank you very much.
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