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I just came across this tube RIAA preamp on which there is a coil on the input capacitor, can someone please explain what's the reason of such thing? The schematic and the actual circuit bellow. enter image description here

enter image description here

The site with the full circuit and details is: http://www.colvero.com/valvulados/Pre_Amplificador/index.htm it's in portuguese thou.

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  • \$\begingroup\$ It doesn't seem to make sense - espiras = coils/turns/windings in case anyone needs a translation. \$\endgroup\$
    – Andy aka
    Jun 10, 2020 at 15:30
  • \$\begingroup\$ That's right, the portuguese for coil/turns it's espiras, indeed. \$\endgroup\$ Jun 10, 2020 at 15:33
  • \$\begingroup\$ But, then again, why would one wind 4 turns over a capacitor? \$\endgroup\$ Jun 10, 2020 at 15:34
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    \$\begingroup\$ Maybe it's a bit of (capacitive) negative feedback to keep the amplifier from oscillating when the input is left open? \$\endgroup\$ Jun 10, 2020 at 15:40
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    \$\begingroup\$ @SpehroPefhany Something like that. Note that the underlying foil capacitor has an outside foil connected to one of its leads. In a sense, this capacitor has a sorta-polarity - which way you install it matters. I'd be interested to know if it matters which way you wind the coil. \$\endgroup\$
    – glen_geek
    Jun 10, 2020 at 15:45

1 Answer 1

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Those few turns look to be a "gimmick" capacitor. Should behave as small (a few pF) capacitor between plate and grid. Provides some degree of negative feedback at high frequencies. Given the relatively high Miller capacitance of the triodes I'm not sure it has a lot of effect. What effect it has will also be dependent on the impedance connected to the I/P of the circuit. Interesting though.

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