10k Resistor input impedance on FET

[preciousmolina666]

after opamp that has very low output impedance. my next stage will be jfet 2n5458 (gain of 10). Is it necessary to put 1M resistor input impedance on jfet? cant find schematic online lower value. noise and snr is what im concern

[EBK]

Shouldn't be necessary.

But, I'd like to see a schematic to make sure I correctly understand what you are asking.

[preciousmolina666]

i hate noise really do

[merlinb]

10k is fine, it won't cause extra noise when driven from the low source impedance of the opamp.

[antonis]

In theory, you can use as low value resistor as marginally not overloading op-amp output..
(e.g. for a +/- 10mA max output current, +/- 4.5V output swing & no feedback loop current 450Ω should be OK - but, plz, DONT do it..!!  )

IMHO, you could use a dual low-noise op-amp and omit discrete JFET..

[PRR]

You need the cap to block the opamp DC, because it is wrong for the FET.

You need a resistor to give zero V DC bias to the FET. In this case it mostly affects the cap size. 1Meg will not "add hiss", because the low-low opamp impedance (~~131r) will bypass resistor random noise. Then C17 may be 0.01uFd for the same response. This may be a smaller/cheaper part.

[preciousmolina666]

1Meg will not "add hiss", because the low-low opamp impedance (~~131r) will bypass resistor random noise.

is this applicable in transistor or vacuum tube? if i changed the FET? 

IMHO, you could use a dual low-noise op-amp and omit discrete JFET..

i wanna try/hear what FET or transistor distortion sounds like

[antonis]

Separating transistors and FETs confuses me a bit..
(you probably mean BJTs & FETs..) 

NE5534 is an FET input op-amp (i guess..  ) so there isn't any difference from 2N5458 discrete FET amp..
(from input impedance & bias point of view..)

You could try a dual Bi-Fet op-amp (Fet in - BJT out) or even a dual Fet-Fet one with the same 2nd stage gain configuration..

[bool]

NE5534 is a BJT NPN-input opamp.