increasing gain in preamp fet / op

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Hello


I'm looking at ways to increase the clean gain of channel green in the schematic below,it does not have to be variable though it would be good to dial it in then set with fix resistors.
I thought the fet would be best as its later in the circuit. I looked for info on the net but can only find stuff for the designs where signal comes out from drain where this design is from the source. and of course alot of formulas i cant understand. I am guessing there is away to increase the gain on the fet using the resistor network R4/R5 in the second schematic copy but have no idea where to start and if the divider values must stay equal or if R3/1M should change as well.


the other option I though was to add gain to the first op stage though im not sure what values to use for R1 or R2 or if R1 should connect to ground or v-ref or some where else



i think the last op stage with the switched phase is to complicated for me to increase the gain on. I have tried with out any luck.


Any advice on where to start would be appreciated, thanks
 

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the other option I though was to add gain to the first op stage though im not sure
what values to use for R1 or R2 or if R1 should connect to ground or v-ref or some where else

R1 must go to a capacitor to ground. The positive capacitor terminal goes to the resistor.

The gain factor of that stage is (1+R2/R1). For example, let R1=10k and C =100uF.

If R2=90k, the gain is x10, or 20dB.

The source follower fet stage won't add any gain, leave it out.
 
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This gives 10dB
hello steveu,


do you think the phase switching will still work now the resistors to the inverting input and non inverting input are different values or should i change the 220k on the non inverting input to 68k as well to keep them matched? here is adjusted schem
 

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The 220K to the positive input and switch just needs to be high enough so that it doesn't load the JFET buffer (or Green return if you remove the buffer), but not so high than it drops much when the switch is open.
BTW, Gain+ is (220+100)/100 AND Gain- is 220/68,
so if you wanted 20dB
then you get 220K, 22K, 24.45K but 20dB could mean clipping, especially with only 9V power.
You may want 2x9V = 18V for 6dB more headroom. I could suggest a bunch of things but if it works for you, then it's fine.
 
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