fet input for power amp?

[greenmanhumming]

I saw this DI circuit by JPK

and i wondered could this basic topology be modified to make an instrument input with gain into a balanced 10k line level amplifier input? I'd like to power from 36v rail, ideally.

 

[chr1s]

If you run it with 36 V you have to increase R6 and R8 which also increases output resistance. Also R7 might be chaged for the different bias.
The circuit doesn't amplify (except for the 6 dB from changing to symmetric) so you need an additional gain stage . (two if you put them after the JFET)


Chris

 

[greenmanhumming]

yep i realised right after I posted that there can't be any gain out of that jfet configured as a phase inverter like that.

higher output Z is fine.

but i think any additional gain would have to be afterwards since the levels required would likely clip that stage, still its a nice simple DI...

 

[ppa]

this DI , as it now is, attenuates the input signal of circa 19.5 dB, but it can be modified for 36V supply and also for having a bit of gain to drive 10K input , but It may need a new stage added and/or that this DI should be very near to amp input.  

 

[ppa]

the schematic can be modified, but the DI msut be connected with very short wires to amp input.
the following devices are changed:

R3= 680K
R8=0 (short circuit)
r7= 1k
R9= 4.7k
C3= omitted
R6= 4.7K or less
R10= 220k
C4= 22uF 50V
C5= 470uF 50V

For long wires to amp input it should added a new buffer stage that I will put in this topic.
 

 

[greenmanhumming]

I see what you mean, but you have to sacrifice the balance output to get some gain.

thats the way I'm going anyway in this case

 

[ppa]

this is the upgrading for having 14 dB of gain with this schematic.



You have asked a simple schematic and I've done this one  

BTW, I'm going to add another stage for having a low impedance balanced output, so you can use normal 600 ohm cables.  

Wait my new schematic, please.