Sadowsky Preamp - OpAmp

[baouftou]

Hello and thanks for the wonderful work you provide to us for free!!!

I have built 4 sadowsky preamps using Geiri's and JohnK's layouts found here on tagboard.
They are okay but the sounds they produce is gainy and distorted. I've tried to mingle with changing the JFets alot but at the end i find myself stuck. Another problem with them is that they are working with 2n5457 and J201 of which i'v tried both with little differences comparing to the desirable outcome.
So my request is:
Could you please redesign the layout to work with op amps?

Thank you very much!!
baouftou

[Neil mcNasty]

Redesigning the layout for opamp is a bit too much to ask for, as this requires a total reinventing of the circuit, since we are talking two totally different approaches to circuit design.
Nothing would be similar, and it is not as simple as just replacing the transistors with an opamp...

What you can do instead, is to socket the 10K's going to the Drain of the transistors, so that you can find the proper values that will bias them correctly.
Or you could replace them with two 25 or 50K trimmers (wired as a variable resistor), with two leads going to the points where the resistors used to be.
JFet's are notorious for being inconsistent and your problem sounds like a bias issue.
So by slightly adjusting the values, you should be able to get it cleaner.

Another approach would be to get some SMD versions of the transistors with a converter board, since the SMD's are much more accurate in value, compared to the old (trough hole) ones that only has the low quality/off spec available on the market these days.

Hope this helps solve your issue...

[j-pee]

as far as I understand biasing JFETs now, there are two routes to go about it...

a:  using a trim pot, finding a setting at which the VOLTAGE is a given number at the JFET1s drain...

b:  measuring Vgs off and Idss values of a JFET, and then
                   b/1:  choosing ones with desired values, or
                   b/2:  changing certain resistors (at the Source or the Drain)... using calculations, or calculators... (as Neil is suggesting)


in any case, when trying different JFETs, it would be cool to record (and share) the info of their Vgs (off) and IDss values...
so others would know, too...
not to mention that our case (now your case) could be better understood.... and you be helped...
for example, it could be that all those JFETs you tried were pretty similar...

PS:
I do not exactly know yet how it practically works, though! :)