Jfet Boost to test my Jfet transistors

[JackSkellington]

Hello,
I have on my breadboard this:



Just some difference because I miss some parts.
R1 is 2.2M
R2 is 12k
C3 is 33uF

I might have just a bit of more boosting. Over 20db.
Sooner or later I want to build it to use it, but even to test some jfet transistors.

The transistors I'm using now is ok, and I'm testing the circuit on a solid state amp.
It works, but it distorts a bit, I assume it was a clean boost.
I just want to know if it is ok.

Thanks.

[mauman]

Yes, this looks OK.  It's a Tillman preamp/boost, with a few values changed, and with an added bypass capacitor C3 that increases the boost. This circuit inverts the polarity and will boost up to about 20 dB as you said.  With the component values you give us, the low-end corner (-3dB point) is about 18.5 Hz, and the response is flat to beyond 100 kHz.  Using a +9V supply and a J201 FET, the output waveform is clean for a relatively small input signal (below 200 mV peak to peak) and gradually distorts as the input level approaches 500 mV P-P.  You will certainly hear the distortion by the time you input 1V P-P.  The hard clipping limits are around 3V positive and 4.8V negative, so the maximum output regardless of input will be around 7.8V P-P.

[JackSkellington]

Thanks for the theory.
So, if all this, in the real life, sounds like a mild overdrive is ok.

I have to find my old Fetzer Valve.



That is not so different, too. The input cap is replaced by a 68k (that could tame the input signal), and I guess I used a J201. Drain voltage set at 4.5v.
I remember it sounded totally clean.