At your risk.
Now that it is inside a nice aluminum box, frequency response tests are being made using an inverse RIAA network by Lipshitz.
There was +3dB at mid bass!
-20dB at 20KHz!
It looks like the high impedance 470K pot loaded by the input capacitance of the JFET creates this high cut.
So almost like cheating the 100 Ohms resistor was changed to 12K.
Now we have -3dB at 20KHz. There is about +1.5dB at 4KHz.
For correcting the mid bass heaviness 15nF was changed to 20nF.
Now mid bass is flat and we have -3dB at 20Hz.
Even before the changes the preamp sounds very good with a feeling of 2 octaves above CDs, YouTube videos etc. After the changes about 3 octaves higher treble sparkle and no bass heaviness.
What I do like about the sound of this preamplifier is the smoothness of the treble on violins. I think this is achieved by the fact that there is no negative feedback, no active negative feedback filter but a passive instead. So transients will not instantaneously clip (see Matti Otala transient intermodulation distortion papers).
I also like that each stage is at low gain so there is headroom at the input.
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