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"These are the lowest distortion big power transistors I have seen.  To give you a sense of the difference, this graph shows the distortion between two different transistors operated under identical conditions in an F2 amplifier. Here we see the popular IRFP240 N channel Mosfet (top curve) versus a SemiSouth SJEP120R125 N channel JFET, both operated in Common Source mode, biased at 10 Volts DC and 2 amps by a constant current source and driving an 8-ohm load without feedback. 

"Zowie! The JFET is about 20dB better. At the higher power level in this particular circuit, you will see that the difference declines. That is simply the effect of the triode-like character of the JFET at lower voltages across the device – you can take care of that with a couple of extra volts on the supply. And with a 1200-volt rating, you have some margin to burn, in fact you could stick this in where a 300B used to go. Not that I would do such a thing. Also, the distortion remains much lower at high frequencies as well. This part has about half the input capacitance and about 2/3 the reverse capacitance of the IRFP240 and the value is more linear so in typical amplifiers, we see about a quarter of the distortion due to junction capacitances."

Here's the distortion into 8 ohms at 1Khz from 0.1 watts to 30 watts.

Here's the distortion into 4 ohms at 1Khz.

Here's the distortion into 8 ohms at 1 watt from 20 to 20Khz.

Here's what that second harmonic distortion looks like.

Here's the frequency response from 10 to 100Khz

Here's the square wave at 20kHz.

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