Here's a circuit for a simple 5X line amp that has good distortion in simulation. Dissipation in the ZVP3306 output fet is ~0.5W, right at the reasonable limit for the e-line package.
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I replaced the input BSS159 with a cheap jfet (J112) and got distortion results that were somewhat higher( 0.0004% instead of 0.00006%), but dominated by 2nd harmonic instead of 3rd. I'll post results when I have a chance to fiddle with the circuit a little more. Still, not bad for a circuit having only 3 active parts.
Noise performance of MOSFET is not very well characterized in simulations as well as datasheets. It would be be interesting to discover the difference between the JFET and the BSS159. Increasing the gain of the circuit would help determine that in an actual build. LF noise is supposedly worse in MOSFET, but how much worse?
Remember that this is a 5X line amp, not an RIAA input stage. Also, mosfets are better than they used to be. The designer for the Creek RIAA amplifiers actually used a depletion mode mosfet in one of his circuits
In reply to Mark, here are the results with 0.4V excitation (2V out) - still PDG for a circuit with only 3 active elements.
In reply to Mark, here are the results with 0.4V excitation (2V out) - still PDG for a circuit with only 3 active elements.
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Yes but 0.1V in, 0.5V out is on the lowish side for line level. Distortion can rise rapidly with level and its best to look at the reasonable worst case situation I think, and here the performance is still pretty good for such a simple circuit - whether the real devices live up to this is another matter of course.
I design my amplifiers for full output at 1V input, so not so far off... The modest amount of gain is for amplifiers that are more sluggish.
Since this circuit doesn't have a whole lot of open loop gain, it is reasonable to expect the THD to rise with increasing drive level. 3 X THD for 4X output drive level is not too bad.
Since this circuit doesn't have a whole lot of open loop gain, it is reasonable to expect the THD to rise with increasing drive level. 3 X THD for 4X output drive level is not too bad.