I've been working on the inverted cascode idea on and off for the last year but Marks reports of its sound as being slightly poor on tone compared to a SUT and JCs use of a SUT input have rather put me off building it (plus not having had the time until this autumn...) so I've been thinking about what to build - a FET/valve cascode a la Nick (and others!) a PNP bipolar inverted cascode or something different???
Back when I had an audio amplifier company we made a mc head amp that used common-base input circuitry and it sounded extremely good - in my head its the reference sound for mc head amps or SUTs and has never been equaled (but long term acoustic memory is unreliable so take this with a pinch of salt). Anyway it made me look at common base input phono amps and, of course, the famous Leach design... This inspired me to think of how I would build something like this now... Well I have some nice low noise JFETs which should be better than using bipolars so how about a JFET version - biasing would be easier if I used selected matched Idss complimentary pairs... Nice so I came up with a design for the front end as attached...
It uses two 9V batteries for power. The interesting thing is that it looks like the signal reaches the output through the two lots of capacitors... and whilst these caps are in the current loop of the signal path they experience no voltage modulation! The batteries are completely floating and the top and bottom signal lines move up and down in step with each other leaving no voltage modulation of the caps
Since it is voltage modulation of capacitors that causes distortion in capacitors we essentially get a free ride through the caps
(I'm sure there are some very low level effects due to the very small current modulation of the cap but all the normal distortion generating mechanisms for caps are voltage driven)
The two resistors from the top and bottom rails are very high value and serve only to define the midpoint between the two rails - signal wise the caps completely bypass them - the Gm of the FETs works with the output resistor to develop the output signal voltage and that drives the next stage. Input impedance looks like about 20-25 ohms for the FETS I have (LSK170 and LSJ74) Idss looks like 6mA or so.... For more gain and lower noise one could add parallel comp. pairs to the amp - the price being current consumption and therefore battery life...
Next step will be to use this to feed a Pentode input RIAA stage - so another Gm amp...
Oh this isn't new config - its heavily based on the Leach mc pre and the two comp FETs are used by Nelson Pass and many, many others in this sort of arrangement but it the first time I've used it for an mc frontend
It should sound very natural and it should have very low distortion - we'll see...
more to follow as I design the rest of the phono amp.
Its not built yet so if I've dropped any clangers please shout!
J
edited to correct drawing!