SIT Variations

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Hoping to revive interest in the 2SK82 SIT , attached are some pics and schematic of my efforts. The circuit is by no means original, borrowing heavily from Andrea Ciuffoli's Power Follower 99 design, sans cap multiplier and active CCS. Having built a Hammond 193 inductor version of L'Amp, I was intriqued by the concept of a single stage, single device amplifier. This concept has been beaten to death (see the OTA and D'Lite threads) but as far as I can tell there hasn't been a 2SK82 implementation.
The circuit as presented is circumscribed by my self- imposed restrictions ie, only one active device per channel with a minimum of components and is not for everyone.
There is no voltage gain and efficiency is very poor - about 3 to 5 watts of usable power and a low output impedance preamp able to swing significant voltage is required. What the circuit offers is a level of transparency I have not encountered before and I have been sufficiently impressed to invest the effort to house my original breadboarded design into something half-way presentable.
For those who care of such things, there is no switch-on thump - the device ramps up nicely and input signal phase is preserved.


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Hi Zen Mod,

I kinda suspected that the proliferation of chokes would appeal to you !
Having ruled out an active CCS, my only alternative was a plain old resistor acting under ideal constant voltage conditions. But then again no active voltage regulation for me so the next best thing was passive choke regulation.
You will notice that even though there is a choke preceding the first cap, this is not a traditional choke input configuration. For one thing the choke value is way too small and a properly sized choke would drop too much voltage. This first choke functions as a kind of 'shock-absorber', buffering the first cap from the charging surges - I use this often in my tube designs.
The power supply as configured performs far better than I expected with the added benefit of extremely low hum and noise level - at the lower limits of my measurement capabilities. No doubt the over-sized Antek transformer with static shield helps.
In operation there is total silence even when I stick my head into my 100db plus efficient horns !
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The 2SK82 has less transconductance surely. When R085 looks like 4-5 Siemens, right? That would give better THD and Zo to those amps shown here. Still R085, PapaFET, and 2SK82, are all depletion mode with "ohmic" ID/VDS curves, correct? I was meaning to ask how close R085 to PapaFET may be. There are things to differ like Ciss, gm, ID/VDS intrinsic linearity. I would expect the custom PapaFET to be the best tweaked one for audio.
diyAudio Chief Moderator
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Which one type exactly is on the curve tracer chart Michael? R085? Can't see the few amps region curves in so high amperage parts datasheets well anyway. They look very high gm at the many amps too but they should cut a lot of throttle when down low.
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