Probably lacking "stones", even though the triode is excellent.
I'm having PC trouble and using my wife's laptop. Currently, I can't upload graphics. Scan the archives for my posts about the tweaked RCA phono preamp. That shows how to DC couple an enhancement mode MOSFET to a triode.
As you will be driving a LOW impedance load, a LARGE O/P coupling cap. is needed. A 15 muF. metallized polypropylene part bypassed by a 1.5 muF. PPFX MultiCap allows for an obscenely low 2KOhm downstream load. Running an IRFBC20 with 25 mA. of drain current will get you an O/P impedance well under 100 Ohms. A 1 KOhm net downstream load, which is worse than obscene, is quite acceptable. 😀
The 2kΩ load, I assume you are refering to the MOSFET? And the large coupling cap is to go between the tube and the MOSFET? So no DC coupling?
When I was building the F4 (actually, still am), I contemplated building the JFET input stage into the line stage instead of the power amp. It is basically a the B1 buffer. But I decided against building funky stuffs to minimise interoperability problems. 😀
If the cable capacitance is the only issue, I can consider switching the output, and/or try to limit the cable length. I haven't build this so all options are on the table.
When I was building the F4 (actually, still am), I contemplated building the JFET input stage into the line stage instead of the power amp. It is basically a the B1 buffer. But I decided against building funky stuffs to minimise interoperability problems. 😀
If the cable capacitance is the only issue, I can consider switching the output, and/or try to limit the cable length. I haven't build this so all options are on the table.
The big coupling cap. connects the MOSFET to the O/P. Dig into the archives for the phono preamp schematic. Except for the gate stopper, the FET's gate is connected directly to the triode's plate. That most definitely is DC coupling.
- Status
- Not open for further replies.