You would need p-channel jfets, so that narrows your choices considerably. I'm also running the input fet diff pair at 10ma/side for higher gain/less distortion, which makes it more difficult for a p-channel jfet. Your only viable choices would be J175, or high-ranked 2SJ/LSJ75. If you flipped the whole schematic around and went with n-channel inputs and P-channel output devices, that may open up your options.
Me, I'll just stick with mosfet inputs, as they are readily available, and I can run as much tail current as I like...
Me, I'll just stick with mosfet inputs, as they are readily available, and I can run as much tail current as I like...
I now have a lineup of small chassis w/heatsink as options for housing the amplifier. I plan to use my first series of boards that are small enough to fit inside the case, with room for a power supply. This time I plan to use a 24V/5A switching adapter to power the positive rail, with a switching DC-DC inside the case to provide the negative rail. The result should be light in heft, but powerful in output.
Here's the DC-DC converter for generating the negative rail for the Mini-Me. This started out as a general purpose converter for generating B+ (2 flavors) Bias, and filament for tube amplifiers, with input from a 24V switching adapter.
Here, just the filament supply is populated, as it has the largest rectifiers. The transformer is wound to generate a -24V rail from a +24V supply.
Here, just the filament supply is populated, as it has the largest rectifiers. The transformer is wound to generate a -24V rail from a +24V supply.
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I took a little time yesterday to revamp one of my DC-DC converter designs to dedicate it to high power negative rail duty. The result is a converter with smaller board area and less height to better fit available small die cast boxes for convenient shielding. Boards are on order.Some fairly nice Alps pots are also on order for front panel duty.
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Here is an alternate -24V DC-DC converter I'm trying as a speculative exercise. The setup is essentially a current fed Royer self-oscillating converter. I have the thing operating on the bench presently as a 3-dimensional cat's cradle of parts. If I can improve the MOSFET switching times, this option might be a viable alternative for a DC-DC converter for generating a -24V supply. Since the transformer is allowed to saturate, it can be very compact. Also, both the transformer and inductor can be constructed at home using available ferrite and powdered iron toroids. I may try using some fancy-pants square loop ferrite material to speed up the switching times, though if I can get adequate performance with readily available materials by tweaking the circuit values, I may not bother, since the square loop material is hard to get.
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