I was going to add this to the vexing hum thread, but that's about run its course. I made an LTSpice simulation of the OP's circuit, but I replaced the tubes with 2N5485 NJFETs and used a 48 VDC power supply (although it will simulate at 280 VDC). On a frequency sweep from 20-20kHz, it has 41.49 dB gain at 20 Hz, leveling at 41.76 dB at 200 Hz, and fairly flat with just a little droop at 20 kHz. Not bad. It works well even on a 9 VDC supply for
20 mV in and +/-2 volts P-P output. The ascii file is at the end of this post.I also designed an amplifier with a single MOSFET, FDS6961A, and it has
40.84 dB gain at 20 Hz, flattens at 40 Hz to 40.80 dB, and just starts to roll off at 30 kHz to 40.77 dB. That's at 12 VDC supply, but it's OK on 9 VDC with 40.5 to 40.45 dB. This ascii file is also at the end of the post. I did tweak it a bit to get the flat response, but it's a very simple circuit and will handle 25 mV for +/- 2.5 volts P-P output.For all the trouble of a three tube preamp with transformer power supply and power cords and all that, this amplifier could be built in a box about the size of a pack of cigarettes with a 9V battery or an external 9-12 VDC wall wart. Why tubes?
Paul
========================================================================================= The original preamp using JFETs:
Version 4 SHEET 1 880 712 WIRE 256 -112 -144 -112 WIRE 256 -64 256 -112 WIRE 256 64 256 16 WIRE 208 128 128 128 WIRE 128 144 128 128 WIRE -144 160 -144 -112 WIRE 256 192 256 160 WIRE 128 288 128 224 WIRE 256 288 256 272 WIRE 256 288 128 288 WIRE 368 288 256 288 WIRE 528 288 512 288 WIRE 256 304 256 288 WIRE 0 400 -64 400 WIRE 112 400 64 400 WIRE 256 400 256 384 WIRE -64 448 -64 400 WIRE 112 464 112 400 WIRE 128 464 112 464 WIRE 528 464 528 288 WIRE 112 512 112 464 WIRE 256 528 256 496 WIRE -144 640 -144 240 WIRE -64 640 -64 528 WIRE -64 640 -144 640 WIRE 112 640 112 592 WIRE 112 640 -64 640 WIRE 256 640 256 608 WIRE 256 640 112 640 WIRE 528 640 528 544 WIRE 528 640 256 640 FLAG 528 640 0 FLAG 528 288 Vout FLAG -64 400 Vin SYMBOL voltage -144 144 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 9 SYMBOL res 240 176 R0 SYMATTR InstName R1 SYMATTR Value 680 SYMBOL res 240 288 R0 SYMATTR InstName R2 SYMATTR Value 220 SYMBOL res 240 512 R0 SYMATTR InstName R3 SYMATTR Value 680 SYMBOL res 240 -80 R0 SYMATTR InstName R4 SYMATTR Value 220 SYMBOL voltage -64 432 R0 WINDOW 3 -75 232 Left 0 WINDOW 123 24 44 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value SINE(0 20m 600 1u 0 0 1000) SYMATTR Value2 AC 1 SYMATTR InstName V2 SYMBOL res 112 128 R0 SYMATTR InstName R5 SYMATTR Value 220 SYMBOL res 224 448 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 220 SYMBOL res 96 496 R0 SYMATTR InstName R7 SYMATTR Value 330k SYMBOL res 464 272 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R8 SYMATTR Value 220 SYMBOL res 512 448 R0 SYMATTR InstName R9 SYMATTR Value 330k SYMBOL cap 448 304 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName C1 SYMATTR Value 0.33µ SYMBOL cap 0 416 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName C2 SYMATTR Value 0.1µ SYMBOL njf 208 64 R0 SYMATTR InstName J1 SYMATTR Value 2N5485 SYMBOL njf 208 400 R0 SYMATTR InstName J2 SYMATTR Value 2N5485 TEXT -176 688 Left 0 !.tran 1 startup TEXT 40 696 Left 0 !;ac oct 5 20 20000
======================================================================================= My new preamp using a MOSFET (it's actually a dual in an SO8 package:
Version 4 SHEET 1 880 704 WIRE 256 -112 -144 -112 WIRE 0 64 -16 64 WIRE 96 64 64 64 WIRE 192 64 96 64 WIRE 96 112 96 64 WIRE -144 160 -144 -112 WIRE 192 160 192 64 WIRE 256 192 256 -112 WIRE 256 288 256 272 WIRE 368 288 256 288 WIRE 528 288 512 288 WIRE 192 320 192 240 WIRE 256 320 256 288 WIRE 256 320 192 320 WIRE 0 400 -64 400 WIRE 96 400 96 192 WIRE 96 400 64 400 WIRE 256 400 256 320 WIRE -64 448 -64 400 WIRE 528 464 528 288 WIRE 96 480 96 400 WIRE 208 480 176 480 WIRE 256 512 256 496 WIRE 336 512 256 512 WIRE 96 528 96 480 WIRE 256 528 256 512 WIRE 336 544 336 512 WIRE -144 640 -144 240 WIRE -64 640 -64 528 WIRE -64 640 -144 640 WIRE -16 640 -16 64 WIRE -16 640 -64 640 WIRE 96 640 96 608 WIRE 96 640 -16 640 WIRE 256 640 256 608 WIRE 256 640 96 640 WIRE 336 640 336 608 WIRE 336 640 256 640 WIRE 528 640 528 544 WIRE 528 640 336 640 FLAG 528 640 0 FLAG 528 288 Vout FLAG -64 400 Vin SYMBOL voltage -144 144 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 9 SYMBOL res 240 176 R0 SYMATTR InstName R1 SYMATTR Value 27k SYMBOL res 240 512 R0 SYMATTR InstName R3 SYMATTR Value 100 SYMBOL voltage -64 432 R0 WINDOW 3 -75 232 Left 0 WINDOW 123 24 44 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value SINE(0 25m 600 1u 0 0 30000) SYMATTR Value2 AC 1 SYMATTR InstName V2 SYMBOL res 192 464 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 10 SYMBOL res 80 512 R0 SYMATTR InstName R7 SYMATTR Value 680k SYMBOL res 464 272 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R8 SYMATTR Value 220 SYMBOL res 512 448 R0 SYMATTR InstName R9 SYMATTR Value 330k SYMBOL cap 448 304 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName C1 SYMATTR Value 1µ SYMBOL cap 0 416 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName C2 SYMATTR Value 1µ SYMBOL nmos 208 400 R0 SYMATTR InstName M2 SYMATTR Value FDS6961A SYMBOL res 80 96 R0 SYMATTR InstName R10 SYMATTR Value 470k SYMBOL res 176 144 R0 SYMATTR InstName R2 SYMATTR Value 27k SYMBOL cap 0 80 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName C3 SYMATTR Value 100µ SYMBOL cap 320 544 R0 SYMATTR InstName C4 SYMATTR Value .003µ TEXT -176 688 Left 0 !.tran 10 startup TEXT 224 664 Left 0 !;ac oct 5 20 30000