Can anyone explain to me why an FFT on the output from this circuit shows a comb of frequencies starting at 2 kHz but when R25 is disconnected, that all goes away.
Version 4.1 SHEET 1 3020 1796 WIRE -176 -288 -240 -288 WIRE -32 -288 -96 -288 WIRE 128 -288 -32 -288 WIRE 224 -288 128 -288 WIRE 400 -288 304 -288 WIRE -32 -256 -32 -288 WIRE 128 -256 128 -288 WIRE -32 -160 -32 -192 WIRE 48 -160 -32 -160 WIRE 128 -160 128 -192 WIRE 128 -160 48 -160 WIRE 48 -128 48 -160 WIRE -240 -80 -240 -288 WIRE -176 -80 -240 -80 WIRE 832 -80 -96 -80 WIRE 1152 -80 1088 -80 WIRE 2160 -80 1232 -80 WIRE -240 16 -240 -80 WIRE -176 16 -240 16 WIRE -32 16 -112 16 WIRE 1088 16 1088 -80 WIRE 1152 16 1088 16 WIRE 1296 16 1216 16 WIRE 2480 64 2416 64 WIRE 2608 64 2544 64 WIRE -240 112 -240 16 WIRE -176 112 -240 112 WIRE -32 112 -32 16 WIRE -32 112 -96 112 WIRE 0 112 -32 112 WIRE 96 112 80 112 WIRE 208 112 176 112 WIRE 256 112 208 112 WIRE 400 112 400 -288 WIRE 400 112 320 112 WIRE 432 112 400 112 WIRE 528 112 512 112 WIRE 640 112 608 112 WIRE 688 112 640 112 WIRE 832 112 832 -80 WIRE 832 112 752 112 WIRE 1088 112 1088 16 WIRE 1152 112 1088 112 WIRE 1296 112 1296 16 WIRE 1296 112 1232 112 WIRE 1328 112 1296 112 WIRE 1424 112 1408 112 WIRE 1536 112 1504 112 WIRE 1584 112 1536 112 WIRE 1728 112 1648 112 WIRE 1760 112 1728 112 WIRE 1856 112 1840 112 WIRE 1968 112 1936 112 WIRE 2016 112 1968 112 WIRE 2160 112 2160 -80 WIRE 2160 112 2080 112 WIRE 2416 160 2416 64 WIRE 2464 160 2416 160 WIRE 2608 160 2608 64 WIRE 2608 160 2544 160 WIRE -240 224 -240 112 WIRE -176 224 -240 224 WIRE 208 224 208 112 WIRE 256 224 208 224 WIRE 640 224 640 112 WIRE 688 224 640 224 WIRE 832 224 832 112 WIRE 880 224 832 224 WIRE 1008 224 960 224 WIRE 1088 224 1088 112 WIRE 1088 224 1008 224 WIRE 1152 224 1088 224 WIRE 1536 224 1536 112 WIRE 1584 224 1536 224 WIRE 1968 224 1968 112 WIRE 2016 224 1968 224 WIRE -32 240 -32 112 WIRE -32 240 -112 240 WIRE 400 240 400 112 WIRE 400 240 320 240 WIRE 832 240 832 224 WIRE 832 240 752 240 WIRE 1296 240 1296 112 WIRE 1296 240 1216 240 WIRE 1728 240 1728 112 WIRE 1728 240 1648 240 WIRE 2160 240 2160 112 WIRE 2160 240 2080 240 WIRE 2208 240 2160 240 WIRE 2336 240 2288 240 WIRE 2416 240 2416 160 WIRE 2416 240 2336 240 WIRE -176 256 -240 256 WIRE 256 256 224 256 WIRE 688 256 656 256 WIRE 1152 256 1088 256 WIRE 1584 256 1552 256 WIRE 2016 256 1984 256 WIRE 2416 272 2416 240 WIRE 2464 272 2416 272 WIRE 224 288 224 256 WIRE 656 288 656 256 WIRE 1552 288 1552 256 WIRE 1984 288 1984 256 WIRE 2608 288 2608 160 WIRE 2608 288 2528 288 WIRE 2704 288 2608 288 WIRE 2464 304 2432 304 WIRE 2432 336 2432 304 WIRE 2704 336 2704 288 WIRE -448 368 -448 336 WIRE -448 368 -496 368 WIRE -240 368 -240 256 WIRE -176 368 -240 368 WIRE 400 368 400 240 WIRE 400 368 -96 368 WIRE 1088 368 1088 256 WIRE 1152 368 1088 368 WIRE 1728 368 1728 240 WIRE 1728 368 1232 368 WIRE -496 384 -496 368 WIRE -448 400 -448 368 WIRE -240 416 -240 368 WIRE 1088 416 1088 368 WIRE -32 432 -32 240 WIRE 880 432 -32 432 WIRE 1008 432 1008 224 WIRE 1008 432 960 432 WIRE 1296 432 1296 240 WIRE 2208 432 1296 432 WIRE 2336 432 2336 240 WIRE 2336 432 2288 432 WIRE 2704 464 2704 416 WIRE 2768 464 2704 464 WIRE 2800 464 2768 464 WIRE 2704 496 2704 464 WIRE -240 528 -240 496 WIRE 1088 528 1088 496 WIRE 2704 624 2704 576 FLAG 224 288 0 FLAG 656 288 0 FLAG -496 384 0 FLAG -448 256 vcc FLAG -448 480 vee FLAG -144 208 vcc FLAG -144 272 vee FLAG 288 208 vcc FLAG 288 272 vee FLAG 720 208 vcc FLAG 720 272 vee FLAG 48 -128 0 FLAG 2768 464 vout FLAG -240 528 0 FLAG 1552 288 0 FLAG 1984 288 0 FLAG 1184 208 vcc FLAG 1184 272 vee FLAG 1616 208 vcc FLAG 1616 272 vee FLAG 2048 208 vcc FLAG 2048 272 vee FLAG 2432 336 0 FLAG 2496 256 vcc FLAG 2496 320 vee FLAG 1088 528 0 FLAG 2704 624 0 SYMBOL res -80 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 10K SYMBOL res 96 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 15k SYMBOL res 528 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 15k SYMBOL res -80 -96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 10K SYMBOL voltage -448 240 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value 15 SYMBOL voltage -448 384 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V4 SYMATTR Value 15 SYMBOL OpAmps\\LT1679 -144 240 R0 SYMATTR InstName U1 SYMBOL OpAmps\\LT1679 288 240 R0 SYMATTR InstName U2 SYMBOL OpAmps\\LT1679 720 240 R0 SYMATTR InstName U3 SYMBOL res -80 352 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL res -256 400 R0 SYMATTR InstName R1 SYMATTR Value 510 SYMBOL diode -48 -256 R0 SYMATTR InstName D1 SYMATTR Value BAV99HY SYMBOL diode 112 -192 M180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D2 SYMATTR Value BAV99HY SYMBOL res -80 -304 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 47k SYMBOL res 320 -304 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R8 SYMATTR Value 47k SYMBOL res 976 208 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R9 SYMATTR Value 10k SYMBOL res 976 416 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R10 SYMATTR Value 90k SYMBOL cap 752 96 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMATTR Value 10n SYMATTR SpiceLine V=25 Irms=0 Rser=0.0262 Lser=534p mfg="Würth Elektronik" pn="885012008030 WCAP-CSGP 1206" type="NP0" SYMBOL cap 320 96 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 10n SYMATTR SpiceLine V=25 Irms=0 Rser=0.0262 Lser=534p mfg="Würth Elektronik" pn="885012008030 WCAP-CSGP 1206" type="NP0" SYMBOL res 192 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R11 SYMATTR Value 910 SYMBOL res 624 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R12 SYMATTR Value 910 SYMBOL res 1248 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R15 SYMATTR Value 10K SYMBOL res 1424 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R17 SYMATTR Value 15k SYMBOL res 1856 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R19 SYMATTR Value 15k SYMBOL OpAmps\\LT1679 1184 240 R0 SYMATTR InstName U4 SYMBOL OpAmps\\LT1679 1616 240 R0 SYMATTR InstName U5 SYMBOL OpAmps\\LT1679 2048 240 R0 SYMATTR InstName U6 SYMBOL res 1248 352 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R14 SYMATTR Value 47k SYMBOL res 1072 512 M180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R13 SYMATTR Value 2.2k SYMBOL res 2304 224 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R21 SYMATTR Value 10k SYMBOL cap 2080 96 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C3 SYMATTR Value 10n SYMATTR SpiceLine V=25 Irms=0 Rser=0.0262 Lser=534p mfg="Würth Elektronik" pn="885012008030 WCAP-CSGP 1206" type="NP0" SYMBOL cap 1648 96 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C4 SYMATTR Value 10n SYMATTR SpiceLine V=25 Irms=0 Rser=0.0262 Lser=534p mfg="Würth Elektronik" pn="885012008030 WCAP-CSGP 1206" type="NP0" SYMBOL res 1520 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R18 SYMATTR Value 910 SYMBOL res 1952 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R20 SYMATTR Value 910 SYMBOL res 1248 -96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R16 SYMATTR Value 10k SYMBOL res 2304 416 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R22 SYMATTR Value 250k SYMBOL OpAmps\\LT1679 2496 288 R0 SYMATTR InstName U7 SYMBOL res 2560 144 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R23 SYMATTR Value 10k SYMBOL cap -112 0 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C5 SYMATTR Value 10p SYMATTR SpiceLine V=25 Irms=0 Rser=0.3913 Lser=584p mfg="Würth Elektronik" pn="885012008019 WCAP-CSGP 1206" type="NP0" SYMBOL cap 1216 0 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C6 SYMATTR Value 10p SYMATTR SpiceLine V=25 Irms=0 Rser=0.3913 Lser=584p mfg="Würth Elektronik" pn="885012008019 WCAP-CSGP 1206" type="NP0" SYMBOL cap 2544 48 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C7 SYMATTR Value 10p SYMATTR SpiceLine V=25 Irms=0 Rser=0.3913 Lser=584p mfg="Würth Elektronik" pn="885012008019 WCAP-CSGP 1206" type="NP0" SYMBOL res 2688 320 R0 SYMATTR InstName R24 SYMATTR Value 600 SYMBOL res 2688 480 R0 SYMATTR InstName R25 SYMATTR Value 600 TEXT -536 600 Left 2 !.tran 0 10 0 100n startup TEXT -536 640 Left 2 !.options plotwinsize=0 numdgt=15 TEXT -248 -400 Left 2 ;1KHz 150dB based on