low freq detector

merco wrote:

You have a DC-coupled amplifier (U1 & U2) with a gain of about 800 followed by a threshold detector. With the circuit as you posted it, the non-inverting input of U1is sitting at one or two millivolts above ground, due to the 100k to +5 and the 33 ohm to ground through the signal source. When you put a capacitor in series with the signal source, that input jumps up to an average level of +2.5V, determined by the two 100k resistors to +5V and ground. Now, you've just changed the input of your 800 gain amplifier by 2.5V. The output wants to go to

2kV; it can get as close as about +4.5V, where it saturates and won't let any signal through. What your circuit needs is a virtual ground at +2.5V; all the DC referenced parts up to the comparator inputs should use that as a reference. You can get a 2.5V reference by using a voltage divider and feeding it to a spare opamp connected as a voltage follower. Note that you should try to balance the resistances seen by the plus and minus inputs on a DC amp with high gain, at least on the input stage, because the bias currents are nearly equal, and equal resistances will produce equal offset voltages at both inputs, which will cancel out. Following is a modification to your circuit using a 2.5V virtual ground at points where needed- note that the circuit works again. (Now it won't work if you remove the input capacitor, unless you reference your input to the +2.5V) Version 4 SHEET 1 1528 680 WIRE 288 -224 -640 -224 WIRE 656 -224 288 -224 WIRE 1056 -224 656 -224 WIRE 1360 -224 1056 -224 WIRE -176 -144 -416 -144 WIRE 1360 -112 1360 -224 WIRE 256 -96 224 -96 WIRE 272 -96 256 -96 WIRE 384 -96 320 -96 WIRE 640 -96 592 -96 WIRE 752 -96 704 -96 WIRE -416 -80 -416 -144 WIRE 592 0 592 -96 WIRE 624 0 592 0 WIRE 752 0 752 -96 WIRE 752 0 704 0 WIRE 224 16 224 -96 WIRE 256 16 224 16 WIRE 384 16 384 -96 WIRE 384 16 336 16 WIRE 1360 32 1360 -32 WIRE 1456 32 1360 32 WIRE -640 48 -640 -224 WIRE 1360 48 1360 32 WIRE 1056 64 1056 -224 WIRE 656 80 656 -224 WIRE 1024 80 944 80 WIRE 288 96 288 -224 WIRE 592 96 592 0 WIRE 624 96 592 96 WIRE 1088 96 1072 96 WIRE 1184 96 1088 96 WIRE 1296 96 1264 96 WIRE 224 112 224 16 WIRE 256 112 224 112 WIRE 752 112 752 0 WIRE 752 112 688 112 WIRE 1024 112 752 112 WIRE 384 128 384 16 WIRE 384 128 320 128 WIRE 496 128 464 128 WIRE 624 128 496 128 WIRE 1456 128 1456 32 WIRE -304 144 -544 144 WIRE 16 144 -240 144 WIRE 160 144 96 144 WIRE 256 144 160 144 WIRE 272 144 256 144 WIRE 592 176 592 96 WIRE 224 192 224 112 WIRE -544 208 -544 144 WIRE 96 208 96 144 WIRE 160 208 160 144 WIRE 944 208 944 80 WIRE 496 224 496 128 WIRE -640 368 -640 128 WIRE -544 368 -544 288 WIRE -544 368 -640 368 WIRE -416 368 -416 0 WIRE -416 368 -544 368 WIRE 96 368 96 272 WIRE 96 368 -416 368 WIRE 496 368 496 288 WIRE 496 368 96 368 WIRE 1360 368 1360 144 WIRE 1360 368 496 368 WIRE 1456 368 1456 208 WIRE 1456 368 1360 368 WIRE -176 416 -176 -144 WIRE 160 416 160 288 WIRE 160 416 -176 416 WIRE 224 416 224 272 WIRE 224 416 160 416 WIRE 592 416 592 256 WIRE 592 416 224 416 WIRE 944 416 944 288 WIRE 944 416 592 416 FLAG 288 160 0 FLAG 656 144 0 FLAG 1056 128 0 FLAG -640 368 0 SYMBOL voltage -544 192 R0 WINDOW 123 24 132 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value2 AC .1 SYMATTR InstName VMic SYMATTR Value SINE(0 1m 4) SYMBOL res 208 176 R0 SYMATTR InstName R4 SYMATTR Value 6.8k SYMBOL res 352 0 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 330k SYMBOL voltage -640 32 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL res 480 112 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 6.8k SYMBOL cap 480 224 R0 SYMATTR InstName C5 SYMATTR Value 1=B5 SYMBOL res 576 160 R0 SYMATTR InstName R7 SYMATTR Value 22k SYMBOL res 720 -16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R8 SYMATTR Value 330k SYMBOL cap 704 -112 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C7 SYMATTR Value 100n SYMBOL cap 320 -112 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 100n SYMBOL res 1440 112 R0 SYMATTR InstName R15 SYMATTR Value 10k SYMBOL Opamps\\\\LT1212 1056 32 R0 SYMATTR InstName U3 SYMBOL npn 1296 48 R0 SYMATTR InstName Q1 SYMATTR Value 2N2222 SYMBOL res 1280 80 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R12 SYMATTR Value 47k SYMBOL res 1344 -128 R0 SYMATTR InstName R13 SYMATTR Value 10k SYMBOL Opamps\\\\LT1212 288 64 R0 SYMATTR InstName U1 SYMBOL Opamps\\\\LT1212 656 48 R0 SYMATTR InstName U2 SYMBOL res 112 128 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 33 SYMBOL cap 80 208 R0 WINDOW 0 -41 28 Left 0 WINDOW 3 -44 52 Left 0 SYMATTR InstName C2 SYMATTR Value 220=B5 SYMBOL cap -240 128 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C1 SYMATTR Value 1000=B5 SYMBOL voltage -416 -96 R0 SYMATTR InstName V2 SYMATTR Value 2.5 SYMBOL res 144 192 R0 SYMATTR InstName R2 SYMATTR Value 6.8k SYMBOL res 928 192 R0 SYMATTR InstName R3 SYMATTR Value 100k TEXT -98 -158 Left 0 !.tran 5 TEXT -96 -184 Left 0 !;ac oct 100 0.01 1000000 TEXT -104 -128 Left 0 ;MCP6294 - Max 412

-- John

"merco" wrote in message news: snipped-for-privacy@h48g2000cwc.googlegroups.com...

John F answered your question so I didn't look any further into your circuit.

Those mic's can give 1mV with normal speech type levels of sound. Be wary though, of unscreened mains pickup which can easily reach 10 times that level. This spice list shows the connection to this type of microphone. Mic' resistors from 1k to 3k9 are usually fine. (ignore the generators below it, just added noise source.)

Version 4 SHEET 1 1528 680 WIRE 288 -224 -640 -224 WIRE 656 -224 288 -224 WIRE 1056 -224 656 -224 WIRE 1360 -224 1056 -224 WIRE -176 -144 -416 -144 WIRE 1360 -112 1360 -224 WIRE 256 -96 224 -96 WIRE 272 -96 256 -96 WIRE 384 -96 320 -96 WIRE 640 -96 592 -96 WIRE 752 -96 704 -96 WIRE -416 -80 -416 -144 WIRE 592 0 592 -96 WIRE 624 0 592 0 WIRE 752 0 752 -96 WIRE 752 0 704 0 WIRE 224 16 224 -96 WIRE 256 16 224 16 WIRE 384 16 384 -96 WIRE 384 16 336 16 WIRE 1360 32 1360 -32 WIRE 1456 32 1360 32 WIRE -640 48 -640 -224 WIRE 1360 48 1360 32 WIRE 1056 64 1056 -224 WIRE 656 80 656 -224 WIRE 1024 80 944 80 WIRE 288 96 288 -224 WIRE 592 96 592 0 WIRE 624 96 592 96 WIRE 1088 96 1072 96 WIRE 1184 96 1088 96 WIRE 1296 96 1264 96 WIRE 224 112 224 16 WIRE 256 112 224 112 WIRE 752 112 752 0 WIRE 752 112 688 112 WIRE 1024 112 752 112 WIRE 384 128 384 16 WIRE 384 128 320 128 WIRE 496 128 464 128 WIRE 624 128 496 128 WIRE 1456 128 1456 32 WIRE -304 144 -544 144 WIRE 16 144 -240 144 WIRE 160 144 96 144 WIRE 256 144 160 144 WIRE 272 144 256 144 WIRE 592 176 592 96 WIRE 224 192 224 112 WIRE -544 208 -544 144 WIRE 96 208 96 144 WIRE 160 208 160 144 WIRE 944 208 944 80 WIRE 496 224 496 128 WIRE -640 368 -640 128 WIRE -544 368 -544 288 WIRE -544 368 -640 368 WIRE -416 368 -416 0 WIRE -416 368 -544 368 WIRE 96 368 96 272 WIRE 96 368 -416 368 WIRE 496 368 496 288 WIRE 496 368 96 368 WIRE 1360 368 1360 144 WIRE 1360 368 496 368 WIRE 1456 368 1456 208 WIRE 1456 368 1360 368 WIRE -176 416 -176 -144 WIRE 160 416 160 288 WIRE 160 416 -176 416 WIRE 224 416 224 272 WIRE 224 416 160 416 WIRE 592 416 592 256 WIRE 592 416 224 416 WIRE 944 416 944 288 WIRE 944 416 592 416 FLAG 288 160 0 FLAG 656 144 0 FLAG 1056 128 0 FLAG -640 368 0 SYMBOL voltage -544 192 R0 WINDOW 123 24 132 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value2 AC .1 SYMATTR InstName VMic SYMATTR Value SINE(0 1m 4) SYMBOL res 208 176 R0 SYMATTR InstName R4 SYMATTR Value 6.8k SYMBOL res 352 0 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 330k SYMBOL voltage -640 32 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL res 480 112 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 6.8k SYMBOL cap 480 224 R0 SYMATTR InstName C5 SYMATTR Value 1µ SYMBOL res 576 160 R0 SYMATTR InstName R7 SYMATTR Value 22k SYMBOL res 720 -16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R8 SYMATTR Value 330k SYMBOL cap 704 -112 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C7 SYMATTR Value 100n SYMBOL cap 320 -112 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 100n SYMBOL res 1440 112 R0 SYMATTR InstName R15 SYMATTR Value 10k SYMBOL Opamps\\\\LT1212 1056 32 R0 SYMATTR InstName U3 SYMBOL npn 1296 48 R0 SYMATTR InstName Q1 SYMATTR Value 2N2222 SYMBOL res 1280 80 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R12 SYMATTR Value 47k SYMBOL res 1344 -128 R0 SYMATTR InstName R13 SYMATTR Value 10k SYMBOL Opamps\\\\LT1212 288 64 R0 SYMATTR InstName U1 SYMBOL Opamps\\\\LT1212 656 48 R0 SYMATTR InstName U2 SYMBOL res 112 128 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 33 SYMBOL cap 80 208 R0 WINDOW 0 -41 28 Left 0 WINDOW 3 -44 52 Left 0 SYMATTR InstName C2 SYMATTR Value 220µ SYMBOL cap -240 128 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C1 SYMATTR Value 1000µ SYMBOL voltage -416 -96 R0 SYMATTR InstName V2 SYMATTR Value 2.5 SYMBOL res 144 192 R0 SYMATTR InstName R2 SYMATTR Value 6.8k SYMBOL res 928 192 R0 SYMATTR InstName R3 SYMATTR Value 100k TEXT -98 -158 Left 0 !.tran 5 TEXT -96 -184 Left 0 !;ac oct 100 0.01 1000000 TEXT -104 -128 Left 0 ;MCP6294 - Max 412

thanks to all, i'm going to test the new circuit !

ok, i understood what you said about "vitual ground", it's correct... but the simulation doesn't work how i espect.

My original schematic produce a pulse if VMic is below 4Hz (every amplitude from 1mV to 1000mV !); new modified circuit doesn't . Using 2.5V virtual ground the signal is equally amplified and i can't detect if the signal freq is below 4Hz (try changing the signal freq VMic, and amplitude).

The original "strange" circuit has this magic feature that the modified doesn't: how could be is possibile to get it working as the original ?

merco wrote:

I forgot the diode drop threshhold that you originally had on U3. The circuit as I gave it to you will run if it sees a very low level of signal there. Take the virtual ground off of the inverting input of U3, and try 100k to plus 5 and 120k to 0V. That will give it a threshold of just over 2.7V. The input to that non-inverting input is centered around 2.4V, so when the signal there is greater than 0.6Vp-p, it will trigger. That, plus the frequency selective circuit of R6 and C5, make it trigger around 4Hz again. The reason the input is centered around

2=2E4V instead of 2.5V is the imbalance in resistors seen by U2. You might be able to play with that circuit so it's better balanced. Here's the modification:

Version 4 SHEET 1 1528 680 WIRE 480 -224 -640 -224 WIRE 944 -224 480 -224 WIRE 1056 -224 944 -224 WIRE 1360 -224 1056 -224 WIRE -176 -144 -416 -144 WIRE 1360 -112 1360 -224 WIRE 256 -96 224 -96 WIRE 384 -96 320 -96 WIRE 640 -96 592 -96 WIRE 752 -96 704 -96 WIRE 944 -96 944 -224 WIRE -416 -80 -416 -144 WIRE 592 0 592 -96 WIRE 624 0 592 0 WIRE 752 0 752 -96 WIRE 752 0 704 0 WIRE 224 16 224 -96 WIRE 256 16 224 16 WIRE 384 16 384 -96 WIRE 384 16 336 16 WIRE 1360 32 1360 -32 WIRE 1456 32 1360 32 WIRE -640 48 -640 -224 WIRE 1360 48 1360 32 WIRE 480 64 480 -224 WIRE 480 64 288 64 WIRE 656 64 480 64 WIRE 1056 64 1056 -224 WIRE 656 80 656 64 WIRE 944 80 944 -16 WIRE 1024 80 944 80 WIRE 288 96 288 64 WIRE 592 96 592 0 WIRE 624 96 592 96 WIRE 1184 96 1088 96 WIRE 1296 96 1264 96 WIRE 224 112 224 16 WIRE 256 112 224 112 WIRE 752 112 752 0 WIRE 752 112 688 112 WIRE 1024 112 752 112 WIRE 384 128 384 16 WIRE 384 128 320 128 WIRE 496 128 464 128 WIRE 624 128 496 128 WIRE 1456 128 1456 32 WIRE -304 144 -544 144 WIRE 16 144 -240 144 WIRE 160 144 96 144 WIRE 256 144 160 144 WIRE 592 176 592 96 WIRE 224 192 224 112 WIRE -544 208 -544 144 WIRE 96 208 96 144 WIRE 160 208 160 144 WIRE 944 208 944 80 WIRE 496 224 496 128 WIRE -640 368 -640 128 WIRE -544 368 -544 288 WIRE -544 368 -640 368 WIRE -416 368 -416 0 WIRE -416 368 -544 368 WIRE 96 368 96 272 WIRE 96 368 -416 368 WIRE 496 368 496 288 WIRE 496 368 96 368 WIRE 944 368 944 288 WIRE 944 368 496 368 WIRE 1360 368 1360 144 WIRE 1360 368 944 368 WIRE 1456 368 1456 208 WIRE 1456 368 1360 368 WIRE -176 416 -176 -144 WIRE 160 416 160 288 WIRE 160 416 -176 416 WIRE 224 416 224 272 WIRE 224 416 160 416 WIRE 592 416 592 256 WIRE 592 416 224 416 FLAG 288 160 0 FLAG 656 144 0 FLAG 1056 128 0 FLAG -640 368 0 SYMBOL voltage -544 192 R0 WINDOW 123 24 132 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value2 AC .1 SYMATTR InstName VMic SYMATTR Value SINE(0 1m 4) SYMBOL res 208 176 R0 SYMATTR InstName R4 SYMATTR Value 6.8k SYMBOL res 352 0 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 330k SYMBOL voltage -640 32 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL res 480 112 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 6.8k SYMBOL cap 480 224 R0 SYMATTR InstName C5 SYMATTR Value 1=B5 SYMBOL res 576 160 R0 SYMATTR InstName R7 SYMATTR Value 22k SYMBOL res 720 -16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R8 SYMATTR Value 330k SYMBOL cap 704 -112 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C7 SYMATTR Value 100n SYMBOL cap 320 -112 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 100n SYMBOL res 1440 112 R0 SYMATTR InstName R15 SYMATTR Value 10k SYMBOL Opamps\\\\LT1212 1056 32 R0 SYMATTR InstName U3 SYMBOL npn 1296 48 R0 SYMATTR InstName Q1 SYMATTR Value 2N2222 SYMBOL res 1280 80 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R12 SYMATTR Value 47k SYMBOL res 1344 -128 R0 SYMATTR InstName R13 SYMATTR Value 10k SYMBOL Opamps\\\\LT1212 288 64 R0 SYMATTR InstName U1 SYMBOL Opamps\\\\LT1212 656 48 R0 SYMATTR InstName U2 SYMBOL res 112 128 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 33 SYMBOL cap 80 208 R0 WINDOW 0 -41 28 Left 0 WINDOW 3 -44 52 Left 0 SYMATTR InstName C2 SYMATTR Value 220=B5 SYMBOL cap -240 128 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C1 SYMATTR Value 1000=B5 SYMBOL voltage -416 -96 R0 SYMATTR InstName V2 SYMATTR Value 2.5 SYMBOL res 144 192 R0 SYMATTR InstName R2 SYMATTR Value 6.8k SYMBOL res 928 -112 R0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL res 928 192 R0 SYMATTR InstName R9 SYMATTR Value 120k TEXT -98 -158 Left 0 !.tran 10 TEXT -96 -184 Left 0 !;ac oct 100 0.01 1000000 TEXT -104 -128 Left 0 ;MCP6294 - Max 412

-- John

i'll try, thank you .

well... ok now seems better ... but is not like the original... If u try (see schematic) with a signal of 10mV and 10Hz freq, you get wronlgy spike output: in this case maybe the source is very near to the microphone so 10mV could be a reasonable level for VMic ? It Is not sure that 1 mV is max amplitude for the input signal...

Version 4 SHEET 1 1528 680 WIRE -640 48 -640 -224 WIRE -640 368 -640 128 WIRE -544 208 -544 144 WIRE -544 368 -640 368 WIRE -544 368 -544 288 WIRE -416 -80 -416 -144 WIRE -416 368 -544 368 WIRE -416 368 -416 0 WIRE -304 144 -544 144 WIRE -176 -144 -416 -144 WIRE -176 416 -176 -144 WIRE 16 144 -240 144 WIRE 96 208 96 144 WIRE 96 368 -416 368 WIRE 96 368 96 272 WIRE 160 144 96 144 WIRE 160 208 160 144 WIRE 160 416 -176 416 WIRE 160 416 160 288 WIRE 224 16 224 -96 WIRE 224 112 224 16 WIRE 224 192 224 112 WIRE 224 416 160 416 WIRE 224 416 224 272 WIRE 256 -96 224 -96 WIRE 256 16 224 16 WIRE 256 112 224 112 WIRE 256 144 160 144 WIRE 288 96 288 64 WIRE 384 -96 320 -96 WIRE 384 16 336 16 WIRE 384 16 384 -96 WIRE 384 128 320 128 WIRE 384 128 384 16 WIRE 480 -224 -640 -224 WIRE 480 64 288 64 WIRE 480 64 480 -224 WIRE 496 128 464 128 WIRE 496 224 496 128 WIRE 496 368 96 368 WIRE 496 368 496 288 WIRE 592 0 592 -96 WIRE 592 96 592 0 WIRE 592 176 592 96 WIRE 592 416 224 416 WIRE 592 416 592 256 WIRE 624 0 592 0 WIRE 624 96 592 96 WIRE 624 128 496 128 WIRE 640 -96 592 -96 WIRE 656 64 480 64 WIRE 656 80 656 64 WIRE 752 -96 704 -96 WIRE 752 0 704 0 WIRE 752 0 752 -96 WIRE 752 112 688 112 WIRE 752 112 752 0 WIRE 944 -224 480 -224 WIRE 944 -96 944 -224 WIRE 944 80 944 -16 WIRE 944 208 944 80 WIRE 944 368 496 368 WIRE 944 368 944 288 WIRE 1024 80 944 80 WIRE 1024 112 752 112 WIRE 1056 -224 944 -224 WIRE 1056 64 1056 -224 WIRE 1184 96 1088 96 WIRE 1296 96 1264 96 WIRE 1360 -224 1056 -224 WIRE 1360 -112 1360 -224 WIRE 1360 32 1360 -32 WIRE 1360 48 1360 32 WIRE 1360 368 944 368 WIRE 1360 368 1360 144 WIRE 1456 32 1360 32 WIRE 1456 128 1456 32 WIRE 1456 368 1360 368 WIRE 1456 368 1456 208 FLAG 288 160 0 FLAG 656 144 0 FLAG 1056 128 0 FLAG -640 368 0 SYMBOL voltage -544 192 R0 WINDOW 123 24 132 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value2 AC .1 SYMATTR InstName VMic SYMATTR Value SINE(0 10m 10) SYMBOL res 208 176 R0 SYMATTR InstName R4 SYMATTR Value 6.8k SYMBOL res 352 0 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 330k SYMBOL voltage -640 32 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL res 480 112 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 6.8k SYMBOL cap 480 224 R0 SYMATTR InstName C5 SYMATTR Value 1=B5 SYMBOL res 576 160 R0 SYMATTR InstName R7 SYMATTR Value 22k SYMBOL res 720 -16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R8 SYMATTR Value 330k SYMBOL cap 704 -112 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C7 SYMATTR Value 100n SYMBOL cap 320 -112 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 100n SYMBOL res 1440 112 R0 SYMATTR InstName R15 SYMATTR Value 10k SYMBOL Opamps\\\\LT1212 1056 32 R0 SYMATTR InstName U3 SYMBOL npn 1296 48 R0 SYMATTR InstName Q1 SYMATTR Value 2N2222 SYMBOL res 1280 80 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R12 SYMATTR Value 47k SYMBOL res 1344 -128 R0 SYMATTR InstName R13 SYMATTR Value 10k SYMBOL Opamps\\\\LT1212 288 64 R0 SYMATTR InstName U1 SYMBOL Opamps\\\\LT1212 656 48 R0 SYMATTR InstName U2 SYMBOL res 112 128 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 33 SYMBOL cap 80 208 R0 WINDOW 0 -41 28 Left 0 WINDOW 3 -44 52 Left 0 SYMATTR InstName C2 SYMATTR Value 220=B5 SYMBOL cap -240 128 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C1 SYMATTR Value 1000=B5 SYMBOL voltage -416 -96 R0 SYMATTR InstName V2 SYMATTR Value 2.5 SYMBOL res 144 192 R0 SYMATTR InstName R2 SYMATTR Value 6.8k SYMBOL res 928 -112 R0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL res 928 192 R0 SYMATTR InstName R9 SYMATTR Value 120k TEXT -98 -158 Left 0 !.tran 10 TEXT -96 -184 Left 0 !;ac oct 100 0.01 1000000 TEXT -104 -128 Left 0 ;MCP6294 - Max 412

{snipped}

The circuit you started with is not a very good frequency discriminator, because the cutoff depends on a single pole R-C filter. It depends strongly on amplitude as well- if it cuts off at 4Hz at 1 mV amplitude, you will still get an output at 10Hz if you increase the amplitude grossly. You could have a better frequency cutoff with a multipole filter, adding one or two more opamps, or just more poles in your existing circuit. Try googling for "active filter". You should look at your application to see whether the circuit serves the purpose. In any case, you have a starting point for playing with it.

-- John