3 Channel Spectrum Analyzer

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This 3 channel 15 LED spectrum analyzer is the perfect addition to any audio amp project. It produces fantastic displays on three LED bars that can be individually adjusted for any particular frequency range. The circuit will take line level output from most any audio source, and operates on 12V DC. This means that it can even be run in a car.

Schematic

Parts

Part	Total Qty.	Description	Substitutions
R1	1	100K 1/4W Resistor
R2	1	820K 1/4W Resistor
R3, R14, R16, R18	4	2.2 Meg 1/4W Resistor
R4, R5, R6	3	22K Pot
R7, R8, R9, R25, R27, R29	6	10K 1/4W Resistor
R10, R11, R12	3	680 Ohm 1/4W Resistor
R13. R15, R17	3	580K 1/4W Resistor
R19, R20, R21	3	39K 1/4W Resistor
R22, R23, R24	3	47K 1/4W Resistor
R26, R28, R30	3	33 Ohm 1/4W Resistor
C1, C5, C6	3	0.012uF Polystyrene Capacitor
C2, C9, C10, C11	4	3.3uF Electrolytic Capacitor
C3, C4	2	0.0022uF Polystyrene Capacitor
C7, C8	2	47nF Polystyrene Capacitor
C12, C13, C14	3	0.47uF Electrolytic Capacitor
C15, C16, C17	3	22uF Electrolytic Capacitor
D1, D2, D3	3	1N4002 Silicon Diode
D4, D5, D6, D8, D8	5	Green LED
D10, D11, D12, D13, D14	5	Amber LED
D16, D17, D18, D19, D20	5	Red LED
U1	1	LM3900 Quad Op Amp
U2, U3, U4	3	AN6884 Bar Graph IC
MISC	1	Board, Wires, Sockets For ICs

Notes

The circuit expects line level inputs. If you connect it to speaker level, you will have to readjust the circuit every time you change the volume.

After the circuit is connected, apply power and signal. Now adjust the pots until the corresponding group of LEDs reacts.

Thanks to Alterac for correcting some errors on the schematic that originally appeared on this page. Read all about it in the 3 Channel Spectrum Analyzer Project thread in the forum.

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Comments

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jp 3 Channel Spectrum Analyzer Wednesday, July 08, 2015 9:09:46 PM

I appreciate all of the work that went into this; could I take out the led drivers without messing with the rest of the circuit? Also, what is the value for capacitor18?

Daniel 3 Channel Spectrum Analyzer Wednesday, August 14, 2013 11:46:35 PM

Hello peoples, I was wandering if this circuit would work well with LM3915 instead of AN6884. I think it would be neat to have 10 LEDs per channel.

Foolshatch 3 Channel Spectrum Analyzer Thursday, May 10, 2012 9:52:04 PM

Error in my last message-- I did not meant LM3900 but those led drivers. @Audioguru: About LM3900, that can be replaced with any quad (or single op-amp). About rare led drivers concern is the same thing -- whichever comparator or opamp will do in this schematic. there are possible to make with widely available components. When replacing OP amps, just pinouts needs to be checked. And LED Drivers... its easy to convert it for opamps/comparators (in that case couple ref-level resistors needs to be added only).

Foolshatch 3 Channel Spectrum Analyzer Thursday, May 10, 2012 9:45:03 PM

Nice. instead of LM3900 can be any cheap quad op-amp or also quad comparator in this schematic (though with some minor modifications in schematic - resistors for ref levels).

anonymous 3 Channel Spectrum Analyzer Tuesday, December 20, 2011 10:55:15 PM

can i add another 2 channels for this project to make it 5 channels? how to do it?

lukas 3 Channel Spectrum Analyzer Tuesday, October 11, 2011 3:12:26 PM

can i add one led on each ouput from AN6884 ? so it will be 2x5 LED in each collum is it posssible ? thanks

anonymous 3 Channel Spectrum Analyzer Sunday, January 30, 2011 7:16:37 PM

I found this reading the forum.. FROM: audioguru Posted - Jan 02 2011 : 10:18:10 PM If you add more frequency bands then you need to re-calculate the bandwidth of the existing bands to make them narrower. The "multiple feedback bandpass filter" design is in Google but uses a "normal" quad opamp instead of the weird LM3900. C18 is simply a supply bypass capacitor. Its value depends on how much ripple is on your power supply voltage. If your power supply already has a big filter capacitor or you are using a 12V battery then C18 can be from 10uF to 100uF. I think the CanaKit is the same circuit.

Aikousha C18 is still missing from the parts list Monday, January 17, 2011 8:07:24 AM

What is the value of C18? --Thanks

Cristian 3 Channel Spectrum Analyzer Thursday, March 04, 2010 7:39:06 PM

hey i got two question...im noob at this but, how do you connect audio input to this????? and what is the value of C18?? i really want to know how to add input though, like an aux cable from an ipod, or something like that.

timpati 3 Channel Spectrum Analyzer Sunday, February 15, 2009 11:17:20 PM

firstly, thanks for this useful circuit. i saw its schematic but there is not value of the c18. i think it might be between 100-1000 microfarads

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jp	3 Channel Spectrum Analyzer	Wednesday, July 08, 2015 9:09:46 PM
I appreciate all of the work that went into this; could I take out the led drivers without messing with the rest of the circuit? Also, what is the value for capacitor18?
Daniel	3 Channel Spectrum Analyzer	Wednesday, August 14, 2013 11:46:35 PM
Hello peoples, I was wandering if this circuit would work well with LM3915 instead of AN6884. I think it would be neat to have 10 LEDs per channel.
Foolshatch	3 Channel Spectrum Analyzer	Thursday, May 10, 2012 9:52:04 PM
Error in my last message-- I did not meant LM3900 but those led drivers. @Audioguru: About LM3900, that can be replaced with any quad (or single op-amp). About rare led drivers concern is the same thing -- whichever comparator or opamp will do in this schematic. there are possible to make with widely available components. When replacing OP amps, just pinouts needs to be checked. And LED Drivers... its easy to convert it for opamps/comparators (in that case couple ref-level resistors needs to be added only).
Foolshatch	3 Channel Spectrum Analyzer	Thursday, May 10, 2012 9:45:03 PM
Nice. instead of LM3900 can be any cheap quad op-amp or also quad comparator in this schematic (though with some minor modifications in schematic - resistors for ref levels).
anonymous	3 Channel Spectrum Analyzer	Tuesday, December 20, 2011 10:55:15 PM
can i add another 2 channels for this project to make it 5 channels? how to do it?
lukas	3 Channel Spectrum Analyzer	Tuesday, October 11, 2011 3:12:26 PM
can i add one led on each ouput from AN6884 ? so it will be 2x5 LED in each collum is it posssible ? thanks
anonymous	3 Channel Spectrum Analyzer	Sunday, January 30, 2011 7:16:37 PM
I found this reading the forum.. FROM: audioguru Posted - Jan 02 2011 : 10:18:10 PM If you add more frequency bands then you need to re-calculate the bandwidth of the existing bands to make them narrower. The "multiple feedback bandpass filter" design is in Google but uses a "normal" quad opamp instead of the weird LM3900. C18 is simply a supply bypass capacitor. Its value depends on how much ripple is on your power supply voltage. If your power supply already has a big filter capacitor or you are using a 12V battery then C18 can be from 10uF to 100uF. I think the CanaKit is the same circuit.
Aikousha	C18 is still missing from the parts list	Monday, January 17, 2011 8:07:24 AM
What is the value of C18? --Thanks
Cristian	3 Channel Spectrum Analyzer	Thursday, March 04, 2010 7:39:06 PM
hey i got two question...im noob at this but, how do you connect audio input to this????? and what is the value of C18?? i really want to know how to add input though, like an aux cable from an ipod, or something like that.
timpati	3 Channel Spectrum Analyzer	Sunday, February 15, 2009 11:17:20 PM
firstly, thanks for this useful circuit. i saw its schematic but there is not value of the c18. i think it might be between 100-1000 microfarads