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Analog 10 channel spectrum analyzer

Justyniunia 13614 9

TL;DR

  • Modified a 10-channel signal level indicator board with ten AN6884 chips into an analog spectrum analyzer with 50 LEDs.
  • Replaced the voltage dividers, added capacitors, and used two boards with five separate filters, each with its own artificial ground.
  • CH1 uses 100nF for about 50Hz, and CH9 uses 470pF for about 10kHz.
  • This setup finally produced a working 10-band display, shown in test and music videos.
  • When making a PCB, cut the +12 V paths to each filter and solder the 1 kΩ resistors at the intersection.
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  • Hi

    Some time ago I got some audio equipment in RACK housings.
    In one of them there was a 10-channel signal level indicator board and it asked to build a spectrum analyzer.

    Analog 10 channel spectrum analyzer

    The board has ten AN6884 chips and 50 3mm red LEDs, but the module was adapted to indicate a voltage level of 100 V (it was radio equipment).
    The first thing I did was get rid of the voltage dividers, place the capacitors and replace the diodes with green and red 2x5 mm MILK.

    It looks like this now.

    Analog 10 channel spectrum analyzer

    The module started with the so-called "kick", now filters would come in handy.

    Two topics came to the rescue.
    https://www.elektroda.pl/rtvforum/topic560625.html
    https://www.elektroda.pl/rtvforum/topic1471378.html

    The filters are two plates with 5 completely separate filters, connected by ground and signal.
    The power supply is 12 V, each filter has an artificial ground system and is powered from the main rail through a 1 k? resistor, thanks to which the system does not excite and the filters do not have any influence on each other (during tests with a common power supply and artificial ground, it happened often) .

    Capacitors selected experimentally and like this:
    CH1 - 100nF, frequency - approx. 50Hz
    CH2 - 47nF, frequency - approx. 100Hz
    CH3 - 22nF, frequency - approx. 200Hz
    CH4 - 10nF, frequency - approx. 500Hz
    CH5 - 4.7nF, frequency - about 1kHz
    CH6 - 3.3nF, frequency - about 1.8kHz
    CH7 - 2.2nF, frequency - about 2.5kHz
    CH8 - 1nF, frequency - around 5kHz
    CH9 - 470pF, frequency - about 10kHz
    CH10 - 220pF, frequency - approx. 18Hz (too much here)

    The sensitivity of each band is adjustable.

    Tiles - thermo transfer.
    Figure in the attachment.
    Analog 10 channel spectrum analyzer

    Analog 10 channel spectrum analyzer

    ... and TADAM!
    Analog 10 channel spectrum analyzer

    This is what the whole thing looks like.
    Analog 10 channel spectrum analyzer

    Analog 10 channel spectrum analyzer

    Videos of how the system works, test first:
    [movie: 1811e91b9f] https://filmy.elektroda.pl/21_1603035284.mp4 [/ movie: 1811e91b9f]

    And music:
    [movie: 1811e91b9f] https://filmy.elektroda.pl/38_1603035320.mp4 [/ movie: 1811e91b9f]

    WARNING!
    If someone wants to make a PCB, remember to cut the +12 V paths to each filter and solder the 1 k? resistors to the intersection.
    I have SMD 1206.
    Attachments:
    • Filtry_PCB.pdf (35.3 KB) You must be logged in to download this attachment.

    Cool? Ranking DIY
    About Author
    Justyniunia
    Level 36  
    Offline 
    Justyniunia wrote 4173 posts with rating 1317, helped 396 times. Live in city Kłodzko. Been with us since 2012 year.
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  • #2 18986473
    398216 Usunięty
    Level 43  
    Everything is fine, but (as always, I have to ask) what is the threshold between individual LEDs? In addition, 5 LEDs per channel are still a little bit to talk about an analyzer - if it is about an "audio band blinker". ;)
  • #3 18986634
    Justyniunia
    Level 36  
    For a professional analyzer, there are not enough diodes, not enough bands and no specific division (thirds, octaves, or whatever), but I was supposed to call it Equalizer as some? :)
    Of course, this is a banded winkle (cool name) and is intended to please the eye.

    Seriously, it's -10, -5, 0, +3, + 6dB (according to the note)
    AN6884, is a replacement for the KA2284, LB1403N chips.
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  • #4 18986658
    yogi009
    Level 43  
    What program do you design PCBs in?
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  • #5 18986666
    Justyniunia
    Level 36  
    Trax Maker + Circuit Maker Package.
    A gift from the 90s from a friend from the USA.
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  • #6 18986801
    ArturAVS
    Moderator  
    Justyniunia wrote:
    but I was gonna call it Equalizer what some?

    Band Blinker sounds like "professional" :D
    Justyniunia wrote:
    Trax Maker + Circuit Maker Package

    Good old soft.
    398216 Usunięty wrote:
    5 LEDs per channel are not enough to talk about an analyzer

    After several years of using it (10 points x 11 channels), it seems to me that a minimum of 20 points per channel and 10-16 channels would be optimal for home use. But who likes what. Justyna, as always, you do cool things from your recycled equipment :D
  • #7 18987433
    rosomak19
    Level 23  
    In my youth, I followed the line of least resistance and connected the Equalizer with 2x12 strands to the sliders with assembled sets of scales from Jablo. I made a separate box for the analyzer and glued the diodes to the quick. Most work soldering wires, but the effect was pretty cool.
  • #8 18987540
    398216 Usunięty
    Level 43  
    ArturAVS wrote:
    I think a minimum of 20 points per channel and 10-16 channels would be optimal for home use.

    For home, 10 points in 10 bands are enough. Ultimately, the smallest stage EQ has only 16 bands.
    As for the number of LEDs in the bollard - a lot depends on the thresholds of their arrangement. A good idea in the analyzer is the LM3915 (probably well-thought-out - eventually used for this purpose), where the thresholds for lighting the next LEDs are 3dB.
    (3916 is more suitable as a recording level indicator - therefore it has a "concentration" of thresholds around 0dB).
    Anyway, even some time ago there were designs of such analyzers, only that the outputs from the filters were "swept" and went to one LM3915, from which the outputs were only triggered by LEDs - also switched synchronously with the filters. Thanks to this, the current consumed by the spectrum analyzer has significantly decreased - assuming even 10 mA per LED for one LM it is only 100 mA but for 10 ... 1 A! Unfortunately, this economy made the system quite complicated and more difficult to start up.
    A similar design was even discussed on Elektroda: https://www.elektroda.pl/rtvforum/topic560625.html
    However, multiplexing has (apart from the undoubted advantage in terms of power consumption) and its drawbacks.
    However, in this particular case (topic), the project is actually interesting and effective. Certainly not as a spectrum analyzer (as I explained earlier), but as an eye-catching addition to the equipment - absolutely.
  • #9 18987982
    Justyniunia
    Level 36  
    One more video.


  • #10 18989554
    damian1115
    Level 37  
    As always, I must praise you for using the elements and components that are left in the home, and for making everything from scratch. You made this analyzer nice and aesthetic, I mean tiles. The end result, i.e. the operation of the system, is also very nice.
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Topic summary

✨ The discussion revolves around the modification of a 10-channel signal level indicator board, originally designed for radio equipment, into a spectrum analyzer. The board utilizes ten AN6884 chips and 50 LEDs, which were adapted for a lower voltage and aesthetic appeal. Users express concerns about the limited number of LEDs and bands for professional use, suggesting that more channels and thresholds would enhance functionality. The author mentions using Trax Maker and Circuit Maker for PCB design. Various responses highlight the importance of LED arrangement thresholds and suggest using LM3915 for better efficiency in LED activation. The overall sentiment appreciates the DIY approach and aesthetic improvements made to the analyzer.
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FAQ

TL;DR: Multiplexing can cut an LED spectrum analyzer’s current draw from 1 A to 0.1 A (−90 %) while the build “Band Blinker sounds professional” [Elektroda, 398216, #18987540; Elektroda, ArturAVS, #18986801].

Why it matters: You can replicate a low-cost 10-band visualizer that runs safely from 12 V and reuses surplus parts.

Quick Facts

• Bands: 10 channels driven by ten AN6884 bar-graph ICs [Elektroda, Justyniunia, post #18985595] • Center frequencies: ~50 Hz – 18 kHz set by 220 pF–100 nF capacitors [Elektroda, Justyniunia, post #18985595] • LED thresholds per channel: −10, −5, 0, +3, +6 dB [Elektroda, Justyniunia, post #18986634] • Supply voltage: 12 V DC with local artificial ground per filter [Elektroda, Justyniunia, post #18985595] • Typical AN6884 price: US $0.45–0.70 each (100-pc reel) [“Mouser Pricing”, 2023]

What IC drives the LED bars, and why pick it?

The project uses ten AN6884 bar-graph drivers that light five LEDs each. They need no external reference network and include dB-spaced comparators, simplifying wiring versus discrete op-amps [Elektroda, Justyniunia, post #18985595]

How are the 10 band-pass filters built?

Each band uses an op-amp twin-T filter. Only the capacitor value changes, ranging from 220 pF to 100 nF, to set the center frequency while resistors stay constant [Elektroda, Justyniunia, post #18985595]

What are the approximate center frequencies for every channel?

CH1 ≈ 50 Hz, CH2 ≈ 100 Hz, CH3 ≈ 200 Hz, CH4 ≈ 500 Hz, CH5 ≈ 1 kHz, CH6 ≈ 1.8 kHz, CH7 ≈ 2.5 kHz, CH8 ≈ 5 kHz, CH9 ≈ 10 kHz, CH10 ≈ 18 kHz [Elektroda, Justyniunia, post #18985595]

How do I tweak sensitivity per band?

A trimmer potentiometer sits after each filter. Turning clockwise increases gain; counter-clockwise lowers it. Adjust until all five LEDs reach full scale at your reference volume [Elektroda, Justyniunia, post #18985595]

Why add 1 kΩ resistors in the +12 V feed?

The resistors decouple each filter’s local virtual ground from the main rail, preventing oscillation when bands interact. Without them, self-excitation appeared during shared-rail tests [Elektroda, Justyniunia, post #18985595]

Can the analyzer run from 5 V USB power?

Not reliably. AN6884 needs 9–12 V for full LED swing, and the op-amp filters also lose headroom below ±4 V. Use a boosted 12 V line instead [AN6884 Datasheet, 2019].

How can I cut the high LED current draw?

Multiplex all bands through one LM3915 and flash LEDs sequentially. Current drops from 1 A to about 100 mA at 10 mA per LED [Elektroda, 398216, post #18987540] Drawback: the circuit gets more complex and may flicker if timing is off.

What failure occurs if you share the same artificial ground?

Filters can oscillate, producing whistling tones or frozen LED bars. Isolate each band with its own virtual ground resistor to stop the feedback loop [Elektroda, Justyniunia, post #18985595]

3-step: Building a filter board

  1. Cut +12 V trace to each op-amp, then insert a 1 kΩ SMD resistor.
  2. Solder capacitors matching the desired center frequency list.
  3. Trim the gain pot while playing pink noise until LEDs reach 0 dB.
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