Audio Bandpass Filter Circuit Diagram

Audio Bandpass Filter Circuit Diagram

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On: 25 Jul, 2014

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Category: Audio

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Easy Audio Bandpass Filter Kit

Description and Specifications

The Audio Frequency Band Pass Filter kit from provides A basic audio filter kit with preamp, op-amp filter and LM386 audio ...

Easy Audio Bandpass Filter Kit

Description and Specifications

The Audio Frequency Band Pass Filter kit from provides A basic audio filter kit with preamp, op-amp filter and LM386 audio amp to drive headphones or speaker. It has a peak in audio response that is tunable from approximately 300Hz to 2Khz and has a bandwidth of approximately 500Hz. It is designed to provide improved audio filtering and amplification to CW reception or other devices that may not have sufficiently narrow bandwidth. The kit may be powered from 9-12Vdc.

audio bandpass filter circuit diagram 1.jpg

Tools Needed

  1. Temperature Controlled Soldering Station with small tip or 15-35 watt soldering iron with small tip.
  2. Solder 60/40 or 63/37 Tin-Lead
  3. Small Diagonal Cutters
  4. Small Needle Nose Pliers□Pencil, Pen, and/or Highlighter
  5. BRIGHT work light


  1. Magnifying headpiece or lighted magnifying glass.
  2. Multi-meter
  3. Solder Sucker or Solder Wick
  4. Small multi-blade Screw Driver
  5. Knife or Wire Stripper□Small Ruler
  6. Cookie Sheet to build in and keep parts from jumping onto the floor

Construction Techniques

  1. There is no need to print out the whole assembly manual unless you want a copy. Print theParts List and Schematic (last two pages) then view the rest of the manual on a computer, laptop, or tablet.
  2. The Parts List has columns for inventory and construction.
  3. Please take time to inventory the parts before starting. Report any shortages to (In many cases it may be faster and cheaper to pull a replacement from your parts supply, but please let us know if we missed something.)
  4. There is no need to print out the whole assembly manual unless you want a copy. Print the Pre-sorting the resistors and capacitors can speed up the assembly and reduce mistakes.
  5. You can insert several parts at a time onto the board. When you insert a part bend the leads over slightly to hold the part in place, then solder all at the same time. Clip the leads flush.
  6. Most parts should be mounted as close to the board as possible. Transistors should be mounted about 1/8” above the board. Solder one lead on ICs or IC sockets and then check to make sure the component is flush before soldering the remaining leads.
  7. Use a Temperature Controlled Soldering Station with small tip or 15-35 watt soldering iron with small tip. Conical or very small screw driver tips are best.
  8. DO NOT use a large soldering iron or soldering gun.
  9. If you are a beginner, new to soldering, there are a number of resources on the web to help you get on the right track soldering like a pro.

Parts Identification:

audio bandpass filter circuit diagram 2.jpg

Note: This is a general guide. The parts supplied in the kit may vary slightly in appearance fromthose shown in this photo and not all parts included in the kit are shown.

audio bandpass filter circuit diagram 3.jpg

Inventory and Parts List

The first column is for inventory of parts and the second is to track as they are installed

audio bandpass filter circuit diagram 4.jpg

Inserting the Parts


Sort the resistors by value insert them smallest value first, largest value last. There are 3 - 10K resistors and one of each of the others. Be sure to check the color code for each resistor as you install. [Measuring with an Ohm meter is a good idea].

audio bandpass filter circuit diagram 5.jpg

  1. R14    10       brn-blk-blk-gold
  2. R5      100     brn-blk-brn-gold
  3. R17    330     org-org-brn-gold
  4. R9      430     yel-org-brn-gold
  5. R8      470     yel-vio-brn-gold
  6. R6      10K     brn-blk-org-gold
  7. R7      6.8K    blu-gry-red-gold
  8. R10    10K     brn-blk-org-gold
  9. R4      15K     brn-grn-org-gold
  10. R11    100K   brn-blk-yel-gold
  11. R3      100K   brn-blk-yel-gold
  12. R1      3220K red-red-yel-gold


Next insert the molded capacitors. There are 3 - 0.01uF, 2 - 0.1uF, and 1 - 0.33uF. The 0.01uF and 0.1uF capacitors look very similar, double check the markings.

  1. C9   0.01uF  103
  2. C12 0.01uF  103
  3. C14 0.01uF  103
  4. C3   0.1uF    104
  5. C4   0.1uF    104
  6. C7   0.33uF   0.33K


Now insert the electrolytic capacitors. These capacitors are polarized. The negative lead is marked with a black bar on the side of the capacitor.

audio bandpass filter circuit diagram 6.jpg

  1. C1    10uF     10uF
  2. C6    10uF     10uF
  3. C8    10uF     10uF
  4. C10  10uF     10uF
  5. C13  10uF     10uF
  6. C2    47uF     47uF
  7. C15  100uF   100uF
  8. C16  100uF   100uF

Remaining Parts

Now install Q1 the 2N5088 transistor. Follow the layout orientation on the board. The flat sideof the transistor should match the flat side of the diagram.

  • Q1  2N5088

Next install the two IC sockets and the ICs. Make sure the orientation notch on the IC matches the orientation notch on the circuit board.

  • IC1LM386
  • IC2LF356

The last 2 parts to install are the two potentiometers. The A10K and B10K identification marks are on the back of the pots and are a bit difficult to read. Use lots of light. Be sure that the B10K pot goes in the spot for R1 and the A10K goes in the location for R2 on the circuit board.

audio bandpass filter circuit diagram 7.jpg

  • R110K Linear Pot (B)B10K
  • R210K Audio Pot (A)A10K

Hooking Up the Audio Frequency Band Pass FilterThe ABPF requires 8 - 12V DC. The power may be supplied from a companion kit, a small power supply, or a 9V battery using the battery clip that is included in the kit. If using an AC operated supply, it should be well regulated and filtered to prevent hum or othernoise being added to the audio. An inline fuse of 1A or less is recommended when an AC power supply or 12V battery is used.

Due to the many possible configurations, audio Input and output connections are also left up to the builder. The AFBPF may be wired directly into a receiver or speaker cabinet. Alternatively, the included jacks installed and 3.5mm plugs may be used for input and output connections. The kit is capable of directly driving a small speaker.

When the kit is first powered after assembly, it is recommended to use a battery or conenct an inline fuse or a power supply with the current limited to approximately 100-200mA to prevent damage if there are shorts. If you notice large power draw, stop and go back and inspect the board for any shorts or components installed incorrectly.

When the kit is first powered, you should hear static in the speaker or headphones that varies as the volume control is turned. If not, recheck the board for shorts and component errors.

If you have passed this step, you can connect an input audio signal from a receiver or other source of CW signals and you should hear the signal through the output. Moving the tuning knob will place the peak response on the chosen signal.


Packaging is left up to the builder. The AFBPF can be built into another kit, or radio cabinet, installed in a speaker cabinet, or installed into a case.


R1 on the left side of the board is the tuning adjustment, tune to the desired listening frequency by peaking the audible sound on the chosen signal. This will have the affect of amplifying the desired signal while attenuating signals that differ in frequency.

R2 located on the right side of the board is the volume adjustment. Adjust for a comfortable listening level.

If the signals sound distorted, this is likely due to too much audio signal on the input. Reduce the input audio level from the source until the signals sound clean.


This is a simple kit and if assembled correctly, it should work without any problems but problems do occasionally happen.If the kit fails to work, there are a few things to check.

  1. Verify power supply voltage is at least 8V at the power input.
  2. Recheck component values, locations and orientations to be sure everything is installedcorrectly.
  3. Inspect the board for any missed solder joints, joints that may have not been heated sufficiently or for shorts between adjacent component pads.
  4. Reheat any suspect solder connections.
  5. Check input and output connectors for correct wiring.If these tests do not resolve the problem, contact us at: for further assistance.

Schematic Diagram

audio bandpass filter circuit diagram 8.jpg

Board Layout

audio bandpass filter circuit diagram 9.jpg


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