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Equalizer box

Discussion in 'General Electronics Discussion' started by Vitautas, Mar 19, 2012.

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  1. Vitautas


    Mar 19, 2012
    Hello. This is my first post.

    I got an idea to build a graphic eq powered by 12 volts battery. A very simple one, only 3 bands (low, middle, high). The box would consist of one input, one output, and three potentiometers for adjusting the gain of different bands.

    So I googled for some schematics and found this circuit.

    I need some help.
    What does "audio in" and "audio out" exactly mean? Could I directly connect this eq between my laptop and headphones?

    Could I use TLC274CD instead of LF351?
    Are there any critical components that I should be careful about?
    If I would like to add the power switch, where should I place it?

    I know it's much to ask. Thanks.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Audio in may be connected directly to line out of your laptop, although a potentiometer at this point would be helpful to allow adjustment of the signal level. Use a logarithimic potentiometer suitable for audio applications. For a first try you can strat without a pöotentiometer, adjusting th elevel by the volume control of the laptop.
    The audio out is most probably not suitable for direct attachment of a headset. You should insert a headphone amplifier (maybe a kit?).

    Almost any OpAmp should be usable in this application. The TLC274 will do, but if you still can switch to another OpAmp, you should look for a low noise type, not a precision type.

    The power switch should be placed in the power supply (+12V).

  3. Vitautas


    Mar 19, 2012
    Thank you Harald.

    Would the MCP606 be more suitable?
    From the "audio out" I get mono signal yes? So I need to amplify this small mono signal up so that my headphones could make some hearable sound.

    What about this one:

    So basically, I just add this to the existing circuit and that's it?
    But why are there 2 outputs?
    And now I need two supply voltage sources: +12 v and +3 v. How to solve this?
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    First about the amplifier:
    The MCP606 is a bit too slow. Consider a sine wave of 15 kHz (about the upper end of the audible spectrum) and an amplitude of 3 V peak. This signal has approx. 0.1 V/µs slew rate when crossing zero. The MCP606 can follow only at 0.08V/µs and will thus distort high frequencies. Your OpAmp should ideally offer at least 1V/µs slew rate to be on the safe side. Take for example the MAX4476 or MAX4477 (no, I'm not getting paid for promoting Maxim). The 4477 has 2 OpAmps in one package (called dual OpAmp) which allows you to build two identical filters with one IC. So you can go stereo! I suspect your audio out is stereo, too. You would have to use stereo potentiometers to adjust left and right channel filters simultaneously.

    Now to the headphone amplifier:
    I think this is the same circuit you reference:
    There are a few explanations on the site I found. Most important is that this amplifier can be operated from 12 V, too.
    Or you can power the equalizer with voltages down to 6 V. A good intermediate value would be 9 V. You can then use a 9 V block battery.
    This amplifier can be built twice for stereo.
    The "two outputs" are just one. You are supposed to connect the headphones parallel to R3. Thus V+ would be the common return for a stereo amplifier. Or you could resort to an integrated solution like this: or
    or others.

    I hope this helps

  5. KJ6EAD


    Aug 13, 2011
    If you use the search term "headphone equalizer" you can see lots of options. If your intent is to only use this with a computer, there may be some software solutions available.
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