A Simple Equalization Circuit
A Simple Equalization Circuit

A friends’ birthday’s coming up, and two of us have gotten together to give him a great present.

A guitar with two on-board speakers

A guitar’s the gift, but we can’t just give the birthday boy a used guitar! We decided to put a speaker into the body of the guitar, but in testing (before drilling) we found that the bass strings overtook the treble. We had two choices: a simple equalizer, or two speakers.

After we chose to use two speakers, we tried a few things to get the desired effect. The simplest approach, the approach used in mass-produced dual-speaker systems (think plastic boombox), is to use a filtering capacitor for the bass speaker, and feed the plain signal into the treble speaker.

This approach was dismissed for two reasons:

Our second point became moot in the end, as we ended up using three op-amps to make the 1x buffer/ 2x equalizer, and an additional two op-amps to drive the speakers themselves. We could have used the fourth op-amp as an adder, and fed the output to one speaker. However, the clarity would have suffered so we went ahead with a two speaker design.

An equalization circuit employing a TL074 and two LM386 ICs
The equalization circuit implemented

  1. Gift guitar gains speakers, rocks out - machine quotidienne

    on September 29, 2010 at 15:22

  2. Gift guitar gains speakers, rocks out | Jayesh Jain

    on September 29, 2010 at 15:50

  3. Dave

    on September 29, 2010 at 15:53

    Are you really running U1 open loop, with NO gain limiting feedback?
    I’d expect almost any input voltage to slam it to the + or – rail, causing severe clipping.

  4. matti

    on September 29, 2010 at 16:20

    Yeah, you’re right, I do have a gain pot attached to the input on U1. I’ll amend the schematic, although I forget what value I loaded the input with, maybe 1k? with the value of the pot and R_Load, a resistor value that must be arrived at by experimentation (use a temporary potentiometer). My R_Load ended up being about 100k.

  5. Jeffrey Alsip

    on October 3, 2010 at 21:32

    Can this be configured to work as a preamp, when the guitar is plugged into a real amplifier? Maybe a switch to put the onboard amp in line with the regular output?

  6. matti

    on October 6, 2010 at 22:03

    Details are fuzzy; I think I wired the regular guitar out directly to the preamp out (before the high/low equalizer); so the output jack was fed both. As far as I can remember, it was very quiet without the preamp engaged. A resistor-divider or a simple SPDT switch would’ve worked; but I didn’t include them in the build as the giftee didn’t have a guitar amp.

  7. miguel

    on August 27, 2013 at 10:01

    i was wondering if i may use this idea for a school project and if i can get a copy of the schematics and material used as well as dimensions for the speaker and positions…?

  8. matti

    on August 27, 2013 at 17:14

    Permission granted!

    As far as schematics; a detail missing on the schematic I’ve posted (but pictured in the completed unit below the schematic) is the TPS6375 negative 5 volt supply and the LM7805 positive 5 volt supply for the quad op-amp. It’s the separate board that’s pictured.

    I can’t say much in way of BOM, just look up the datasheets for the LM7805 and TPS6375 and use the components in their reference schematics. All other parts are listed on my schematic.

    If you want to change which frequency is attenuated, change the values of the 333 capacitor and 10k resistor that feed the op-amps after the buffer. The 470/1k resistor divder after the lower op-amp merely cuts the volume in half for the bass speaker.

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