Mini Headphone Amp /w Bass Boost

I listen to music when I commute using the subway. Since it's very noisy in the subway the bass sound of the music tends to be masked. So I made a small headphone amplifier that can boost bass sound as needed.

I listed out my requirements as below, and started designing.

• Use two AA or AAA batteries (I didn't want to use 9V batteries)
• Long battery life
• Decent sound quality - doesn't have to be audiophile-grade, since it'll be used mostly in subways
• Switchable bass boost

I found a suitable amplifier IC named LM4880. This IC was developed for portable audio devices, and used to be widely used in computer sound cards. It only requires 2.7V to operate, is yet capable of driving 8 ohm speakers.

> LM4880 Datasheet

According to the datasheet, you need DC blocking capacitors in the output, which is typical of single supply amplifiers. However you can modify the circuit to use dual supply to eliminate the need for these large capacitors.

As you can see, using each battery as positive and negative power supply (dual power supply), you can eliminate the output capacitors. This is a big win for portable design, not to mention the cost saving (high quality, large capacitors required for amplifier output are not cheap).

Direct coupling (like this circuit) of amplifier output and the speaker/headphones improves sound quality by eliminating sound degrading capacitor in the output path.

LM4880 allows setting of gain by using external components to set negative feedback - much like typical op-amp circuit. I decided to include timing components (combination of capacitors and resistors) to give the amplifier bass boost. The topology is called "shelving filter".

I used a 3 position switch to switch between boost off, and two levels of boost.

You can customize the component values to change boost frequencies and the amount of boost, to suit your phones and music you listen to.

If you want to know how to calculate the frequency response of shelving filter please see this page: http://linkwitzlab.com/filters.htm#5

I have tried a few different values until I found the sound I liked.

I made a few versions of prototypes - AA/AAA batteries, with/without volume control, and slightly different circuit configurations...

The final version of the circuit schematics is attached here.

If you want to put one together, you can order the PCB from OSH Park: https://oshpark.com/shared_projects/WNPwUUfw

BOM

• IC1: LM4880 or LM4881 (SOIC8)
• R2(x2), R3(x2), R6(x2): Res, metal film, 27k ohm (0603)
• R1(x2), R4(x2): Res, metal film, 62k ohm (0603)
• R5(x2)*: Res, metal film, 1k ohm (0603)
• R7: Res, 1k ohm (0603)
• R8: Res, 1M ohm (0603)
• R9: Res, 330k ohm (0603)
• C1(x2): Cap, Film or Ceramic(C0G), 0.1uF (1206)
• C2(x2): Cap, Film or Ceramic(C0G), 68nF (1206)
  
• C3: Cap, Ceramic, 1uF 6.3V (0603)
  
• Cs1,2,3,4: Cap, Ceramic, 100uF 6.3V (1210)
  
• Cb: Cap, Ceramic, 0.1uF 6.3V (0603)
  
• VR1: Pot, Dual, 100k ohm B taper or 10k ohm A taper
• SW1: DPDT Slide Switch
• SW2: DP3T Slide Switch
• CN1, CN2: 3.5mm Stereo Jack

(* optional components)

I recommend using metal film (thin film) resistors instead of thick film (carbon) resistors for lower noise.

Capacitors that audio signal goes through should be film or C0G type ceramic. Avoid other types of ceramic capacitors (such as X5R, X7R, etc.) for audio path, as they will distort sound. I tried both film (Panasonic ECH-U series) and C0G ceramics and found no difference in sound quality. Ceramic capacitors however pick up vibrations as noise (only audible if you tap the capacitor directly) - some of you might not like that.

I used the gain setting of -7.22db, which is unusual, but I was using in-ear phones that are very sensitive and only needed small power to get loud enough volume. However I recommend unity gain (0db) setting for most cases - simply swap R1 and R2 as described in the schematic.

This simple headphone amplifier delivers punchy sound and long battery life. You can use rechargeable AAA batteries for the best economy as well.