Description

The DMOY is an audio headphone pocket amplifier that is largely based off the CMOY pocket amplifier. The power section, however, is updated based off Sijosae's discrete rail splitter design. Although it drains more power than the simple resistor based splitter, it allows more current output which is necessary if you want to drive high impedance headphones such as the HD-650's correctly. The amplification section also includes a static bass boost via a low-pass filter in the feedback section of the op-amp. It can be switched on or off. With op-amps such as the OPA2134PA, it can run off a 9V battery and fit snug within a mint tin (such as an altoids box).
The photo on the right is a finished version of the DMOY.

Parts List

The parts for the DMOY can be found at almost any reputable electronics store. Nevertheless, it is convenient to shop for the parts online. I've put together the list of parts I've used. Since I've purchased the parts from Digikey, I will also list the digikey catalogue number for easy access.
The photo on the right links to a .png version of the schematic. The pdf can be found here. Note that the switch for the bass boost should ideally be one dpdt switch but I was too lazy to add one into the schematic so I used two seperate spst switches. The same goes for the pot - it should have 2 channels.

Part Number | Quantity | Description | Digikey |
---|---|---|---|

C1 | 2 | Power Capacitor - electorlytic 330uf or more | 493-3180-ND |

D1 | 2 | Power Diode - general purpose diode | 1N4003-E3 |

Q1 | 1 | Power NPN BJT - small signal NPN transistor | 2N3904TFCT-ND |

Q2 | 1 | Power PNP BJT - small signal PNP transistor | 2N3906D26ZCT-ND |

R1 | 2 | Power Rail Split Resistor - 4.5kohm | 4.53KXBK-ND |

R2 | 2 | Power Drain Resistor - 10ohm or less | P10.0CACT-ND |

RLED | 1 | Power LED Resistor - 2kohm | RNF14FTD2K00CT-ND |

LED | 1 | Power LED - 10mA LED | 160-1710-ND |

C2 | 2 | Input Capacitor - film 1uf or more | BC2076-ND |

CBB | 2 | BB Filter Capacitor - film 0.1uf | BC2055-ND |

R3 | 2 | OP-AMP Resistor 1 - 100kohm | P100KCACT-ND |

R4 | 2 | OP-AMP Resistor 2 - 3.30kohm | P3.30KCACT-ND |

R5 | 2 | OP-AMP Resistor 3 - 10kohm | P10.0KCACT-ND |

RBB | 2 | BB Filter Resistor - 10kohm | P10.0KCACT-ND |

OPAMP | 1 | Operational Amplifier - 2 channel rated for < 4.5V input | OPA2134PA-ND AD823ANZ-ND |

SW1 | 1 | Power Toggle Switch - spst | 360-1788-ND |

SW2 | 1 | Bass Boost Toggle Switch - dpdt | 360-1835-ND |

Vpot | 1 | Volume Potentiometer - 10kohm variable resistance | P2U4103-ND |

VK | 1 | Volume Knob | 226-1033-ND |

BS | 1 | Battery Strap - 9V battery strap | 232K-ND |

CJ | 2 | Connection Jack - stereo 2.5mm connection jack | CP1-3513-ND |

Don't be afraid to find your own parts from other sources such as mouser. The parts description gives you a general idea of what parts you need but feel free to experiment. If you choose to buy your own parts, it's a good idea to choose electrolytic capacitors for the power and film capacitors (with a maximum tolerance of 1%) for the input/bass boost. The resistors should ideally be metal film and rated for 1/4W (0.25W). It is far better to hand match the values for matching parts than to order really expensive parts with low tolerances. This is easy to do with resistors as they are cheap but not so easy to do with the capacitors as they are a bit more pricy (which is why you want a small tolerance on the input capacitors). Also note that the transistors in the power supply must be matched.

If you don't understand anything in the previous paragraph don't worry about it - just choose any of the parts that you ordered and assemble the amplifier. They should be reasonably matched straight out of the package.

Gain

There are a few things to take into consideration here. The first is the gain of the op-amp. The circuit utilizes a non-inverting op-amp configuration, which has the following gain (in the ideal condition) with respect to the beforementioned part numbers while the bass boost switch is in the off position.
The gain istelf is 'how many times louder' you will hear the signal. The gain with the listed parts is 4. This means you will hear the signal 4 times louder through the amplifier. That being said, the gain can be modified by changing the values of R4 and R5. To modify the gain to be 11, for example, you can simply change the value of R4 to 1.00 kohm. This would make equation 1 as 1 + 10kohm/1kohm = 11. Your amp will then be able to go to 11.

If the bass boost is in the on position, the gain then becomes slightly more tricky to solve for since it involves complex numbers and impedances. If complex numbers scare you, feel free to skip to the next section with the knowledge that the bass boost will cause an increase in gain for low frequencies but add a little distortion. In the case of a complex impedence, equation 1 above can be modified to be.

Where Z5 is the total impedance of the feedback loop as shown in equation 3.

Where j is a complex number and omega is the angular frequency. What complex numbers introduce are an associated magnitude and phase. With respect to equation 2, the magnitude would be the actual gain you would hear and the phase would be how much shift there would be. The phase is an unfortunate by-product of this bass-boost method that does actually modify the sound you would hear.

With a little algebra, you can seperate the real and imaginary parts of equation 2 and plot the resultant magnitude and phase. The magnitude and phase of equation 2 can be found with equations 4, 5, and 6.

The gain shows that the 'bass' (the frequencies < 100 Hz) has a higher gain which means it is amplified more than the other frequencies. This equates to a 'boost' in those frequencies. The phase is a little more tricky to visualize. It's a term people will throw around liberally, but not fully understand what is actually physically happening because of it. Physically, the phase will translate to a time shift of your signal. For example, if you had a simple sine wave with a frequency of 100 Hz and you had a phase shift of 360 degrees (or 2pi radians) you would 'physically' hear the singal 0.01 seconds late. Now, if you had different delays at different frequencies you could describe the sound as being 'muddy' as you would be hearing some parts of the song later than you should while other parts of the song are playing.

This delay can be characterized through a simple ratio analysis shown in equation 7 below. I have plotted out the delay for the beforementioned parts.

This means that at lower frequencies, there will be a 'delay' in the sound you hear by about 0.0005 seconds. Personally, I can't hear this minute delay but perhaps the super discerning audiophile may be able to. The gain/phase (and delay) can be all modified by changing the values of R4,R5,Rbb, and Cbb. I have included a MATLAB/Octave script here if you want to play around with those values and see how the graphs change.

Of course, the above analysis is a purely theoretical one. In reality, the input source, input capacitors, and op-amp all have complex impedances that cause phase and magnitude distortions. The parts, however, are chosen to minimize the distortions in the amplifier itself to un-audible regions of the frequency domain thus the above analysis is a good approximation to the actual characteristics of the gain.