More pictures at the end of the document.

Remarks

This design was for an individual project in an undergraduate lab session. With the limitations of using components from lab and keeping cost down many things were compromised. The overall sound quality and repeatability of performance will suffer from low quality parts including resistors, capacitors, and type of printed circuit board used. The battery performance will suffer because of the low efficiency op amps; the power rails are ran off of an LM324, which are power hungry op amps in a comparator configuration. This configuration was chosen to minimize the number of components and overall size of the end circuit and also to keep refrain from ordering a compatible high efficiency quad op amp. With all of these restrictions, this project has become a prototype design project.

The goals in mind:

Fit in an Altoids box:

The design started with the challenge of fitting the circuit in an Altoids box. I have seen many projects online about different ‘Altoids Box Projects’ but wanted to make my own. The altoids box proves to be difficult because of the rounded corners and Tin bottom. A printed circuit had to be made to accomplish this goal.

Solid design/schematic:

The signal handling is based largely on the NWAVGUY’s schematic for his ‘Objective 2’ headphone amp design. NWAVGUY used two 9-volt batteries, a 200mA LED, a DC supply option, and an added gain switch; all features I have left out of this design to save space.

Handle a large variety of headphones (in ear/over ear/on ear):

There are HIFI headphones that are extremely current hungry for example the 38 ohm planar HiFiMan HE-4/HE-5LE and headphones like the Beyer DT880-600 that need large amounts of voltage for being dynamic/planar headphones (Design Process NWAVGUY).

Ability to add a charging port (future).

Design

The following steps were taken to design the headphone amplifier.

Power

The first step was to design the power handling stage. The Objective 2, also referred to as the O2, uses a dual supply off of two 9-volt batteries and has a DC supply that will charge the rechargeable 9-volt batteries when not in use. To fit the circuit in an Altoids box the DC supply option was taken out and one of the two batteries were removed.

Virtual Ground

To accomplish a clean output signal dual rails were created off of a single battery. Using a voltage divider and a two-stage voltage follower from an LM324 to source enough current and create a virtual ground.

Low Battery

To detect a low battery a comparator circuit was used and implemented with an op amp. The operational amplifier chosen has four channels and handles plenty of current for this application. The circuit from the Objective2 amplifier was modified to set the cutoff voltage to 7.0 Volts (see calculations section). So when the circuit is running off of a 9-volt battery, and the battery runs low the rails will be cut off to protect the sensitive circuitry to follow.

Switching

MOSFETs were used to switch the output rails from on to off. The source of the MOSFET was connected to the input rail, drain was connected to the output rail, and the gate was connected to the respective outputs from the op amps.

When the battery supplies sufficient voltage (above 7.0 volts) the P-MOS (on the top rail) gets a low voltage make V_SG greater than its threshold and the N-MOS (on the bottom rail) gets a high voltage, making V_GS greater than the threshold. When the output rails are supposed to be cutoff the gate voltage on each MOSFET are equal to the source voltage, so V_SG =V_GS = 0.

Completing the Power circuit

Electrolytic and ceramic capacitors along with a power switch were added to the design to stabilize the key voltages and complete the circuit.

The capacitors from the output of the op amp to the rail stabilize the gate voltage so the MOSFETs do not turn off more than they need. The capacitors from the rail to ground (input or output) are used to supply momentary voltage to the rails when an unbalanced output is pulled from the rails

Signal

The signal handling was straightforward. To supply the added voltage gain for the voltage hungry headphones a gain stage was the first stage the signal went through. For current hungry headphones a current stage was added before the output.

Voltage Gain

A non-inverting amplifier configuration was used to give a voltage gain and save the phase angle. The capacitor to ground was used for coupling to provide better response to lower frequencies. The capacitor in the feedback loop is to kill any DC signals and very low notes (less than 10 Hz). Note: only one channel shown.

Current gain

Two stages of current gain were added per channel using a high current op amps in a voltage follower configuration. Pull down resistors were added at the non-inverting input to kill any amplification on no signal inputs (ie noise) and to reduce the no signal or ideal output.

Volume Control

The volume control was achieved by putting an audio potentiometer between the voltage gain stage and the current gain stage. The input was fed to one sided of the POT, the output was taken from the wiper of the POT, and the other end was connected to ground. As the potentiometer is turned from lowest audio output to loudest output the resistance is decreased allowing more voltage to go to the current gain stage. This is shown in the schematic section.

Schematic

Full Schematic: Note the Battery on the left is represented as a capacitor so the leads of the 9-volt connecter can be properly soldered.

Close up of the power handling:

Close up of the signal handling:

View of the routed PCB board:

Pseudo ground planes were added to the signal side to help clean any capacitive noise in the output signal.

Parts list

Power Circuit

5 * 330 KΩ resistor

1 * 120 KΩ resistor

1 * 560 Ω resistor

4 * 100 μF electrolytic capacitor

5 * 0.1 μF ceramic capacitor

1 * Red LED

1 * DPDT Latching Switch

1 * 9-Volt battery clip

1 * LM324 operational amplifier

1 * BS170 N channel MOSFET

1 * ZVP3306 P channel MOSFET

Signal Circuit

2 * 120 Ω resistor

2 * 10 KΩ resistor

2 * 1 KΩ resistor

2 * 330 KΩ resistor

1 * 10 KΩ Dual Audio Potentiometer

4 * 220 pF ceramic capacitor

2 * 1 μF ceramic capacitor

2 * 3.5mm Headphone jack

2 * NJM4556AD High current Operational Amplifier

1 * NJM2068D Low Noise Operational Amplifier

Calculations

Low power ratios:

Low battery voltage = 7.0

Negative terminal to the first op amp:

This voltage is slightly greater than the V_d of the RED LED causing the output rails to be zero.

Frequency Response:

The following MultiSim test was performed to visualize the pass band of the amplifier.

The amp has great pass band region. The top graph shows the output at the potentiometer at zero percent, and the bottom graph shows when the POT is at 100%.

The output current was measured using a moderate load from a 300mV signal.

Measurements

The amplifier was built and tested as follows. Pictures of finished product are at the bottom of the page.

Frequency response

The output response was measured at maximum volume at different frequencies. The following O-Scope shots detail the results.

At 20 Hz:

At 100Hz:

At 1KHz:

At 10KHz:

At 20KHz:

At 10Hz:

G=V_o / V_i =2.485 V/V

At 100Hz:

G = 2.507

At 1KHz:

G=2.507

At 10KHz:

G = 2.507

At 20KHz:

G = 2.507

It is easy to see that the amplifier has only amplifies the signal and does not attenuate it.

Idle Input Condition

The output of the amplifier was measured when ground was wired into the input. This test shows the output of the Amp when no input is connected or when the music is paused. Note the scaling factors no the probes (Bottom left):

The input signal is right around ground with added noise from the environment. The output signal in red shows the ideal condition response. The ideal response ~= 35mV. The input signal is often around 350mV so the added distortion to the signal can be generalized and approximated to be ~ 10%.

Conclusion

The amplifier works and sounds good. The testing results from the frequency response is promising but the idle condition test yielded a very poor result. Many things will be changed in the next version:

PCB Routing:

The PCB Routing could have been much more efficient. The PCB can also be much smaller when a professional service is used to create the circuit. The battery had to be stood upright to fit in the enclosure. If the board was shortened the battery could be flat to fit under the lid.

Cost:

Smaller resistors and more efficient op amps can be used to save space and power.