B² Spice

Case Study: Modeling Headphone Amplifiers in B² Spice

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Introduction       

Considering that almost all consumer audio electronics (VCR’s, CD players, receivers, and walkmans) come with a headphone jack, why would anyone want to build an external amplifier? The reasons are varied. Some dislike the poor sound coming from the existing headphone jack, which usually results from cheap op-amps and poor-quality electrolytic coupling capacitors. Others wish to play their headphones louder than their existing headphone amplifier can support: portable CD and MP3 players have only so much battery voltage available, much of which is often consumed by the voltage drops internal to the op-amps. For example, even an op-amp that swing within a volt of its power supply rails, can only put out half volt peaks, if the rails are +/-1.5 volts. Still others prefer the sound that the vacuum tubes bring to (or is it “reveal to”) home musical reproduction. Regardless of the motive, SPICE assists in the design of a quality headphone amplifier.

The first step is to determine what our design goals are. It doesn’t take much to drive a dynamic headphone to painful levels (electrostatic headphones are an altogether different story, of course), usually only a few volts and milliwatts are needed (2V and 100mW at very most). And since the headphones had already received an adequate (or close to adequate) signal amplitude from the preexisting amplifying device (the CD player, radio, etc) , the headphone amplifier doesn’t need to provide much gain: 2 to 4 times the input voltage is sufficient. Furthermore, the headphone presents a fairly benign load that runs from 16 to 600 ohms and is fairly immune to the amplifier’s output impedance; thus, the amplifier will need to provide no more than 100mA of peak current swing. 

On the other hand, headphones readily reveal a small amount of hum and noise that would pass unnoticed if reproduced from a loudspeaker. Furthermore, an amplifier DC offset of 1 volts, which wouldn’t hurt a loudspeaker, would easily destroy a headphone’s delicate voice coil. So, to recap: our goals are a quiet, DC offset free, 2 volt and 100mA peak output, low-gain amplifier.