Pocket Headphone Amplifier by OPA134

The OPA134 is wired as a non-inverting amp with a gain of 11. At this gain, the output impedance of the amplifier is less than 0.2 ohms throughout the audio range. The high-pass filter C1-R2 at the input blocks DC current and has a corner frequency of about 15Hz. Substituting a 1uF capacitor will lower the corner frequency to 1.5Hz. However, 1uF capacitors tend to be too large for the recommended enclosure. Instead, if a lower corner frequency is mandatory, try increasing R2 to 1M (and scale R1 accordingly). You could omit C1 entirely, if DC input protection is not important. I recommend leaving C1 in the circuit. R5 is an optional load resistor, which reduces residual hiss when the amp is driving low impedance headphones such as the Grados (32 ohms). Because the voltage drop across R5 reduces the maximum output of the amplifier, I recommend trying a 50 ohm resistor first (or as low as 30 ohms). If there is still residual hiss with low impedance phones, then increase the resistor value to 100 ohms

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http://www.headwize.com/projects/cmoy2_prj.htm
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