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Headphone amps for dynamic standards
Hi Guys

The every-day working man's headphone has always been the dynamic type, where the transducers are simply small dynamic loud speakers. These will be 8R up to 600R.

Traditional headphone outputs from power amplifiers were simply a set of 100R-1W dropping resistors feeding a stereo jack that had closed contacts to disable the main speakers. This meant that whatever the big PA was, it would be idling however hot it normally would, providing the usual voltage swing at much reduced current.

Preamps might have a headphone output that was fed from the main output, again with closed switches on the headphone jack, or from an auxiliary output dedicated to the task. If the latter, there would generally be a headphone Level control.

Dedicated headphone circuits were often just a couple of transistors before ICs became dominant, in which case a generic opamp circuit would drive the headphones. In all cases, there would be a low-value resistor added in series with each cup. This actually reduces THD and stabilises the driver from the cable impedance. At the lowest impedance, parallel opamps or a buffer help for high-SPL listening. 1mW at 8R is only 90mV at 9mA. 1mW at 600R is 770mV at 1m3A. These are RMS; peak values are 40% higher. Nested feedback circuits or discretes can push THD to inaudibility.

Most opamps can drive 1k loads, with more now capable and rated for 600R. The ubiquitous 5532 is rated for 600R loads yet it can drive 30R cups through 50R resistors quite well. For most people who only casually use headphones, this is adequate. The nice thing about opamps and most similar discrete circuits is the high power supply rejection ratio (PSRR), which is the ability to ignore supply noise. As a headphone driver with modest or unity gain, the PSU can be quite "cheezy" and still be hum-free.

Of course, with the low power requirement of headphones, we can be inventive and creative  and use tubes, transformers, or hybrid circuits as we see fit. Whatever your aesthetic is, just go for it.

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