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Tone Control: Thin and Rich
Tone does not mean the same thing "Tone" usually means when followed by "Control," as it does not refer to more (or less) low or high frequencies, but rather to something like sustain in a musical instrument. Some coupling capacitors, resistors, and some tubes are said to have a big tone. Maybe it is the result of beneficial microphonic resonances. Maybe adding small loudspeaker inside the chassis would allow an adjustable amount of mechanical feedback. Or maybe a hint of frequency selective positive feedback is the answer.
Headphone Listening
It's a common sight, the bass-boost control on portable radios and CD players. So many users leave this boost permanently in place that I am surprised that it is offered as an option and not hardwired into the circuit; but as a nod to the absolute sound, the boost remains an option. And although the applied boost is too heavy handed, we can readily discern that it is a move in the right direction, as a flat frequency response in a headphone sounds wrong, too thin, too bright. How is this possible?
The answer lies in the fact that we do not hear with eardrums alone. Our ear's shape and our haircuts alter the sound before it enters the ear cannel. And our bodies hear-feel (somatosensory perception) the deep bass notes.
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Our head is not large enough to prevent one ear from hearing anything that the other ear hears, creating a blended sound at each ear, involving frequency tailoring and phase shifts, all of which headphones prevent. Because headphone listening lacks these components, we need a set of controls specific to headphone use. If nothing else, a stereo-blend control and a simple bass boost would help make headphone listening more enjoyable.
The circuit above shows a simple headphone amplifier for 300-ohm headphones. It includes a fixed +6 dB bass boost, image width control, and a phase selection switch with mute in the center position. (Driving lower impedance headphones requires paralleling several output triodes or using a White cathode follower.)
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