I would like to use a pocket-size MP3 player to feed audio signals direct (no added amp) to a solenoid type coil. The coil resistance itself would be negligible. What is the minimum series resistance required to avoid overloading the player's output? I am thinking maybe
That's probably a good starting point, since 32R headphones are fairly common. A better MP3 player might be able to drive a set of 8R headphones to a reasonable volume level, although the power consumption might be higher. It'll depend a lot on the player.
Suggestion: generate a few near-full-scale sinewave signals (say, 20 and 1000 and 10000 Hz) of a minute or so length each. MP3 encode them at a high quality setting. Load them onto your player. Play them, while loading the headphone output down through resistors of various values, and monitor the voltage across the resistors with an o'scope. Find the smallest-value resistor for which the output signal neither drops too much in amplitude, nor distorts.
I used a cheap MP3/Ogg/FLAC player as an easy source of some low-frequency calibration tones, when I wanted to confirm the correctness of the CTCSS sub-audible access tones in ham radios belonging to some of my friends. My old service monitor doesn't have a CTCSS encoder/decoder... but by feeding a known-good 100 Hz tone into the horizontal input of its monitor scope, feeding the decoded CTCSS tone to the vertical, and looking at the resulting Lissajous oval on the screen, I could tell people just how much their tone frequency was off, or if it was distorted.
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Dave Platt AE6EO
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