Headphones, powered speakers, never a bare speaker
Line level drives a powered speaker's input with room to spare. Headphones work too, at a moderate volume: the board's 470 Ω and a 32 Ω earpiece share the 2.1 V, and the earpiece gets about 0.13 V, worked out. A bare speaker gets almost nothing, because moving one is the TK100 amplifier's job.
A divider you did not build
Whatever you plug into the jack sits in series with the board's 470 Ω, and the two share the chip's voltage. The load gets 2.1 V × R / (470 Ω + R), where R is its impedance. That is a voltage divider, and it decides how loud everything is.
The numbers in the figure are worked out at full scale from typical impedances, not measured.
Powered speakers: the job it was made for
A powered speaker's input, or a stereo's AUX or LINE IN, is typically a load of kilohms. It gets nearly the whole 2.1 V and turns it into loudness with its own amplifier. The kit brings a cable for each:
- 3.5 mm to 3.5 mm for a powered speaker, a desk speaker set, or anything with a 3.5 mm AUX input.
- 3.5 mm to RCA for a stereo or amplifier with a pair of RCA line inputs.
Headphones: moderate, not loud
Headphones are 16 to 32 Ω. Against 470 Ω a 32 Ω earpiece gets 32 / (470 + 32), about 6 % of the voltage: about 0.13 V RMS at full scale, worked out. That is plenty for earbuds and modest for big headphones. For loud, use powered speakers.
Never
- Never a bare speaker. 2.1 V behind 470 Ω cannot move one. The kit's speaker belongs on the TK100.
- Never a speaker amplifier's output. The terminals where speakers connect on an amplifier are an output; plug the TK103 into the amplifier's line input instead.
- Never an amplified signal into the jack. It is an output too.
When it does not work
It will barely whisper: 4 Ω against the board's 470 Ω gets about 18 mV at full scale, worked out. The speaker's plug is made for the TK100 amplifier's white socket, and that is the block that drives it.
Raise the level in the sketch first: the tone sketch plays at 3000 out of 32767, under a tenth of full scale, to be gentle. At full scale 32 Ω headphones get about 0.13 V RMS, enough for earbuds and modest for big headphones. For loud, use a powered speaker.
Quiet, on purpose. Still, hold the headphones away from your ears the first time any sketch runs, until you have heard the level: a sketch with a larger number in it is louder with no warning.
Not through the jack. The TK100 takes I2S, not an analog signal: wire it to the ESP32-S3 on its own pins, which the book's pin map keeps free, and send it the same samples.
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