Interfacing Line in (audio in) and line out (audio out) on Teensy 4.0

Not open for further replies.


Any idea how can I interface a line in and a line out on a Teensy 4.0? I want to use jacks both for line in and line out in order to connect a microphone and ear headphones.
Note that the mic input on the teensy audio board is much *lower* impedance than the line-in. But follow these rules and all will be fine:

1) match the input gain to the voltage of the source - low voltage (most microphones) needs high gain
2) use an input impedance at least several times higher than the output impedance of the source
Microphones are high impedance

Dynamic are 200 to 600 ohms or so, electret mics are typically a few k due to the FET buffer load.
I wouldn't call that high impedance - in audio 47k is the standard load for line signals for instance.

Most line-outs are ~100 ohms or so due to the resistor on their outputs which serves to prevent
oscillation driving capacitive loads. However most opamps driving line outs aren't able to handle
heavier than 1k loads without overload distortion appearing at higher signal levels, so its wise
not to load a line-level signal heavier than about 2k if you care about SQ.

Driving a headphone from lineout will create quite significant distortion at higher volume levels as
the opamp's output current limiter operates, unless its a high-current opamp designed for the purpose,
or a very high impedance set of 'phones.
It does stereo in and out but only one MIC level input. With a two-channel preamp you could address the gain issue and use both line-level inputs... but listen to experts on how and not me. ;)

Also, the ADC on the audio board is OK quality but not very impressive specs. There are third-party audio boards with better specs.
Last edited:
Not open for further replies.