Re: [EE] DAC-to-headphone-level output amp

Neil <[email protected]> wrote:
> I'm designing a one-off device for myself that will generate audio from 
> a microcontroller and I want to send that audio to a small BLE audio 
> transmitter (something I picked up on Amazon).
> The BLE transmitter has a 3.5mm headphone plug and they specify it's for 
> "Aux" level devices, but works very well when plugged into my PC or 
> cellphone, so I take that to mean headphone level.

"Line" level ("Aux" level) and "headphone level" are essentially the same
thing -- about 1 Vrms -- with the difference that a device intended to drive
headphones is designed to handle the low load impedance (high current).
If not, a "headphone amplifier" is required -- which is really just a
more-or-less unity gain amplifier with higher output current capability.

> FWIW, the audio signal is mono, but I want to drive both left and right 
> channels.

There should be no problem connecting your output directly to both inputs
of the BLE device.

> Alternately, I'd be happy if there's some simple monolithic chip that 
> would do this for me (take a 0V-3.3V audio signal from a uC DAC and 
> convert it to headphone level audio).

You just need to AC couple the signal -- put a capacitor between the DAC
and the BLE inputs. a 3.3 Vpp signal corresponds to about 1.2 Vrms (sine
wave), so this should be just right for a "line" or "aux" level input.

The capacitor value will be a function of the lowest frequency you want to
transmit and the actual input impedance of the BLE device.
For example, 100 Hz and 10 kohms would mean you need a capacitor of
1 / (2 pi R f) = 0.16 uF

-- Dave Tweed