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

OK, so this does make sense, considering that the headphone (speakers) 
would have a very low impedance.
Some more research tells me that the *output* impedance should be 
100-600 ohms and also that my uC of choice (STM32) has a built in op-amp 
with an output impedance of only 10-ohms. Not sure if this would cause 
any issues, or if I should add a series resistor, but I will experiment 
with this. Thanks.



On 12/19/2024 7:08 PM, Dave Tweed wrote:
> "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