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

There's no need for matching between the output amp and the load unless you
want high power, and that's unlikely to be available anyway.
3V at 100 ohms would be 90mW already, and that seems a lot for a
headphone.  You also need to look at the current that the opamp can
support.
Very likely you'll want a series capacitor so that you can tie the other
side of the speakers to ground without drawing continuous DC current.

The inputs on the transmitter likely need to be biased to VCC/2 which could
be done inside the chip, but also will want a blocking capacitor for the
same reason.
The chip is unlikely to have a large capacitor internally.  A module
might.  Or it might want differential input.  As always, see data sheet!

On Sun, Dec 22, 2024 at 9:56 PM Neil <[email protected]> wrote:

> 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
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