[sdiy] simple output capacitive coupling

[Mattias Rickardsson]

Den 3 sep. 2016 3:29 fm skrev "Chris McDowell" <declareupdate at gmail.com>:
>
> howdy list,
>
> I'm working on a small device based on the STM32F334 (a nice and friendly
mcu, btw), with a stereo 1/8" output. What I'm pondering is the best way to
have this device not hurt anyone's headphones. It's not at all intended for
headphones, but I know folks will plug their ipod or iphone headphones in
there. I'm not concerned at all about the performance if someone does this,
otherwise I'd use a proper headphone amp. But I'd love some advice on the
best way to go about this! Here's what I have at the moment:
>
>
https://cdn.shopify.com/s/files/1/0176/0012/files/Screen_Shot_2016-09-02_at_8.05.04_PM.png?12557696908300460869
>
> Now, I don't know if those 51R resistors are at all necessary, and I do
realize they'll attenuate the output by about half if someone does use low
impedance headphones.

Keep them. Imagine someone inserting a plug in the jack, shortening the
connectors for a brief moment.

> Also, I'm using the built in DACs, and attenuating for a max output of
0.825V. This is intentionally on the quiet side, as there is no volume
control (this device is for an electronic band to include with their album
bundle, it's a bit cowboy and we're okay with that) BUT! is a 412mV bias
even enough to hurt the headphones? As I'm used to making eurorack modules,
I never AC couple any outputs, and would love to just leave it off of here
too. But since that's coming from laziness, I figured I'd ask for some
advice.

You should definitely AC couple it, and add a large resistor to ground
after the capacitor. Unless you want to drain your battery? ;-) Or hurt
people's hearing when switching on/off or plugging/unplugging.

The few millivolts bias error in your eurorack modules are rarely a
problem, but here you have a real offset.

Speaking of the offset - before the OP-amp there, wouldn't you want to
attenuate the signal towards a fake ground at half the supply instead of
ground? (I guess you only have positive supply and ground in your system.)
To get it symmetrical and nice for your OP-amp, which then even doesn't
have to be rail-to-rail.

> The skinny: I'd like a buffered output to be compatible with headphones,
without working too hard :)

Even tiny circuits need some extra work, partly because they need to be
tiny. :-)

/mr
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