[sdiy] simple output capacitive coupling

Ove Ridé nitro2k01 at gmail.com
Sat Sep 3 07:42:29 CEST 2016

Consider the impedance of each source. Mixer etc will be >100 kOhm typical
whereas headphones will be single digit ohms. For high impedance input,
your  series resistor is (read, should be) negligably small compared to the
input impedance. For headphones, and dead shorts, this resistor acts as a
current limiter. DC offsets aren't the only thing that can kill headphones,
but excessive amplitude can too. Calculate the current that your voltage
would produce the series resistor+typical headphones (4 ohm is a good
start) and choose the value such that the headphone's nominal power isn't
exceeded. This is best down with P=R*(I^2) since there will be a voltage
drop across the resistor, and your original voltage won't reach the
headphone element.

On Saturday, 3 September 2016, 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.
> 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.
> The skinny: I'd like a buffered output to be compatible with headphones,
> without working too hard :)
> Cheers,
> Chris


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