[zer061zer0]

Hi guys,

I was wondering if anyone hav any idea if this is possible. Using a Lm3886, which i have gotten from chipamp to have a dual output, 1 to driving speakers and another one to drive my headphones,600ohms.

Anyone know how I can include a voltage divided to drive the headphones without cooking them

 

[balou]

the lm3886 is just a mono chip - you would need at least two of 'em. you'll find a reference implementation on national's product page: http://www.national.com/pf/LM/LM3886.html

 

[zer061zer0]

but how do i calculate the amt of output, 28w to 8ohms, how much would be sufficient for 600ohm

 

[AudioCats]

P=U^2/R, so:
* to push 28W into 8 ohms load the needed voltage is 15V rms.
* To get 100mW into 600 ohms, the needed signal voltage is 7.7V rms, about half of what the 3886 can produce.
so to protect your headphone, put a 560 ohm resistor in series with the headphone. That way you will Never burn your phones.

But in reality, a 220 ohm resistor will work good enough. It also allows you to use lower impedance phones.

 

[tyre]

err, nevermind.

 

[zer061zer0]

oops, I made a mistake here, its:

68W cont. avg. output power into 4 at VCC = ±28V
38W cont. avg. output power into 8 at VCC = ±28V
50W cont. avg. output power into 8 at VCC = ±35V

taken from the link by Balou.

Audiocats, what is the minimum amout of output required to push the 600ohms sufficiently, any numbers or range. And also what is "U" as depicted in ur formula.

 

[Clutz]

Oops. I made an error! AudioCats is correct, please ignore me!

 

[AudioCats]

u is the output voltage, R the load resistance.

normal headphones have 100mW power handling capability, some can stand more for short durations, but not all the time. So to be safe, stick with 100mW.

50mW is probably all you will need, normal portable devices are only 30mW per channel.

 

[zer061zer0]

I am a little confuse due to the units, I am unable to arrive at 220 or 560ohms of resistors to add.

I think I am confused by the units. Say take my Xiangsheng708B its say its output at 500mv-250mv, how do I know at what impedance phones will result in what sort of outut from the amp.


I was actually looking at the 01 amp, where its says 1000mw for 32-64 ohms and 120mw for 600ohms before arriving at the idea of using this board http://www.chipamp.com/lm3886.shtml.

Can I say that all I have to do will be to wire them up to a swtich with an output to speakers and another to headphone with 220ohms resistors to each channel.

 

[AudioCats]

220 is what you usually see in a power amp's headphone jack.two 220R (L/R) are used for headphone protection. So 220 is good enough.

 

[zer061zer0]

audiocats, can u point me to any source where by i can read up to try to understand more about impedances and the calculations.

Besides mw is there any other factors that decide whether if the amp could drive a headphone adequately.