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Old 21st June 2012, 07:09 PM   #1
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Default Wiring headphone out

Hi
I wanna ask a question, if u wont mind

Im currently buliding a lm1875 with dual mono construction....

I wanna make 2 different output, headphone and binding post....

So my question would be :

1. I want to put 100r resistor at my headphone out ??? Is it safe ?
2. Can i use dpdt switch for headphone and speaker out ???? Without any protection circuit ???


Thank you very much... Hope you can answer my noob question
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Old 22nd June 2012, 02:23 PM   #2
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I am not able to directly answer your question. But here is FWIW.

I once build a "speaker binding post to headphone adapter". Basically speaker cable on one end and female TRS jack in the other end, with a network of resistors (with both shunt and series resistors). With series resistors you will be sacrificing damping factor.

I don't remember the exact values I used, but I used it to step down a 100Wrms and I ended up with 120ohms output impedance. The headphone that I intend to use has an impedance of 120ohms. It appears to work well.

So I suppose it depends on the headphone you intend to use (sensitivity, max power handling and impedance), and the output power of your amp.

With the DPST switch, you might get a pop if you flip the switch while the amp is powered. May be a make before break switch would work, but I really don't know.

Another way is to build a headphone amp and use it to drive both the headphone and the speaker amp (effectively working as a pre amp, which is what I did).
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Old 22nd June 2012, 02:43 PM   #3
AndrewT is offline AndrewT  Scotland
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The resistance/s you put across the speaker terminals must not be allowed to overheat, nor should they apply excessive loading on the amplifier.

I suggest you try three different adapter circuits for driving your headphones.

1.) the simple 100r in series with the headphones.
2.) a resistive divider using two resistors. 100r in series to the headphone TRS and a 10r across the TRS to audio Ground.
3.) a higher impedance resistive divider again using two resistors. 330r in series to the TRS and a 100r across the TRS.

These will all sound different. Choose which you prefer. Or even have 3 TRS outputs each loaded differently and to suit your (listening) mood.
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Old 22nd June 2012, 02:51 PM   #4
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Hi navyblue and andrew

Thank you for replying....

What is the different using resistor on ground and not ????
Whats the effect ??
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Old 22nd June 2012, 02:53 PM   #5
AndrewT is offline AndrewT  Scotland
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Old 22nd June 2012, 02:55 PM   #6
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I mean, i wont to blow out my headphone....
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Old 22nd June 2012, 03:24 PM   #7
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What is the power output of your amp?

What is the impedance, sensitivity and max power handling of your headphone? Some headphone can be driven to deafening level with an iPod, while others basically demand an amp designed for speakers.
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Old 22nd June 2012, 07:01 PM   #8
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Overdream, it might be beneficial if you told us which particular headphones you intend on using.

Quote:
Originally Posted by AndrewT View Post
1.) the simple 100r in series with the headphones.
2.) a resistive divider using two resistors. 100r in series to the headphone TRS and a 10r across the TRS to audio Ground.
3.) a higher impedance resistive divider again using two resistors. 330r in series to the TRS and a 100r across the TRS.
Since most modern headphones are designed for voltage drive, I'd personally go for something along the lines of option 2, and if you are using really sensitive headphones like some modern IEMs, with a smaller shunt resistor to ground. The smaller this shunt resistor in the L-Pad the lower source impedance your headphones see, and you get better electrical damping.

Some headphones can benefit from current drive, and deliberately increasing the source impedance like that of sticking a 100R series resistor in front of it tends to increase the bass a bit. There's really no right or wrong, just equalize to your liking.
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Old 22nd June 2012, 07:21 PM   #9
raul_77 is offline raul_77  Europe
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Headphone Adaptor for Power Amplifiers
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Old 22nd June 2012, 10:53 PM   #10
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Quote:
Originally Posted by raul_77 View Post
I wouldn't recommend a T-type attenuator. There are only a handful of headphones sounding best with a non-zero output impedance like the 120 ohms this one aims for (some older/smaller AKGs and Beyer DT831/931 come to mind). Generally, an ordinary voltage divider is a better option.

The choice of resistor values really depends on the headphones to be driven, as mentioned. Some are happy with most anything under 100 ohms (or even more), while others would like to have less than 10 ohms; a few should see less than 1 ohm even and are almost impossible to drive properly via ordinary resistor dividers.

If the headphone out is important, best use a dedicated headphone driver that enables low-impedance output. Maybe something like a pair of OPA551s wired up as a buffer, which can be used on the same supplies as the LM1875 (though I'd soften those up with some series resistors of maybe 47 ohms to prevent excessive heating when driving low-impedance loads).

I assume the '1875 would be run at about 26 dB of gain, so 10 dB of attenuation (with, say, a 6k8/3k3 divider in front of the HP drivers) yields a total of 16 dB. More attenuation may be needed for sensitive headphones. Use a 10k pot as a headphone level trim instead of the fixed divider for more flexibility.
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