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Old 31st May 2005, 10:09 PM   #1
svokke is offline svokke  Belgium
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Default opamp/BJT headphone amp

I am busy making this headphone amplifier as my first solid state project.
Click the image to open in full size..
What do you think of this circuit and how do I start choosing a suitable transistor/bias point?
What parameters do I have to look for?

Regards, Svokke
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Old 1st June 2005, 04:03 AM   #2
jcx is online now jcx  United States
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1st order of business is deciding what impedance load you want to work with and how much power you need; dynamic headphones range from 16 Ohm to 600 Ohm, sensitivity from slightly less than 90 dB/mW to over 100 dB/mW

HeadWize faqs points to all sorts of good relevant headphone information - the link to Rane under headphone power requirements gives the sensitivity of many headphones

I always have to point out the difference between LOUDNESS and HEADROOM - read http://headwize.com/articles/hearing_art.htm twice, once for avoiding hearing damage from average LOUDNESS and again for the required dynamic peak HEADROOM of real music - that you should want to cleanly reproduce, but only for seconds per hour of listening

taking some dynamic range values from the headwize article you can see that while 10 mW average power into most headphones could lead to hearing damage with extended exposure, it wouldn't be totally unreasonable to want 100-500 mW peak power to reproduce natural music peak levels without clipping

Once you have load Z and peak pwr you can choose a bias current and current sink circuit as well as transistor sizes and if heatsinking is necessary
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Old 1st June 2005, 04:48 AM   #3
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Hi Svokke,

So to pick up where jcx left off, and assuming you're happy with 100mW of class A power into (the minimum load) 16 ohms then you'll require a standing current of around 120mA from you're current source and a thermal load on your transistor of 1.8W so you'll need to heatsink a BD139 perhaps to the casework/chassis.
Perhaps a BD140 in the current source. Might be a good idea to put a current limiting resistor in just in case of shorts.
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