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Old 19th November 2009, 12:39 AM   #5971
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Default Somewhat off topic Headphone amp in service manual

I decided to build the headphone amp from the service manual, since the parts are minimal. It works just dandy, however the 50 watts of resistors making my 16ohm source resistor still run about 120c. The heatsink for the mosfet runs cool. Any idea what the ideal resistor wattage is here? Looks like a heatsink type for sure...
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Old 19th November 2009, 02:28 AM   #5972
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Your resistor should dissipate 16 watts only.
Did you measure 16 volts across it?
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Old 19th November 2009, 05:52 AM   #5973
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I've replaced the one channel that burned out. I made sure my soldering was very good.
Every thing seemed great. Then the newly installed channel was starting to have distortion. I check the dc offset on the terminals and it was all the way up to 500mV!
The resistors on R11 and R12 were very low, under 30mV. I've hit a wall. This situation is very similar to before, when the same channel burned out when I messing with the pots trying to correct the problem. All new parts and pcb were installed. I would really appreciate any help.

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Old 19th November 2009, 07:20 AM   #5974
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i would suspect the speaker or it's cable.
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Old 19th November 2009, 10:07 AM   #5975
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Quote:
Originally Posted by bobodioulasso View Post
Your resistor should dissipate 16 watts only.
Did you measure 16 volts across it?
The measurements are exact to what's posted. Someone did their homework.
The 16 ohm resistor from source to ground just seems to need a be a large power resistor.
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Old 19th November 2009, 10:50 AM   #5976
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Some oscillation?
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Old 19th November 2009, 04:41 PM   #5977
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Quote:
Originally Posted by Tea-Bag View Post
The measurements are exact to what's posted. Someone did their homework.
The 16 ohm resistor from source to ground just seems to need a be a large power resistor.
The 16 ohm resistor needs to be a power resistor, just as Nelson writes it in the text. Pity though he didn't specify a Watts rating for it. Nelson, could you shed some light onto this matter?
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Old 19th November 2009, 04:47 PM   #5978
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Just work it out
Quiescent is 16 watts dissipation.
With the FET fully on nearer 25 watts or more peak.
So a 20 watt resistor will be fine.

And for driving a pair of headphones that IMO is obscene, but there we go.
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Old 19th November 2009, 06:27 PM   #5979
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Quote:
Originally Posted by Mooly View Post
Just work it out

And for driving a pair of headphones that IMO is obscene, but there we go.
I was hoping for some help there.

I have 45 watts of sand-cast wirewounds bundled together, putting off an enourmous amount of heat. a Finned heatsink mountable seems the way to go, but what wattage? Right now just a little clip lead project to get an idea of the sound of the unit.
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Old 19th November 2009, 06:34 PM   #5980
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Let it get cold, short circuit the input to ground and see wether it heats as much. That is a way to check for oscillation.( unless you have a scope)
If there is less heat this way, the problem is : it oscillates.
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