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Old 6th November 2012, 12:22 AM   #1
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Default Class A mosfet headphone amplifier. How does it look?

Hi. I just made a class A amp and tested it on my breadboard. It sounded as good as my ears could hear, the only drawback was a little background noise. I have now made a complete circuit and I'm planning on etching the board to get a decent result out of it. I would like to get some feedback, is this a decent way of doing it? Can changes be made to up the quality even more?

The circuit can be found here: http://i45.tinypic.com/2ufde6v.png

I use LM317's to set the bias current to around 250mA. This combined with a supply voltage of over 30V will make it drive pretty much all types of headphones.

The amp will have an optional bass boost function. C5 and R3 takes care of this in combination with R5. C4 is there to limit the higher frequencies to avoid the sound getting too intense at high volumes.

R4 and R1 sets the gate voltage, but I'm a little worried that my current setup will have too little gain for high impedance headphones. I have tested the setup with the values in the schematic and it is plenty loud enough for my akg k-701, but my headphones has an impedance of 62Ohms, I think the gain might be too little for a 600Ohm headset

To avoid the popping sound when the amp is turned on, the outputs goes through their own relays. The relays are controlled by IC3. R16 and R15 sets a bias voltage on the inverting input. C10 is then gradually charged after the supply voltage is applied and when it's voltage reached the bias set by R16 and R15, the output vill go HIGH thus turning on Q3 and the relays, this will take something like 2 seconds I believe. I'm not going to use the OP275, I will probably use an LM393 or something like that.

The input is a 3.5mm minijack and the output is a 6.35mm jack. I didn't need to put these in the schematic, but I need them for the board layout.


How does this look?
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Old 6th November 2012, 03:11 AM   #2
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Pretty Cool !!!

jer
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