Headphone Amplifier Schematic
How to increase sound quality on this cascode amplifier?
http://www.tubecad.com/2009/09/25/srpp+%20mosfet%202.png
that is the circuit i am using for my mosfet cascode amplifier. and what i have found is that at a high volume on the input of the circuit coming from my mp3 players headphone jack (at level 23 out of 25) it distorts the sound really bad! now, i know how this thing operates thanks to a youtube video on power amplifiers (it’s like a huge hour long lecture from this indian professor dude) but one thing he never explained is how to determine distortion or how to make the circuit have low distortion. what i am trying to accomplish is a loud volume but basically no distortion. like less than 1% at full power. i made everything in the schematic just like it says except for the 1M resistor. i couldn’t pull together 1M but i got to around 780K.
The 1M resistor probably isn’t critical, but it costs peanuts compared with the other components you have there, so why improvise?
If you overdrive an amp like that you will get sudden huge distortion. The sound quality won’t get worse gradually as you increase the volume.
Check the obvious things like the voltages and the quiescent current (1.2 amps is big, but if that’s what the designer says it is, that’s what it should be).
Another possibility is that your power supply isn’t up to delivering 50V at several amps, so peak signals will clip earlier when the supply voltage sags. Or, you haven’t got enough smoothing in the power supply to handle low frequencies at full power. If it’s an unregulated supply you will need a seriously large smoothing capacitor – I would guess something like 20,000 to 50,000 uF.
Final comment – the schematic shows an 8 ohm speaker. You don’t find many of those around these days. If you are using a 4 ohm, the amp will clip before you get to the specified max input voltage because the amplifier gain will be wrong for the lower output impedance. (And if you get really unlucky, the output current will be high enough to fry the FETs, or the power supply)
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