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Old 23rd November 2013, 10:17 PM   #31
Johno is offline Johno  Australia
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Perhaps head phone load at gain = 4 exceeds the output rating, unless you have a finger (around 1k at that voltage) in series with the load.

And now you know why it is good to minimise as well as balance the input resistances to op amps. Note that while fet inputs have high DC resistance they also have large input capacitance, it is the capacitive current with AC signals that acts like an input bias current and causes amplitude distortions when unbalanced rather than DC offsets.

I hope you are now learning to carefully evaluate what people here say.
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Old 23rd November 2013, 10:54 PM   #32
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While you have had lots of advice on resistor values around op-amps no one has mentioned the power supply.
Make sure you use star grounding or you will get massive hum.

I threw together a pcb for a 6 channel mixer and when I built it the hum was at an unbearable level.
I found the hum was coming from smoothing capacitors charging up and causing a pulse on the ground line.
I re-laid out the pcb with the power supply separated and it worked a treat.
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