I am going to design an amp using dual 9V batteries as power supply, I am concerning if one of the battery died, will the amp or headphone be demaged? Do I need to add a protection circuit like O2 does?
How to use an output cap to protect the amp/headphone?
What happen if one of the 9V battery dies if thete is no protection circuit?
Use a single supply (2x9V in series) circuit, with input and output blocking capacitors.
No protection problems, can be discrete or op amp.
An example: https://i.stack.imgur.com/P1gYC.png
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Wire it in series with the headphones, it blocks any DC. Value depends on headphone impedance.
I reckon you'd notice it sounding distorted first and turn it off. Its definitely a problem if you've powered the amp up, with cans connected but aren't listening, then the remaining battery could cook your headphones' voice coils.
@rayma's proposal is the best one - don't use a 'balanced' supply (i.e. plus/minus 9V) rather use two batteries in series to give 18V, with caps at in and out. Then you'll avoid the problem and not need any protection circuit.
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For DC offset, it won't be an issue since I am going to implement dc servo. But those op amp I am going to use are dual power supply op amp, can't be drived with 18V.
So, the conclusion is, I still need a protection circuit, right??
So, the conclusion is, I still need a protection circuit, right??
Your DC offset servo won't work properly when one battery goes down.
I'm only aware of one opamp that needs a ground pin, so chances are you're going to be fine with your opamp choice running on a single supply, provided you bias your audio symmetrically between those two potentials - i.e. around +9V - with some kind of supply splitting circuit.
So no, you don't yet need a protection circuit.
I'm only aware of one opamp that needs a ground pin, so chances are you're going to be fine with your opamp choice running on a single supply, provided you bias your audio symmetrically between those two potentials - i.e. around +9V - with some kind of supply splitting circuit.
So no, you don't yet need a protection circuit.
The easiest way is to use capacitors (AC coupling). Capacitors has audio marks and larger capacitors are less distortion.
In fact, with the phones under 100ohm I have tried, the poor quality 2200uF capacitors (from China) have also achieved good results, although the sound is reduced in a little more detail.
Type of cap would be electrolytic, value depends on the impedance of your headphones. For 60ohm, I use 1500uF myself. Lower impedance needs a higher value.
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