[lcp12345]

I have made several amp. But, I don't know how large the DC offset will damage the headphone. My figure about this was come from buliding the first PPA amp which was less than 20mV. But, my friend has a amp with 60mV. His IEM has no problem after using for several months.

Please let me know the figure!

Thanks!

 

[jbloudg20]

The lower the better. There really is no exact number, but you should aim for it to be less than 10mV.

 

[Seaside]

OK, Here's what I heard.

Headphones are AC device.

They have a coil and magnet inside it, the AC current that goes to the coil makes diaphragm move back and forth. The energy that was put into the coil dissipated as a form of sound, and you listen that.

If its DC, the coil won't make diaphragm move back and forth. It just try to move it only one direction, thus no sound. the energy is not able to disspate as a form of sound, and that makes the coil gets hot. When something gets hot inside the closed enclosure like earphones, demage will occur.

That's how it works. Any DC offset more than 20mV can possibly demage your phones over time. If the DC offset is like... let's say... 300mV, it will kill your small earphones in a few min. That's why people try to keep it as low as possible as close as 0.

 

[balou]

Piezo speakers don't care about dc voltage. I once exchanged a piezo speaker with a dynamic speaker, and it got very hot. Only after quite some time here on head-fi I found out why it got hot...

Next time I use that ancient laptop, I'm gonna put a cap between the speaker and the wiring

 

[ericj]

Well, it comes down to basic electricity and math.

A positive DC offset will move the headphone voice coil forward, which may affect the sound, especially if it means that the coil reaches maximum excursion during regular listening. Same deal with negative DC offset and minimum excursion.

You can use Ohm's Law to calculate how many mw of heat your voice coil has to dissipate for a given DC offset, but i'll give you a hint: You'll fry 16ohm coils much faster than 600ohm coils.

Handy javascript ohm's law calculator:

http://ourworld.compuserve.com/homep...en/ohmslaw.htm

compare the watt number that calculator gives you based on the offset voltage and the nominal impedance of your headphones with the power handling rating for your headphones.

Oh, and yes, piezo elements are capacitative, and don't conduct DC at all.

 

[Seaside]

Since, eric doesn't comment anything on my post, I assume my explanation is correct.

 

[kipman725]

any kind of DC ofset is bad as it will reduce the dynamic range of the speaker by forcing it forward (or backward) by a constant amount from its ideal resting position.

 

[Pars]

Quote:

Originally Posted by Seaside /img/forum/go_quote.gif Since, eric doesn't comment anything on my post, I assume my explanation is correct.

Well, correct as far as it went... Eric is right that the lower the driver impedence, the higher the current will be thru the driver coil, hence more likely to be damaged. Ohms law...

 

[lcp12345]

Quote:

Originally Posted by Seaside /img/forum/go_quote.gif OK, Here's what I heard. Headphones are AC device. They have a coil and magnet inside it, the AC current that goes to the coil makes diaphragm move back and forth. The energy that was put into the coil dissipated as a form of sound, and you listen that. If its DC, the coil won't make diaphragm move back and forth. It just try to move it only one direction, thus no sound. the energy is not able to disspate as a form of sound, and that makes the coil gets hot. When something gets hot inside the closed enclosure like earphones, demage will occur. That's how it works. Any DC offset more than 20mV can possibly demage your phones over time. If the DC offset is like... let's say... 300mV, it will kill your small earphones in a few min. That's why people try to keep it as low as possible as close as 0.

Your explanation really make sense and informative. Yes, I will try to minimize the DC offset from my DIY amp. I have observed some opamp(5534) can get higher DC offset value vs the other.