Behringer HA400 headphone amp putting out 5.5 volts of DC offset. Are my headphones damaged?

[Michael777]

I noticed a deafening "pop" whenever I plugged headphones into my HA400 amp, so I decided to take some measurements.

It turns out that this thing pumps out 5.5 volts DC, regardless of input. Here's what a 1kHz sine wave looks like:

https://i.imgur.com/pGlsQzx.png

I disconnected the amp and ran the same test with the sound card output, which is centered on 0 volts as expected:

https://i.imgur.com/3KfM7YW.png

I verified this with a multimeter, and also got the same result testing with a second HA400, so it's not just a faulty unit.

So, this thing has been continuously dissipating hundreds of milliwatts of power in my headphones, which I leave plugged in. Given that they've been plugged in for months, is this likely to have damaged the headphones, by heating up the coil?

If there is damage to the headphones due to the constant heat, how would I tell/what would this sound like?

I'm obviously throwing this amp away, I've had it for years but never realised it could be actively damaging whatever's plugged into it.

 

[GREQ]

I'd be curious to find out what the amps are.

Multiply the amps by the volts and you've got the wattage.
I'm guessing the amps is somewhere in the region of 0.1-0.3.

I wouldn't worry too much about it yet.
Theoretically only excessive wattage is what will cause real damage; which is normally immediately apparent due to extremely distorted audio.

 

[Michael777]

Thanks for the response GREQ.

With a 47 Ohm resistor as the load, I've measured the current as ~100mA. The voltage across the resistor was 5.0V, which means the power was 500mW.

I also tried a 20 Ohm load, and saw that the voltage dipped to 1.6V (I guess the amp was overloaded). The current was about 80mA, so the power in this case was only ~130mW.

My concern is that leaving the headphones connected for months has caused some subtle degradation to the sound quality. My main headphones are the HD 599, which at ~50 Ohms (resistance measured at DC) will have been constantly taking the full 500mW. How much power would you consider dangerous in this situation, or is it hard to know?

 

[GREQ]

In this case its pretty safe to say that no damage has occurred, or will occur.

Those measured wattage values are totally safe, and well under what most people here would call a 'strong amp'.

My audio-gd NFB11.28 amp will push around 1800mW into a 50 ohm load, and I've never had a burn out on any headphone....

Technically I could burn out a headphone by cranking the volume for an extended period of time, but under normal listening volumes it just won't happen.

Damaged/burned coils will only cause a noticeable degradation. There won't be anything subtle about it