Budget Headsets

[Brian]

I'm currently knee-deep in the final testing of the BlueCom system and have discovered something that might be more widely relevant.

 

I've been having problems with severe distortion on the headphone out. One of our design points was the output should be capable of being LOUD for use in noisy environments. Testing with SMH210/DMH210 units worked fine but the output was horrible on some more budget units.

 

The SMH units are 400R, the budget units 200R so I was suspecting current limiting in the driver stage but when I swapped the headsets for a 200R resistor for ease of measurements the problem disappeared. Odd.

 

So then I opened the budget units up...

 

 

Yes, that's a pair of diodes strapped across the driver to limit the voltage. No other protection so those diodes will clamp the output of anything driving them. Which explains why our 1A driver stage was getting hot.

 

 

So, if you've got some headsets which don't seem to be loud enough you too might be suffering from a misguided attempt to comply with sound level regulations.

 

 

[E2A]

They measure out as 2.4v zeners with a 0.75v forward voltage.