headphone preamps, suddenly no bass?

[Chris Strellis]

Hi Michael,

It could well be a capacitor.  You'll have one in series with the input to act as a DC blocker.  In actual fact it forms a High Pass filter generally with a cut-off around 5 to 10Hz.

Also, and I think that this is the culprit, there may be a series output capacitor.  Headphone amps tend to be Class A amplifiers for best quality, the power is low and so is the distortion.  However, the audio will sit on a DC voltage of maybe 10V.  The series output capacitor (between the drive transistor output and the headphone sockets) will remove this voltage to leave an AC audio signal.  It generally is a bipolar type capacitor around the 100uF range.  Again it forms a High pass filter with the load.  If this value has drifted low with age and abuse then the bass will get rolled off.

There's a schematic here of one using IC amps (principle is the same).

http://www.minidisc.org/schem.gif

The caps I'm on about are C3 and C7.

Just a thought, is any one channel (L or R) worse than the other?  I'd be surprised both caps would have gone the same way.  Do you hear any big clicks when you plug in the headphones or input sources.  This would indicate a DC offset.

Cheers

Chris