Looking for an amp for my Sennheiser IE8

The first step is to determine whether the hiss is being generated in the amp, or is coming from the source. If you unplug the source and the noise goes away, you’ve found your culprit: the source is just noisy. If the amplifier has a volume control at its input and changing the volume level results in a corresponding change in the noise level, that’s another strong indicator that the noise is coming from the source. The only thing you can do about this (short of getting a better source) is to reduce the amplifier’s gain, if you can. That way, you don’t magnify the source’s noise any more than you have to.

Reducing the gain can also help if the noise is being generated within the amplifier, because many of the noise sources within a headphone amplifier are directly affected by the amp’s gain. The optimal gain is where the volume control is at about the 3 o’clock position when the music is as loud as you normally listen. This gives you a little bit of extra control to turn the music up for quiet passages or the occasional rock-out session. If you’re using less than half of your volume control’s range, the gain is too high: not only does that raise the noise floor unnecessarily, it makes the volume control "touchy".

If you’ve selected your amplifier’s resistor values, another thing to check is whether they are too high. All resistors add noise to a circuit, with higher values being more noisy. This effect is called Johnson noise, after its discoverer. All you can do about it is lower the resistor values, if possible. You can play around with different values and see their effect on noise with my op-amp noise calculator.

If lowering the gain and resistor values is impossible or ineffective, try adding some resistance in series with the amplifier’s output. If the amp design allows it, try putting the resistor inside the feedback loop first. (This is R5 in the CMoy pocket amp, and R8 in the PIMETA, for instance.) Putting the resistor inside the feedback loop is often effective, and has the fewest bad side effects. The incentive is to use as low a value as possible, to avoid side effects. Start with 10†‰Î©, and work your way up to 100†‰Î© at most. If that doesn’t work, try adding the resistance outside the feedback loop: just put the resistor inline between the amp’s output and the headphone jack. This gives a stronger effect, but with more bad side effects.

If you’ve lowered the gain and added up to 100†‰Î© outside the feedback loop and the noise is still there, give up on this tack. The problem is probably something more serious, like a broken component, interference, or oscillation. Read on for discussion of these topics.