Headphones Sounding Bad? It Could be Your Headphone Amplifier!
It's all about control! The movement of headphone transducers must be well controlled in order to produce high-quality audio. It is easy to build a headphone amplifier that produces sound. It is an entirely different matter to produce an amplifier that is clear, clean, and enjoyable.
The voltage sensitivity of the headphones must be well-matched to the output voltage capabilities of the headphone amplifier.
The headphone amplifier must be capable of providing enough output voltage to drive your headphones to a comfortable listening level. If your headphones have a low voltage-sensitivity (common with many high impedance headphones), your portable devices may have difficulty providing enough voltage to cleanly drive your headphones to a suitable listening level. Benchmark HPA2™ headphone amplifiers are designed to provide up to 17 Vpp, more than enough to drive headphones with the very lowest voltage-sensitivities.
On the other hand, if your headphone amplifier produces too much output voltage, you may find that you have to turn the volume control to a very low setting. This excessive use of the volume control usually causes a significant loss of audio quality. SNR, THD, and L/R balance can deteriorate at low volume-control settings.
It is important to be able to use a volume control setting that is near the top of the range. Audio performance usually deteriorates as the volume control is turned down. To achieve this, you will need to select a headphone that has a voltage sensitivity that is well matched to the capabilities of your headphone amplifier. If you are using a Benchmark product with an HPA2™ headphone amplifier, this is never an issue. The HPA2™ is equipped with jumpers that allow customization to match the output level to the sensitivity of your headphones.
If your headphone amplifier cannot produce enough output current, the amplifier will begin to produce distortion. This is a common occurrence with many headphone amplifiers. It is almost universal with portable devices.
The Benchmark HPA2™ is actually a small audio power amplifier. It can produce up to 800 mA. This is far more current than can be produced by typical headphone amplifiers and it is more than enough to simultaneously drive 2 pair of 60-Ohm headphones.
A more detailed discussion of the voltage and current requirements of headphones can be found in this application note:
Headphone amplifiers need power and accuracy to achieve control. These characteristics are closely related to the damping factor of the headphone system. Damping factor is the ratio of the headphone impedance to the output impedance of the headphone amplifier. A high damping factor is an indication that the headphone transducers will be tightly controlled.
Damping factor reaches a maximum when the output impedance of the headphone amplifier approaches 0 Ohms. It is impossible to build an amplifier with a perfect 0-Ohm output impedance. However, it is possible to achieve 0.1 or even 0.01 Ohms with careful engineering. These very low-impedance designs are often called "0-Ohm" headphone amplifiers.
Benchmark products are equipped with HPA2™ "0-Ohm" headphone amplifiers. The HPA2™ has an output impedance of about 0.01 Ohms. These "0-Ohm" amplifiers offer outstanding control.
"The 0-Ohm Headphone Amplifier" white paper by John Siau shows how the performance of headphones can improve dramatically when they are driven from a "0-Ohm" source. The paper includes measurements that show that distortion is reduced as damping factor increases. The measurements also show that frequency response is flatter when headphones are driven by a "0-Ohm" amplifier.
The same principles apply to loudspeakers and power amplifiers. We plan to address the specifics of these devices in future application notes.