If you are using ear buds with your iPod or MP3 player, you are getting the equivalent of sonic garbage. Nor does using a high-quality pair of headphones do much to improve it. You need to use a good-quality external headphone amplifier and a good-quality set of headphones and then you will be really travelling (riding, walking, whatever) in style.
So why put up with sonic garbage? You can have much better sound quality. The headphone amplifier described here has low noise and distortion, as well as a long battery life.
So why are these portable players so poor? While the digital-to-analog converter (DAC) in your music player may be quite good, in many cases it is let down by a feeble headphone driver. This not only limits the maximum volume but can also introduce a lot of distortion even at lower volume levels.
With an external amplifier, the headphone driver in the music player is no longer required to supply high currents into a low impedance. It only has to provide a signal voltage into a high impedance load (in this case, about 5kΩ). The external amplifier takes on the more demanding job of driving the low (and variable) impedance headphones to a sufficient power level.
There are a lot of different music players out there and it is not possible for us to try them all but from the tests we have run, it seems that the majority of even better-quality players can benefit significantly from an external amplifier such as the circuit presented here.
While various different styles of headphones and ear-buds are available, from this point on we shall simply refer to them as “headphones”.
Fig.1: a comparison of the distortion from an iRiver iHP-140 MP3 player with and without our headphone amplifier, both channels driven. For both 32Ω and 16Ω loads, the distortion is lower when using our amplifier up to around 15kHz. Between 1kHz and 10kHz, the reduction in distortion with the external amplifier is dramatic, in some cases by an order of magnitude.
Fig.2: a comparison of the distortion from an iPod Nano 8GB with and without our headphone amplifier, both channels driven. In the case of a 32Ω load, the distortion with the external amplifier is the same or better and again the largest gains are between 1kHz and 10kHz. For 16Ω loads, the same applies except that the iPod output is slightly better between 30Hz and 120Hz.
Fig.3: this shows the distortion when driving small Apple ear-buds (both channels) from the headphone driver in an MP3 player and then the distortion from the same player via our amplifier. The reduction in distortion is clear from DC up to 12kHz. Above 12kHz, the light loading on the player’s output with the external amplifier allows its distortion to rise sharply (a quirk of the player).
Fig.4: the Total Harmonic Distortion plus Noise (THD+N)over the audible frequency range, for our amplifier only. In the critical mid-band region of 300Hz-3kHz, the distortion is below 0.005% for 32Ω and below 0.01% for 16Ω. For higher load impedances, the performance is even better although maximum power drops. The high-frequency distortion for 600Ω rises quickly due to the high output voltage.
Fig.5: this graph shows the THD+N at 1kHz for common load impedances over the full power range. The distortion falls as power climbs because the rising signal amplitude swamps the noise signal. More power can be delivered into lower load impedances. Most MP3 players can only deliver up to about 20mW whereas this amplifier will deliver 60mW and more in most cases.
Fig.6: the frequency response for our amplifier is essentially flat over the range of audible frequencies (note the vertical scale). The 0dB voltage level was not changed for the different load impedances so this also demonstrates the low output impedance of the amplifier, ie, changing the load impedance barely has any effect on the voltage level delivered to it.
To see how much of an improvement this headphone amplifier can provide, refer to Fig.1. This is a graph of total harmonic distortion and noise (THD+N) against frequency for an iRiver iHP-140 music player. This is an older model with an internal 40GB hard drive and we tested it because it has a reputation for reasonable sound quality (and we had one handy).
The red line shows the distortion from its line output. Not all portable players have a line output but if it is present, it usually provides the lowest distortion signal. As can be seen, the performance of this unit is quite good, with distortion below 0.01% up to 4kHz and 0.015% at 10kHz.
However, if we connect a load to the headphone output (to simulate headphones), the distortion is considerably higher. The green line shows the distortion into a 32Ω load and the blue line into a 16Ω load, which is considerably worse. Most ear-buds present a 16Ω impedance or thereabouts. In that case, THD+N at 1kHz is above 0.07%.