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High-Performance Stereo Headphone Amplifier, Pt.1

If you can't afford a high-performance amplifier and loudspeakers, you can still have the best possible hifi sound, with this headphone amplifier and a set of high-quality headphones.

By Nicholas Vinen

Features & Specifications

Main Features
• Suits 8Ω – 600Ω headphones and ear-buds
• Very low distortion and noise
• Plugpack-powered (no mains wiring)
• Short-circuit protected
• Can also drive efficient 8Ω loudspeakers

Specifications (Figs.3-7)
Rated power: 100mW (8-100Ω), 25mW (600Ω)
THD: 0.0006% @ 1kHz; 20Hz-22kHz bandwidth
Signal-to-noise ratio: -113dB unweighted; 20Hz-22kHz
Frequency response: ±0.15dB, 20Hz-20kHz
Channel separation: -73dB @ 1kHz
Maximum power: 4.25W (8Ω), 3W (16Ω), 1.5W (32Ω), 800mW (60Ω), 80mW (600Ω)
Class-A power: 18mW (8Ω), 36mW (16Ω), 72mW (32Ω), 80mW (600Ω)
Music power: 4.25W into 8Ω, both channels driven (see text)

Yes, we know that the Ultra-LD amplifier modules described elsewhere in this issue are “over the top” for many people, especially those living in small home units and those who have to worry about sound levels annoying their neighbours.

But why not listen via a good pair of headphones? Spend a few minutes looking around the internet and you will find all manner of hifi headphone amplifiers that claim to have top-notch performance. In most cases, there is little or no performance data to prove it. Before spending upwards of $1000 on a headphone amplifier we’d want to know just how good it is!

Click for larger image
Fig.1: this block diagram shows the basic arrangement of the headphone amplifier. It incorporates RF filtering, a stereo preamplifier, stereo amplifier, output isolation filters and a regulated power supply.

Our new headphone amplifier has a performance virtually the same as our benchmark 20W Class A Stereo Amplifier (May-September 2007). Its distortion at 100mW is lower than that from even the best CD and BluRay players. So essentially what you hear is what is recorded on the CD – no more and no less.

This project does not supersede the Portable Headphone Amplifier for MP3 Players (April 2011) since that one is small, light and battery-powered. That design was intended for use “on the go” and to give much better sound than normally available from iPods and MP3 players.

This new headphone amplifier will also drive 8Ω loudspeakers and has a music power of 4.25W for both channels driven. This is more than adequate if you have reasonably efficient loudspeakers in your study, office or bedroom.

It is housed in a half-size 1U steel case just 210mm wide, 49mm high and 125mm deep and is powered by an AC plugpack (no 230VAC mains wiring). The interior of the case is filled by the PCB which accommodates all the components. There is no other wiring to do; just assemble the PCB, fit it into the case and you’re finished.

Circuit features

Click for larger image
Fig.2: the complete circuit of the Hifi Stereo Headphone Amplifier. The stereo preamplifier section is at upper left and is based on three low-noise dual op amps (IC1-IC3). This stage provides a variable gain of 0-15 depending on the setting of VR1 which functions as the volume control. The two identical power amplifiers are shown at right and these drive the headphones via RLC filters (for stability) and a 6.35mm jack socket. The linear regulated power supply is at lower left and this derives regulated ±12V rails from a 12V AC plugpack.

Fig.1 shows the block diagram of the unit, while Fig.2 shows the complete circuit. It looks huge, doesn’t it? That’s partly because it shows both channels. It can be split into two sections, with the preamplifiers and power supply on the lefthand side and the power amplifiers on the righthand side.

The preamplifier for each channel is based on three op amps so three LM833 dual op amps are used. The preamp configuration is a classic Baxandall design. The preamplifier is inverting and has a gain range from zero to -15.

The reason for such a wide range in gain is that we have to provide for a large variety of headphone impedances and sensitivities. 8Ω headphones require a much lower voltage swing for the same power compared to 600Ω phones. Driving 8Ω headphones from a CD player (typically 2V RMS) may require a gain of 0.25 or less while using 600Ω phones with a line level signal (0.775V RMS or sometimes less) could require a gain of several times.

The Baxandall preamplifier circuit has the advantage that it varies its gain according to the setting of potentiometer VR1. As a result, the residual noise level is kept low at the low gain settings most commonly required. Like a traditional preamplifier, its gain can go all the way down to zero and up to some fixed number, in this case, 15.

Another advantage of this circuit is its log-like gain curve from a linear potentiometer, which generally have superior tracking compared to log pots. All but the most expensive “log” law potentiometers actually use a dual linear taper and so they don’t really have an accurate log response either.

The two power amplifiers on the righthand side of the circuit are loosely based on the 20W Class-A Amplifier but with smaller output transistors and heatsinks. The power amplifiers invert the signal again, so the unit’s outputs and inputs are in-phase. Since there is so much gain available in the preamps, the power amplifiers operate at unity gain (ie, -1). This improves the noise performance and maximises the feedback factor, keeping distortion exceedingly low even with run-of-the-mill output transistors.

Because the headphone connector is a jack socket, the outputs can be briefly short circuited if the plug is inserted or removed during operation. As a result, the design incorporates short-circuit protection to prevent any damage.

Our noise and distortion figures are quoted at 100mW for 8-32Ω and 25mW for 600Ω. With efficient headphones, this is enough to generate very high sound levels. For most headphones, a typical listening level is 0.5-5mW.

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