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High-Performance Stereo Headphone Amplifier, Pt.2

Last month we introduced our new Hifi Headphone Amplifier which features very low distortion and noise. It can even drive efficient 8-ohm speakers. This month, we show you how to build and test it.

By Nicholas Vinen

The assembly of the Hifi Stereo Headphone Amplifier is straightforward, with all the parts mounted on a single PCB coded 01309111 and measuring 198 x 98mm. Apart from the PCB, there is no other wiring.

Fig.9 shows the parts layout on the board. Before starting assembly, it’s a good idea to test-fit the larger components (eg, the jack socket, heatsinks, RCA sockets and so on) to check that their mounting holes are large enough.

Click for larger image
Fig.9: follow this PCB overlay to assemble the headphone amplifier. Note that the specified case has no ventilation, so we recommend the use of 105° electrolytic capacitors for long-term reliability.

That done, begin by installing the 10 wire links using 0.7mm-diameter tinned copper wire or component pigtail off-cuts (don’t forget the one near CON3). Once those are in, install the resistors, noting that two (both 100Ω just below the RCA sockets) have ferrite beads on their leads. Check each resistor with a DMM set to Ohms mode before soldering it into place.

Follow with the 14 1N4004 diodes, taking care to ensure they are all correctly orientated. In each case, the stripe faces to the left or the bottom of the board. The four BAT42/BAT85 small-signal Schottky diodes (D15-D18) near IC1 (upper-left) can then go in. Their orientations vary so take care.

If you are using sockets for IC1-IC3, install them now with the notches to the right as shown. Alternatively, you can solder the ICs direct to the board with the same orientation.

The MKT and ceramic capacitors are next on the list, followed by the 20 small-signal transistors. There are four different types so be sure to install the correct type at each location. Use a small pair of needle-nose pliers to crank the transistors leads so that they mate with the board holes and take care to ensure that each transistor is correctly orientated.

The two 500Ω trimpots and the fuseholder clips can now go in. Check that the small fuse clip retention tabs are towards the outside before soldering the fuse clips, otherwise you won’t be able to fit the fuse later on.

That done, fit PCB pins at test points TP1-TP4 plus another two to support the tinplate shield between inductors L3 & L4. Once they’re in, fit the electrolytic capacitors but leave the two 2200µF filter capacitors out for the time being.

Installing the LED

Before installing the 3mm blue LED, you need to bend its leads down by 90° exactly 4mm from the rear of the lens. Be sure to bend the leads in the right direction though, so that the longer anode lead goes through the hole marked “A” on the layout diagram.

Once the leads have been bent down, solder the LED in place with the horizontal section of its leads 6.5mm above the PCB surface (a cardboard spacer can be used to set the height). This ensures that it will later line up with its front panel hole and will be in line with the centre shaft of the adjacent switch (S1).

Winding the inductors

The two air-core inductors (L3 & L4) are wound on small plastic bobbins. It is much easier to wind them if you make a winding jig, as shown in the adjacent panel.

To wind the first coil, first secure the bobbin to the jig with one of its slots aligned with the hole in the end cheek. That done, feed about 20mm of a 1m-length of 0.8mm-diameter enamelled copper wire through the hole, then carefully wind on 20.5 turns before bending the end down so that it passes through the opposite slot in the bobbin. Trim the “finish” end of the wire to 20mm (to match the start end), then secure the winding with a layer of insulation tape and remove the bobbin from the winding jig.

A 10mm-length of 25mm-diameter heatshrink tubing is used to finally secure the winding. Slip it over the outside and gently heat it to shrink it down (ie, be careful to not melt the bobbin).

The second coil is wound in exactly the same manner. Once it’s finished, scrape the enamel off the leads on both inductors and tin them before fitting them to the PCB.

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