• Circuit Maker

A simple but ingenious alternative for home practice - a headphone socket to fit to your amplifier.

Ever wished you could practice your keyboards without disturbing your neighbours but were prevented by the lack of a headphone socket? Well, here's a simple circuit idea to let you do just that.

Many power and instrument amplifiers lack headphone sockets, but the necessary attenuator-cum-socket is easily added. Headphone power handling, sensitivity and impedance varies widely, but the attenuator values given below will suit most of the common models, including high impedance types from Beyer, Sennheiser and AKG. Note that, in general, low impedance cans (headphones) require more power, and accordingly, potentially large amounts of power dissipation have been allowed for in the attenuation resistors wired across the higher powered amplifiers. One advantage of high impedance headphones is that attenuation resistors need only be cheap 1⁄2 watt or 1 watt carbon types, almost regardless of the amplifier's power output.

Resistors may be-carbon or wirewound, but above 1 watt, you'll find wirewound types smaller, and therefore easier to mount. The values given are standard, but not all the E12 values are easy to obtain in wirewound format. If you need to use several resistors in series or parallel to make up the value/wattage given in the table below, wire these on a subsidiary heavy-duty tagboard, connected to the socket via flying leads.

Resistor wattages are given assuming music signals; continuous tones at high levels may cause overheating, but it's likely that the headphones will expire first. If in doubt, uprate the wattages given, by 30% to 50%. The nominal maximum power applied to the cans (2W for 8, 15, 16 ohms, 200mW for 200, 400 & 600 ohms and 100mW for 2000 ohms) is available only at the power amplifier's maximum output, so excessive power shouldn't be available under normal circumstances.

Headphones are universally wired for stereo, but instrument amplifiers have only a single channel output. In this instance, each side is powered via a separate attenuator, and connection to the (mono) amplifier output is made via a stereo socket, so allowing standard stereo cans to be plugged in without further ado (Figure 2). Headphone sockets wired across power amp outputs cannot in general be arranged to switch the speaker in and out, as R2 remains in circuit, and would overheat when the headphone jackplug is withdrawn. Switching out the speakers/power amp is more easily arranged at line level, particularly with high impedance headphones, which don't normally require attenuators in this position. However, we've opted for connection to the power amp outputs, as this is easier for readers without technical know-how to implement, so figure 3 shows on SPDT switch arranged to select either speaker or cans. This should be a heavy duty toggle or rocker type, rated at 5 amps minimum, and preferably 10 to 15 amps for use with amplifier outputs in excess of 100 watts.

Lastly, if you're stuck for panel space, or would rather avoid drilling holes in your cabinet, look for redundant sockets you can replace. For instance, consider replacing two (mono) send-return facility (see jackfile 3, December HSR for send return wiring using a stereo socket). With this rearrangement, a spare hole will then be made available in the panel for the stereo headphone socket.