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Circuit Maker

Building A Headphone Attenuator

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.

Figure 1. Connecting a headphone attenuator to a stereo power amplifier.
(Click image for higher resolution version)

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 ½ watt or 1 watt carbon types, almost regardless of the amplifier's power output.

Figure 2. Feeding stereo headphones from a (mono) instrument amplifiers output.
(Click image for higher resolution version)

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.

Figure 3. Wiring a speaker/headphone selection switch.
(Click image for higher resolution version)

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.

TABLE 1. Resistor values for various amplifier output powers.
nominal amp power into: Headphone Impedance
  8ohm   15 or 16ohm   200ohm   400ohm   600ohm   2000ohm  
8ohms 4ohms 2ohms RMS output voltage R1 R2 R1 R2 R1 R2 R1 R2 R1 R2 R1 R2
30w 60W 120W 15V 22R omit 27R omit 150R 220R 120R 390R 100R 620R 100R 2k2
  5W   3W   ½W ½W ½W ½W ½W ½W ½W ½W
60W 120W 240W 22V 18R 8R2 22R 15R 270R 220R 270R 390R 330R 620R 560R 2k2
  17W 2W 11W 1W 1W ½W ½W ½W ½W ½W ½W ½W
100W 200W 400W 28V 24R 8R2 33R 15R 390R 220R 430R 390R 470R 620R 1k 2k2
  17W 2W 9W 1W 1W ½W 1W ½W ½W ½W ½W ½W
150W 300W 600W 35V 33R 8R2 47R 15R 470R 220R 560R 390R 680R 620R 1k5 2k2
  17W 2W 11W 1W 2W ½W 1W ½W 1W ½W ½W ½W
250W 500W 1kW 48V 56W 8R2 68R 15R 680R 220R 820R 390R 1k 620R 2k2 2k2
  21W 2W 15W 1W 2W ½W 2w ½W 1W ½W ½W ½W

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Jan 1984

Donated & scanned by: Stewart Lawler

Feature by Ben Duncan

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