Headphone Adaptor for Power Amplifiers

This simple project is nothing more than a handful of resistors and a double pole, double throw switch, but will reduce the output of almost any amplifier to a nominal level of 5V RMS, and maintains the recommended 120Ω source impedance (IEC 61938).  This is designed to suit most headphones currently made, as they are generally designed to operate from that impedance.  There is also a second version, which is designed to ensure that the headphone output impedance is around 2 ohms or less.  This arrangement is more likely to suit headphones that are intended for use with personal media players, most of which have a low output impedance.

Naturally, neither approach is always suitable, as many manufacturers appear to have chosen not to adopt the standard for one reason or another.  The circuit will suit most headphones well regardless.  If you find a serious degradation in sound quality (possible but unlikely), you can either build the second version shown, or you can build Project 113.

The maximum output level of 5V RMS (Version 1) was chosen to ensure that the power amplifier will not clip when driving the headphones, but is much too high for normal listening.  As always, you may make changes to suit your preferences, but be aware that nearly all headphones are capable of sound levels that will cause permanent hearing damage, so always be mindful of this.

Whether you use Version 1 (120 ohm output impedance) or Version 2 (2 ohm output impedance) depends on your preferences, the recommendations of the manufacturer of your headphones, and how much power you are willing to dissipate to get your headphone output from a power amplifier.  There are probably as many opinions as there are headphone users as to the 'best' source impedance, but ultimately it's down to what you prefer.  Some 'phones don't care much at all about the impedance, others may behave very differently.

Warning: This unit (either version) is not designed to be used with bridged amplifiers! If there is a warning on your amp that states that the -ve speaker terminals must not be grounded, then you must not connect this adaptor, or the amplifier and/or headphones may be damaged.  If in doubt, find out first from the manufacturer or distributor - assumptions can be very costly!

The project could not be simpler - it basically consists of a switch to disable the main speakers, and the attenuator to set the correct level and impedance.  Figure 1 shows the circuit diagram of a single channel, and this is duplicated for the second channel.  Note that not everyone agrees about the use of 120 ohm source impedance for headphones, and it has been abandoned by all manufacturers of portable media players.  This was done because the low supply voltage causes headphone amplifiers to struggle to provide enough level when fed via a 120 ohm resistor.

Figure 1 - Schematic for Left Channel, Version 1

The only hard part in all of this is choosing the resistor values that will give you as close as possible to the correct voltage and impedance for all typical amplifier powers.  Table 1 saves you the tedium of working this out, and all attenuators use standard value resistors.  The nominal voltage and actual output impedance are also shown, and as you can see, the variation is very small.  Exact impedance is possible, but requires odd value resistors.  The table shows standard E12 resistor values (12 values per decade) in all cases.

The table shows the nominal amp power (8 ohms), and the values for R1 and R3 (marked with * in the schematic).  The actual voltage available to the headphones is also shown (Vout) as is the maximum power for R1 and the recommended power rating for that resistor.  R2 is fixed at 120Ω for all power levels.  Should you need more (or less signal) for your headphones, you may simply use the values for the next lower (or higher) amplifier power.  For example, if your amp is 60W and you want less level for the headphones, use the values for a 100W amp.  If you want more level, use the values shown for a 40W amp (note that the dissipation of R1 will be increased in this case).

The following unit achieves much the same result, but is designed to ensure that the output impedance is kept to around 2 ohms.  The maximum output level available has been reduced, because a level of 5V would cause dissipation in R2 of up to 10W which is excessive.  The amplifier load would also be greater, and total resistor dissipation would be excessive.  While the 'new' level is set at 2V RMS (at peak amplifier level), this still requires that R2 is rated for at least 2W.

R1 is determined by the amplifier power.  Headphone power is limited to around 100mW for 32 ohm 'phones.  This will be more than enough, even for listening at very high levels.

Figure 2 - Schematic for Left Channel, Version 2

For high power amps, the power rating for R1 gets up to 40W, but that's only if the amp is run at full power with sinewaves.  For normal listening you may be able to reduce the power rating to roughly half the value shown in the table.  Also, consider that the values shown assume that you need around 2V of signal to the 'phones - in reality you will be unlikely to need more than around 1V, and even that will be very loud with most headphones.  1V will provide 31mW, corresponding to an output of 115dB SPL with 100dB/1mW 'phones - more than enough cause permanent hearing damage.

The power into 32 ohm headphones is based on the nominal impedance, and the voltage at the output.  The latter was calculated with a 32 ohm load present, so is fractionally lower than calculated with no load.  Output impedance is determined by the parallel combination of R1 and R2, assuming that the amplifier has a near zero output impedance.

The R1 values in brackets are those for standard values, and the power ratings in brackets are also standard (or are easily made using standard resistors).  You can use resistors in series or parallel to get the desired value and power rating.  Some of the high power versions need lots of watts for R1, and to get (for example) 47 ohms at 40W, the easiest is to use 4 x 47 ohm 10W resistors in series parallel (two parallel connected strings of two resistors in series).  For all amps above 20W, the divider reduces the load on the amp compared to using speakers, so it may sound a little 'cleaner'.  However, for any competent amp the difference will be vanishingly small.

Note that wirewound resistors are required in all versions, and these generally have a fairly wide tolerance.  You may need to get more than you require and select them so that left and right channels have the same attenuation (±0.5 dB or better).  The absolute value is not as important as the balance between channels.  This is especially important if your amplifier lacks a balance control.

Feel free to change the value of R2 and re-calculate the value and power rating for R1 if you wish to modify the levels and/ or impedances presented to the amp and headphones.  Calculations involve basic theory only.

To construct either version of the circuit, you will need a double pole, double throw switch to disconnect the speakers, assuming that this is not already available.  Do not be tempted to use a rotary switch, unless it is rated for the maximum amplifier output current - most are not.  A heavy duty toggle or rocker switch is recommended, with a minimum current rating of 10A.  Some amplifiers use a switched jack socket that disconnects the speakers when the headphone plug is inserted - if you want to use that arrangement you'll need to work it out for yourself.  I don't recommend it because some jack sockets may short the speaker lines when the plug is partly inserted (e.g. while inserting or removing the plug).

As shown, when the speakers are disconnected, the headphone adaptor is connected and vice versa.  This prevents power being fed to headphones for no good reason, and also prevents extraneous sound when you are listening to the speakers.  The entire adaptor may be installed in a separate box, with a speaker switch, headphone socket(s) and speaker in and out connectors.

This approach is assumed in the schematic, and will generally be the easiest way to provide headphone capabilities for an amplifier that does not have this ability.  If more than one set of headphones is required, you must use a separate attenuator for each output - do not simply parallel headphones.  This is not required for the Version 2 circuit, but consider that paralleled headphones may cause some mutual interference.

The 'tip' of a stereo phone plug is the Left channel, the ring is the Right channel and the sleeve is Earth (Ground).  If your amplifier has a balance control, you can check that the jack(s) are correctly wired by using the balance control to mute one channel.  I suggest that you use standard ¼" (6.35mm) jack sockets rather than miniature types which are not as robust.

Before connecting the unit to your amplifier, make sure that there are no wiring faults that present a short to the amplifier terminals.  This can be tested with a multimeter, and you should also verify that the switch connects and disconnects the headphone attenuators and speakers in the correct manner.

The real test is to connect your amplifier and headphones, and verify that the level is correct, and that everything works as it should.  This must not be done until you have checked your wiring thoroughly, and verified that there are no shorts - especially across the speaker leads!

Note: In use, make sure that the amplifier volume is set low to start with.  Headphones vary considerably in impedance and sensitivity, and it is virtually impossible to determine the correct setting in advance. It is very important that you always maintain a safe listening level - as stated above, headphones can produce extremely high SPL (Sound Pressure Level) - more than sufficient to cause permanent and irreparable hearing damage! Please see the table in Project 113 for maximum levels over time.  At 97dB SPL, maximum listening time is 30 minutes in any one day.

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