Power Amplifier Speaker Output Headphone Adapter DIY



I was wondering if it was possible to drive a pair of regular dynamic headphones from a power amp such as my Class A F5. Sometimes I have periods of time without a pair of speakers such as if I have sold them and looking for something new to replace them with. In the meantime it would be nice to use a pair of headphones which quite frankly are hard to beat with most speakers. It seems that you need to spend quite a few more in pennies on the speakers to match even a modest pair of cans and once you are in the realms of higher end headphones then most speakers don't come close to the same levels of enjoyment and immersion in the music.

I found a neat little project over at ESP. It is nothing more than a 6 resistors, 3 per side to give the correct voltage levels and impedance to the headphones. You could probably solder these resistors in-line with the headphone to speaker banana leads and heat-shrink them but I used a small box, proto-board to hold the resistors and some nice nylon sleeving.

In theory, using a decent power amplifier such as a headphone amp should beat the usual puny op-amp and power IC circuits with the ability to drive even the lowest impedance difficult headphones which some amps struggle with. The circuit can be adjusted to suit headphone impedance and power amplifier output wattage by changing resistor values. The values I used below are suitable for amplifiers in the 25W range and headphones down to 32R with everything on the side of caution, giving good volume range. The series resistance is needed to limit current draw and the higher values of R also lower the noise floor to levels you will not hear in headphones were even the slightest hiss or hum would be exposed. Lower values of R would give the amp a better matched load but noise levels then increase so it is a trade off. It is all down to experimentation of values to get the best result.

 Warning : Do not use IEMs or ear buds !!

There are also commercial versions of these adaptors and the ART Headtap is one such unit.

Here is the original table from ESP to choose the correct resistor values for the input and output resistor. The parallel 120R resistor in the circuit does not change value. You only need to select the value of the input and output resistor. I would use 2W or 3W resistors regardless as they are cheap and not large. So as you can see for my 25W power amplifier I selected the values for a 30W unit to stay on the cautious side. The idea of this circuit is that it maintains the recommended 120Ω source impedance (IEC 61938) for headphones. Some other circuits doing the same thing abandon this idea of fixed 120R and instead set the source impedance to much lower values. I am not sure if these sound better since I have not tested them.

Your Amp Power at 8Ω R input (R) R input (W) R output (R) Zout (R) Vout (RMS)
10 W 100Ω 1W 47Ω 102Ω 4.8
20 W 180Ω 1W 47Ω 119Ω 5.2
30 W 270Ω 1W 39Ω 122Ω 4.9
40 W 330Ω 1W 33Ω 121Ω 4.8
65 W 470Ω 1W 22Ω 118Ω 4.7
100 W 560Ω 2W 22Ω 121Ω 4.9
150 W 680Ω 2W 18Ω 120Ω 5.7
250 W 1kΩ 2W 12Ω 119Ω 5.1

 

Speaker Output Headphone Adapter
Speaker Output Headphone Adapter
Speaker Output Headphone Adapter
ART HeadTap