Headphone output

Ever thought about adding a headphone output to a tube power amplifier? I did…

The important aspects are:

1. protecting your ears (priceless) and the (usually expensive) headphones
2. protecting your power tubes and transformer by maintaining the correct output impedance
3. obtaining a good output sound level and quality
4. easy switching between speakers and headphones

A common tube amplifiers puts out between 5 and 50 watts of power. The maximum power that headphones can take is 0.1 – 0.5 watt (check the manufacturer’s specs). Obviously we need to limit the power available to the headphones. A T-PAD attenuator will do that just nice. There are online t-pad calculators.

We need to figure out how much power attenuation we need.
My EL84 PP amp produces 12 watt and my Sennheiser HD-650 can take a maximum of 0.5 watt. The minimum needed attenuation is given by:

attenuation = 10 * log (12 / 0.5)  = 14 dB

We can increase that figure to 20 dB, for safety and for compatibility with lower impedance headphones.

The input impedance (that the amp sees) should be equal to the speaker impedance – 8 ohm. We can choose a slightly higher value too, like 10 ohm.
The output impedance – headphones are generally designed to operate from a 120 ohm impedance. If you get too much / too flabby bass, then you can reduce the impedance.

The calculator recommends R1 = R3 = 8 ohm and R2= 2 ohm for a 8ohm output impedance, R1=2 ohm, R2 = 6.2 ohm, R3=116 ohm for 120 ohm output impedance and R1=R2= 4 ohm and R3= 47 ohm for a 8-ohm input, 50-ohm output impedance.
R1 and R2 should be power resistors and they will get pretty hot! For example, if R1=R2=4 ohm, each resistor will dissipate 6 watt of heat and should be minimum 10w rated.

We should also protect the headphones against high current bursts, with a fast-blow fuse. Its resistance could increase as it heats up, but we can safely ignore that.

Let’s say we need a fuse for a R = 300 ohm, P = 0.5 watt headphone. Ohm says:
P = U * I = I*I * R
I = sqrt(P / R).  So at full power the headphone will pass a current I = 40 mA, and we can use a 40mA fast blow fuse in series with R3.

There must be a switching system to select between headphone or speaker output (but not both!), something like a DPDT switch or a relay. I wouldn’t rely on a headphone jack’s switches. And the amp should be turned off while switching to avoid thumps. 🙂