• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL84 Headphone Amp

I have a few EL84's and am considering using them to build a headphone amp. No driver tube, just one EL84 per channel. Any reason something like this would not be decent? (see attached loadline). I'd use a couple of inexpensive Edcor OPT's. My headphones are Grado SR80.
 

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Markw51,

Questions:

Grado SR80 impedance?
Grado SR80 sensitivity (like mW at 100dB sound level)?
Which Edcor OPT (Primary impedance, secondary impedance, maximum quiescent DC current in the primary)?
How many volts does your signal source have? (voltage that goes to the EL84 control grid, g1).

Engineering always needs numbers to decide feasibility; and to be able to design properly.
Designs done in the dark will probably have the lights continue to be dark.
There is no magic here, look to the witch doctor, entertainer, or politician for magic.

I am too lazy to read the Grado site: and too limited to know which Edcor OPT you have in mind.

Please help us help you.

A triode wired EL84 with moderate plate+screen current has approximately 1700 Ohms plate/screen impedance.
The mu is approximately 19.
This are the only numbers we have to start the feasibility study and design work.

Have fun researching, designing, building, and listening!
 
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5k to 8 Ohm is a step-down turns ratio of Square Root of (5000/8) = 25:1
With the triode wired EL84, the tube gain is 19 x (5000/(5000 + 1700)) = 14.
Total gain = 14 / 25 = 0.56

1 Volt from the iPhone will give about 0.56V to the headphones.
More than 0.56 at mid frequencies, but less than that a low frequencies because of the output transformer's primary inductance.

((0.56V)Squared) / 38 Ohms = 8 milliwatts
LOUD!, +99.8 dB + 9 dB = 108.8 dB

Just an estimate.