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A 12 Volt Tube Headphone Amplifier at “Bring a Hack 2017”

Editor’s note: At the end of the MakerFaire in San Mateo, there is a “Bring a Hack Party “. That is where Ron Quan showed his tube headphone amplifier.

Here’s quick circuit for driving high fidelity headphones whose impedances are > 50 Ω .

The circuit uses one 12DS7 tube per channel with a regulated 12 volt DC supply at 2 amps or more. A switching 12 volt power supply (at > 20 kHz) worked fine for me.

My input audio source was a portable CD player with a volume control.

I chose these two headphones:

  1. Koss KTXPro1 (60 Ω )
  2. Sennheiser HD280 Professional (64 Ω )

Of course other headphones will work.

Figure 1

Stereo 12DS7 tube amplifier for headphones.

Stereo 12DS7 tube amplifier for headphones.

Figure 2

Schematic for one channel of the headphone amplifier.

Schematic for one channel of the headphone amplifier.

Note that the 2200 uf capacitor’s “+” terminal can power the second channel amplifier. Also, the 150 Ω plate resistor should be 1/2 watt or more. All other resistors can be 1/4 watt, including the 4.7 Ω cathode resistor.

Also, the 100KΩ input resistor may be replaced with a 100KΩ potentiometer for controlling the volume.

4 comments on “A 12 Volt Tube Headphone Amplifier at “Bring a Hack 2017”

  1. DLA
    June 2, 2017

    Ron – I enjoy your articles and your books.

    I did have a question on this tube circuit.

    Seems that the “mu” is quite low, and with 50 Ohms loading the 150 Ohm load resistor – maybe only an overall voltage gain of about -mu/4 = around -4 ?

     

    Is that correct, or did I miss something ? I looked at the 12DS7 tube curves and ascertained this.

     

    – Dave Albean

  2. DLA
    June 2, 2017

    Also wondering – what is the perveance value for that tube ? Can't seem to find it anywhere and not sure how to glean it from the tube datsheet curves.

  3. rnquan
    June 2, 2017

    Hi Dave,

    Thank you for your kind comments.

    Yes, you are absolutely correct. The gain is low, but fortunately we do not need that much power into the headphones for a comfortable listening volume. Usually something in the order of 1 milliwatt will be sufficient.

    From my measurements using a 12 volt supply, the open circuit gain (e.g., no load) is about 1.63. It turns out that the 12DS7 also has a “low” internal plate resistance that reduces the gain.

    With a 300 ohm load resistance the gain drops to about 1.22. For a 50 ohm load, the gain is 0.55. This implies that the output resistance is close to 98 ohms.

    If the input signal is 1 volt peak to peak, with a 60 ohm headphone such as the Koss KTXPro, then the signal into the headphone will be about just a bit higher than 0.55 volts p-p or about 3 milliwatts. When I listened to a 1500 Hz tone under these conditions, the volume was already way too loud for comfort.

    One thing to keep in mind, designing this stereo headphone amplifier was just a fun exercise to see if I could avoid using output transformers.

    Thanks.

    Ron

    June 02, 2017

     

     

     

     

     

  4. Yura
    September 3, 2019

    Hello. How did this model end on 12DS7 tubes? For some reason the volume on the transformers is low. Sorry for the translation.

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