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Ronald Quan

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

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rnquan
6/2/2017 9:54:31 PM
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Re: Pretty low gain ?>
Hi Dave,

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

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DLA
6/2/2017 5:16:10 PM
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Newbie
Re: Pretty low gain ?>
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.

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DLA
6/2/2017 11:05:05 AM
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Newbie
Pretty low gain ?>
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

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