Miniature tube headphone amp

I’ve always thought certain sound pressure levels need to be achieved in order to be engulfed in music. This effect is most noticeable when music comes from loudspeakers and that is because when sound is played at low levels, the perception is affected by ambient noise. Even late at night when things are quiet and you don’t want to disturb people around you, there is still noise. A clock on the wall, a pet, you computer, a cooling fan, some kind of noise will disturb your listening experience. Things are not the same with headphones though. You put it on your ears and outside world is gone.

There are plenty of good headphones on the market, giving good level of detail, good frequency response, and good sound-stage. But there are some drawbacks to using headphones. You will not feel the same impact and if you don’t chose your headphone carefully you will feel fatigue after long listening time. Another thing to consider is that with speakers you can move them around the room to get the response that most suites your tastes, something that you can’t do with headphones. This is way the amplifier that drives them plays a huge role in a headphone setup.

Everyone who has a passion for audio knows about the qualities of tube amplifiers. The problems with these amps is that they are big and heavy, pretty hard to build and operating at dangerous high voltages. This usually discourages most Do-It-Yourselfers. However Mark Houston built a headphone amplifier that is battery powered and it’s small enough to fit in a pocket.

Mark used Raytheon JAN6418 Tubes in his project. These miniature tubes are pentodes made in USA and you can find them in a kit sold by Oatley Electronics. The amplifiers uses one tube per channel in common cathode configuration followed by a buffer to be able to drive lower impedance headphones. The buffer is a low distortion IC PT2308, a class AB CMOS headphone driver.  This IC has an impressive SNR of 110dB so it’s well suited for this application. The output impedance of tubes in common cathode is high so higher value resistors in the driver’s feedback loop are needed. The 6418 tubes need 1.2V to heat their filament. A 5V regulator chip heats the two filaments connected in series through a 270 ohm resistor.

This amplifier can also be used as a preamp. Mark did some modifications to component quality, replacing the PT2308 with a OPA2134 IC for example. There are also some scope traces he shows in his article measured with a load of 47k. I would’ve been curious to see how it performs at much lower loads. All being said, Mark is pleased with the sound, and with tubes you can not possibly go wrong, can you?

Miniature tube headphone amplifier: [Link]

August 21st, 2009

Hi-Fi Tube Amplifier

Hi-Fi Tube Amplifier

Here is a project for music lovers. Although electronic tube or valve era is long passed, these devices are still used in high quality audio applications. It is true that you won’t wont find them in every home because their regarded as obsolete or too exotic. Guitar players are the ones who know and appreciate their sound as many famous guitarists use tube amps to drive their instruments. Stereo amplifiers built with tubes for hi-fi applications are still being made today and their cost can be pretty high as these amps are though to deliver high quality sound.

The reason tubes have a specific sound has to do in my opinion with the fact that tube distortion is mainly second order which is more music-friendly than the 3rd order distortion that solid state devices have. Some say this is because in tubes you control the flow of electrons and only electrons carry your information unlike transistors.

Depending on their output stage configuration tube amps can be single ended ( using only one tube working in class A) or push-pull ( like with transistor using two tubes working either class A or class AB). Because of the high output impedance of tubes they need an impedance transformer to drive the speakers. This is also the reason of their low damping factor which in combination with their usually low power output makes these amps suitable for certain types of speakers. From my experience you will need speakers with big woofers and very efficient (high SPL per watt). Also i have found that single ended amplifiers ( in class A of course) with high anode voltages applied are best when tube-like sound is desired.

Returning to the project at hand, it is a push-pull design using 6v6 pentodes connected as triodes. If you get an ultralinear output transformer you can use the 6v6 as pentodes.  The input stage using a 5965 double triode is as simple as it gets. It amplifies the signal in order to attack the final stage and provides phase inversion necessary for push-pull. The power supply uses semiconductor rectification which i find a bad thing for a tube project. There are tube diodes available for this task.

As with most audio high fidelity projects  the quality of the sound is in strong relation with the quality of the build. If you do this project just for fun then you will be happy with it just working, if you build it to experience tube sound you should take greater care at layout and components you use. Using multiple transformers is a bad thing as they introduce noise. Using one transformer with many output voltages is alot better. And screening of the transformers is a must. In the project link you will find many advices on how to build it even though the builder didn’t follow all of them.

Another thing, and a reason that keeps many diy-ers away from tubes, is that you will be dealing with high voltages. Be very careful with that. And to close this presentation with another advice don’t use it to amplify your iPod.

Hi-Fi Tube Amplifier: [Via][Link]



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