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]

October 28th, 2008

MOSFET Headphone Amplifier

MOSFET Headphone Amplifier

As we all know headphone amps are not cheap to buy. I think this is the main reason that drives the hobbyist to make their own amps. Most people are familiar with the popular C’Moy headphone amp which is cheap to make and it sounds nice.

But there also other designs available, for example this class A MOSFET headphone amp. The author designed and build it because he’s 32 ohm Grado SR80 headphones didnt’t sound good on it’s computer soundcard.

An IRF610 MOSFET was used by the author but there is a wide variety of FET devices that can be used instead. Unlike the C’moy amp this amp was primarily designed to sit on a desk, it’s bigger the MOSFET needs heatsink cooling so this wouldn’t fit the portable category of amps.

A LM317 regulator is used for the constant current source, and the current is limited to about 250mA. In the end the sound is better than on the sound card and it was a small investment as most parts were salvaged.

It’s not too difficult to build something similar but it will sure help to have some experience with similar circuits.

MOSFET Headphone Amplifier: [Link]

February 25th, 2008

RC4560-based headphone amplifier

This SMD op-amp project uses the TI RC-4560 dual op-amp chip and a salt water-etched PCB. The builder got the op-amp as a free sample from Texas Instruments and scavenged all of the SMD resistors from old hard drive electronics. The results are not pretty, but they’re functional. The circuit used (with some changes) is the Chu Moy design found here. The CMoy circuit is not SMD.

RC4560-based headphone amplifier

RC4560-based headphone amplifier [VIA]

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