Last year, I made this hybrid vacuum tube headphones amplifier project. The basic idea is based on the pictures on the internet that a headphone amplifier is using a vacuum tube with only 2 pieces of AA batteries. It is visually (from the internet photos) can drive the full size tube on it. I curious because there is no proven or any review about this vacuum tube amplifier in a working condition yet.
Then comes to the conclusion when I built one here, it simply doesn't works at all. The power from 2 AA batteries cannot heat the tube heater, thus increasing the voltage doesn't help it at all. Because it facing with power supply current issue, not the voltage issue.
I changed the layout by simply using 12 volt 2 ampere external switch mode power supply. Then the final layout is using Nichicon UKA as upgraded capacitors here and I also using the 3 batteries 18650 in series with total 12,6 volt 2.200 mAh here. It is now a working hybrid vacuum tube headphone amplifier.
For high efficient earphones, this amplifier can drive properly. My Sony MH-1C sometimes can sound wonderful with this amplifier but for more complex music that require more power to attack, this amplifier still feels lack of power to drive this earphone. The sound in a simple music when it drive the high sensitivity earphone, can sounds detail and wonderful.
So I guess I need to rework this vacuum tube amplifier again. The candy tin as the amplifier case previously is too small and cannot fit so much components in it. Especially for the buffer stage that using an op-amp for both channel also not deliver so much power to drive an earphones.
The concept of this new layout is to double up the output current by adding parallel buffer stage. This idea is the same as the O2 amplifier here. Each dual op-amp will do for single channel and the output are combine by using 1 ohm resistor. This resistor is to improve the op-amp stability. Then it goes as per initial vacuum tube hybrid headphone amplifier schematic.
To start with this new layout, I need to remove the important parts on previous amplifier circuit, then move to the new layout.
The casing for this new layout will be a copper casing dismantled from Nakamichi DAC-101 project before.
The new PCB is require for this new layout. It should be more bigger to accommodate dual op-amp and WIMA MKP10 input capacitor.
I try to populate the components first before connecting them by point-to-point.
I also make a new board for the vacuum tube socket. This socket will be much simple when attach to the main board compares to previously using the wires for each pins.
I bought the new capacitors for this project. This is Panasonic FC 220uF capacitor which is affordable, yet high quality. This capacitors will be place in the tube heater regulator circuit and DC filter output.
The voltage regulator for the tube heater is required to make the output DC voltage at 6,3 volt. It is using LM317 adjustable regulator.
Then I start to assembly the components.
Actually I really like this new layout, especially the vacuum tube socket attach along with the main board.
They are look nice indeed.
This new board is now completely finish. I double checked the wiring and make sure all connection are correct.
Then comes to the best part of all which is attaching the vacuum tube on the board and put them inside the copper casing.
With this new amplifier layout, the dual op-amp at the buffer output really give significant impact to the power output. This vacuum tube amplifier really drive my earphone really well. The sound quality with this new layout really improve so much, the clarity, the 3D sound-stage are much more better now. I can feel the vacuum tube sound which very rich and at the same time neutral. This amplifier far from "warm" sounding vacuum tube amplifier and no even a dull sound coming from this amplifier. This is also the most detail sounding amplifier I ever experienced.
Right now I am using my Sony MH1C during this test which I think really suitable with this hybrid vacuum tube amplifier. The Sennheiser HD600 headphone also working very well with this amplifier.
I can say, this vacuum tube headphones amplifier is worth to build. The sound signature from this amplifier is tube alike compares with the common solid-state amplifier.
This amplifier sound signature really depends on the vacuum tube itself, which playing major role coloring the sound in the good way.
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