On the previous post, I failed to make this hybrid vacuum tube amplifier works by only using 2 AA batteries as the power source. This is because not enough power to crank up the tube heater.
So going to the Plan-B, which is back to the basic schematic. This hybrid vacuum tube amplifier will using 12 volt DC wall adapter and with all components are all the same. The voltage inverter module will be remove and now using LM317 voltage regulator for the tube heater instead.
I remove all module and PCB on the tin first before make some changes.
I am using LM317 for regulate the voltage input from 12 volt to 6.3 volt. The original schematic only use 4,2 ohm to get 300mA current which is also okay, but I did different approach by using the same schematic as per LM317 datasheet.
The tube heater is the most important part and need to have special attention. It not only effect to the vacuum tube lifespan, yet the tube heater also a noise generator if the voltage in is not clean enough. I also put more capacitor to reduce the voltage ripple on it.
The LM317 is quite hot during operation and should be put on the heat sink.
As you can see from the schematic bellow, I adding 2 pieces electrolytic capacitor at the voltage input on the LM317. The more capacitance, the more this circuit can reduce the noise from the power source better.
I using the power adaptor taken from unused Huawei modem DC adaptor. The power rating is 12 volt 1.5 ampere, very capable for this project.
I also make another hole at the other side of the tin to accommodate the power adapter plug. It is a standard DC plug for the voltage input.
All voltage are confirm in specification, which the 6.3 volt for tube heater and the 12 volt for the amplifier circuit is taken directly from DC power adapter socket as per schematic.
During the initial test, the vacuum tube glow bright. This amplifier play the music very well. The voltage didn't drop at all. Nice!
The vacuum tube are naturally catch the outside noise, so when put all components inside the tin, I always connect the signal ground to the heat sink and the chassis.
I am testing this amplifier using iPod as the music source.
I using the 30 pin Apple dock to true line out adapter to this amplifier.
The earphone in this test is using Sony MH1C.
I cannot believe that the sound characteristic in this amplifier is very similar to what I got in the Bottlehead Crack amplifier.
The treble are very extended but not edgy, the vocal presentation is warm and forward and the bass can going deep enough but not much boomy sound.
All music presentation are very dynamic with the speed that can catch with all music genre. The sound-stage is wide and the stereo imaging also well deliver.
When the vacuum tube gets hotter, the sound more detail and the high frequency get even more extended.
I also can closed the tin and stack to the iPod like this.
Okay, that's are the positive sides. You might be wondering is there any cons from this amplifier? and yes, there is a minor I think.
The first thing is the idle noise. It is heard in my Sony MH1C. It is a slightly hiss sound during idle. But once the music play and the volume is turn up, I really doesn't notice it anymore.
Second, this amplifier not produce so much power. It cannot drive high impedance headphones like Sennheiser HD600. It sound awful when it required more power to drive the big headphones. That is why I keep using this Sony MH1C when combine with this hybrid vacuum tube amplifier.
Then the last things I notice is the background sound from this vacuum tube amplifier is quite noisy. Not as clean as the solid state amplifier for sure. I don't know if I change the power supply to the linier transformer, it might be improve.
This pocket size vacuum tube amplifier right now is not portable as the initial planned, instead it is a desktop amplifier using the wall power adaptor. But it is so much reliable and sound very good.
I cannot complaining at all with the sound quality I got here. Although this amplifier can only drive sensitive earphones or in-ear monitor type earphones, but it is very worth on every time and penny I spent when building this project.
Disclaimer: Any statement and photos in this article are not allowed to copy or publish without written permission from the writer. Any injury or loss from following tips in this article is not under writer responsibility.
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