Please note that vacuum tube brand may change, depends on its availability. But it is limited to RCA, Raytheon, or other company made in USA.

What is it?

High resolution audio (24bit/96kHz) capable USB bus-powered vacuum tube headphone amplifier

Why did you make it?

I have made plenty of "Nutube" related items, and realized it is "twin triode" again!. Historically, plenty of twin triodes were made and I quickly checked for some other substitution which has similar characteristics as the Nutube. Nutube has low heater voltage and it is almost "vacuum tube designed for battery operation", and I found very nice candidate 3A5. Both Nutube and 3A5 are directly heated tube and similar characteristics can be expected. Naturally I changed heater circuit to adjust 3A5 operation and got to know "3A5 is wonderful"!!!! No way to avoid this one!

What makes it special?

• Wonderful rendezvous between 1950's USA made vacuum tube with sophisticated 21st high-resolution DAC technology!

• High resolution DAC (Asahi Kasei AK4490) with USB-I2S receiver (SA9023). This nice combination provides maximum 24 bit resolution with 96 kHz sampling rate.

• The AK4490EQ is a new generation Premium 32-bit 2ch DAC with VELVET SOUND™ technology, achieving industry’s leading level low distortion characteristics and wide dynamic range with six types of 32-bit digital filters, realizing simple and flexible sound tuning.

• The function of AK4490 can be controlled through Arduino sketch, together with on-board Arduino boot loader loaded Atmel ATMega88PA. The default firmware sketch is uploaded to my Github. Detailed function implemented in AK4490 is explained in the datsheet. In order to upload your own audio setting written by Arduino sketch, USB-UART interface (CP2102) is included in this item. Yes we have a joy to tune sound!!

• Full USB bus power. Just connecting the USB cable (mini USB) is enough to drive everything. Careful PCB layout design with several DC line filter, no annoying noise related with USB line. On-board 1.6 MHz (no audio interfere) voltage booster creates high voltage for Tube and Op-amp operation.

• Outstanding excellent vacuum tube, 3A5, provides wonderful sound. You may think it is outdated? No, I bet you will be so impressed by its wonderful mellow sound!

• Texas Instruments's SoundPlus™ High Common-Mode Rejection, Low Distortion Differential Line Receiver INA1650 is applied between AK4493's differential output to Nutube amplifier part for minimize noise and maximize AK4493's sound.

• No need to install driver for Windows/OSX. Just connect cable and set the sampling rate/bit resolution to 96 kHz/24bit is enough to enjoy High-resolution sound!

• The fully completed and tested PCB board is fit in the fully drilled metal enclosure. USB-UART interface (CP2102) is included for uploading your own customized Arduino firmware.

iPhone (or any smartphone and iPad) operation is NOT supported with this amplifier due to power draw increase. Only PC/Mac operation is supported.