C2C Tube DI project with some analysis

[beasleybodyshop]

Hey everyone! I built a few of these tube DI boxes awhile back and, while not tube amp per se, I think it's something that everyone should at least build once.

This particular one comes in the form of a PCB project from Conspiracy to Commit Electronics:

https://c2celectronics.com/product/nobelium-pcb-set/

Seems based upon the Noble Tube DI that's been quite popular the last few years.

It runs a pair of 12AU7s at proper tube voltages, using a little switch mode circuit that boosts 9v to 200vDC or so using a 555 timer. Heaters run on raw 9vDC. I know, that sounds like heresy but I've been using this thing for almost a year with no issues!

The build was pretty straightforward - If you've built a tube amp then really nothing different here, except perhaps making sure the right angle headers that hold the tube socket board are installed correctly. The documentation on C2C is decent and walks you through any installation pitfalls.

Here is what mine looked like when I was finished:

VTDI.jpg

I have to say it sounds really good. I got curious so I went and ran some analysis on it with an AP555 to get some specs:

Unagi Transformer Equipped Unit: Measured @ 600R Zin
Noise of output: (EQ @ noon)
1/4":
Volume off: -44.174dB
Volume Max: -24dB
DI:
Volume off: -43.5dB
Volume Max: -44.243dB
Maximum output before clip (roughly 10% THD)
1/4": +5.332dBu
DI: +5.5dBu
SNR:
1/4": 52dB
DI: 49dB
Lowest THD before Clip: +2dBU Output knob @ Noon

Untitled picture.png

FFT @ 1Khz

Untitled picture2.png

Phase relationship between 1/4" and XLR outputs:

phase.png

Frequency Sweep of EQ:
Treble Sweep w/Bass off
1/4" Out

EQ 1.png

XLR:

EQ2.png

Comparison:

EQ3.png

Bass Sweep w/Treble off
1/4"

EQ4.png

XLR:

EQ5.png

Comparison:

EQ6.png

[beasleybodyshop]

Sweep with both knobs in same position
1/4"

EQ7.png

XLR:

EQ8.png

Comparative:

EQ9.png

Sweep of High Pass filter:

HP sweep.png