Low cost DIY tube pre-amp / buffer kit

While trawling the PRC eBay, I came across a low cost DIY tube pre-amp kit which was advertised as a possible tube buffer as well. Definitely easier and cheaper than try to procure a X10-D from the used market, as those goes quite quickly once listed.

The completed DIY unit mounted using magnetic feets on top of a cheap DVD player

Received the DIY kit with all parts as per advertised which does not include the 12VAC 0.8-1A power adapter, potentiometer knob, mounting feet nor the plastic casing.

The good news is the PCB is well adorned with marking of component position and values. And the potentiometer has a built-in ON/OFF switch.

Bad news is the PCB is extremely compact, with most of the critical components located very close to the others. Would help if you used a soldering iron with a fine or small tip.

Close-up of the DIY kit; 2 of the audio path critcal capacitor on top with the other 2 on the bottom side of the PCB (not visible)

One minor minus point is that the potentiometer does not came with a knob and volume at the lowest setting was not balanced. Hence once switch on, you need to turn the volume up slowly until you could hear a balance volume on both LHS & RHS speakers.

Another minor minus point is that there were no labelling for the INPUT and OUTPUT terminals on the PCB or the optional casing. It's only stated on the instruction sheet (provided in Chinese), with schematic of the PCB. The design uses the X10-D methodology for doubling the input voltage but implemented for a lower cost 7-pin valve instead.

The major minus point is the location and available space for the 4 critical capacitors on the audio path - 2 for input and 2 for output. Due to the severe space restrictions, your choice of capacitors are severely limited by physical size, lead length and available values (if you wish to keep everything on the PCB). In addition, the compact PCB makes swapping capacitors a "pain in the neck".

How does the default kit sound when used as a tube buffer?

Reproduction is surprisingly clear with good details BUT with a heavy HF bias. The default Chinese 6J1 provided seem to be adequate for the moment.

Am still performing substitutions and listening to the new components (once they have settled) on the audio critical paths. Once done, will then decide if the supplied Chinese 6J1 x 2 should be swap out.

Post to be updated at a later date.