Well there is an answer in this, a cheap tube and low voltage supply or what we call as B+. To add some warmth in clinical sound produced by CD player and our gainclone, a topology called buffer is all we needed. Buffer does not amplify, it's just make driving your amp easier by providing better impedance matching, also the gain of this buffer is actually less than 1.
About a year ago Mr. Arif from Tubelover introduces tube buffered Gainclone, he gave us the schematic during DIY audio Blind Test gathering at Mr. Didik W. home. There are high voltage tubes that can sound very nice in low voltage, such as 6DJ8, 12AU7, and the Russian 6N1P. This 6N1P is the cheapest and widely available so I started to think about this buffer when friends at Solfegio Forum are looking for cheap and 'safe' tube gear. So here are the story.
Schematic from Mr. Arif (many thanks for sharing pak !) it is his hand writing :)
My translation of the schematic and board layout
Green is the input signal, yellow is the buffer output. Clearly the tube does something to the original signal, it's adding harmonic peaks at 2kHz (even harmonic), 3kHz (odd), 4kHz (even) , 5Khz (odd) and 6kHz (even).
What does this means, it's means that we get distortion on the output. Notice the higher 2nd harmonic at 2kHz relative to the other, people says that this is the 'nice' sounded even order distortion, just like what happen in nature, so we tend to call it pleasant sounding. The not so pleasant sound are the odd harmonics.
So simulation show quite promising figures. Will the simulation results reflected to real build? I don't know so let's build this buffer and fast forward to the final one after days of tweaking :)
My buffer is using 6N1P tube, 6.3VAC heater, why tweaking? after initial listening using a DVD player it's really needed that, the buffer sounded mediocre but it have potentials. First the output capacitor, final value is 6.6uF, then power supply electrolytic, also the type and brand of the drop resistor for heater (from 12VAC) has significant influence to sound.
I did tweaking after hearing and 'measuring' the DVD with and without buffer. I've found a nice software called AudioDiffMaker , its do the comparison of reference signal against a 'tweaked' signal. In this case I compared direct DVD out to Buffer out. I did 2 signals, a 500Hz 0dB sinusoidal from my Denon test CD, and music signal. Here are the screen shots.
|500Hz direct DVD out|
|500Hz Buffer out|
|500Hz Difference DVD-Buffer|
I did another signal, this time a real music signal. here are the differences, and yes you can hear them as well
|the difference, both channel|
Check here for details & friends building their first tube buffer. Thanks Bill, thanks pak Arif. And to my friends..built it, tweak it and enjoy it