Minggu, 30 Januari 2011

Entry to wonderful world of tube audio

When we first think about this 'ancient' amplifying device, at least 2 things comes into beginners mind, lethal high voltages and expensive tubes. Both of them drives many first time DIYer away. I must admit it's not easy for most of us to start with glowing tubes after the comfort of making a Gainclone or chip amp.

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
But before I built the buffer I would like to see how it's perform by simulating it, I want to know what is the 'harmonic distortion' figure say at 1kHz sinusoidal input signal. Fisrt the input-output figure,

Traces : green is input signal 2Vpeak -peak (just like most CD-DVD player output), yellow is the buffer output, note that it's not 2Vpp anymore. Now let's see the harmonics of the same signals,

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.

AudioDiffMaker form
500Hz direct DVD out
500Hz Buffer out
500Hz Difference DVD-Buffer
Here we see again the harmonics of 500Hz signal, what to understand? first check out 2 small peaks after large 500Hz peak. Direct from DVD shows 2 small peaks upwards, out of buffer 2 small peaks one upward (even 2nd harmonic) and the next downward (odd 'bad' harmonic), so at least the tube somehow alters the peaks. As for the rest the AudioDiffMaker sort out the difference and in term of signal it is very large difference and you can hear it!.

 I did another signal, this time a real music signal. here are the differences, and yes you can hear them as well

the form

the difference, both channel
So finally Bill Waslo, the developer of AudioDiffMaker gave us the tool to check whether you have 'improved' your system using new cable or 'voodoo' tweaks :). As for me, the software will not tell you how good the sound is and I have tweaked my buffer to a level that I missed something when I'm not using it :) some day I'll measure it again with AudioDiffMaker.

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


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