Updated: 24-Jan-01
This is the right question to ask. Why use Tubes? Let me explain…

The wonderful DHT Triode nr 30

For those being Tube lover already, this must be an open door. Also, I will not re write all the thousands of pages being written already on why tubes are conceived superior over discrete solutions when it comes to music reproduction. Never the less I like to spend a few lines to why specially in this case I am pleating for Tubes in the stage after the DAC in my design. Normally you will see lots of (sometimes fast) OPAMPS  for applying filtering functions and gain. All these OPAMP designs share the same basics: lots of silicon in the signal path and lots of feedback to obtain sharp roll off filters and extreme low distortion figures, as well as extreme dynamic dB numbers. The only reason for this, is pure marketing. No one needs 120dB dynamics in his living room or .000001% max distortion. Believe me, the background noise in your living room is quickly 40 to 50 dB. Assuming you play at a max level of 100 dB (I normally don't...it is already very loud!!), and assuming you can hear details 20 dB below noise levels, you need max 70 to 80 dB dynamics. Distortion at 1% is for normal people unhearable. Well, you would say, who cares, those OPAMP numbers are better, don't they? Yes, indeed, but only with pure sinewaves. If you, just like me, like to listen to music instead, you will listen to transients and noise like signal products. and here is where the transistor adds the problem. The transistor (inside the OPAMP as well) starts conducting after a short while. This is called the avalanche effect. When you add many feedback in the signal path, you are never fast enough to apply the correcting signal to the entry point of the signal. Not to mention the frequency corrections in the feedback. This all results in sharp SSSSSSSS's and lost spatial stereo stage as small transient phase correlated signals are defining the depth in the image.

The signal from my DAC design goes unfiltered into a transformer, which does the first filtering by its nature. Secondly comes the tube stage. A non-feedback tube stage is stable from its own and has a frequency response of aprox 100kHz. This is exactly the kind of filtering we need to get the high frequency components out of the DAC signal WITHOUT any transient or feedback problems! the result is smooth, analog like reproduction of the music. Numbers look poor, but we all know, that there is no correlation between typical spec numbers and music reproduction.....

With OPAMPS it is impossible to have no feedback as you will have an amp with a gain of 100dB and a cut of frequency of 10Hz. Not very good eh?

See the description TAB for explanation of the tube stage itself.   Tube Stage

Happy Listening and Building !!!!

Doede Douma

IMPORTANT: The information provided on this page is intended as guide for DIY activities and therefore free to copy and or publish. If any one wishes to use any of the information from my WEB site, please make sure to refer and footnote to my URL Link as source! Doede Douma