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previous: Dave Harris "Squashed, asymmetrical and shifted... -- 8/6/1997 3:17 AM View Thread

Re: Differential amplifiers?

8/6/1997 9:45 AM
Erik M. Wood
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Re: Differential amplifiers?
Dave, I know it's strange to think that the "squashed asymmetrical duty cycle" is counterproductive to a good sounding signal, but it really has to do with the king of distortion produced by the relative technologies. If you were to measure the THD of a tube preamp and a transistor preamp, very often you will find that the measurements are not that far off from one another. The real difference is in what kind of distortion is produced by these two technologies, and that is simply inherent in their nature. Single ended tube triode stages overload so as to generate a predominance of even harmonics. Each harmonic relative to the fundamental frequency is specifically responsible for a particular characteristic of the timbre of the sound you hear . Distortion of the even harmonic variety is much more consonant than odd harmonic distortion -that which is predominantly generated by transistors. THis is because the lower order even harmonic series is more closely musically related to the fundamental than the odd harmonic series. It's easy to prove this by simply working out the harmonic series above any arbitrary frequency. All you have to do is work out the ratios of the diatonic notes of a scale to your fundamental note and compare the accepted consonances and dissonances of the notes represented by each harmonic. As a result, to our ears, a tube amp in the early stages of overload actually sounds cleaner and more pleasing than a transistor amp distorted the same amount (THD measurement). The shifted duty cycle and assymetrical clipping are what produce even harmonics. In a transistor, these two actions do not occur. Instead, the duty cycle is nearly perfect, and the clipping occurs the same amount, top and bottom. This is responsible for the predominance of odd harmonics. Now this is not to say that tubes only produce even harmonics while transistors only produce odd harmonics. It is simply the fact that each technology in it's most basically designed circuit will produce a greater amplitude of one kind over another. To go into the details of it all is beyond the scope of what I have time to type. And I'm sure I've not been exceedingly clear with what I mean, but look up the website I mentioned and it will explaing it better than I can.  
-Erik.  
 
The address to the website article is:  
http://www.access.digex.net/~mmilbert/tstxt.htm

 
Replies:
John Martin FYI: There is a good article in t... -- 8/6/1997 11:51 AM
Dave Harris Hi Erik,I have read the Rus... -- 8/7/1997 4:21 AM