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previous: R.G. For common mode rejection: Assuming... -- 8/4/1997 5:26 PM View Thread

Re: Differential amplifiers?

8/5/1997 9:56 AM
Erik M. Wood
Re: Differential amplifiers?
Thanks for the reply. I am beginning to grasp the concept of differential amplifiers. In essence, they look to be the same as the push pull amplifier stages we see so often in power amplifiers. The only difference is the biasing, it seems. The principles seem to be very similar. Now, I was planning to dabble with differential stages for a mic preamp I am making out of an old Bogen PA amplifier, but I find that I must now reconsider the idea of fully balancing it end to end. If the differential amplifier cancels out asymmetrical distortions (e.g. even harmonics), it will significantly cut down on what makes single ended triodes sound so desirable. Overload of a single ended stage contributes to that tube sound we know and love. I don't know if you've read it, but there is an article which was presented to the AES back in 1971 by recording engineer Russel O. Hamm. In it he examines the electrical causes of and physiological responses to tube distortion and transistor/op-amp distortion. In it, he finds that the overload characteristics of a tube in a mic preamp (guitar amps, which are designed to be overdriven are another story) are usually found where there is a mechanical-electrical interface, such as with a transducer (microphone, pickup, speaker, etc...) and a tube stage. This makes sense as real world dynamic ranges of sound as they are produced live, are much wider than the capabilities of the amplifier to accurately reproduce them. Thus, we get squashing of an overloaded input signal, say from a mic. This squashing of the signal accompanied by the asymmetrical clipping and the shifted duty cycle characteristic of a single ended tube stage would not occur with a differential input stage. I'm sure you already know all this, I'm just typing out loud. So, what I think I'm going to do is balance the input with a transformer into a single ended amplifier stage, then run the single ended output into a differential stage since it would not experience the same dynamics as the previous one. The signal would already have the desired squashed, asymmetrical, and shifted duty cycle characteristics, so the benefits of the differential amp would be harmless to the good stuff while still cancelling out further Common Mode signal.  
In case you were wondering, the address to that article is:  
Thanks again for your reply, RG.  

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