Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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RE: This is interesting

Posted by morricab (S) on August 24, 2007 at 02:02:05

In Reply to: RE: This is interesting posted by andy_c on August 23, 2007 at 10:38:27:

"At any rate, one thing that's shown in that thread is that for the crossover distortion of a solid-state output stage,"

It was my understanding that negative feedback was found to not successfully address crossover distortion and that other distortion reducing techniques (like High idle bias) were required. Is this right or not?

You mention in your other posts that amps with gross non-linearity open loop did exhibit distortion behavior like Baxandall's examples. Why do you think that is and what percentage of commercial designs do you think would fall into this category?

Finally, at the amount of average power needed to listen to music the distortion levels of well designed tube amps are quite low (well under 1%) and of generally low order harmonics. It could be argued then that if an SS amp is sonically benign at these low powers (say 1-5 watts) then the well designed tube amp is equally sonically benign by virtue of have low and largely inaudible distortion. However; listening tests don't seem to bear this out and even at low to moderate volume levels the sonic differences can be heard. It leads me to conclude that even though the levels of the distortion may be absolutely low they may still be audible.

The character of an amp type seems to be dictated by two things primarily, 1) Topology and 2)Execution of design. For example many tube amps seem to have poor bass control and sound "sloppy" or "wooly" in the bass. It has often been attributed to low damping factor but I have now heard some tube amps that have relatively low damping factors (less than 10) that have fantastic bass control. Also, an article in Audioholics regarding damping factor made it pretty clear that high damping factor is not really necessary for accurate amplifier response. So I started looking at other factors and the one thing the two tube amps I heard with good bass control have in common is almost no increase in distortion at low frequencies. Most tube amps have a HUGE increase in distortion in the bass due to output transformer saturation. I think it is this distortion increase and not low damping factor that is largely responsible for the bass problems most tube amps have. So it is not so much a topology problem as an execution problem and using suitable output transformers, which oddly most don't seem to do.

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