|
Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
|
|
|
Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
|
In Reply to: Mostly agreed. However, I have yet to find an algorithm that does pleasing distortion like a soft clipping tube will. posted by Ugly on July 26, 2008 at 16:49:20:
While tube amps generally progress more gradually towards clipping (depending on how much negative feedback is employed...the more feedback the more they behave like a typical SS amp in this regard) once they clip they clip hard. However; two things become important 1) At what point into clipping (ie. how many +db) does the sound become objectionable and 2) How quickly does the amp stop clipping once the input signal that caused the clipping ceases?
Based on a paper I found regarding microphone preamps they found that tube microphone preamps have about 10db SUBJECTIVE clipping headroom vs. transistor based ones. In other words both preamps were objectively clipping on an oscilloscope and then they continued to overdrive them until the listeners found the sound objectionable. They were able to go 10db further with the tube based preamp.
Their explanation for this is that once a tube is overdriven it begins to naturally compress the signal. This means that the sound is getting less loud with each input increase. They noted that high order harmonics are present on both types of preamp. An increase in high order harmonics is perceived like loudness increase. The speculated that the two effects (compression vs. increasing high order harmonics) was resulting in the higher threshold for the tube preamp. As the transistors do not compress when overdriven the objectionable harmonics are perceived earlier and found to be more annoying at a lower state of being overdriven.
Now most hifi sources and preamps are not overdriven but amps are. If we take this 10db advantage and apply it to power amps we see that a 30 watt tube amp then has the SUBJECTIVE headroom like a 300 watt SS amp (10db is 10 times the power), not so puny then from a subjective standpoint. So, while a 30 watt SS amp whimpers and dies on a low sensitivity speaker it may not be the case for a 30 watt tube amp.
Next is the recovery from clipping. Obviously the less time an amp is clipping the less likely that clipping is going to be audible or at least seriously objectionable. A zero feedback amplifier will respond instantly (or as fast as the power supply can replenish) to the signal in the sense that once the clipping signal passes it will stop clipping. A high negative feedback amplifier will continue to clip for perhaps hundreds of miliseconds or in the worst case begin to oscillate. The feedback slamming it into hard clipping the instant it is slightly overdriven (thus the very abrupt change from ultra low distortion to hard clipping). This is very specific to feedback amplifiers because a transistor amp with little or no feedback will have more progressive distortion characteristics.
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- RE: Mostly agreed. However, I have yet to find an algorithm that does pleasing distortion like a soft clipping tube will - morricab 08/7/0807:41:47 08/7/08 (1)
- Yes this is what I meant by "soft clipping" thanks for clarifying. - Ugly 13:43:13 08/7/08 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
