|
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: Re: On a different note: Response to Morricab on Amplifiers posted by morricab on February 14, 2007 at 03:28:39:
"Wrong again. Sure a single ended Class A amp cannot have crossover distortion...its single ended but a complementary design can still have some distortion at zero crossing if the transistors are not perfectly matched (and they are never perfectly matched). If they were perfect complements of each other and running true Class A then there would be no crossover distortion but we are talking about real amplifiers, right?"Okay.
"Wrong again. Please look at page 3 of the article.
http://www.st-and.demon.co.uk/AudioMisc/distortion/page3.html
He states that asymmetric waveforms can heat transistors differently for each half of the wave cycle leading to dynamic differences in their bias. This leads to asymmetry in the output devices thus crossover distortion. I don't know what effects lead to asymmetry in SET amps or amps without feedback but it seems that music itself can lead to asymmetry in complementary Class AB amplifiers and that asymmetry is distortion. Bipolar transistors are especially temperature sensitive.
So the point is simple here, even with a PERFECTLY matched class AB design that measures Perfect with sine waves it will likely have distortions with real music due to asymmetry of the bias of the devices."
You have your knickers in a twist, Certain non-linearities, such as what you have noted cause a departure from symmetry (see the previous page of the link you posted", however the The transfer curve of a complementary P-P circuit is symmetric. On the other hand, the transfer curve of a SE Class A is asymmetric, non-linearity simply results in yet more asymmetric distortion, its asymmetric transfer curve means, that is it unable to cancel out any distortion components, harmonic or anharmonics, in a zero feedback design, the non-linearity is even worse, as the open loop linearity is simply nowhere near what is achievable with a closed loop design. This where LeSurf original article comes in, a single-ended design is inherently less linear than a P-P design if both both are run in Class A, the SE with zero feedback even less linear than than an equivalent than equivalent P-P design with negative feedback, musically complex signal introduce new anharmonics components that an asymmetric design cannot cancel out. As you note, a Class AB circuit introduces the possibility of a new distortion products, however good design can reduce crossover distortion to barely traceable levels. Also a good design will ensure that a design the active devices are run at optimal temperature. At any rate, I saw a funny link for you to peruse, which mentions the some of the distortion products that may contribute to the 'unique tone' of zero feedback SE amplifiers. Be that as it may, the original still, a zero feedback SE amplifier is definitely less linear than a complementary Class A P-P circuit with negative feedback.
"So the point is simple here, even with a PERFECTLY matched class AB design that measures Perfect with sine waves it will likely have distortions with real music due to asymmetry of the bias of the devices. "
Strictly speaking, this is a non-sequiter issue and the statement is not clear at all, at least since the document you quote a very important assumption that is "quiescent bias current level of the AB output is small enough to be ignored" in all scenarios, firstly because Class AB circuit with negligible current defeats the whole point of having a Class AB circuit in the first place, in a decent Class AB, these distortions mechanisms (which includes crossover distortions) will be vastly reduced, because of the optimal biasing, for one bias current is never negligible.
Music making the painting, recording it the photograph
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- Re: On a different note: Response to Morricab on Amplifiers - theaudiohobby 02:43:18 02/15/07 (1)
- Re: On a different note: Response to Morricab on Amplifiers - morricab 07:57:13 02/15/07 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
