Slewrate induced distortion
BTW IMO its best practice to slow down an amp with a single
pole to avoid SID in any case!
Normally you encounter SID in multistage amps somewhere in
the driver or predriverstage where global feedback does not
influence the open loop gain, this stage is overloaded.
To get rid of this phenomenon people try to use NDFL
(nested differential feedback loops) but there you build up
multiple pole systems which tend to be not that stable as a
single pole system. Even an Aleph amp has at least 4 poles
which influence phase and thus the point where NFB switches
to PFB which leads to oscillation.
By carefully adjusting the major lopass pole you set the point
where amplification drops down that gain is below 1 before the
point of PFB occurance is reached.
Uli
Nelson Pass said:
BTW IMO its best practice to slow down an amp with a single
pole to avoid SID in any case!
Normally you encounter SID in multistage amps somewhere in
the driver or predriverstage where global feedback does not
influence the open loop gain, this stage is overloaded.
To get rid of this phenomenon people try to use NDFL
(nested differential feedback loops) but there you build up
multiple pole systems which tend to be not that stable as a
single pole system. Even an Aleph amp has at least 4 poles
which influence phase and thus the point where NFB switches
to PFB which leads to oscillation.
By carefully adjusting the major lopass pole you set the point
where amplification drops down that gain is below 1 before the
point of PFB occurance is reached.
Uli
Distortion is whole family of strange beasts....
In signal theory, the term "Linear Distortion" is used on a circuit that does not have flat amplitude response and/or constant phase.
The easiest way to envision linear distortion is to draw the first 3 ( fundamental, 3rd and 5th) of asquare wave on a piece of paper, and then try to alter the amplitude relations, or phase shift one of the harmonics....
Nature is a cruel beast.......
In signal theory, the term "Linear Distortion" is used on a circuit that does not have flat amplitude response and/or constant phase.
The easiest way to envision linear distortion is to draw the first 3 ( fundamental, 3rd and 5th) of asquare wave on a piece of paper, and then try to alter the amplitude relations, or phase shift one of the harmonics....
Nature is a cruel beast.......
That's the simplest way, but from experience I can say that
amps which are near to oscillating, but not actually oscillating,
can sound just awful, like some op amps run at unity gain.
It is reaonable to expect that in order to get good sound, you
need a phase margin quite a bit greater than that which merely
prevents outright oscillation.
amps which are near to oscillating, but not actually oscillating,
can sound just awful, like some op amps run at unity gain.
It is reaonable to expect that in order to get good sound, you
need a phase margin quite a bit greater than that which merely
prevents outright oscillation.
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