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:Many years ago I learned not to send really fast amplifiers
out to customers. I don't like for them to have problems due
to crappy cable and poor grounding, so 100 KHz is my default.
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.......
uli said:
... 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.
Even if I do not fully understand what you say, it is interesting. Do you mean that Aleph amp has many poles, but Aleph is internally compensated so to have a single pole?
poles
Every gain stage creates a pole caused by the non infinite speed
of the gain device!
In case of Aleph you get one pole formed by Q12/C6, one by
Q5/Q6/C7 one byR7/C8 and the last is formed by R3/R5/R7/C5.
The major pole (which determines the frequency range) is R7/C8.
Uli
Every gain stage creates a pole caused by the non infinite speed
of the gain device!
In case of Aleph you get one pole formed by Q12/C6, one by
Q5/Q6/C7 one byR7/C8 and the last is formed by R3/R5/R7/C5.
The major pole (which determines the frequency range) is R7/C8.
Uli
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,
In reality, what is a simplest method to check that this point is reached?
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.
Hi Uli, Nelson
Can it be the point where we get clean, regular and without overshoots square wave at 1kHz?
Regards
...you need a phase margin quite a bit greater than that which merely prevents outright oscillation.
Can it be the point where we get clean, regular and without overshoots square wave at 1kHz?
Regards
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