Thanks @clio09. That partial-quote is but one example of Roger’s depth of knowledge and wisdom in amplifier design, and why his passing is such a loss to hi-fi.
- 16 posts total
- 16 posts total
Ralph (@atmasphere), what do you think of Mr. Putzeys’ statement that ...
... the idea that feedback causes TIM is probably most noteworthy for being not just wrong, but also the exact opposite of the truth. TIM happens in the input stage. An increase in global feedback makes the input stage work less hard. That causes a disproportional reduction in TIM.@almarg Bruno is spot on here. The problem is that certain elements of the amplifier design can lie outside the feedback loop, but nonetheless be affected by the operation of the feedback. When you're dealing with something like this you get distortion.
Feedback has mostly been used in the last 40 years to decrease output impedance so that the amp can behave as a voltage source. That is why in most cases there is brightness/harshness that isn't part of the original signal since the feedback needed to get the amp to behave this way is always going to be less than what is needed for the feedback to have the ability to correct for distortion it adds.
One issue that's not seen a lot of attention is the simple fact that a speaker does not need more than about 20:1 for damping and many need considerably less. If overdamped, you get a phenomena often called 'tight bass' which is pleasurable but a coloration. So right here you can see that adding the feedback that is really needed for the amp to do its job right is overkill for the needs of the loudspeakers.
I've seen nothing to address this; audio is still a developing field.
Thanks, Ralph ( @atmasphere ). So would it be fair to say that feedback can cause TIM if the amount of feedback that is applied is insufficient to prevent it, but in many designs it is not practical to apply that much feedback "due to all the frequency poles which limit phase margin and decrease Gain Bandwidth Product." (Quoting from one of your earlier posts).
If so, I believe that would reconcile Mr. Putzeys' statement with the seminal work of Dr. Matti Otala, mainly in the 1970s, that was referred to by @tomic601 in an earlier post.
Thanks! Best regards,