Poles or feedback 101 - diyAudio
 Poles or feedback 101
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 15th January 2012, 02:45 PM #2 diyAudio Member   Join Date: May 2007 Are you asking about stability theory for closed loops, or applying that (already understood) theory to amplifier design? Don't forget that harmonic distortion is usually accompanied by intermodulation too, as they have a common origin in nonlinearity. You might not hear the second harmonic of 15kHz, but you will hear the 1kHz second-order IM when 15kHz and 16kHz are put through the amp together.
 15th January 2012, 02:58 PM #3 diyAudio Member   Join Date: Dec 2009 Location: Md Both. Just learning the amp bits. Ah, IM. That makes perfect sense. I am trying to assimilate a century of amp knowledge. Lots of non-obvious details. Self assumes you know half of it already. Would not the IM artifacts be the harmonics of the difference? Not 1K but 2 , 3 etc? I need to WIKI that. So, this means we still need to strive for absurd bandwidth and the feedback poles are primarily for stability of each given stage?
 15th January 2012, 04:14 PM #4 diyAudio Member   Join Date: May 2007 You can get IM at harmonics of the difference and lots of other frequencies too - higher order IM. For example, 4th order will give (among others) 2x16-2x15=2kHz. To understand how this works you need to revise your trigonometry, as it is all about multiplying sines and cosines together. In a typical amp most of the poles are not in the feedback but the forward path. Typically, each stage contributes (unavoidably) at least one pole. However, once you close the loop they are all in the loop together. I'm not aware of any need for absurd bandwidth, either open or closed loop.
 15th January 2012, 07:17 PM #5 diyAudio Member   Join Date: Dec 2009 Location: Md Let's see, Oscar Has A Heap of Apples. Darn, where is the free version of PSpice.... Looking at my DH 120 as that is handy to study, I see a hiogh pass, and a low pass in the feedback (C4, C7) C5, C6 and D14 all of which I do not understand the function of as part of the input amp. And of course the mystry to me is R10 which consumes 1.8W all the time and will toast the board.
 15th January 2012, 07:50 PM #6 diyAudio Member   Join Date: May 2007 R10 is involved with bias sources for CCS, but may also play a role in diode clamps which prevent BJT collector saturation. D14 may be one of these clamps. The DH120 is quite complex, so you may better starting with a simpler circuit such as Self's 'blameless'.
 15th January 2012, 08:59 PM #7 diyAudio Member   Join Date: Dec 2009 Location: Md Yup, solid advice there. I am looking at that too. I figure it is about the "classic" of modern thought. A place where you can say, OK you think you can do better, how. On the flip side, its not a bad idea to understand what some of the more clever gentlemen were doing. He must have had a good reason for using a dollar resistor or Hafler would not have allowed it being the master of few components himself. I am assuming Erno designed this one? He used a completely different design in the 200 and the AA series 60, DC100, and servo 100. "When in doubt, plagiarize. Let no ones work evade your eyes. That's why God gave you eyes." Tom Lehrer
 16th January 2012, 12:37 PM #8 diyAudio Member   Join Date: Dec 2009 Location: Md Bouncing between Cordell, Self, and a few WIKI pages is starting to make sense. Lots of details that as Self says, were left out as he expects you to know them already.
 16th January 2012, 12:46 PM #9 diyAudio Member   Join Date: May 2007 I always find when learning new stuff that a certain amount of time is needed for it to sink in, and join on to what I already know. Working harder, reading more etc., does not always shorten this time. Just keep at it, and you will get there.
 16th January 2012, 01:51 PM #10 diyAudio Member   Join Date: Dec 2009 Location: Md Terribly impatient. Easily distracted. Access to the kinds of experience this forum provides is almost addictive. What is actually hardest is backing up to concentrate on the basics when you have some knowledge of them. It is too easy to skip over lightly as you want to get to the parts that are new. Bad idea, but that's how I seem to work. I also have a tendency to pick out a detail to understand that is not the focus of the current lesson. I have been driving professors crazy for years.

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