Friends,
I've started writing the occasional short piece, a column really, for the weekly free AudioXpress newsletter The Audio Voice.
Several people suggested I post them here at diyaudio as they may be usefull to members.
So, here's the first one, on feedback. Please let me know what you think of the idea to post more similar pieces, but also if you have comments on the contents.
The free TheAudio Voice is published every week on Thursday. Subscribe to the newsletter - it's free and there are always unsuspected gems about audio!
There's a sign-up button here.
Your opinion is appreciated. And if you have suggestions for future columns - let me know!
Jan
I've started writing the occasional short piece, a column really, for the weekly free AudioXpress newsletter The Audio Voice.
Several people suggested I post them here at diyaudio as they may be usefull to members.
So, here's the first one, on feedback. Please let me know what you think of the idea to post more similar pieces, but also if you have comments on the contents.
The free TheAudio Voice is published every week on Thursday. Subscribe to the newsletter - it's free and there are always unsuspected gems about audio!
There's a sign-up button here.
Your opinion is appreciated. And if you have suggestions for future columns - let me know!
Jan
Jan, with all due respect, I think the style of the article is too academic and I am not sure if it would attract general public readers. Of course everything is correct.
Sometimes trying to simplify an idea may lead to a misunderstanding of the idea, so I think that Jan's more in-depth approach is necessary to get a good understanding of the topic. Some stuff simply isn't easy. Disclaimer: I am an EE, retired now. As for attracting the general public, how many of that cohort would be interested in EE topics anyway? If the idea were to be oversimplified for that, then it would leave the EE types hanging.
I believe that anyone intelligently enough to be interested in audio electronics is able to get a good grasp on feedback by taking time to 'get' such a short column. By definition, audio enthousiasts are a curious lot and want to know about tech stuff.
Granted, you need to put in some effort, but if you're not willing to do that, what's the point anyway?
I'll post another one next week, and hope to get more comments by then.
Jan
Granted, you need to put in some effort, but if you're not willing to do that, what's the point anyway?
I'll post another one next week, and hope to get more comments by then.
Jan
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Jan, your introduction on the subject of negative feedback is quite good, I think lots of users on this forum can benefit from it.
The article is being praised by those who already understand. Perfectly fine if that's the target audience but don't kid yourself that you have reached any musicians , trades people, doctors, history teachers and other diyers without a career investment in math who will continue to ignore your forum conversations in favour of those threads rehashing the supremacy of zero feedback, non-oversampling and fatter speaker cables.
I'would had been really happy to find your article when i was a sound engineer student more than 20 years ago and asked myself about feedback: i think it gives an easy to follow approach to the phenomenon and will tease the curious one to know more about it.
I like it.
Thank you Jan.
I like it.
Thank you Jan.
It is a good article as far as it goes. One thing I learned about negative feedback, and correct me if I am wrong, if you start with a single stage amp and from the input signal to output you measure its total harmonic distortion, then apply negative feedback and the total harmonic distortion amount is reduced. But the original output distortion spectrum before feedback is 2nd and/or 3rd order harmonics, with the negative feedback the total distortion product maybe reduced but now you have added 4th and 5th order harmonics, at a lower level but there, and around you again and now you have an even smaller amount of 6th and 7th order harmonics added to the output signal. In the early days of transistor amps they applied large amounts of negative feedback and had much lower distortion than the tube amps but they sounded lifeless. That lead to designers of amps trying to minimize the amount of distortion generated in a circuit first and then using minimal levels, or even no global feedback, and although they measured worse they sounded far better.
Jan, how about to add a real life situation, when input signal is not only fast, but also too big in amplitude, resulting in output saturation and loss of feedback proper action? This might be of readers interest. Suggestion of input signal limiter?
Feedback is not sending the signal “round and round”. It compares actual input value with actual phase shifted divided output value. But you are right in another point, it changes distortion profile and pushes energy into higher harmonics.
According to Feldtkeller's analysis from 1936, that's mainly an issue at low levels of feedback. Mind you, he was looking at third-order distortion.
Thanks for the article link!
Just a small remark: it's (hopefully) not carbon monoxide in your blood but carbon dioxide!
Just a small remark: it's (hopefully) not carbon monoxide in your blood but carbon dioxide!
In this particular case about feedback, I would have used the more standard symbols from control theory.
They are just much easier to understand and follow for the average reader compared to these specific LTSpice blocks. (they actually always confuse me)
I am aware that they do the same, but in my opinion anything that's control loop related, is just easier with adding/summing/subtract blocks, in combination with proportional blocks (= gain), integrator, differentiators and other transfer functions.
This just gives a much better overall understanding, also about the fact that these things are not just only related to electronics.
Which gives the opportunity to use more practical things as an example.
Making it a lot less abstract.
By default, those can't be found in LTSpice, but the famous "zzz library" has them.
Which can be found here, if I am not mistaken http://www.bordodynov.ltwiki.org/
Also, what I was missing, is that if you talk about feedback one also has to (briefly) mention feed forward as well.
Because feedback is not the only to prevent things from crashing into other things.
In general that's already way to specific and going deeper in very specific details.
I don't see how that's useful for an article that's talking about generic details?
In practice there can be a billion things why such a system doesn't work linear anymore (which is the correct term).
A pure theoretical feedback loop just compares the output with the input and does everything to make the two equal.
Nothing more and nothing less.
All other things are just shortcomings of the practical system.
According to this article by Nelson Pass https://www.passlabs.com/technical_article/audio-distortion-and-feedback/ , Peter Baxandall did also write about re-entrant distortion. Mentioned writing from Nelson Pass ,that addresses distortion and feedback, might be easier to pick up for the not initiated reader compared to the one from Jan Didden.
It happens at any loopgain level, more or less. The more loopgain, tighter feedback, the better and effect is lesser. However, it is always here. You may have pure H2, after application of feedback it will be reduced but also and always higher harmonics are created, though very low in level. Easy to show in simulation.
