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 Post subject: Re: Inductive Feedback
PostPosted: 29 Nov 2018, 12:54 
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Joined: 09 Oct 2012, 19:43
Posts: 338
Location: Vancouver Canada
I would think using a passive spkr to monitor an active spkr and provide feedback to the amp would result in a time lag that would be like a cct to monitor and prevent a balloon from popping that only worked after the balloon has popped. It would be an interesting experiment though. Let us know what you come up with. Keep us in the know as it were.


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 Post subject: Re: Inductive Feedback
PostPosted: 29 Nov 2018, 20:22 
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Joined: 26 Dec 2016, 03:46
Posts: 195
Location: Bayarea
To sum it up, regardless of how expensive, you have to have dedicated speaker that you know the characteristics. Also, you have to find a way to bypass the crossover. AND you have to do it on individual tweeter, mid and woofer. I don't see the way you can do it with just monitor the speaker input terminal to do feedback.

Question is why do you want to do that? Do you have the knowhow to do that? There is a lot of software and DSP involve. I don't think you can use simple analog feedback to do this. You are talking about precision control here.

Let's just look at it a little deeper. I don't know DSP, my stupid method will be:
1) Signal return back from the speaker current. Digitized with ADC.
2) Create a look up table in EPROM to map the signal from the ADC to output a correction signal to compensate the distortion.
3) Feedback to the original input signal to the amp to modify the signal to drive the speakers.
4) Now you have to find a way to stabilize the "system" as the feedback is from the speaker, there is a lot of delay. This will limit the frequency response.
5) To do that, you have to characterize the speaker so you can create the look-up table for the EPROM. That will entail a lot of experiment.

This to me, is a group of engineers working together. In my 30 years working as an engineer and manager of engineering, this is a team project, I never seen one engineer that is expert in analog design ( you need it here) AND a good DSP, programming engineer. Then you have to design the analog to digital, digital to analog interface.

You equipped for this? If you actually succeeded, what market are you going to go into? How much can you sell?


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 Post subject: Re: Inductive Feedback
PostPosted: 30 Nov 2018, 02:15 
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Joined: 09 Oct 2012, 19:43
Posts: 338
Location: Vancouver Canada
No digital involved with the systems I was talking about. Pure analog from one end to the other but yes needing much experience
in analog design. These systems all however used a single full-range driver. Trying to monitor 3 spkrs would bring so many perimeters
into play it would be a nightmare. The bass takes the most power and has the most mass needing to be controlled so maybe just
feedback from the bass spkr would be sufficient. I still think it would be a fun experiment though.
I don't know what was being used as a sensor. Perhaps a second coil wound beside the main spkr coil or maybe a pizzo sensor
of some sort. It may be worth some time looking at patients to see if anything turns up. The systems where said to have "active feedback".


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 Post subject: Re: Inductive Feedback
PostPosted: 30 Nov 2018, 02:25 
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Joined: 09 Oct 2012, 19:43
Posts: 338
Location: Vancouver Canada
Have a look at this write-up. They call it "motional feedback" and show a smaller set of spkrs from Phillips using 2-way x-over. They do say which type of sensing is involved and suggest a sight showing how to do just what you are talking about to your own system.

https://hifipig.com/philips-motional-feedback-speakers/

This should answer many questions you have and even offer some suggestions for an existing system to be modified.


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 Post subject: Re: Inductive Feedback
PostPosted: 30 Nov 2018, 03:50 
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Joined: 26 Dec 2016, 03:46
Posts: 195
Location: Bayarea
laurie54 wrote:
No digital involved with the systems I was talking about. Pure analog from one end to the other but yes needing much experience
in analog design. These systems all however used a single full-range driver. Trying to monitor 3 spkrs would bring so many perimeters
into play it would be a nightmare. The bass takes the most power and has the most mass needing to be controlled so maybe just
feedback from the bass spkr would be sufficient. I still think it would be a fun experiment though.
I don't know what was being used as a sensor. Perhaps a second coil wound beside the main spkr coil or maybe a pizzo sensor
of some sort. It may be worth some time looking at patients to see if anything turns up. The systems where said to have "active feedback".


The compensate for none linear distortion, digital DSP is not as crazy as you think. It is hard to control only using analog. We succeeded in removing the distortion of a sine wave using look up table to make an absolute perfect sine wave. I would not even consider doing in analog domain.

Brand like Anthem already have preamp that has room correction, you go through a procedure to sense the sound at different location and input into the preamp, the DSP will do calculation and do correction to optimize the sound, the delay and everything for the room. They correct at the input level by info from the recording.

I think this method is a lot more reliable, simpler to do speaker correction. You are not limited to particular speaker, just correct from the sound produced. But then, this is NOT a new idea, likely they have patent on this already.

Linearize using current through the speaker or sensing movement of the cone or even using another coil on top of the speaker coil is a lot more difficult. You don't know the non linear characteristics of the speaker current vs the sound, good luck doing this in pure analog domain. Just imagine a kink on the sinewave, How are you going to use analog circuit to get rid of the kink? DSP or look up table can get rid of it just like that.

It's too complicated.


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 Post subject: Re: Inductive Feedback
PostPosted: 30 Nov 2018, 14:38 
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Joined: 26 Dec 2016, 03:46
Posts: 195
Location: Bayarea
laurie54 wrote:
Have a look at this write-up. They call it "motional feedback" and show a smaller set of spkrs from Phillips using 2-way x-over. They do say which type of sensing is involved and suggest a sight showing how to do just what you are talking about to your own system.

https://hifipig.com/philips-motional-feedback-speakers/

This should answer many questions you have and even offer some suggestions for an existing system to be modified.


I read through the article, the feedback was only for woofer. It's a whole lot easier as frequency is slow. If you want to monitor mid and tweeter, it's a different ballgame.

They have sub woofer that is DSP controlled, digital is the way to go.


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 Post subject: Re: Inductive Feedback
PostPosted: 30 Nov 2018, 19:53 
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Joined: 09 Oct 2012, 19:43
Posts: 338
Location: Vancouver Canada
Yup


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 Post subject: Re: Inductive Feedback
PostPosted: 30 Nov 2018, 20:33 
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Joined: 26 Dec 2016, 03:46
Posts: 195
Location: Bayarea
The paper even admitted they get into the price range the comparable cost system sounded better. It's a lot of constrain to make this happen.

Let's just assume you succeeded in making the system, you have to sell the amp with the speakers together, you cannot give the option of people buying your amp only. I have a pair of speakers I like, I am not about to be forced to buy another pair of speakers. The cost is going to be very high.

Technically wise, it's a different world to do this on mid and tweeter, frequency is too high, you introduce more distortion than correcting them.


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