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Inductive Feedback
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Author:  Joe_Limon [ 09 Nov 2018, 16:15 ]
Post subject:  Inductive Feedback

I had an idea regarding amplifier design that I wanted to pitch and get some opinions on (possibility, feasibility, or even advice on pushing forward on this potential project).

I am wondering if it is possible to use current induced by a speaker to estimate its instantaneous velocity by taking a difference between a theoretically perfect current draw and the real current draw of that speaker. Essentially, using the speaker coil as a velocimeter and making the feedback system turn any speaker into a servo driven speaker.

Author:  ILoveHiFi [ 11 Nov 2018, 22:22 ]
Post subject:  Re: Inductive Feedback

That sounds like some sort of massive innovation no one has ever done it so cannot answer you.
This could possibly get you a PHD degree.

Shortly speaking its impossible to do what you are trying to do.

Author:  Yungman [ 14 Nov 2018, 05:30 ]
Post subject:  Re: Inductive Feedback

No idea what you are talking, some drawing might help.

Author:  gofar99 [ 14 Nov 2018, 17:10 ]
Post subject:  Re: Inductive Feedback

Hi, Something maybe similar to what you want to do has been done in the past. Several companies attached sensors to the speaker's cones to measure how they acted when a signal was put to them and used the sensors to provide feedback to the amplifier. A servo type of arrangement. It was supposed to work well at low frequencies and as I recall was used to make compact powered sub woofers.

Good listening
Bruce

Author:  Yungman [ 15 Nov 2018, 19:19 ]
Post subject:  Re: Inductive Feedback

gofar99 wrote:
Hi, Something maybe similar to what you want to do has been done in the past. Several companies attached sensors to the speaker's cones to measure how they acted when a signal was put to them and used the sensors to provide feedback to the amplifier. A servo type of arrangement. It was supposed to work well at low frequencies and as I recall was used to make compact powered sub woofers.

Good listening
Bruce

Do they have any success? How does that sound?

I think this gets way too complicated. I think if one use an amp that has very low output impedance ( damping factor over 1000), very low inductance speaker cable. You really control the speaker already.

It's not practical to monitor the speaker movement, every speaker is different, they have different phase delay. How is the amp going to do the closed loop compensation for stability if they don't even know what speakers the customer is using.

Stability is a major problem with audio amps, making the speaker as variable can really rig havoc to the stability.

I think OP talked about monitoring the current through the speaker. I don't know whether it works, but at least it's a lot more simple. BUT is that accurate? You monitor the current, part of the current flow through the crossover, not the speakers, so you can be fooled.

I am not going to assume anything, OP need to be more specific before we start guessing.

Author:  Joe_Limon [ 27 Nov 2018, 02:35 ]
Post subject:  Re: Inductive Feedback

I know servo woofers are a thing. But they require sensors built into the woofer and custom feedback loops with dedicated amps.

I think the last comment had the best understanding of my idea. I agree regarding the crossover statement making it harder to predict speaker movement. Without data or experience I do not know how serious of a concern this is. If it is major, perhaps it would be best to limit the amp as a headphone amp where the likelyhood of using a crossover is minimized significantly.

Author:  Yungman [ 28 Nov 2018, 02:30 ]
Post subject:  Re: Inductive Feedback

I think even in the head phone, I don't think speaker cone movement is linear to the current through it. remember, the more extreme the excursion, the movement of the cone decrease with the same current change when the cone reach the extreme position vs at rest. The spider gives more resistance when you push close to the limit of the movement. In another words, the movement is no linear to the current, movement get's compressed at the higher end.

Also, cone has harmonics, you cannot count on the cone move in one piece up and down.

Each speaker has it's own characteristics of movement vs current, you calibrate for one does not mean it fits another speaker in the headphone.

this goes the same as real speakers, even if you can bypass the crossover, you have 3 different speakers in a 3 way speaker. You have 3 totally different control, you have to monitor 3 different speakers. Then different brands have different speakers with different excursion of the cone vs current............

I think this will go into a rabbit hole fast. Not any better using a sensor monitoring the movement of the cone.

Author:  Joe_Limon [ 28 Nov 2018, 11:34 ]
Post subject:  Re: Inductive Feedback

Yeah the only way I can see 2 way or 3 way speakers being done is with an integrated amplifier coupled individually to each driver.

To the single driver setup though. The excessive fluctuations away from a baseline curve due to harmonics and mechanical characteristics of the cone/driver/enclosure are specifically want this feedback loop would be designed to mitigate.

Author:  laurie54 [ 28 Nov 2018, 17:49 ]
Post subject:  Re: Inductive Feedback

Years ago now I was the tech for a company which rented out sound and lighting equipment for dances etc. We got in an amp and speakers which came as a set. It was rated at 10,000 watts and used the active feedback system talked about here with sensors in the spkrs sending info back to the amp. I do not remember the name of it but I do remember the line cord was the size of my wrist, the spkr cables (supplied) were the diameter of my ankles, and turned 1/2 way up one could not be in the same room as the spkrs. And the spkrs were not that large being similar in size to the average DJ spkrs that would be placed on raised stands. Certainly not large enough to produce the level of sound pressure any other system on the market could ever hope to achieve. Something like 36" x 24" x 18" each.

So is it possible? yes. Has it been done? yes. Does it work? YES! Is it affordable?????? only if one was willing to take out a second mortgage on your house.

And, while writing this, it seems to me Phillips Co. had an active spkr/amp system at one time as well, aimed more at the home stereo market. Non of these systems have stayed around I think more due to cost, cct complexity, and the fact that it was not a system the average Joe could wire up and throw the power switch on as it required special plugs to ensure only that system's spkr cable would be used which also meant the owner would have to be content with the cables supplied as they become an intricate part of the amp's feedback system and could not be changed or lengthened at all without catastrophic consequences to system and or personal life and limb.

Author:  Joe_Limon [ 29 Nov 2018, 00:23 ]
Post subject:  Re: Inductive Feedback

Yeah, this is precisely why I want to avoid a specific paired amp/speaker combo. I want to make an amp that can turn passive drivers into servo drivers. And so far it seems like the potential market is either integrated amp speakers or headphones due to crossovers.

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