Cool if collectively minds can get to the bottom of economical/practical/useful diy motional feedback with modern materials, techniques, resources and devices.
Experiments also on available economical DVC drivers might be interesting and useful despite their limitations according to the coil winding experiment proposed.
Enquiring minds........encourage you.
Dan.
I'm not saying snake oil but actual broadband FM removal would be a serious task unless of course you assume a perfect distortionless fixed piston or something like that. Then maybe you could compute the instantaneous cone velocity and feed that into an SRC algorithm?
BTW it should be trivial to run jn's experiment, two tones, get some serious displacement going with the lower tone, filter it out at the mic and do a Hibert transform on the 1KHz tone. If I had access to my stuff I would do it right now. I could use the Python script we did for the turntable speed stability the separation of AM and FM of the HT was remarkable in that application.
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I'll scan the relevant bits of my infinity sub service manual when I can. Not sure if I have the final evolution of servostatik or the cost reduced minimal version, but more the merrier for analysis 🙂
I dont think any of the 2 attached document are relevant to jn's idea as they talk about velocity and acceleration. Funny thing - the speak lin 1 document refer to a document authored by a person which I was invited to a dinner party not long ago :-D
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yes, I know..... I already brought up current feedback and DVC methods with schematics. JN's refinement could be attempted.
.....other views and ways and results. A foot note/reference indicates motional feedback goes back a long ways (1961).
There are many papers which i cant put here due to very limited file size allowed here.
-RNM
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The only difficulty I see is the prop delay in air. Significant phase shifts can make the feedback go positive, and if the entire system has gain at the frequency where margin goes to zero, it's over.
My concern with the second coil feedback, is that any delay in the main speaker line can make the system go unstable.. Using either my T-line model or the LCR model, a step response time constant in the drive line due to it's RF Z (or LCR) into a low z load will be made, with it's corresponding phase shift. Granted, for woofers it's not a concern, but if there's 20 or 30 microseconds of settling time in the drive cable, it might go to zero margin within the amps bandwidth, and we are providing direct high bandwidth feedback.
In tests of this dvc, it may be necessary to use a lower RF Z drive cable to eliminate the settling time, making the drive cable something like 15 to 30 ohms, or 150 pf per foot. For the feedback cable, I think a bog 600 ohm mike cable would work.
For the drive, I'd go with 4-6 randomly twisted zips in parallel (twist first, then connect parallel), bring the RF Z down to 15 to 25 ohms. Settling time will then be quite low as it's much closer to the load z. And, also add a zobel at the speaker drive terminals, make it 100/(# of zips) ohms.
Course, you could always just use one foot of cable for initial tests.
jn
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In this post on a german forum a member of Schanks Audio speaks of about 10dB reduction in midrange distortion as per multitone IMD when the Doppler correction is switched on. Weak data, I know, like no SPL given, etc. He then states that this is about the reduction we would typically get when going from 2-way to 3-way with a dedicated midrange, and that a 2-way with correction sounds very much alike a 3-way, cleaner mids overall.
My personal opinion is that those seem to be serious people and that they definitely have measurements in-house that prove the effectiveness of Doppler correction once the driver has been optimized by MFB (morphing to current drive when loop gain reserve drops below 0dB, afaik). If not, they would be living in a bubble...
Wether this is audible is another question, in my view in a 2-way with a small 6.5" or 8" midwoofer driven to large LF excursions it's normally masked by high IMD distortion from other ill-effects being dominant. Once that's fixed, it sure can make a difference. It is a viable solution to a specific problem, only affecting a small class of speakers.
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Doppler is FM, a multi-tone noise floor is a mess of AM and FM. Unless someone produces a technical paper I guess we don't know how the effects are separated.
Probably the 'right' amount of Doppler error is badly needed
to compensate for the microphone's Doppler error.
😛
to compensate for the microphone's Doppler error.
😛
There has been enough armchair engineering. I ordered a dual coil driver and should have some info by Monday on the coupling and what to expect from the connections. I don't have a box so it will be free air for now.
I'm not sure how you would wind the coils side by side and not expect some serious linearity issues not to mention a tendency to tear themselves apart on strong out of phase bass.
FWIW There is delay and phase shift between voltage across the voice coil and its motion. Then between the voice coil and the center of the cone and then between the center of the cone and the periphery. Similar for a dome.
The highest frequency I have been able to close the loop on a headphone driver is about 1500 Hz and that would be a driver with a self supporting coil, no mass and acoustic delays in the 50 uS or less range. I could never get a big driver to work much above about 300 Hz before running out of phase margin.
I don't think energy storage in the speaker cable will be a gating issue.
I'm not sure how you would wind the coils side by side and not expect some serious linearity issues not to mention a tendency to tear themselves apart on strong out of phase bass.
FWIW There is delay and phase shift between voltage across the voice coil and its motion. Then between the voice coil and the center of the cone and then between the center of the cone and the periphery. Similar for a dome.
The highest frequency I have been able to close the loop on a headphone driver is about 1500 Hz and that would be a driver with a self supporting coil, no mass and acoustic delays in the 50 uS or less range. I could never get a big driver to work much above about 300 Hz before running out of phase margin.
I don't think energy storage in the speaker cable will be a gating issue.
My concern is not how fast the coil can physically move. Rather, the frequency response of the coupling between co wound coils. I think that can go seriously high in frequency. So I worry about phase margin well beyond the speaker response range.
Just something to consider.
Jn
The driver is due Sunday (Amazon) so I should be able to get some measurements pretty quickly.
Look at it as 2 systems.
VC-1 and the cone make the driver.
VC-2 and the cone make a microphone.
VC-1 and the cone make the driver.
VC-2 and the cone make a microphone.
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