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Motional Feedback Speaker Project - Circa 1981
Motional Feedback Speaker Project - Circa 1981
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Old 15th March 2019, 08:26 PM   #91
rscamp is offline rscamp  Canada
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Quote:
Originally Posted by forr View Post
I use a very fast and efficient way to model driver transfer functions...
Thanks, forr!
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Old 16th March 2019, 10:49 AM   #92
bentoronto is online now bentoronto  Canada
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Motional Feedback Speaker Project - Circa 1981
Quote:
Originally Posted by Turbowatch2 View Post
...The only “feed back” comes from the voice coil/ series resistor, the other measures are called feed forward, as far as I know, as they do not react to anything the cone/ voice coil will do.

Maybe we should first see what the basic current drive really does in a construction, then add the rest and look at it again. ...
In the end the resistor is not the best way at all. Any real signal sensor should do better, I think about glueing a microphone to the cone for correction, not more expensive in the DIYS world.
Big debate about using the VC to produce its own feedback using Back-EMF versus the various issues using sensors.

A more sophisticated way of using the VC is by locating it in a Wheatstone bridge and feeding the cross-connection into the two inputs of an op-amp.

With a bridge, the theory is that you balance the bridge for the stationary (blocked) impedance of the VC which cancels the resistance and the inductance of the VC. So the out-of-balance feedback voltage which is generated is mostly Back-EMF. In practice, not too much different than Turbowatch2's current feedback with a single series resistor in the minus lead.

How could anybody possibly imagine they could get quality sound from a system as inferior in design concept as a subwoofer without feedback? I really love rscamp's scope traces. Folks should look closely at the over-shoot at the end of the tone-burst with and without MFB.

B.
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HiFi aspirations since 1957. Currently working on motional feedback again... the final frontier in audio (also, see recent post with data on a 17-foot labyrinth sub)

Last edited by bentoronto; 16th March 2019 at 10:54 AM.
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Old 16th March 2019, 05:23 PM   #93
rscamp is offline rscamp  Canada
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Originally Posted by bentoronto View Post
...How could anybody possibly imagine they could get quality sound from a system as inferior in design concept as a subwoofer without feedback?...
Darned if I know.
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Old 16th March 2019, 06:04 PM   #94
rscamp is offline rscamp  Canada
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The "Motional Feedback Speaker - Circa 1981" which hasn't been operated since 2000 is working!

I didn't want to rip a power amplifier out of my sound system so I used an old receiver as the amplifier. Being an integrated amplifier, the volume (amplifier gain) and tone controls are active so first I made sure the transfer function was okay by defeating the tone controls and such and checking with REW. This confirmed there were no amplitude or phase anomalies and left the volume control on the receiver as essentially the feedback loop gain adjustment.

I replaced the 1-1/2A fuses with 1A fuses for a little extra safety margin, plugged in the accelerometer signal to the MFB controller and slowly turned up the amplifier gain. Eventually when exciting the diaphragm with the fingers I began to see the familiar oscillation around 10 Hz. MFB is in da house.

Here's the poor little 8" driver trying to reproduce 16 Hz. The video frame rate makes it look like the amplitude is pulsing but in reality it is steady.
Attached Files
File Type: mp4 MFB First Test 16 Hz.mp4 (4.13 MB, 13 views)
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Old Yesterday, 05:59 PM   #95
rscamp is offline rscamp  Canada
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These charts compare the Output to the amplifier with and without feedback.

In the amplitude response from REW, I didn't match the input amplitudes so these can't be directly compared, but note the relative reduction in Output with feedback where the speaker response would be greatest around 100 Hz and the indication of resonance issues, particularly close to 4 kHz.

In the scope/analyzer view from VA, the loudspeaker was driven with a fairly high amplitude 20 Hz signal. I would guess p-p motion was in the order of 7mm or so. A small change in waveform shape is visible when feedback is applied and this is manifested in the ADDITION of 3rd harmonic energy applied to correct the 3rd harmonic distortion in the driver. I probably could have set these views up better but it is the first time using this software.
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Old Yesterday, 10:25 PM   #96
forr is offline forr  France
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Hello,

I have 280 Mb of documentation on servoed loudspeakers.
Send me a PM if you are interested.
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