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Optimising Bass Microphone Servo control
Optimising Bass Microphone Servo control
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Old 4th December 2017, 07:35 PM   #1
wgh52 is offline wgh52  Germany
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Default Optimising Bass Microphone Servo control

Hello folks,


the pair of 1984 B&M Omega I own already employs electret Microphone feedback for Bass servo control. After all this construction is old and I'm now wondering how I could/may be able to optimise the very simple servo circuit as it does not accomodate for any adjustments.


A little background:
  • 2x 200mm Bassdrivers
  • Each driver runs in its own, closed volume
  • A Microphone is glued onto each driver cone surface
  • Bandwidth: ~ 30 Hz (achieved by servo) - 180 Hz (by 4th Order LR Low-Pass)
  • One 70 W Poweramp drives Bass chassis (parallel)
The servo circuit:


Optimising Bass Microphone Servo control-regelung-original-png


I'm not really sure if the above solution as such is "state of the art", accomodating for all the possib le or necessary servo optimizations, i.e. can achieve what is possible. Therefore would like to gather input for servo bass improvement of my speakers, including how I would confirm by (acoustic and oscilloscope) measurements.


Sincere thanks for the inputs, recommendations and discussion!


Regards,
Winfried
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Old 5th December 2017, 07:22 PM   #2
bentoronto is offline bentoronto  Canada
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Optimising Bass Microphone Servo control
Sorry, I don't understand the device. Is that an accelerometer, not a mic?

What results do you have when the motional feedback is disabled?

It is a basic motional feedback configuration and might be quite wonderful when working.

B.
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Old 6th December 2017, 12:51 PM   #3
wgh52 is offline wgh52  Germany
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Hello Ben,


and thanks for being the first respondent. So much I can say:
  • The circuit has been implemented in 1984 and has worked ever since
  • The Mic is a Mic and senses the sound pressure level while being fastened to the piston and moves with the piston
  • The Mic membrane itself is rotated 90 vs. the piston movement direction, this way it does not (or just minimally) pick up mechanical acceleration.
Optimising Bass Microphone Servo control-mic-platzierung-png
  • The sensed sound pressure is fed back as electrical Mic signal and is compared with / subtracted from the incoming electrical audio signal
  • Differences are this way sensed and corrected "real time": A closed feedback loop.
Optimising Bass Microphone Servo control-tt-regelung-prinzip-2-png


The feedback circuit senses and "compensates" for the driver resonance caused frequency and phase response as well as other non-linearities, i.e. linearises sound pressure frequency response and lowers distortion. Yes, all this within given power, excursion and so on limits. The integrator effectively extends the sound reproduction of the bassbox to a selectable lower bass cut off, defined by the integrator C and the Resistor "bridging" it.

I have not yet tried the cicuit open-loop, may have to do this to find if it still works .

There are a few details I don't understand about the circuit myself, though, e.g. what is the function of the 4n7 C across the microphone terminals in the actual circuit?

Hoping this helps,
Winfried
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File Type: png TT Regelung Prinzip 2.png (91.8 KB, 135 views)
File Type: png Mic Platzierung.PNG (212.7 KB, 74 views)
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Old 6th December 2017, 02:19 PM   #4
bentoronto is offline bentoronto  Canada
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Optimising Bass Microphone Servo control
A simple MF solution. Far harder to use MF as you go up the tone scale. So possibly this system is meant to handle the main task, tightening up the resonance. The capacitor might be there to cancel the mic feedback north of the resonance band.

Using the figure-8 reception of that mic (once I paid 50-cents for that kind of capsule), if it works as you suggest (which still seems unlikely to me from where I sit), is a clever concept but pretty crude.

Odd not to simply use a glued-on device as an accelerometer, like a piezo crystal?

It would be really great if you posted REW FR, HD, and impulse-related curves before and after the motional feedback loop was connected.

B.
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Last edited by bentoronto; 6th December 2017 at 02:21 PM.
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Old 6th December 2017, 05:01 PM   #5
wgh52 is offline wgh52  Germany
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Hi Ben,

Quote:
Originally Posted by bentoronto View Post
...The capacitor might be there to cancel the mic feedback north of the resonance band...
Yes, that may well be the case. Maybe I find out later...

Quote:
Originally Posted by bentoronto View Post
...Using the figure-8 reception of that mic (once I paid 50-cents for that kind of capsule), if it works as you suggest (which still seems unlikely to me from where I sit), is a clever concept but pretty crude...
Well, we could discuss if you would elaborate on your explicit concerns. The geniality of this approach may be in the simple, though... And if a more elaborate feedback loop circuit (which takes more, which ever, considerations into account) would be an improvement, what I'm after here would be achieved.

Quote:
Originally Posted by bentoronto View Post
...Odd not to simply use a glued-on device as an accelerometer, like a piezo crystal?...
Simple answer: Accelerometer feedback control for speakers was a patent by Philips in those days, so not usable commercially for Backes & Mueller. Using the accelerometer feedback, by the way, has it's own set of limitations and complications.

Quote:
Originally Posted by bentoronto View Post
...It would be really great if you posted REW FR, HD, and impulse-related curves before and after the motional feedback loop was connected.
I'm planning that, have to see when I get the chance for measurements.

Regards,
Winfried

Last edited by wgh52; 6th December 2017 at 05:15 PM.
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Old 6th December 2017, 07:29 PM   #6
bentoronto is offline bentoronto  Canada
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Optimising Bass Microphone Servo control
All good. Thanks.

For anything in audio and esp. motional feedback, many trivially different patents introduced for commercial reasons. For example, ACE-BASS texts are hocus-pocus. Likewise, there may be differences between the manufacturer's write-up on your device and how you would describe it in reality.

Great to see your before-and-after REW results, esp. if you can link to your entire REW data file, not just the pretty pictures.

B.
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Old 6th December 2017, 08:26 PM   #7
weltersys is offline weltersys  United States
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Quote:
Originally Posted by wgh52 View Post

There are a few details I don't understand about the circuit myself, though, e.g. what is the function of the 4n7 C across the microphone terminals in the actual circuit?
Winfried,

The capacitor is used to filter the electret microphone (typically omnidirectional) DC power supply voltage out of the audio path.

Art
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File Type: png Electret mic.png (53.8 KB, 19 views)
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Old 6th December 2017, 08:36 PM   #8
bentoronto is offline bentoronto  Canada
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Optimising Bass Microphone Servo control
Quote:
Originally Posted by weltersys View Post
The capacitor is used to filter the electret microphone DC power supply voltage out of the audio path
Not the 4n7. One of us needs to take Electricity 101 again or buy new eyeglasses; not certain which or who.

I'll stick with my loop attenuation guess. Or possibly RF filter.

B.
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Old 6th December 2017, 09:21 PM   #9
weltersys is offline weltersys  United States
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Quote:
Originally Posted by bentoronto View Post
Not the 4n7. One of us needs to take Electricity 101 again or buy new eyeglasses; not certain which or who.

I'll stick with my loop attenuation guess. Or possibly RF filter.

B.
Opthamologist appointment scheduled for 4/19/18, maybe I should schedule it sooner ;^)...
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Old 6th December 2017, 10:08 PM   #10
wgh52 is offline wgh52  Germany
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Never mind folks! I'm glad the misunderstanding got resolved.

I follow the RF blocking theory to a good extent, but the 4n7 is mylar (not ceramic) and past measurements had shown that it does have an effect on low pass roll-off: Because of an appearent dip around the 180 Hz cross over region I had tried 2n2 to reduce that, which worked, but lost ~6dB woofer output level. ... one of the reasons I'm looking for other/better optimization alternatives

Maybe a possible trick here would be to filter the Mic-signal with a low pass equivalent to the 180 Hz cross-over low-pass, so the feedback circuit does not "hear" the mid-driver taking over at 180 Hz? Would that be "a huge overkill" or wrong or counterproductive or a potential improvement?

Cheers,
Winfried
Attached Images
File Type: png F-Gang Regelung mit Weiche Mic-C Variation klein.png (17.9 KB, 22 views)
File Type: png F-Gang Summe TT&MT 4n7 klein.png (18.8 KB, 17 views)
File Type: png F-Gang Summe TT&MT 2n2 klein.png (22.0 KB, 18 views)
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