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Optimising Bass Microphone Servo control
Optimising Bass Microphone Servo control
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Old 6th December 2017, 09:25 PM   #11
weltersys is offline weltersys  United States
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Originally Posted by wgh52 View Post

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?
The mic works with the inverse-distance law, each doubling of distance looses 6 dB level. At "ground zero" the woofer level is likely far louder at the acoustic crossover than the relatively distant mid-driver. If you lower the mic low pass, it won't "hear" harmonic distortion as well, so can't correct it as effectively.
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Old 7th December 2017, 12:04 AM   #12
bentoronto is offline bentoronto  Canada
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Optimising Bass Microphone Servo control
The 4n7 is clearly operating in the audio range. As you enlarge the operating band, you risk entering a zone of instability. Manufacturers of motional feedback systems live in fear of instability and blown drivers: otherwise they would have gone to a smaller filter cap too. Not saying what a DIYer should do with a closely monitored home system, just want to be careful.

This may sound heretical, but multi-speaker FR charts can be misleading due to phase/interference effects and in relation to the REW mic and musical content. In other words, pure swept sine wave results may not correspond to the audible loudness on complex sounds. So the middle FR with the dip may sound better than the 2n2 on the right.

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Old 12th December 2017, 02:22 PM   #13
Sonce is offline Sonce  Macedonia
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Originally Posted by bentoronto View Post
Using the figure-8 reception of that mic (once I paid 50-cents for that kind of capsule),
That kind of electret mic has omnidirectional (kugel) characteristic.
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Old 15th December 2017, 09:58 PM   #14
bolserst is offline bolserst  United States
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Originally Posted by wgh52 View Post
… not really sure if the above solution as such is "state of the art", accommodating for all the possible 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.
Looks like you have an interesting speaker in your hands

To determine what B&M achieved in the circuit and what optimization is possible, you would need to measure the open loop response. To do this, you need to break the feedback loop. The most convenient place to do this is the connection between the preamp and power amp as shown in the attached figure by the red X. Input a low level test signal into the power amp(10 – 50mVrms) from measurement software(ARTA, REW, etc) and measure the electrical frequency/phase response coming out of the preamp where you broke the loop. Dividing this by the input signal will gives the open loop response, which tells you how much feedback is being used and what the gain and phase margin is.

While you have the feedback loop broken, you might also measure the acoustic output of woofers while using the same direct input into the power amp. This will show you what the natural response of the woofer/cabinet is before feedback is applied.


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?
The 4n7 capacitors across the microphones form a first order low pass filter with R157, rolling off the feedback signal above about 1kHz.

You mention seeing a 6dB change in woofer level when changing the values of these capacitors. I don’t understand why that would be unless one of the 2 microphone connections was bad before or after the change. (ie factor of 2 = 6dB) Where are the two capacitors physically located? And how are the connections made between mic, capacitor, and preamp board?
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Old 15th December 2017, 11:53 PM   #15
bentoronto is offline bentoronto  Canada
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Optimising Bass Microphone Servo control
I'd add - to the always super help from Bolserst - you might also chart the feedback signal itself. Like most of the signal of interest, it is relative to whatever is also motivating the system.

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