| acoustic1 |
Many moons ago I had a go at building the motional feedback subwoofer designed by Russel Breden in electronics world magazine, feb 97. It didn't work :(
I was told that this may be because the circuit diagram in the article is wrong, and not being an electronics wizz I have no idea how to make it work.
Does anyone have plans for this subwoofer which they know work? I've bought the expensive DVC woofers and it would be a shame to waste them! |
|
|
| forr |
This was a very interesting project. As far as I know, there was no correction or update. Have you any more details about what could be wrong in, the schematics ? Does youy speaker work open loop ?
In AudioXpress, Dan Ferguson published a project with a similar idea without knowing previous Breden's work and using a Dayton dual voice coil driver. This may help. |
|
|
| peranders |
Elektor had long time ago a motional feedback project.
Send me message if you want the article. |
|
|
| bobo1on1 |
Check this post from me: http://www.diyaudio.com/forums/show...7378#post987378
It's a working schematic for feedback with a microphone.
For use with a secondary voicecoil you should leave R11 and R12 out.
Connect the secondary to the two outer legs of a 10 potentiometer, connect the middle leg at the point where R11 and R12 are connected to C2.
Now one of the outer legs should be connected to ground so that the signal is in phase with the input signal.
Now you have to tune it, turn R4 all the way to the left or the right so that pins 1 and 2 of R4 are basically connected together.
Turn the circuit on and play a signal through the woofer.
Now turn the potentiometer connected to the secondary voicecoil so that the signal at the middel leg has about the same amplitude as the input signal.
Now you can start turning R4 until you start to get oscillation, then turn it back so the oscillation stops, now it ready for use.
If you have a very powerfull amplifier, connect a 60 watt lightbulb in series to protect the woofer when setting the feedback level.
This should work, assuming the signal from the secondary voicecoil represents the cone's acceleration.
I you don't understand something about it, just ask, or you might be building a one time smoke-machine.
Pherhaps you could send me the electronics world article? |
|
|
| Puggie |
| someone on here did a motion feedback system using a conductive foil inside the VC former and a metal ring ontop of the polepiece to make a basic air cored capacitor that changed value as the VC former moved, anyone remember and have a link to the project? I've looked for it a couple of times but not managed to find it. |
|
|
| Puggie |
| Thanks (you don't want to know how many searches I did without finding that) |
|
|
| bobo1on1 |
If you look at the schematic in the pdf, you can see they used a 4th order lowpass in the feedback path, that's not a very good thing to do imho, because the phase shift of such a filter is more than 90 degrees.
A first order lowpass should be used so that a higher level of feedback can be achieved. |
|
|
| acoustic1 |
| quote: | Originally posted by forr
This was a very interesting project. As far as I know, there was no correction or update. Have you any more details about what could be wrong in, the schematics ? Does youy speaker work open loop ?
In AudioXpress, Dan Ferguson published a project with a similar idea without knowing previous Breden's work and using a Dayton dual voice coil driver. This may help. |
Which Issue of Audioxpress? |
|
|
| johnnyx |
| If you want to use your DVC woofers, have a look at this thread for ideas. |
|
|
| forr |
Hi Acoustics1,
Your information is right, there are some errors in Breden schematics.
There are two C8 : an electrolytic one in series with R11 and one in parallel with R21. This second one belongs to the integrator circuit. An adeqaute value should probably rely between 10 nF and 100 nF.
I found the connection of stability capacitor C16 (270 pF) to the low impedance outpout of A6 through C7 rather strange. I would suggest the insertion of a resistor (1 kOhm maybe) between the node (C7 - R10) and (C16 - inverting input of A5). |
|
|
| bobo1on1 |
If one was to use an accelerometer, what range should the accelerometer be capable of measuring?
They come in ranges of 1,7G to as high as 250G.
If you can't buy an accelerometer anywhere like me, you can buy a wiimote or the nunchuck attachment for the wiimote, it contains an ADXL330 accelerometer which is capable of measuring +/- 3.6g.
Soldering to it might be difficult, since it comes in a 4 mm × 4 mm × 1.45 mm LFCSP package. |
|
|
| moamps |
Another good article on servo subs was published in the December 2006 issue of audioXpress: "Simple Servo Sub 100/120 Modification" by Bill Waslo. The author is our member "bwaslo" so you may wish to contact him for more details if he doesn't drop by in the meantime.
Regards,
Milan |
|
|
| forr |
Looking at different ways of servoing a driver, I found that velocity feedback has an interesting feature. At resonance, the velocity is in phase (or at 180°) with the input voltage.
It has been said that using one of the coil of a Dual Voice Coil driver as sensor looses efficiency due to the halving of Bl. This effectively looses the reference efficiency as defined by Richard Small.
But remembering that Duddley Harwood was complaining about too powerful driver motors, it occurs to me that subs are dealing with low frequencies and that there are good reasons to have an underdamped resonance because it enhances the local efficiency around the resonance. The feedback will bring back the desired damping.
Ideally, the Qt of a driver in a closed box sub should be around 1.1, because it is the value for which the efficiency is the highest in the resonance zone.
Considering the average spectral distribution of audio signals, I wonder if there exists an ideal resonance frequency in the same manner as there is an ideal value for Qtc.
Another interesting feature of using one of the coils of a DVC driver is that its output voltage is quite high and can be directly fed to the power amp. The Rythmik audio subs of C.-S. Ding are using this idea. |
|
|
| bentoronto |
Motional feedback is the way to go. It has been the progress of humankind to complete the feedback loop - in this case, right out to the speaker and beyond.
But very hard to do and not possible commercially. Feedback requires precise alignment of phase (you gotta be 180 degrees off, eh) and your means of sensing error has to good. Given how speakers change with the weather, furniture, etc. tricky to keep stable, except for a DIY person.
Doing it acoustically or using a 50-cent accelerometer (or home-brew capacitor made by gluing stuff to a dust cap) seems challenging. But if you are using speakers where coil/cone motion is fairly well related to sound wave, you have a fighting chance. That mean you have to use a sealed box (which includes, I think, the sealed box in a corner horn). But then with MF you can pump up the bass with less distorition in order to compensate for loudness shortcomings.
I don't know if using a second voice coil is more trustworthy than doubling back an error signal from the first voice coil. But I've been using a very well-performing MF system with my Klipschorn for 20 years solid-state and 20 years before that with tubes. It is a bridge design where the speaker VC is one leg of a bridge - not unlike some current feedback circuits.
Can't recommend it highly enough to bass lovers for cleanliness as well as tight bass.
Basic reference was, I think, J. Audio Eng. Soc, maybe 1954, RCA labs.... pre-internet, I know. |
|
|
|
