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Old 25th August 2014, 06:22 PM   #1
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Default Direct Servo Subwoofer Amp... What is different?

I am trying to find a schematic of a direct servo subwoofer amp so I can try to understand it's design better.

I guess the idea is there is another coil in the sub driver that senses something and sends a feedback signal to the amp so it corrects differences between the original signal and what the driver is doing.

Sounds kind of like negative feedback to me. I don't really know because I can't find any design schematics of this so it seems like just a bunch of hype to me. I know the patent is from the early 70's at least so if it was worth doing I don't see why it wouldn't have caught on.

I recently saw pics of the internals of a plate amp with this technology and there didn't seem to be much to it. I also can't find any measurements of subs with the circuit out vs in.
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Old 25th August 2014, 06:49 PM   #2
sreten is offline sreten  United Kingdom
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There are various ways of doing it from cone accelerometers
to a simple pressure sensing device in sealed sunwoofers.

Yes it is a feedback loop, and its main problem is harsh
clipping rather than gradual overdrive. The loop would
make it flat to DC, so you have to add an active roll-off.

It doesn't suit anything other than sealed subs.
It doesn't really do anything in sealed subs that
can't be done well another way.

Still it can be made to work well with low distortion,
but with a lot of details tagged onto the basics.

It can also work well with wacky bass drivers
with odd drive systems that distort a lot,
like DC motors and drive belts and the like.

rgds, sreten.
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
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Old 25th August 2014, 07:30 PM   #3
forr is offline forr  France
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The velocity signal fedback by the sensing voice coil lowers the Q of the system (amp + driver + box) so its slope looks like as 6 dB/o in the resonance region.
Adding a 6 dB/o bass boost extends the overall response in the bass region.

You refer to the Rythmik Audio direct servo subwoofer.
The amp is certainly conventional. What is interesting is the driver with its velocity sensor. The concept, known since long, is the most simple of all servo drivers technologies.
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