Speaker Feedback & Active Amplification

[Bing Fung]

It was said by a coworker today that an active subwoofer is a better design than a passive.

I think that statement may have been taken out of context as most active subs do not employ the type of design principles that he was stating, I think.

It was said that and active sub that employs an amplifier that is dedicated to sense the feedback EMI generated by the voice coil and respond accordingly, is a much better system than a passive driver with a "X" amplifier.

If this is so important, why is it not a design consideration in most speaker designs (like our DIY subs), and how does this affect most of our current systems where we just buy a brand of speakers that we like and mate them to the amplifier of our choice?

Is this why the Meridian Active speaker systems are so costly to design and produce?

I'm told that Makie also uses this on some of their systems.

I understand the theoretical implications, but at what price point or sonic level does this really have an effect on.




Bing

[ThomasW]

If by active sub you mean one with a built-in (integrated) electronics then your co-worker is wrong. The goal of most of that type of construction is to create an "plug & play", no brainer install. Also the mfgr is able to force the consumer to purchase the "pkg" from him

In the case of designs like the Velo subs; the servo-feedback integrated approach makes since, because the signal paths are short and custom amps can be used. But servo feedback designs have their own inherent problems, one being the "limiting" circuit created to protect the driver from being over driven.

The Paradigm or Meridian active speakers are expensive for several reasons 1)limited demand, 2)significantly higher start-up and design costs, 3)and of course snob appeal.

My objection to this design concept is that like anything intergated, if one component in the unit dies then you lose the entire unit to the repair shop. Also one can't simply upgrade just the amp or XO or speaker. The entire unit must go in order to create any upgrade.

Also most of the theoretical ideals fall by the wayside because most mfgrs do make custom amps, but they take advantage of the situation by cutting corners with the circuit/parts/etc, so keep the costs low.

I don't know about the price point issue, because exact comparsions between intergrated designs and discrete components are difficult to make.




theAudioWorx
Klone-Audio

[JonMarsh]

I agree with ThomasW in his points; my only additions regard the concept of servo-feedback-
it can be used to lower THD and extend bandwidth, but only to a point- the bandwidth extensions are similar to what you can get with a Linkwitz transform approach, (which doesn't require servo feedback), and the distortion reduction only applies within the linear travel region. It's like changing from a tube amp to a solid state amp- the servo unit will act like a solid state amp, with lower distortion until it starts to hit the limits, but then *boom*- it's like hitting a brick wall, and distortion goes up hard and fast. With a non-feedback subwoofer, or low feedback tube amp, distortion gradually rises with increasing output level. That's part of how we subconsiously have a feel for whether we're playing a system loud or not, and how close to the limits we're getting.

That whole thing about rising THD and IM correlating with how "loud" we percieve a system to be playing is tricky. The spooking thing Tom and I found about my homemade X1 klones is that because they move so much air and are so clean (typically under 0.1% THD at 100 dB output for 1 speaker), you tend to not have a good feel for how loud you're listening during an extended session- until at the end when you notice you're ears are ringing and to glance in shock at the volume control settings you've been using!

Myself, I understand why designers have tried to optimize the performance of small boxes, and fool us into thinking they're doing the same things acoustically as big boxes- and servo bass is one way to do it. For those who like to mix and match, it's a little limiting. On the otherhand, it's totally pre-configured, and with built-in protection to keep you out of trouble. And hey, if I had cubic bucks, sure, it would be cool to have something like an Aerial SW12 sub, which is one of the nicer complete configuration units I've seen, and *doesn't* have servo feedback either, but has a very nice amp and a plethora of options.
Maybe that's why our latest black ops project is the "Aerial Stryke". Ooops! I'm going to be in big trouble with ThomasW if he finds out I let that slip!

Regards,

Jon




Earth First!
_______________________________
We'll screw up the other planets later....

[Bing Fung]

Thanks guys, I did state that what he was talking about was more like a servo system. If it was not, it likely would not be an important design parameter.

I too feel the separates as you state, are the best alternative.




Bing