[Daygloworange]

Reminds me of the active suspensions and traction control   employed on Formula 1 cars in the early 90's before it got banned from the sport. 

Cheers

[rythmik]

Brian,

Thanks a lot for this thorough discussion. Very interesting and I am certain that the Servo system is excellent and contributing to 'true' reproduction.
I investigated just those possibilities that you put forward, changing the Qes-value to alter Qts and Bl values and there will be a great potential to fine tune
performance. Incidentally your second picture of cone displacement is an exact illustration to what happens in an OB if speaker parameters are right ie a 'good' combination of Qts and Bl values. It will be very interesting to follow the developments of this design.

/Erling

BTW, using high Qts drivers to implement OB speakers is not new. If you asked Bob Carver, he would tell you he has done that 20 years ago. I will give him credit for that.  What he has done was he used driver with very small magnet to achieve high Qts (I believe it is greater than 1.5)  in order to compensate the cancellation loss from open baffle. The name of the speakers were "Amazing", or something like that. But the intrinsic ringing from the Qts is not gone simply because the cancellation loss. All metal cone midrange driver has this nasty ringing. Some junior engineers would suggest adding a notch filter. But does that completely resolve the problem? You bet not. I am not saying servo would address this mid range metal cone issue. I am merely illustrate EQ/filtering does not eliminate high Q from the system. When the distortion hit that ringing frequency, you will hear it as emphasized and takes longer time to go away. Our contribution is adding the multi-dimension to this so called Q-value (which in many people's mind is still a single value)  and demonstrate that there are several Q values.   Our contribution is to change the intrinsic Qts value of the system such that it has no spot to ring and any distortion if there is, we will make them go away several times faster than a nonservo system so that you don't even realize it is there . That is why people's impression of these servo subs has been very consistent.  They are just so unlike other subs.  

I have one customer keep asking for the distortion numbers. I don't understand. Every piano, even from the same manufacturer has different harmonic contents. Doesn't harmonic distortion from the system just like that? Changing the piano from one to another, but still as real as a piano.  We have been barking on the wrong tree for so long it is going to take a long time to undo that.  Our problem is not that it does not sound like a particular piano.  Our problem is that we can still tell the sound from the sysem is not like ANY real piano.  All these boomy sound is not a solely reflection of harmonic distortions, instead, it is the harmonic distortion lingering longer than what they really should be so that it creates a very unnatural sound and we perceived as boomy or slow sound.  Don't get me wrong, our subs are low distortion. Their mechanical distortion is 3x smaller than a nonservo sub. But there are other technologies that also claim low distortion. So what do we offer that they don't.  A bigger picture will suffice.

[rythmik]

Reminds me of the active suspensions and traction control   employed on Formula 1 cars in the early 90's before it got banned from the sport. 

Cheers

That is a better way to explain the whole thing. Thanks. Any sensor feedback system would create results like that. We all call it active control. I am not sure how faster the system on Formula 1 respond to sensor reading as some of them may not require faster response time.  In our servo it's instantaneous as it needs to react very fast.   

[rythmik]

Thanks, Brian

Your answer makes sense, no money for nothing. But I agree with your earlier reasoning about the OB-response and the Qts and Bl values.
My own experiments suggest that for OB use a Qts-value ranging 0.8 - 1.2 would be alright if at the same time the Bl-value is below 10. This would ensure
a relatively flat bassresponse even from pratical OB-sizes and thus require more modest EQ.

Eminence Alpha 15 as proposed by Martin King for OB use has almost perfect parameters for this application. The SW-12-16FR will have ideal IB parameters.

/Erling

I keep pondering why you mentioned BL value below 10. But it just occurs to me you are referring to the stall effect. BL does not need to be smaller than 10. It is the Q value that is important. This is because BL can scale with Re. As a matter fact, for the same driver, (BL)^2/Re is almost a constant. So for a BL value of 10 at 4ohm Rdc is same as BL=14 at 8ohm Rdc.  Danny's drivers are at 14ohms with a BL value of 14. So its BL value is definitely lower than 10 when you scale the resistance down to 4ohms.  So why 14-ohms, not standard 4 ohms. It is more convenient just to parallel more drivers as you want. 4ohms driver or 8 ohms driver are very difficult to configure between 1-4 units.

[chrismercurio]

A few questions....

Has top end response been tested on the subs? I'm asking how far beyond 160hz the woofers can be used? 

Now that an OB woofer has been developed, what are the parameters for Danny's OB line source? 

Thanks,

Chris

[scorpion]

Brian,

Take a look at my simulation with MJK's spreadsheet in this thread, which will illustrate my argument: http://www.audiocircle.com/index.php?topic=51217.40   

/Erling

[justinc2]

Im very interested in these OB subs.

Let me know if this is correct.

3 16ohm subs can be hooked up to 1 of the gr-plate amps in parallel on an open baffle and will be flat to 14hz with the servo?
What is the max spl this setup will provide at 14hz?
What size baffle is recommended?

[rythmik]

Brian,

Take a look at my simulation with MJK's spreadsheet in this thread, which will illustrate my argument: http://www.audiocircle.com/index.php?topic=51217.40   

/Erling

Erling,

If I am not mistaken, your approach is very similar to Carver's approach. Did a google search on "carver amazing" and found http://www.stereophile.com/floorloudspeakers/290carver/. It is an old article decribing the rationale used by Carver.

[EDIT] If you understand what I have stated earlier about the divergence of Q values, whatever you come up as the greatest and flatest response, I can implement that in the FR, independent of my physical parameters. However, the physical parameters should not have Q value higher than 1.0. This is because this

Now that hot spot is at the passband and one will experience bottoming far more often than a 10hz bottoming or 2hz bottoming unless he reduces the power which affects the output below and above your fs frequency.  That is what Carver has not told us.  You cannot see that in the frequency response. Come think of it, the divergence of Q value along 3 different axises (physical, instrinsic, frequency response) enable us to do a lot more. We have a switch let us switch between 14/20/28hz roll-off, we have another switch provides damping with Q value of 0.5/0.7/1.1.  Your recommendation of 1.1 is in there, anyone can try that.  However, my goal is to do something differently.  As with a lot of technologies in the past, what Carver used was handicapped by the following two objectives: 1) no EQ is required and 2) efficiency needs to match the rest of the speakers. Another example is Isobarik. I kept asking people to give me an efficiency comparison between a sealed enclosure of two drivers and an Isobarik with the same amp output and enclosure size. If one can show me mathematically or by simulation that the latter produces more output with the same wattage and enclosure size,  I will change my dual driver kit to target only for Isobarik.

How loud this system can play down at 14hz? That is related to physical parameters and that we have done the best. Personally what I think is important is how loud we can play at 20hz.  If you can use a wider baffle, it helps a lot.  Let us say at 20hz, each woofer can put out 15m one way excursion (peak is definitely higher than this), this is about 102db for a 12" woofer without considering cancellation loss. If you have 3 of them, that is 10db increase. But now you have to subtract loss. If we assume the baffle is large enough, your 3db shelving down due to cancellation starts at 60hz. When it is down to 20hz, it is roughly 3x loss which is 10db. So you get back to 102db, same as sealed enclosure with one driver.

If you know how to solder, you can even experiment after you bought the kit because the shelving function involves only one cap and one resistor.  Based on the math, one can see the loss at 14hz is about another 3db as compared to sealed.  I have a lot of customers use a single 12" in sealed box and are happy with the output.  If one is a purist with deep pocket, you can drive 4 units with a single 370WRMS amp. It can extract every juice out of the driver.  We have plan these ideas from the very beginning, considering how customers can upgrade, what wattage we target at,...ect. When we consider bottoming, we consider the situation even when users accidentally turn up the volume or the movie contains unexpectedly loud scense so that the amp is already into clipping.  Our goal is not hear the driver bottom out.  As a matter of fact, the foundation of this OB is actually based on our sealed box results. I was hoping I can increase the fs of the driver higher than 24hz by reducing even cone weight. I have done my best. Anyway, in our sealed servo sub, the driver after loading into the enclosure has a fs of 40hz and Q value of 0.9-1.0. It plays down to 14hz at moderate volume. If one really wants to play loud, set the roll-off to 28hz and damping to low. If that is still not enough, we have larger driver 

[klh]

Can your servo technology be applied to other subs? I have two high excursion 18" drivers in an IB and would like to protect them from bottoming out. I love the drivers and don't want to swap them out, I just want to protect them. Put differently, for my situation, can your product help me more than a high pass filter?

[rythmik]

Can your servo technology be applied to other subs? I have two high excursion 18" drivers in an IB and would like to protect them from bottoming out. I love the drivers and don't want to swap them out, I just want to protect them. Put differently, for my situation, can your product help me more than a high pass filter?

Unfortunately it is not a band aid solution. Servo can only work with drivers designed for servo from the very beginning.

More highpass filter is not gonna help you. The Samson amplifier has filter built-in already. I am pretty sure you have tried that.  Adding more filtering cannot stop bottoming because filtering only stop th input signal, but amp clipping can also generate low frequency signal that filter does not have any effect at all. A lot of bottoming is acutally induced by amp clipping.

Two more effective approaches are 1) buy more units (but they are no longer available), and 2) add serial resistors to reduce the output at low end.   So for your situation, the best bet is add a 4ohm 50W resistor put in series with the driver. You need one resistor for each driver. The fundamental problem is the driver is low Q and you thought you can get a lot of excursion based on the capability of the drivers, in reality, it is only giving you 1/3 of what it is capable of.  If you get more than 1/3, you run in the risk of bottoming out the driver.   

[klh]

So a 4ohm 50W resister will shelf the response at 20Hz, right? Although the output of my current setup is a little less than I desire, it's not bad. Could I put in a resister that would shelf the response at 15Hz? This way I don't give up too much of the ultra low frequencies I aimed for when I set up the IB while at the same time protecting the drivers from that same low end response. This would seem like a nice compromise that would give me more mid to upper bass without giving up too much lower bass. Sure, I won't get as much mid to upper bass as I could with a shelf at 20Hz, but it would be better than my current situation. Does this seem reasonable? How could I go about getting a 15Hz shelf?

[scorpion]

Brian,

I suppose the Servo will counter excessive cone-movement and ringing or other anomalies in the cone movement, is there a mean to
estimate the amount of feedback generated with different Q-values ? That would be interesting. It would be interesting to test with your other bass-units
in OB also, in all cases with and without dipole loss correction. That could perhaps in some way answer the Q(ts)-question.

/Erling

[klh]

So a 4ohm 50W resister will shelf the response at 20Hz, right?

Does it primarily shelf the output or does it primarily limit excursion (which ultimately limits output as the frequency decreases)? Or, am I missing it completely?

[rythmik]

So a 4ohm 50W resister will shelf the response at 20Hz, right?

Does it primarily shelf the output or does it primarily limit excursion (which ultimately limits output as the frequency decreases)? Or, am I missing it completely?

It is more than a shelving. Adding a resistor reduces the output below and above fs frequency. The only thing that is not changed is the output at Fs.  There is no better solution.  The driver is just not designed for OB or IB, period.  Adding a resistor is not the best solution either. It is like widen the magnet gap in the motor.  This reminds me a story. A manufacturer used a very large motor to design a driver. After it is done, it was found the Qts was too low, which is not suitable for sealed box. So guess what the manufacturer did to resolve this problem? Widen the gap so that it weakens the motor in order to increase Qts. Poor consumers!!! The consumer could have paid less if the manufacturer used a smaller magnet.  Anyway, since you already pick up the drivers, all you can do is a fix. Adding a resistor limit the excursion below 20hz. The downside is the reduction of output.  Even though it has similar effect as filtering, the main difference is the resistor is after the power amplifier, so it also protects you from the amplifier misbehaving. 

[rythmik]

I have shipped a few amps starting last week. There is an IQ test on figuring out the wiring    Here is the cheat sheet. There are two pairs of wires. One from the servo board and one from the power amp board. The pair with a red insulated faston terminal on the wires should come from the power amp board, and that goes to the driver coil. The other pair goes to the sensing coil.  In short, the pair of wires with red insulated faston terminal goes the driver coil.  I would recommend to install this pair of wires first.  Then install the sensing coil wires next.

[sts9fan]

I have never used or built a sub so be gentle. 
I am pretty sure I want to add a sub or two to my system.  If I want stereo subs I will need four channels correct?  One for the main coil and one for the sensing?  If I wanted to house the amps separate from the sub how should I wire it?  I would think two sets of binding posts on the amp and two on the sub which I could connect with a run of quadstar or something. 
Is there a way to bypass the crossover on these amps?  If I want to feed it a signal from a Behriger DCX or other such device is that possible or should I not worry?  If I wanted to run two 8ohm subs off the  same amp should I wire the two main coils in parallel and the two sensing coils in parallel?
Finally why does everyone seem to prefer a 4ohm single sub?  Just for the added juice from the amp?

Thanks

Kris

[scorpion]

Hi  Brian,

Wouldn't try the challenge ?

/Erling

[rythmik]

A few questions....

Has top end response been tested on the subs? I'm asking how far beyond 160hz the woofers can be used? 

Now that an OB woofer has been developed, what are the parameters for Danny's OB line source? 

Thanks,

Chris

The design of servo network for 16ohm drivers has been done. The -6db point is at 150hz and that is really the highest xover frequency we will recommend.

All other models are recommend to xover at 100hz or below.

[rythmik]

Hi  Brian,

Wouldn't try the challenge ?

/Erling

Erling,

There is no so-called challenge. It is a done work.  Please do not consider this as some experimental work posted on diyaudio (unless I misunderstand what you mean by challenge). I have been selling these kits for 4 years (the patent has been granted for 10 years) and designed servo networks for no less than 30 configurations (with four different driver manufacturers), including some drivers that do not work so well for servo.  And I learnt my lesson and will not spend time on drivers that I don't think are gonna work, those drivers include those with high inductance, high moving mass, and/or low Vas.

[scorpion]

Brian,

This was a serious proposal. But I can also see your point and I have certainly no intention to take away anything from the good efforts of your work. But with the ongoing debate about high or low Qts-value elements as mostly suited to dipole work in the bass area I saw your units (or perhaps those of GR-Research) as perfect samples for conducting research into this question. There is of course a bit more to it with EQ and overall response.

However, which unit would perform with the least Servo feedback, would have been an interesting question to have an answer to.
This would not be the same as saying that this unit would not perform even better with the Servo feedback in place.

/Erling