Originally Posted by Tong Chia
I have the HGS18MKII and the DD12
I can further explain how our servo subs are different from Velodyne. This is not to talk down the value of your Velodyne subs. I would like to answer a couple of questions other have regarding the comparison fo the two.
1) All accelerometer-based subs are critically stable. They don't perform well when they are driven into clipping. Hook them up without enclosure will further demonstrate that. So the limiter in those subs are the life-saver for them. They needs to be zero tolerant to clipping. So a normal limiter design that is based on the output waveform cannot work at all as this design would potentially allow one clipping to escape and cause the accelerometer servo system to lose control. So it needs to be designed based the input signal strength (instead of the amplifier output waveform) and the big variable in his type of preamp stage limiter (it is actually is an Automatic gain controller, or AGC) is how to predict the ouptut waveform will start clipping based on input signal.
2) The problem using AGC as limiter to prevent clipping is that the servo is also an adaptive EQ. The input/output relation at the power amplifier stage depends on the voice coil temperature. To ensure proper operation under all temperature condition, it is designed for the worse case -- highest temperature. That means when the voice coil is cold, the limiter will cut in prematurely. That would require power amplifier with huge reserve and at low temperature, it is not fully utilized at all.
3) To appeal to customers, Velodyne also use smaller enclosures. Since the wall thickness can take out the effective enclosure volume dramatically in small enclosures, the internal enclosure volume scale down way faster than what the external dimension suggests. For instance, by reducing the external dimension volume by 10%, it may take away 25% of the effective internal volume if one takes account the driver magnet, ... 25% smaller volume means 50% more power is needed. If one adds all these up and you can see why it needs so much more power. I have an ULD15-II. That is an enclosure size I would build and it uses only 400WRMS power (if I still remember correctly). All this changes when Bob Carver introduced Sunfire subwoofers which demonstrates that consumers can be easily led to think higher power means higher output regardless enclosure size. And that had prompted Velodyne to get into this wattage competition. Bob is a brilliant engineer, but his venture of Sunfire subwoofers is not a good one.
To conclude the comparison, here is the measurement for DD12http://www.avtalk.co.uk/showthread.php?t=15242
92db output at 20hz with 1250WRMS. 96db at 30hz.
Here is our 12" driver at ******* (do a google and you will find):
96db output at 20hz with 370WRMS and 105db at 30hz.
Even though DirectServo has higher output, the distortion at 96db is pretty high because the amplifier is already at clipping. However, a fair distortion comparison is the 95db output line of our sub vs DD-12 95db output line. It is pretty comparable. Nevertheless, the key is that our servo design does not need limiter to keep it alive. They are allowed to get into clipping just like all other nonservo subs. That has increased the useful output with HT usage. In other words, we get to enjoy the benefits of servo without any compromise. So what is the benefit of servo? It is the fast response from a low Q/high damped brought in by servo feedback. It is actually 3x lower nonservo subs. In additon, a stable frequency response because of the adaptive EQ inherited in servo operation.
If we compare 96db at 30hz of DD12 and 105db at 30hz of our servo, that is 9db difference which is 9x difference in power. Let us just assume Velodyne prematurely turns on AGC to prevent clipping, that is still a huge under-utilization. AVtalk also has DD15 and the story is the same. I know there is a lot of questions about why the power requirement of our servo subs is so different from others. I hope I have answered to the point.
BTW, directservo technology has been around for 6 years now. You can do a google search for feedback all the way dated back to 2004 or 2003. In the first few years, we only addressed the custom installation market. After the technology is well proven in the field, we begin to offer completed subs mid last year.