Servo Replacement Driver (new guy) - diyAudio
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Old 6th March 2005, 11:00 PM   #1
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Default Servo Replacement Driver (new guy)

Hi all,

I'm new to this lovely forum and I have kind of a tough question.

I have an old M&K Servo subwoofer with a dual voice coil driver that uses one voice coil for the amp and one for the servo. Each driver voice coil measures 4.5 ohms with a meter.

My original driver is bad; the VC is rubbing after foam failure and replacement. M&K does not have any DUAL VOICE COIL replacement drivers.

I really like this sub a lot and want to replace the driver with an APPROPRIATE after market one.

The Dayton 12" DVC looks pretty good, but with only one voice coil being "driven" it will act more like an infinite baffle woofer according to its specs (Ihave no clue as to how the servo affects the T/S parameters). If I parallel the VC's, then I can't use the servo.

The Atlas 12: http://www.ascendantaudio.com/Atlas%2012.htm
looks very interesting with dual coils and the right Re & Qts on the main coil. The second coil (2 ohm) is a smaller voice coil than the main one (4 ohm) and is intended to alter the Q for various applications. Here's my question:

Will the smaller voice coil on the Atlas 12 work in a servo application, or will the voice coil size differences not allow proper function???

Which woofer would you get? Recommendations?

Thanks a bunch from the new guy on the block,

Andy
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Old 7th March 2005, 02:38 AM   #2
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Hopefully I'll get some answers here. I may be wrong, but here are some my own thoughts on the subject.

A direct servo seems to be suitable for controlling driver motion, thereby reducing distortion - thus allowing for a higher Q sealed enclosure that will have more bass (like a Q of about 1), but not have the "flabbiness" (distortion) associated with a high Q alignment: because the driver is being servo controlled, it has similar "overhang" (tightness) as critically damped subwoofer (like a Q of .5-.6).

In other words, you get the deep bass of a high Q sealed alignment without the overhang and flabbiness. The direct servo allows this high Q bass output to be coupled with the control (tightness) of a lower Q alignment, but without the "thinness" in the low bass associated with a lower Q system. So it is deeper and tighter.

So if the above is true, then a Dayton 12" DVC would be a good choice since it will give a Q alignment of about 1 in the M&K's roughly 2.5 cuft enclosure with just the once voice coil hooked up.

I'm a little out of my league with understanding all this servo stuff. Gurus, is this assessment correct, or am I way off base? (No pun intended). Dayton DVC? Atlas 12?

Kind thanks,
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Old 8th March 2005, 04:01 AM   #3
simon5 is offline simon5  Canada
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You could take 0 risks by going for a Adire Audio Shiva, which have both 5.8 ohms coils.
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Old 9th March 2005, 05:06 AM   #4
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20yrs is a pretty long run for a powered sub. And you don't know (do you?) if the M&K amp contains circuitry specifically designed for the performance characteristics of the original M&K driver. As a result there's no guarantee a generic replacement driver will operate properly. So you might want to consider euthanasia for your old warhorse.

This could be your opportunity to upgrade to 21th century technology. Consider gutting the box, then get a replacement amp, driver, and maybe an LT circuit from these folks

http://www.rythmikaudio.com/index.htm
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Old 9th March 2005, 08:58 PM   #5
RHosch is offline RHosch  United States
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That's what I was thinking Thomas.

Just as the BL of the M&K driver was not linear, neither would be the feedback signal generated from the second VC. It is possible (I have no idea how likely) that the M&K amp applied filters to the feedback to "correct" for some of that non-linearity... but that only works when you know the characteristics of the driver used. And that means not only the gross characteristics like Q, Re, L, etc. but also the behavior of non-linearities in the motor. If that is the case (that they have circuitry specifically designed for that driver) then a generic replacement driver could be better or worse than not having the servo control operating at all... and I'm not sure if there is any good way to determine which is more likely without taking the gamble and trying it. Worst case, something goes up in smoke.

The advice of "starting over" is pretty good advice. Certainly safe advice.
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Old 11th March 2005, 03:27 PM   #6
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Okay, I put a Dayton DVC in the box and wired one coil to amp, the other to the servo. No go, baby. The woofer went bananas when the servo circuit tried to do it's compensation thing (the woofer cone started flapping like a mad man). Yes, the phasing was right.

Anyway, I unhooked the servo and paralleled the voice coils. Very tight bass, but not deep enough with it's sealed 2.4L, Q=.5 and F3 of about 50Hz according to my software (no room gain).

So, I hooked up just one coil to the amp (no servo). Much deeper, much better. 2.4L sealed, Q=.92, F3=34Hz (bassbox software, no room gain). Pretty tight, maybe slightly loose for music, but not bad at all. My wife was complaining about pictures rattling all over the house.

By my calculations, I can put a 1.5 ohm resistor across the second voice coil and get a .707 Q alignment for this box with an F3 in the low 30's. From what I've heard out of this sub, so far, this should be a good compromise between extension and tightness. Good idea?

Can that resistor be 1/2 watt since it is only operating in an inductive capacity?

Thanks,

Andy
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Old 11th March 2005, 09:28 PM   #7
simon5 is offline simon5  Canada
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Adire Audio recommends 5W in one datasheet, but you can probably use less, I don't know.
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