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Rebuilding the Enigma...
Rebuilding the Enigma...
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Old 23rd April 2019, 02:12 PM   #1
Brian Steele is offline Brian Steele  Grenada
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Default Rebuilding the Enigma...

So, I loaned out my Enigma subwoofer to my brother a few weeks ago for use in the background music system for his bar, and staff managed to destroy the driver (not too surprising).

I cut out the moving parts for the driver, and it looks like the fiberglass former fractured at one point. I suspect that's from the INF10 driver bottoming out - a particular flaw with that driver, so that's not too surprising. The bottom 2/3rds of the coil also seems to be blackened, and I'm a bit curious as to why the top 1/3rds is not. The driver has a vented pole piece, and the pole piece is t-shaped at the gap, but the driver is ordinary otherwise, e.g. no under-spider venting. It's rated at 100W, and was connected to an amp that also does 100W and, while the coil is a bit blackened, I think the failure was more likely due to mechanical damage from bottoming out too hard.

Rag Bose all you want - they seem to at least get speaker protection right. So I'm thinking about how I can add Bose-like speaker protection to this subwoofer, to make sure that power delivered to it does not exceed say 70W for a continuous period. PE doesn't seem to have circuit-board mountable lamps, and there's not enough information available on their polyswitches (e.g. insertion resistance) to really make an informed choice about them.

Any suggestions? I plan to raise the order for the new driver later today, once I've identified one that sims well in the current box (another big challenge).
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Old 23rd April 2019, 05:01 PM   #2
TBTL is offline TBTL  Germany
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Too bad the woofer has been damaged



You could add a limiter to the amplifier. If you think the limiter will be defeated, mount it inside of the amplifier chassis.
Audio Amp Power Limiter
Mechanical damage can be prevented by using a (at least 2nd order) high pass filter. PA woofers also are more robust.

Is the amplifier still working? The fact that only half of the coil is blackened could mean that it was subject to dc, which could have been caused by a blown amplifier.

Last edited by TBTL; 23rd April 2019 at 05:18 PM.
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Old 23rd April 2019, 06:06 PM   #3
Brian Steele is offline Brian Steele  Grenada
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Quote:
Originally Posted by TBTL View Post
Too bad the woofer has been damaged
Too bad in truth. Apart from that particular flaw that allowed the former to slam into the back plate even though the it had an extended pole piece, that driver had some interesting parameters, and the bandpass speaker was designed with those parameters in mind. The only replacements I've found so far are some Dayton drivers with Xmax around 5~6mm (the original driver had an 8mm Xmax).


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Originally Posted by TBTL View Post
You could add a limiter to the amplifier.
Not possible in this case. The amplifier in question is an old Technics SA-EX700 receiver, which has a built-in powered subwoofer channel. If any limiting is to be done, it has to be done at speaker level.


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Originally Posted by TBTL View Post
Is the amplifier still working? The fact that only half of the coil is blackened could mean that it was subject to dc, which could have been caused by a blown amplifier.

Yup, it's still working, though to be honest I haven't checked it for DC offset. Being a receiver, it probably does a self-check for this before engaging the internal circuit breakers for the speaker outputs anyway.
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Old 23rd April 2019, 06:10 PM   #4
TBTL is offline TBTL  Germany
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I'm surprised the receiver is still working. Hi-fi equipment usually does not have sufficient cooling for PA applications.
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Old 23rd April 2019, 06:33 PM   #5
Brian Steele is offline Brian Steele  Grenada
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Originally Posted by TBTL View Post
I'm surprised the receiver is still working. Hi-fi equipment usually does not have sufficient cooling for PA applications.
This thing is a bit of a beast. 100Wpc into 5 channels continuous (including a subwoofer channel) and has a built-in fan, so cooling isn't an issue. Class H+ output circuitry, and it still runs pretty warm.

It was my first HT receiver, so it's likely going on almost 15~20 years old.
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Old 24th April 2019, 06:58 AM   #6
Brian Steele is offline Brian Steele  Grenada
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Hmm, this looks a bit interesting. Converting the Enigma 4th order BP to a 6th order series-tuned BP, using the Eminence Kappa Pro-10LF. It looks too good to be true, in fact. Not only is the resulting bandpass response wider, it's also at a higher SPL level for the same input voltage.

The Kappa Pro-10LF is a little more expensive than what was prepared to pay, and I had to derive the semi-inductance parameters using a trace of its impedance curve as the ZMA data was not available, but the predicted response of 38 Hz to over 160 Hz looks very promising.

At least that's what Hornresp is predicting. Can anyone cross-check this for me using another software program that can sim series-tuned 6th order BP systems?

Driver parameters are as follows:
Eminence Kappa Pro-10LF
Sd=376.90
Bl=14.60
Cms=3.61E-04
Rms=1.26
Mmd=41.92
Le=0.97
Re=5.47
Pmax=600
Xmax=7.2
Re'=5.17
Leb=0.70
Le=5.67
Ke=0.13
Rss=50.08

6th order (series-tuned) parameters as follows:

V1= 21.9 litres
Vent Area = 91.2 cm^2 (two 3" vents)
Vent Length = 10 cm
V2 = 21.8 litres
Vent Area = 40.6 cm^2 (two 2" vents)
Vent Length = 16 cm long

The image is a comparison of the sim'd response of "Enigma2" against the sim'd response of the original Enigma subwoofer. The second is an export of the Hornresp file, just in case anyone's interested in having a look at the sim.

Of course there are some compromises to the design - particle velocity through the vents, for example. And GD of 22ms just under 40 Hz. But still, it does look very promising...
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File Type: png 20190424-enigma2.png (57.4 KB, 565 views)
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File Type: txt enigma2.txt (1.0 KB, 4 views)
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Old 24th April 2019, 07:22 AM   #7
Brian Steele is offline Brian Steele  Grenada
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Something's definitely a bit weird. Here's the same box, configured as a parallel 6th order BP with the same resonance points. The response is very different. David, any ideas why the big difference?
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Old 24th April 2019, 07:31 AM   #8
TBTL is offline TBTL  Germany
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Is it possible to pick larger ports? At 100 watts, the internal port reaches 40 m/s at 40 Hz. At these velocities the ports stop working.
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Old 24th April 2019, 02:18 PM   #9
Brian Steele is offline Brian Steele  Grenada
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Originally Posted by TBTL View Post
Is it possible to pick larger ports? At 100 watts, the internal port reaches 40 m/s at 40 Hz. At these velocities the ports stop working.
Unfortunately larger vents just wont fit in the box. The 2" vents will flare to around 3.5" at the ends (if I do this, I'll be using Precision Ports), and of course they'll be terminated in the box, so some of the resulting distortion will be filtered by the bandpass filtering of the second chamber and vents, but still, yes, particle velocity is one of the concerns I have about this idea.

The bigger concern I have is if the sim'd FR is correct, because it does look a bit optimistic.
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Old 25th April 2019, 07:36 AM   #10
David McBean is offline David McBean  Australia
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Quote:
Originally Posted by Brian Steele View Post
David, any ideas why the big difference?
Hi Brian,

I was hoping that you could tell me :-). The thread linked below discusses the issue:

Parallel vs Series Tuned 6th order bandpass boxes

As far as I can establish, all of the Hornresp band pass models are working as they should.

Kind regards,

David
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