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It was what I wanted to do since I've seen inside of the minis. It took quite a while but recently I had some time to do it at last. The improvement was almost predictable considering the original parts Magnepan used.
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The schematic I used. It's a .7 style series crossover, with a protection cap and resistors in series to the tweeter.
Edits: 07/22/21 07/22/21
You don't need a "protection' cap in series with a foil tweeter.
With a true-ribbon ... perhaps - although I never had any problems, without one (for 10 years or so) on my true-ribbons.
Removing it will improve your sound - just clip a lead across the 85uF cap to find out. :-))
Andy
The tweeter of mini is a very short true ribbon. I think the 85u is blocking DC and the 1 ohm resistor keeps minimum resistance unless it becomes almost short. Well, in my case amps before the panel are all DC coupled (B4 and V5) so I think some safety is not bad here. Or, if one tries tri-amping, a single ended cap-coupled amplifier for the tweeter might make some sense?
Edits: 07/24/21 07/24/21 07/25/21
A capacitor that large has a sufficient time constant that any "protection" is insignificant.
I agree with Andy. I'd just jumper it out.
Dave.
Aah, thanks for that; I assumed it must be foil-on-mylar.
Yes, that 85uF cap will certainly provide some protection for the ribbon. Unfortunately, it will degrade the sound, though. :-((
Andy
Are you going to replace the DWM crossover as well ?
I'm biamping with First Watt B4 active crossover. I believe DWM's XO can be bypassed without opening it. I had uploaded a picture of its backplate, but the picture looks like to be deleted.
What is the difference in RESISTENCE between that foil inductor and the stock (presumably) Iron core?
That Alone can change the frequency balance and your perceptions.
Too much is never enough
Perhaps, I used CFC14 of which DC resistance is supposedly similar to the iron core inductor. I looked for the datasheet of iron core inductors (too low to measure with a simple multimeter) looking similar to the original and chose the foil inductor with same resistance. Well, resistance of the mid panel is +3 ohms and the difference of resistance between inductors would be less than 0.1-2 ohm. My feeling is the tonal balance is not altered so much. I tried two kinds of hook up wires and they affect the sound noticably. So everything matters in XO of course.
Edits: 07/22/21 07/22/21 07/22/21 07/22/21 07/22/21 07/22/21
You can get a very cheap DSR/LCR meter + transistor meter from Ebay for about $15.
You can compensate for the change in resistance by altering the capacitance or inductance. However as you noted the difference in the resistance is small and, since the variation of the reactive components is 5-10%, only the most anal tweaker would bother with it.
I married the perfect woman - the downside is that everything that goes wrong is my fault.
Edits: 07/24/21 07/24/21
Interesting, thanks for the link!
Inductor in the 1.6 is 15ga and is about 0.40 ohms. You should be able to measure THAT, I'm sure.
Most of the aircores are about 1/2 that and since the driver is about 4ohms, it will tip the respons UP in the bass maybe 1 db or so. I'm not going to calculate just yet.
Other problems with OTS (Off The Shelf) inductors are 2 fold. First? Difficulty getting the exact specified value. Typically you will get a 'dewind' to meet that spec. Second? Virtually ALL commercial inductors are of Sub-Optimum shape. Using WIRE as and example, perfect inductors REGARDLESS of core size, have the same number of TURNS per LAYER as LAYERS. That gives a square inductor......Maximum tween-layer interaction.
Using an online calculator, you can run thru a bunch of similattions fairly quickly and decide what is best.
For example? If your inductor is 16ga, you can use 13ga or 14ga and MORE wire and still end up with the same DCR. Play with core size and the other entries until you get a square inductor. Perfect.
I'd ALSO tend to use PVC sizes as the core. Wax it up so you can remove it easily for putting in the WARM oven to cure the epoxy......
Too much is never enough
Sorry, I mistook 0.4 ohm for 0.04 ohm, anyway DCR is below 0.1 ohm so a simple multimeter is not precise enough but error here is expected to be below 0.1 ohm conservatively, less than 0.05 in fact.
Regarding L, or other values, I don't believe even the factory matches the values so precisely, and I measured L of both, quite similar to the original. Well it'll be good to find a vendor who does pair matching, (Cs were matched) anyway so far so good.
Edits: 07/22/21 07/22/21 07/22/21
How close do you wish for?
If the coil winding machine is set for X number of turns, and is loaded with the correct wire AND had the right core size with ends to define width? Nearly a 100% waste of time to 'match'....
In MULTI-INDUCTOR crossovers, it is much more important to orient all inductors at right angles to one another.
Caps which are typically 20%? Probably NOT a bad idea to get 'em matched or perhaps buy a 10% part, if available. Extra $$$, either way.
When noodling thru inductors, I was able to match the stock inductor for DCR (within 5%) and inductance using an AIR CORE on a standard diameter piece of PVC.....AND have the same number of turns per layer AS layers.
Winding such a coil is its OWN joy. You must count turns and layers as well as have the right amount of WIRE spooled off to begine wiht. Maybe 140 feet or some such. NOT a trivial length. Have the coil former 'greased' or 'waxed' so the coil comes OFF and the Epoxy you dabbed on it doesnt create problems.....THan a SLOW bake in about a 100f oven for a couple hours.
LINK is to about 400feet of 14ga 200c wire. Such a coil will NEVER overload or saturate in Magnepan usage.
The question than becomes 'Will the differences between panels swamp the slight value differences in crossover parts?'
I also wonder if measurment accuracy of an inductor is better than the calculated value, in the REAL World?
Too much is never enough
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