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Acoustannoy 22nd October 2011 06:36 PM

Acrive - Passive Filters for Magnepan MG111a
 
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Many threads and posts have been written about Magnpan active filtering, but I did not know any about active-passive filtering.

I equipped my MGIIIa with a 2 way active passive filter, and the obtained results are the best I had since years, far (far-far...) away from the original filtering, and far better from a basic active filtering based on a modified behringer DCX 24/96.


- Measures shows that the response curve stays flat, phase response is improved in the bass area (no more self), and the impulse response is improved by at least 15%.

+++ Listening is very dynamic and neutral, and the already very good tri-dimensionnal performance and the Maggies is improved !!

- The simulation of the filter was performed with a freeware software. After tests, I choose a 12db/oct in place of the original 12db/oct - 18db/oct. But this is my personnal taste.
- Only good resistors (vishays) and polystyrene caps were used. This is a main issue : with standard components, the result was not so dynamic and neutral.

More generally, I'm surprised that this solution is NOT more often considered, when a high musicality and dynamic are researcherd.
These kind of filter are cheap (about 20Euro), easy to build, performant.
The only obstacle is that you absolutly need a simulation software. Because of the correlation between the differents steps of the filter, the classic RC calculation do not work properly.

Why are they forgotten in the Hifi world... ?
Mathieu

MGIIIa + AR SP16 + Cary valve amp (medium-treeble) + Carver PM 1.5 (bas)

tyu 22nd October 2011 07:08 PM

Thanks vary much for your post .Magnepan made a box for what your doing for the MG3a an it gets bad press now thay have moved away from even biwiring...Thay had a pot so you could set the top to the bottem i think,the rebbons are so good that ever a so so active crossover can make the sound good.

andyr 27th October 2011 10:21 PM

Hi Mathieu,

I don't quite understand what you mean by "2-way active passive"?

The picture on the R (of the XO box) suggests that it is a Passive Line Level XO (PLLXO) - is this correct?

If so then, yes, I guess you could call this "active" in the sense that the amplifiers are directly connected to the speaker ... so it is a passive implementation of an active approach! :D

I have done the same thing with an active XO - except that I went 3-way active. I suggest you would find it difficult to achieve a good result if you tried to install a PLLXO for the (internal) 12dB mid/ribbon XO as well.

However, you could change slopes to all-6dB (given that the 3.7 has this) - this is my next experiment with my Maggie 3-ways (currently, I have stock slopes - 18dB bass LP/12dB mid HP/12dB mid LP/12dB ribbon HP).


Regards,

Andy

Acoustannoy 31st October 2011 03:43 PM

Dear all,

Indeed the new filter is a PLLXO, but only 2 ways, 'cause I see no advantage in having the ribbon tweeter connected directly to a power amp...
I tried also a 18db/oct PLLXO (as suggested in the original Magnepan filter) but there was not any advantage. This must be connected to the fact that, for the same slopes, Passive XO and PLLXO do not have the same phase shift.

Curves show improvement both ion the dynamic range and pahse shift.
The listening feeling confirm both.

I did also made a first attemp of PLLXO years ago, without any good results.
My mistake was to misconsider the importance of the quality of the components.
=> PLLXO must be 'better' than the amplifier and preamplifier which are connected before... Otherwise the passive Magnepan filter gives more transparent results, especialy in the medium range.

I alspo tried active XO -a modified Behringer 3400 and later a DCX2496-, but the sonic result was'nt at my taste.

Now what next ?
- I changed the medium stock selfinductance against a 'flat wire' one, and gained even more dynamic.
- A sub is under construction, using a DCX 2496 + Carver power amp and a 18' JBL louspeaker in 120L B.Reflex. Then I will add a high pass 12db/oct cut in the PLLXO at about 45Hz. I already experienced this on my Acoustat with good results.


Have a nice day everybody !

andyr 31st October 2011 11:54 PM

Quote:

Originally Posted by Acoustannoy (Post 2765027)
Dear all,

Indeed the new filter is a PLLXO, but only 2 ways, 'cause I see no advantage in having the ribbon tweeter connected directly to a power amp...

You need to do the experiment (ie. connect the ribbon directly to an amp) before you will understand the advantage. :)

However, I suspect you would lose too much signal on the mid panel by having a HP and LP 12dB passive line-level filter. IE. active is the only way to go, for 3-way.

However, if you switched to 6dB slopes (like the 3.7 has) you might get a successful result.

Quote:

Originally Posted by Acoustannoy (Post 2765027)
Dear all,
I alspo tried active XO -a modified Behringer 3400 and later a DCX2496-, but the sonic result wasn't at my taste.

I'm not surprised - these are pro XOs. You can build a very good-sounding analogue active XO using Rod Elliott's P09/P81 PCBs.


Regards,

Andy

albertNL 1st November 2011 12:31 AM

Just found this thread. Very interesting!

I own a pair of MGIIIa's and have heard how great they sound with a really good active XO (I have heard them driven through a vintage Levinson LNC-2 with MGIIIa specific frequencies).
I have tried a cheap (Behringer) active XO but the magic that is my Aleph 3 is lost as soon as the Behringer is put between the pre and power amps.
Have heard from the owner of the LNC-2 that the Bryston and other units he tried do not come near the old Levinson.

What are your suggestions for a top-quality active XO? Pref. DIY!

andyr 1st November 2011 12:36 AM

Quote:

Originally Posted by albertNL (Post 2765517)
What are your suggestions for a top-quality active XO? Pref. DIY!

Use Rod Elliott's P09/P81 PCB, with P05 PS - see here:
ESP Projects Pages - DIY Audio and Electronics

Nominally, these are either 12dB or 24dB filters but you can change the bass LP filter to 18dB if you want, quite simply.

Regards,

Andy

albertNL 1st November 2011 12:43 AM

Quote:

Originally Posted by andyr (Post 2765523)
Use Rod Elliott's P09/P81 PCB, with P05 PS - see here:
ESP Projects Pages - DIY Audio and Electronics

Nominally, these are either 12dB or 24dB filters but you can change the bass LP filter to 18dB if you want, quite simply.

Regards,

Andy

I had a look, but these are opamp based. Generally, I am not such an opamp fan and would prefer discrete components. Any non-opamp suggestions?

andyr 1st November 2011 01:06 AM

Quote:

Originally Posted by albertNL (Post 2765529)

I had a look, but these are opamp based. Generally, I am not such an opamp fan and would prefer discrete components. Any non-opamp suggestions?

I agree with you, Albert, in the sense that discrete "sounds like" it would be better sounding ... but I have been looking for a discrete DIY XO for over 10 years and have not found one! :(

Marchand makes analogue XOs - see here:
electronic crossover, active crossover, discrete opamp, custom amplifier, passive preamp

... but I believe these are also opamp-based - except for their XM126 which is tube-based (but quite pricey! :( ).

Regards,

Andy

Acoustannoy 1st November 2011 09:22 AM

PLLXO ?
 
Dear all,

You are not convinced of the advantages of the PLLXO...

On my side, I would not have written the post if the transformation of the MGIII has not been so dramatic.

For about 40$, you can get the resistors (takman or similar), caps (polystyrene) and the casing and RCA connectors.

- Soft Micro cap 10 is free, the biggest work was to optimize the simulation. it took me some weeks but i do not regret it.
In the simulaiton I considered the amps internal resistance to be 30 to 50kohm. This is why I added the 30kohm before the amp, in order to smooth the amp resistance.

- After testing the 0.25 uF cap was replaced with a 0.15uF. Medium is then low-cut at about 450Hz.

Mathieu


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