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Magnepan 2.5R crossover help

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Posted on April 23, 2015 at 06:53:14
kappa546
Audiophile

Posts: 28
Joined: October 6, 2004



I just acquired a pair of 2.5R in really great condition, ribbons works although they look a little saggy in spots. One speaker does have a very small buzz only noticeable when I run tone sweeps.

When I first received them the first thing I noticed was the obvious increase in bass in my room coming from MMGs. But also there seemed to be a lot of information missing in the midrange (crossover region) and some basic measuring confirmed this. After much researching I found a post where someone mentioned switching polarity of the speakers because Magnepan had wired them wrong. I tried this and to my surprise there was an improvement! I did another rudimentary measurement using Foobar2000s Mathaudio Room EQ and there is still quite the null in the ~700-5k region (5-10db drop). I normally EQ my response with a similar cut from 1khz-4khz but not quite so pronounced.

Which brings me to inquire what kind of tweaks to the passive crossover some of you have made. I've seen that some 2.5 seem to have shipped with 25uF caps in the tweeter circuit, mine are the 15uF variety that match the schematics online. I was thinking of trying some Dayton 25uF tweeter caps I already have on hand and maybe move the 15uF cap over to the panel shunt position. Anyone have any experience tweaking these networks or have one they've settled on? Can someone explain to me what the 2uF behind the connection plate is for?

I also need some help figuring out how to bypass the fuse and attenuator. I think I successfully bypassed the fuse on one panel but I want confirmation I did it right. What I did was move the brown wire on the fuse holder to the top where the resistors are. Thanks!

 

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RE: Magnepan 2.5R crossover help, posted on April 23, 2015 at 09:24:19
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 The 25uF sounds like the way to fill in the 1-5khz range with the tweeter

The fuse bypass - to clarify - what you did was desolder the brown wire from the one terminal at the end of the fuse holder and soldered it to the other end where the resistors connect? If so then you probably did it right post a photo and we can tell better..

 

RE: Magnepan 2.5R crossover help, posted on April 23, 2015 at 10:36:31
neolith
Audiophile

Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 Changing the tweeter cap to 25 uf will lower the HP -3db point but when I ran it on my spreadsheet, there was not noticeable change in the output curve. You can see from the Bode diagram that there is a significant drop in the output of the MG2.5 above 200 Hz but then it is somewhat flat. This is the electrical curve only and does not necessarily reflect the speaker output.
As far as polarity, the tweeter is inverted relative to the woofer. If it is wired in the same polarity there is a huge drop at ~1000 hz (blue line).
If one speaker is wired backward compared to the other than as you note you will have a diffuse image but there will not be a dropout.
The 2 uf cap is in parallel with a 2 ohm resistor both of which are in series with the tweeter to raise its impedance which a ridiculously low 1.45 ohms. At the crossover frequency (1000 hz) and lower the added impedance is about 2 ohms (same as the resistor alone) and as the frequency rises it drops reaching 1 ohm at 40khz.





"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: Magnepan 2.5R crossover help, posted on April 23, 2015 at 11:17:07
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 The tweeter is more sensitive than the midbass panel - we guestimated a 5-6 db difference - so the Bode diagram understates the tweeter's contribution since despite the 2 ohm resistor its output should be at about the same level as the midbass panel.
So the lower XO from the higher val cap should be more pronounced than the Bode diagram shows.

 

RE: Magnepan 2.5R crossover help, posted on April 23, 2015 at 14:37:09
kappa546
Audiophile

Posts: 28
Joined: October 6, 2004
 Thanks for the input guys. I've already looked at the electrical response diagrams for the Magnepans and that only tells you part of the story. The bass panels have a severely rising response (or dipole dropoff, take your pic), hence the chosen crossover point by the factory. In my living room I actually have flat-ish output from ~40-50hz up to about 200 (the peaks and valleys in this range are definitely room and placement related), then there is a lot more energy from there peaking at around 500hz. Then it drops from that peak about 10db at 1.2khz and the ribbons start kicking in strong at 4khz. This is just a quick measurement with Mathaudio, haven't done a more comprehensive measurement with REW yet.

I know the resistors are there to bring the ribbon impedance up from 1.45 ohms, but I still don't get the point of the 2uF cap.

When I talked about speaker polarity i meant switching the whole speaker inputs (- at +, + at -), not individual drivers. I know this model has the mylar back so that's probably why it's flipped. The suggestion I read insinuated Magnepan had many 2.5R that were shipped with something wired wrong or labeled incorrectly which caused severe cancellation at the crossover. I can also confirm this with previous measurements (forgot to screen shot it) because the null centered at 1.2khz was like 20db down! This caused a very hollow, far away from the stage type of presentation. Once i flipped polarity at the speakers plugs the music sounded much less hollow. I don't know how the crossover managed this, it's the kind of thing I'd expect if I was flipping the polarity of one driver on one speaker, but not when flipping the whole speaker. I guess I could switch one drivers polarity to see the effects, but I haven't taken the socks off yet and won't really have the time for that until this summer most likely. I have much more experience in active setups than I do fiddling around with passives (that's probably obvious by now), but I won't be taking that step with the Maggies just yet.



 

RE: Magnepan 2.5R crossover help, posted on April 23, 2015 at 14:43:01
kappa546
Audiophile

Posts: 28
Joined: October 6, 2004
 Yes the ribbon is much more sensitive, and with the fuse bypass it's almost overbearing albeit cleaner. I may try a 3-4ohm resistor to further pad the ribbon down, or should I just try 1-2ohm resistors at the attenuator posts? If I tweak this value it shouldn't affect the bass impendance, right?

And yes I moved the brown wire to the top of the resistors.

 

RE: Magnepan 2.5R crossover help, posted on April 23, 2015 at 14:46:16
kappa546
Audiophile

Posts: 28
Joined: October 6, 2004
 To clarify the above post I meant trying a 3-4ohm resistor at the crossover underneath the plate. Would this affect bass panel impedance (and effectively crossover point) or would only the ribbon be affected.

 

I still don't get the point of the 2uF cap. , posted on April 23, 2015 at 16:45:36
neolith
Audiophile

Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 The presence of the cap causes a slight rise (I estimate 0.5db) in the tweeter response over the range 15K-20K (and the rise continues beyond). I am pretty sure you would not notice it's absence but apparently Magnepan felt it was beneficial. It was also used with the 2.6.



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: I still don't get the point of the 2uF cap. , posted on April 23, 2015 at 16:49:30
Davey
 It flattens the response for canine listeners. :)

Dave.

 

RE: Magnepan 2.5R crossover help, posted on April 23, 2015 at 19:08:10
neolith
Audiophile

Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 I mentioned that the Bode diagram was probably not an accurate representation of the actual speaker output, but I fail to see why a change in the cap would be more apparent with a more sensitive speaker. The crossover point would be lower with a more sensitive speaker but the relative shift with a change in caps would be unaffected. Am I missing something?
BTW, I found a small error in my spreadsheet and in fact the output curve is shifted upward in the with the larger the cap. Mea culpa.



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

OOPS!, posted on April 23, 2015 at 19:23:33
neolith
Audiophile

Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 I located a small typo in my spreadsheet which caused it to give an erroneous picture. In fact the output curve is shifted upward with the larger cap. Here is the changed Bode:





"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: OOPS!, posted on April 23, 2015 at 20:21:08
kappa546
Audiophile

Posts: 28
Joined: October 6, 2004
 Yea, a bigger cap lowers the HPF.

 

RE: Magnepan 2.5R crossover help, posted on April 23, 2015 at 21:11:34
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 It will change the impedance of the integrated speaker speaker. Get a few resistors at intermediate values so you can fine tune the tweeter level to your liking. Say 1/4-1/2 ohm intervals.

 

RE: Magnepan 2.5R crossover help, posted on April 23, 2015 at 21:19:06
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 But it would make the difference more noticeable than the bode diagram would indicate. Hopefully it is just enough to partially fill the dip in his response so it would be a bit smoother.

 

RE: Magnepan 2.5R crossover help, posted on April 24, 2015 at 19:14:11
RickeyM
Audiophile

Posts: 2208
Location: East Coast
Joined: March 15, 2003
 At one time I was futzing around with the crossovers in my MG-2.5's (25uF tweeter cap versions) and I had 2.7mH inductors on the woofers. I didn't do any measuring but the mids sounded very nice to me and the speakers survived just fine.

 

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