Magnepan crossover question

[adamct]

I would like to "upgrade" the crossover in my MG12s (hat tip to Eric for inspiring me!). I found this tutorial, which is short, clear and to the point. But here is my question: the author used three different models of caps: Auricap, Northcreek, and Solen. Can anyone venture a guess as to whether these were chosen for a specific purpose, or whether it it was likely done for cost purposes? Would it be OK to keep all of the caps from the same line of the same manufacturer, or are some caps known to be better in certain positions in a crossover vs. other positions? And am I correct that the Northcreek "bypass" cap isn't a type of cap, it is just a Northcreek cap that is being used as a bypass?

Thanks for any advice,
Adam

[adamct]

Thanks, Grainger. After speaking to the very helpful guys at Madisound, I'm going to skip the bypass caps entirely. I'm going to use a ClarityCap for the 50 uf cap for the bass.

There are differing reports on what the stock cap is for the treble. It's either 20uf or 33uf.

As I understand it, the MG12s are rated 4 Ohm, so:

50uf = 800 Hz
33uf = 1200 Hz
20uf = 2000 Hz

I don't really understand the relationship between the crossover frequency for the bass vs. the treble. Does anyone have a guess as to whether 1200 Hz or 2000 Hz would be preferable for the treble cap?

I'm trying to decide between the following caps for the treble cap:

- ClarityCap ESA
- Obbligato Gold Premium
- Mundorf EVO Oil + Aluminum

Anyone have a view on the best cap to use there?

Any advice you have would be greatly appreciated, since PCX's 20% off sale expires today...(that's the reason for the exclamation point - apologies in advance...)

Best regards,
Adam

[Paul Joppa]

The crossover component values, in a decent crossover, will work together with the drivers' impedance and frequency response, neither of which is flat with frequency, to produce the net acoustic response. So don't get too hung up on theoretical values based on perfect response and flat impedance!

For the lowpass function, it is second order, and the corner frequency is inversely proportional to the square root of inductance times capacitance, divided by two times pi. Bottom line, I get 433Hz. But the Q is about 0.5 so it's 6dB down at 433Hz if the woofer impedance is in fact 4 ohms (it probably isn't exactly). My guess is that the woofer is more sensitive, so the crossover also serves to shade in the woofer to create a baffle step correction along with the crossover function. That would make sense because the second order phase shift is twice as much as the highpass first order; by crossing them higher that excess phase shift is reduced.

[saildoctor]

I'm using the Obbligato gold's for many of the caps in my system and think they're nice.  They at least feel substantial with in the anodized aluminum casing they have.  I haven't heard any of the other ones you mention though. 

[earwaxxer]

Yep, Paul took the words right out of my mouth (yea right).. anyway, the take away is that, for some reason Magnepan likes to cross over that bass panel super low. Not sure why. My MMG's were stock crossed over at about 500. I increased that to about 1500hz for a more uniform sound. Its open for experimentation. I even took out the low pass all together at one point. It didnt sound bad, just not ideal.

[adamct]

Eric has been helping me out through private messages, so I'll repeat here what I just wrote him:

"Hahahahahahahhahahahahaha!!!!!! Soooooooo..........get this.

I mentioned that there were different reports on the value of the cap for the treble panel: either 20uf, or 33uf. So I decided to open my MG12s to take a look and see what Magnepan installed in mine. I bought my MG12s used. At the time that I bought them, I didn't know anything about crossovers or mods. So I purposefully looked for a pair that was in stock form. But tonight I saw that the fuse was missing. Damn! I thought....the seller lied to me.

I took the staples from the bottom out, and slid up the sock a bit. I saw a Solen film cap. OK, that's the cap for the treble panel (the cap on the bass panel is an electrolytic in stock form). What value is it? It appears to be 30uf. What the...? So there are three different values out there? 20uf, 30uf and 33uf?

But then I see another cap is in there. OK, what value is that? 20uf. Now I'm confused. I pull the sock up some more. What do I find? 4 caps! There is a 20uf Solen wired in parallel to a Solen 30uf. OK, that's for the bass panel. Then there is a 33uf Solen cap bypassed with a 10uf Crescendo film and foil cap. OK....that's for the treble panel. This clearly isn't a stock crossover. Then the kicker: the inductor is a Jantzen Cross-Coil copper tape inductor!!!

Frankly, I'm not sure it's worth monkeying around with. I've loved the way my MG12s sound from the moment I got them. This crossover sounds brilliant as-is, so I'll leave we'll enough alone."

Apologies for wasting your collective time, but I appreciate the lesson nonetheless...

Humbly yours,
Adam

[earwaxxer]

Adam,

BTW, I was embarissed to look to long at the pictures.  It looked like he was pulling up its skirt!  You don't know what you are going to see in that situation.

Thats funny! It is a bit like that when you "violate" the maggies. I remember when I got rid of that fuse. It was like a mortal sin, or something. No issues running 500wpc of clean power. Then to hack out the "vasculature and the the internal organs". That was heart pounding!

[adamct]

Out of curiosity, is there any difference between bypassing caps, vs. wiring caps in parallel? I ask because the 33uf cap in my MG12s is "bypassed" with a 10uf cap. Doesn't that mean its effectively a 43uf cap, thereby affecting the crossover frequency?

Best regards,
Adam

[earwaxxer]

Yep, bypassing caps is wiring them in parallel and the values are additive. That cant be the tweeter crossover (high pass), to low, they must be using them in combo with the inductor in the low pass filter. Your panel is a two way right?

[galyons]

Bypassing is putting a cap in parallel with another cap.  Typically one uses a bypass cap of 1/100, or so, value of the cap being bypassed. This is typically done to improve high frequency response.

Caps are also put in parallel to hit a target capacitance, which, IMO, is the case with you MG12's.  So yes it is effectively 43uF and that is the capacitance for the required frequency.

Cheers,
Geary

[adamct]

Yep, bypassing caps is wiring them in parallel and the values are additive. That cant be the tweeter crossover (high pass), to low, they must be using them in combo with the inductor in the low pass filter. Your panel is a two way right?

No, it's definitely the tweeter crossover. For the bass crossover, the previous owner installed a 20uf cap in parallel with a 30uf cap (stock Magnepan uses a 50uf electrolytic).

Hmmmm....now I'm wondering if I should remove that 10uf bypass cap, or just leave well enough alone. Like I said, I love the way my system sounds as-is, so maybe if it ain't broke, I shouldn't "fix" it...

[earwaxxer]

50uf electolytic.. No kidding! Thats wild... It get confusing... I like to use the term "low pass" and "high pass". The low pass allows the low frequencies to pass.

[Paul Birkeland]

Hmmmm....now I'm wondering if I should remove that 10uf bypass cap, or just leave well enough alone. Like I said, I love the way my system sounds as-is, so maybe if it ain't broke, I shouldn't "fix" it...

A 10uF cap across a 33uF cap is not a bypass cap, it is acting to increase the capacitance value significantly.

A 0.33uF cap across a 33uF cap would be a bypass cap, in other words, it would have very little effect on the function of the circuit where it was being used.

If you have a 43uF first order crossover capacitor and an 8 Ohm resistive load, you would have a target frequency of 465Hz. 

Decreasing that to 33uF will bump that frequency up to 600Hz, you will hear that, and potentially grossly degrade the performance of your loudspeaker.

[adamct]

I'm going to leave as-is, simply because I love the way my MG12s sound. But to be clear, the stock cap is a 33uf cap. The previous owner then installed a boutique grade 10uf cap in parallel with that cap. I agree it's not a bypass. Which is what led me to speculate whether I shouldn't remove it, and go back to the stock crossover values (the owner didn't change the cap value on the crossover for the bass panel, for example, he just replaced the 50uf eletrolytic with 20uf and 30uf films caps wired in parallel).

Best regards,
adam

[Paul Birkeland]

Ah, yikes, I would attempt to contact the prior owner?  Have you tried the stock value?