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Re: MG-IIb with XO-1 crossover

There are differences between the MG-IIb and the MG-IIIa. The MG-IIb is a two-way speaker (bass and tweeter), whereas the MG-IIIa is a three-way speaker (bass, midrange and ribbon tweeter). The factory crossover for the MG-IIb is very simple, and entirely contained within the panel. The MG-IIIa, because it is three-way, by comparison, has a complex crossover network. It has a low-pass and high-pass filter in the panel (for the midrange to ribbon tweeter crossover) and yet another low-pass and high-pass filter in an external crossover box (for the bass to midrange crossover). So, when I speak of removing the 60 MFD capacitor in the external crosssovers, this does not apply to your MG-IIb's. Go to the pictures area of the MUG site and look at Bob Allman's system. You will see the MG-IIIa external crossover box and how it is supposed to be wired when used with the XO-1.

You did ask for clarification on the MG-IIIa wiring. When the XO-1 is used with the MG-IIIa, the 60 MFD capacitors in the external crossover boxes must be disconnected (not jumpered). (The function of these mid/tweeter high-pass filters are being replaced by the high-pass filters of the XO-1.) The low-pass filter in the external crossover stays in the circuit, because the XO-1 does not perform this function. The short speaker cable between the external crossover to the mid/tweeter input on the panel speaker is removed, and the output of the mid/tweeter amplifier is wired to the panel.

For the MG-IIb, there is no external crossover box and things are much simpler. Consult your instruction manual for the MG-IIb. You will remove jumper bars so that you can add your second amplifier wiring. That's all you have to do. This will disconnect the high-pass filter inside the panel, but leave the low-pass filter in the circuit (for the bass driver).

Now, for the XO-1 with MG-IIb, you will need different capacitors. The high-pass filter is specified at 1600 Hz. Therefore C = 1/(2*pi*1600*R). R is the tweeter amplifier input impedance. For the Bryston 2B, 3B or 4B, it is 50 kohms. Therefore, for the MG-IIb, you would need a 0.002 MFD capacitor ($3.44 each for Multicaps at Michael Percy Audio, The Parts Connection). Since you are using identical amplifiers, the potentiometers on the XO-1 are not needed and should be set at maximum.

One more advantage of the XO-1 when used with the MG-IIb is that the tweeter becomes electrically isolated from the bass driver and bass amplifer. If the single-amplifier system (without XO-1) is driven into clipping due to bass content, additional high frequency harmonics will be generated, which will blow the tweeter fuse and eventually melt the tweeter wire if done repeatedly. With your dual Bryston 3Bs, if the bass amplifier goes into clipping, the tweeter amplifier likely will not, which means that your tweeters are much better protected.

If you do not have an instruction manual for the MG-IIb or XO-1, the people at Magnepan are very kind and will send it to you if you call them.

I hope this helps. Let us know what sonic improvements you find when using the XO-1.


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