Planar Speaker Asylum: Re: MGIIIa with XO-1 crossover vs. electronic crossover by RichardB

I have a situation very much the same as yours. I am in the process of replacing MG-IIB's with mint condition MG-IIIa's and I am investigating bi-amping options. I see you have a Bryston 3B; I am using a pair of Bryston 4B's. I spoke to Wendell at Magnepan, who did not recommend the XO-1 passive crossover solution. Instead, he recommended the Bryston 10B electronic crossover or trading in the MG-IIIa's for 3.6's.

The XO-1 passive crossover splits the stereo outputs of your preamp into two pairs, one pair for the bass amplifier and one pair for the mid/tweeter amplifier. It is a small, black plastic box with six female RCA jacks, two capacitors and two potentiometers. You can buy a used/reconditioned one from Magnepan, factory installed with the correct capacitors, for $50 plus $10 shipping. Or, you can make your own.

The two potentiometers are wired as simple voltage dividers, to decrease the gain of the bass amplifier (for bass to treble balance adjustment if your bass amplifier has a higher gain than the mid/tweeter amplifier). If you use two identical stereo amplifiers, these potentiometers may not be needed (as with my case). If they are not identical, then the bass amplifier must be more sensitive (watts output / volts input), so that the potentiometers can reduce its gain to match the less sensitive mid/treble amplifier.

The two capacitors are wired as simple RC high-pass filters, one for each channel. One end of a capacitor goes to a preamp output, and the other end goes to a mid/tweeter amplifier input. The amplifier input impedance acts as the resistor. For 6 dB/octave slope, the capacitor value is calculated as: C = 1 / (2*pi*f*R). According to the XO-1 and MG-IIIa instruction manuals, the high pass frequency should be set at 500 Hz. This simplifies the formula to C = 0.318/R in Kohms, where R = input impedance of the mid/tweeter amplifier. For 50k input impedance (Bryston 4B), the capacitor value is 0.0064 MFD. You can change the crossover point by varying the capacitor value. Use high quality capacitors, such as Multicap polypropylene ($3.73 each at Michael Percy Audio), or better.

Next you must disconnect the high-pass 60 MFD capacitors in the external crossover boxes of the MG-IIIa's. They will be unnecessary and, according to Magnepan, "may cause the bass amplifier to become unstable or shut down." The output of the mid/tweeter amplifier is wired directly to the panel mid/tweeter connections, and the bass amplifier is connected to the input of the outboard crossovers. (If you buy the XO-1, all of the above is covered in the instruction manual.)

What can you expect from the XO-1 passive crossover? I tried it on the MG-IIb's and find that the bass and dynamic range are considerably improved and the speakers play slightly louder. I haven't tried this on the MG-IIIa's, as they have not yet been delivered. For that, I defer to Bob Allman on the MUG site. Also go to http://portland.maine.edu/~n-audio/BITS0007.txt This is a 1988 review of the MG-IIIa's with the XO-1. They claim, "When switching to a biamp configuration you get a huge improvement in imaging and clarity. I highly recommend that you biamp these if you can afford it all. As I said you get a very noticeable improvement in every aspect of the sound. One very noticeable change you're likely to get with these is actual stage height."

Now, why consider an electronic crossover? You should get the above advantages, plus more. With the XO-1, the only components you eliminate in the standard Magnepan passive crossover are the 60 MFD capacitors and 3.5 mH inductors in the outboard crossover boxes. With the electronic crossover, you can eliminate the entire external crossover boxes, which contain the low-pass filters for the bass drivers. The electronic crossover has a quality high-pass filter for the mid/tweeter AND and a quality low-pass filter for the bass. In this case, you would still have the low-pass and high-pass filters for the midrange and tweeter drivers, internal to the panels. (To eliminate those, you would have to go to tri-amping. Or, you could biamp, but upgrade those components.) The claims for improvement due to upgrading crossover components are well documented by Paco and others on the MUG site. For even more benefits, go to www.sound.au.com/bi-amp.htm This has a very detailed description of bi-amping with an electronic crossover. Here you find out that at 500 Hz, 60% power is needed for the bass and 40% power is needed for the mid/high. This will help you to size your second amplifier. The Bryston 10B is an excellent electronic crossover - but is expensive ($1200+). Marchand Electronics sells one for much less, and even offers a kit version. Probably you get what you pay for, but I would like to hear from anyone who has compared the two. Finally, with an electronic crossover, there is the possibility of introducing coloration of the processed signal, however the Bryston 10B is reportedly very close to neutral.

I hope this helps you. Let us know how it worked out for you.