Passive high pass filter - Marchand XM446XLR-A


So, I am about to set up a new pair of monitor speakers and fill in the lower frequencies with two subs.  The monitors are Aerial LR5s and the subs will be two Aerial SW12s with internal 400 wpc amps.  The LR5s weigh 105 pounds each and play flat to just a bit above 40Hz (-6dB at 35Hz).  The Aerial subs are fast, powerful and musical and should integrate well with the LR5s.  My pre and power amps are solid state, fully balanced and the power amps provide 300/600 wpc into 8/4 ohms.  I will be running separate ICs from the preamp to my power amps and a second set from the preamp to the input on the subs.

My first question is whether I would benefit from using a high pass filter between the preamp and amplifiers to limit the low frequencies going to the LR5s so the amps are not trying to drive them full range.  

I didn't find a DIY design that I was comfortable with but I did run across several posts by people using a Marchand passive high pass filter.  The Marchand XM446XLR-A is fully balanced and has a high pass corner frequency of 80Hz (i.e., the frequency that is down by 3dB).  The Marchand has a standard slope of 24dB/octave but can be made for other slopes.

My second question is whether anyone out there has experience with the Marchand passive high pass filter and is it sufficiently transparent or should I expect to hear unwanted artifacts with the filter in my system?  

Finally, is there a higher quality alternative or should I simply play with the low pass on the sub and forget about filtering the signal to the main speakers?
http://www.marchandelec.com/xm46.html
mitch2
Thanks bdp24, I have calculated the capacitor size needed but everything I have read seems to be for single-ended signals.  If I want to go the single capacitor route with my balanced amp, do you know what I would do differently...i.e., do I put a series cap on both of the differential signal legs?  Is the calculation different for a balanced signal?
Great question Mitch, one I need the answer to as well! I've done it on a single-ended amp (my Bedini 25/25, for use with Quad 57's and subs), but not a balanced amp. Hopefully someone with more technical knowledge than I can help us out. I have a Music Reference RM-200 Mk.2 waiting for the answer! It has balanced/XLR inputs only. I'll be shooting Roger Modjeski an email, so I'll let you know what he says.
Gentlemen, yes, you would need a capacitor on each of the two signal lines in the balanced signal pair. Preferably the two capacitors should have a reasonably tight +/- tolerance on their value (for example 5%), so that they match reasonably closely, although I doubt that the tolerance is particularly critical.

Usually, although not always, the specified input impedance of a balanced input reflects the sum of the input impedances between each of the two signal lines and ground. That appears to be the case for the RM-200 Mk2, since the spec is 30K balanced and 15K unbalanced, the unbalanced spec presumably reflecting the impedance that would be seen if a single-ended signal were provided to the XLR connector. Stereophile measured the actual balanced impedance as 29K, so you would calculate the capacitor value based on a 14.5K input impedance.

Regarding Mitch2’s Clayton amps, you would want to verify that the specified 100K input impedance reflects the sum of the input impedances of the two legs. If so, the value of each of the two capacitors would be calculated based on a 50K input impedance.

Good luck. Best regards,
-- Al

Thanks much Al!
The Claytons only have balanced inputs so I am confident the 100K input impedance is for the balanced condition.
Good point on matching the caps.  I plan to try something relatively high quality like Jupiter copper film caps.
Partsconnexion will match them for $1 per pair.
I plan to try 0.022uf caps for a cut-off frequency of about 72 Hz and then start by low-passing the subs at 80 Hz.
The next cap size up (0.047uf) gives a cut-off frequency that is a little low at 34Hz but may still be an option.
If I wanted something in-between, could I series the 0.022 with a 0.01 cap?
Caps are thankfully 25% off this month, which is good since I need four of them.