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In Reply to: RE: One suggestion posted by E-Stat on July 02, 2009 at 11:45:28
I did consider the crossover point. If I were able to play the system loudly, that would be a major consideration. It was the reason the earlier Polk Advent system used a 175 Hz crossover. The Polks couldn't handle as much power and they also had a more peaked output in the 80-90 Hz region. But in the condo, with my current downstairs neighbors, I really can't play all that loud, and the 80 Hz high pass is OK. Putting the input to the PSB's down 3 db at 80 Hz and 9 dB (an 8:1 ratio)at 40 Hz.
Raising the crossover point by one octave to 160 Hz would be better for high power use; but it would also mean that I'd need to use a graphic EQ in the system to get the bass contour I want. At 80 Hz, the plate amp controls allow me to get what I want.
The required stroke of a given speaker varies inversely as the square of the frequency for constant output level. So doubling the crossover frequency cuts the stroke on the satellite speaker by 4:1. But once you get near the speaker's in-box resonance that changes - particularly if the speaker is ported as the stroke is at a minimum at the port tuning frequency. In this case, I left the ports open as the port tuning frequency is low enough to help prevent overstroke.
Years ago I helped a friend put together a similar system using AR3 woofers and Rogers LS3/5a. There was a first order roll off on the Rogers at 125 Hz and it survived quite well, even though the system was played at very high levels. The bass was driven by a Phase Linear 700 which produces over 450 W/ch into 4 ohms. The mid and highs were driven by a Van Alstine modified Stereo 120. But again, we had to use a Soundcraftsmen EQ to flatten the mid-bass.
One of the saving graces of my den is that the room modes don't cause a lot of problems for me; but it took me two years to come up with the speaker locations which are asymmetric to the long axis of the room.
Your comment on the benefits of active biamping are right on. You can clip the bass amps and the clipping distortion is masked by the clean energy from the satellites. Also, active biamping gives a lot more effective power. For example, if the crossover were at the half power point in the system and the woofers and the satellites had the same sensitivity, a 50 watt amp on the bass and a 50 watt amp on the mids and highs gives undistorted loudness equal to a single full range amp of 200 watts. But the smaller satellite speaker is still only driven by a 50 watt amp. Much easier on the speakers if you drop the tone arm. The higher effective power is one of the reasons why the speakers for rock concerts are biamped or triamped.
I picked some simplifying assumptions for my example, and for typical systems you need 2-3 times the power on the bass that you do on the mids and highs. In my system, I'm at 4:1.
Jerry
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