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Planar Speaker Asylum Welcome! Need support, you got it. Or share your ideas and experiences. |
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In Reply to: Re: Now that is a bigger problem! posted by Blaine H on April 1, 2006 at 06:37:43:
Yes, I have 10 microhenry chokes in series with my ribbon tweeters as part of my upgraded mid/treble crossovers.I also have R-C networks in parallel with the bass panel inputs. These are 56 microfarad capacitors (paper-in-oil, bypassed with smaller silver-mica) in series with 1-ohm resistors and wired onto the ends of the cables that feed the bass panels.
The stock Magnepan low-pass filters, which I use with my bass amps, comprise a series inductor, a shunt capacitor, and another series inductor. The output to the bass panel is the second inductor, so that higher frequency resonances in the bass panel cannot be damped by the bass amp.
My R-C networks provide a path for damping currents to flow at higher frequencies. Electrodynamic damping is where the motor becomes a generator when external forces cause the driver to move (such as airborne or structure-borne tones, or the inertial moving mass of the driver and air to which it is coupled). The generator currents flow and cause magnetic fields which oppose the motion. This kind of damping is very important for cone woofers, and the damping factor of power amplifiers is considered an important criterion for matching amps to speakers. See the URL for an article on this topic published in 1954-5 [!].
Consider how a Magnepan speaker is made. The bass and midrange panels, in the case of the true-ribbon models like ours, share a single sheet of Mylar. The Mylar tension is varied from top to bottom so that the film does not have a single strong resonance frequency. This means part of the bass panel will resonate in the midrange, and any coupling from the midrange panel, or reflected sound from the room, will cause it to ring. The stock low-pass filter will prevent the amplifier from damping this ringing.
This tweak provides a serious improvement in mid-bass resolution. Piano tones reveal the individual piano wires and sound like the instrument is present in the room (on good recordings, of course). Complex voices are rendered more cleanly. A good jazz rhythm section becomes a true delight. An orchestral piece with a lot of rhythm work is easy to comprehend.
I determined the component values by trial-and-error. The 1-ohm of resistance seems to be necessary to avoid over-damping, but 2/3 of an ohm might work just as well. Larger capacitance values would cause an audible suck-out, I believe, but I did not have enough capacitors to confirm this.
This is not as cheap as many of my tweaks, but the cost is still very low for the benefit.
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Follow Ups
- Low-pass R-C; separate from the RF-choke. - Al Sekela 19:02:57 04/01/06 (0)
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