Well, I live and learn...
I went searching and found some cases of series-connected, biased film capacitors in commercial crossovers. I know I have somewhere in my files a series of tests performed on various capacitor types to determine their effect on audio signals. I think at least one test included the effects of DC bias. I'll go take a look.
My previous attempt at drawing the internal equivalent circuit of an electrolytic capacitor was not as successful as I would have liked. The following one is closer to the real topology:
Code:
+----->|-----+-----|<-----+
| | |
-----+ | +-----
(+) | | | (-)
+-----||-----+-----||-----+
High C, low V Low C, high VIn a typical electro, the oxide layers start to break down and pass a high current at a volt or so reverse bias, represented by the forward voltage drop of the "diodes".
The "reverse" capacitor has a much thinner (ideally nonexistent) oxide layer than that formed on the "forward" capacitor. It typically has a capacitance 50 or more times higher than the "forward" capacitor, but only a volt or two breakdown voltage.
As for knowing when to change the battery, it depends on the leakage current through the capacitors. I would expect them to last for their rated shelf life or longer. Assuming standard 9 volt 216 type batteries, you could measure them in-circuit with a digital voltmeter (1 megohm per volt or more sensitivity) and replace them when they show a noticeable drop, say to 8 volts.
I'm planning to go 'a step beyond' battery power, with solar cell power if I ever get my ultimate hi end speakers built. In fact, I've already got a couple of 9V RS solar cells that put out several volts with room illumination at night. I may back up the solar cell with a supercap, though, to hold up the voltage when there is no light.
Incidentally, for the biased caps I'm planning to use some ASC GLY513's that are metallized polypropylene, so they should have low enough leakage current to work with this scheme.
Incidentally, for the biased caps I'm planning to use some ASC GLY513's that are metallized polypropylene, so they should have low enough leakage current to work with this scheme.
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Quick Basics
In the following notes I will try to give you some answers:
To maintain constant output as frequency is lowered requires increasing driver volume displacement at a geometric rate. The necessary air volume displacement, [Vd], is proportional to 1/ (f^2). In other words, the air volume displacement increases by 12 dB per octave with decreasing frequency. This relationship does not depend on the type of enclosures used. So, if we need for example 50 cm3 of volume displacement at 100 Hz for 110 dB SPL, we need 1250 cm3 at 20 Hz for the same SPL. Additionally each doubling of the SPL (+6 dB) requires twice the volume displacement.
Above system resonance, [fc], It is not unreasonable to expect linear excursion from the driver up to the limit characterized by [xmax]. The drivers lower bound, then is determined by the equation [Vd] = [xmax]*[Sd], noting that increased efficiency will be acquired by increasing [Sd] (rather than [xmax]) so that the mismatch between the acoustical impedance of the driver and the air surrounding it is reduced. However, as driver cone dimensions increase, the onset of breakup modes and output beaming will occur at a lower frequency. Bottom line is: what you will be exploring is the tradeoff between bandwidth and efficiency while you spelunk the lower bound of program signal reproduction. If you attempt to squeeze appreciably more than a frequency decade from a driver here, expect higher audible distortion when the system volume is turned up to meet the demands of a large party or other similar venue. Note also that room gain and modes need to be addressed to optimize performance in the first decade. In this setting use of multiple woofers is preferred and their placement in room corners will increase output by 18 db.
For the second decade, the voice range, use of a smaller driver is required, but with a not too-small [xmax], as the lower crossover point will be somewhere in the 200-500 Hz. range. Here driver bandwidth is more important than driver efficiency, as we try to stretch bandwidth to cover most music fundamentals and the range of the human voice. An upper crossover point of 6-7 kHz would seem ideal for this purpose. In this region human hearing sensitivity is in decline.
Regards,
WHG
In the following notes I will try to give you some answers:
To maintain constant output as frequency is lowered requires increasing driver volume displacement at a geometric rate. The necessary air volume displacement, [Vd], is proportional to 1/ (f^2). In other words, the air volume displacement increases by 12 dB per octave with decreasing frequency. This relationship does not depend on the type of enclosures used. So, if we need for example 50 cm3 of volume displacement at 100 Hz for 110 dB SPL, we need 1250 cm3 at 20 Hz for the same SPL. Additionally each doubling of the SPL (+6 dB) requires twice the volume displacement.
Above system resonance, [fc], It is not unreasonable to expect linear excursion from the driver up to the limit characterized by [xmax]. The drivers lower bound, then is determined by the equation [Vd] = [xmax]*[Sd], noting that increased efficiency will be acquired by increasing [Sd] (rather than [xmax]) so that the mismatch between the acoustical impedance of the driver and the air surrounding it is reduced. However, as driver cone dimensions increase, the onset of breakup modes and output beaming will occur at a lower frequency. Bottom line is: what you will be exploring is the tradeoff between bandwidth and efficiency while you spelunk the lower bound of program signal reproduction. If you attempt to squeeze appreciably more than a frequency decade from a driver here, expect higher audible distortion when the system volume is turned up to meet the demands of a large party or other similar venue. Note also that room gain and modes need to be addressed to optimize performance in the first decade. In this setting use of multiple woofers is preferred and their placement in room corners will increase output by 18 db.
For the second decade, the voice range, use of a smaller driver is required, but with a not too-small [xmax], as the lower crossover point will be somewhere in the 200-500 Hz. range. Here driver bandwidth is more important than driver efficiency, as we try to stretch bandwidth to cover most music fundamentals and the range of the human voice. An upper crossover point of 6-7 kHz would seem ideal for this purpose. In this region human hearing sensitivity is in decline.
Regards,
WHG
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