kelby
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Post by kelby on Oct 31, 2016 23:05:13 GMT -5
I'm considering using a BAS-X A-700 to power a pair of 3-way speakers with a miniDSP for an active crossover. Do the BAS-X series amplifiers have DC protection? I wouldn't want to fry expensive drivers that have no passive filter to protect them.
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Post by Chuck Elliot on Nov 1, 2016 5:28:06 GMT -5
Typically, the method of doing this is to include an in-line capacitor on the mid-range and/or tweeter. The woofer doesn’t need much protection.
The value of the capacitors is determined by a simple 1st order high-pass filter 1.5-2.0 octaves below the actual high-pass crossover of that leg.
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KeithL
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Post by KeithL on Nov 1, 2016 11:35:37 GMT -5
The BasX amps do all have our standard (quite good) protection circuitry. However, suspenders and a belt never hurt anyone. Typically, the method of doing this is to include an in-line capacitor on the mid-range and/or tweeter. The woofer doesn’t need much protection. The value of the capacitors is determined by a simple 1st order high-pass filter 1.5-2.0 octaves below the actual high-pass crossover of that leg. |
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Post by Wideawake on Nov 1, 2016 12:09:22 GMT -5
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KeithL
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Post by KeithL on Nov 1, 2016 13:42:38 GMT -5
The capacitor goes in series with the tweeter... usually in line with the "hot lead"... and, in this application, you wouldn't want to add a resistor or any other components. Adding a capacitor in series with other components does cause phase shift; the amount of phase shift depends on the value of the capacitor and the other components - and it varies with frequency. But, here's the deal.... When you use a capacitor in a crossover, and you're using it to help pass certain frequencies while blocking others, you will be using a capacitor whose impedance at the crossover frequency is more or less similar to that of the driver. In this situation, the capacitor will cause significant phase shift, which must be taken into account. However, when you use a capacitor simply to block DC, you're going to be using a proportionally larger value, which will have a much lower impedance at the crossover frequency, and so will cause much less phase shift at the crossover frequency. In theory, you would ideally use a HUGE capacitor that would act as high-pass filter at some ridiculous low frequency like 0.1 Hz, and so would block DC, while having no effect at all in the normal audio band. However, because high-quality high-value capacitors tend to be quite large and expensive, you'll settle for one that acts as a high-pass filter significantly below the crossover frequency - so it will have relatively little effect at the crossover frequency and above it. (You're much better off using a high-quality capacitor that sets the "filter cutoff" at 1/4 or 1/2 the crossover frequency than you are using a much larger one of lower quality.) The other thing is that all crossovers create significant phase shift, including electronic and DSP-based ones, and drivers themselves, as well as enclosures, often also have a lot of phase shift. (Remember that drivers are inductive, and the driver-enclosure combination can have inductive and/or capacitive characteristics). Many speakers with passive crossovers have an overall phase shift of as much as 720 degrees over the full audio range. When compared to this, the 10 or 20 degrees of phase shift caused by a high quality capacitor used to block the DC is relatively insignificant. Some people try to avoid or minimize phase shift, while others prefer to design for a smooth consistent phase shift across the audio band. If you're really worried about phase shift, and you're up to correcting for it, then you're going to have to compensate for the phase shift of the driver itself, and you can just lump the electrical characteristics of the blocking capacitor into the deal when you're making those calculations. If not, then it really isn't enough to worry about. (Electrically, over the audio frequency range, the blocking capacitor is just "a crossover component that doesn't do very much".) |
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Post by Wideawake on Nov 1, 2016 14:55:02 GMT -5
@keithl, thanks for that very clear and detailed explanation!
Googling for high quality capacitors brings up tons of links but it mainly boils down to selecting one from a reputable manufacturer. Are you at liberty to share some brands that make high quality capacitors?
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Lonnie
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Post by Lonnie on Nov 1, 2016 16:45:21 GMT -5
Yes, the amps have really good protection from a lot of things.
However, back in the day when I was building show cars we used light bulbs to protect the drivers. The type and voltage of the light bulb would depend on how much power you were running, but the basic idea was to put a 12volt light bulb in series with the tweeter. At low to moderate levels the light bulb would pass the signal through to the tweeter, but if you pushed it hard enough to actually energize the filament, the resistance of the filament would rise exponentially and lower the level of the signal going through the voice coil. Works well and doesn't change the phase angle.
Lonnie
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KeithL
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Post by KeithL on Nov 1, 2016 17:13:56 GMT -5
I'm not big on "fancy audiophile capacitors". In the normal range of values used for a crossover, or for what you're planning, you should stick with metallized polypropylene. They really are significantly better than some of the cheaper alternatives - and they don't cost that much more. However, I certainly wouldn't go for anything "fancier" than that. @keithl, thanks for that very clear and detailed explanation! Googling for high quality capacitors brings up tons of links but it mainly boils down to selecting one from a reputable manufacturer. Are you at liberty to share some brands that make high quality capacitors? |
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Post by jmilton on Nov 1, 2016 20:51:21 GMT -5
Lightbulb. That's brilliant!
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Post by leonski on Nov 2, 2016 0:10:08 GMT -5
Look at the PARAMETRIC EQ of the MiniDSP.
Some have 5 channels PER INPUT and OUTPUT.
Some have 10, like the 2x4HD.
For 3-way speakers? You'll probably start looking at the 2x8 which will leave an extra output per side FOR A SUB, to be added later!
Do you have the calibrated mic? you'll need that and a fresh install of REW.
You should be able to provide a steep low cut at maybe 10hz to 15hz or perhaps lower. I'd ship off a quick note to either the MiniDSP support staff or join their forum and throw it to the crowd.
If you want to spend MORE money, Solen makes good caps as does Clarity. If you are wealthy, you could always go for Mundorf or Duelund.
For proper implimentation you'll need to know the INPUT and OUTPUT impedance of the devices you wish to insert the capacitor between.
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Lonnie
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Post by Lonnie on Nov 2, 2016 8:45:22 GMT -5
Lightbulb. That's brilliant! Glad you like it. It really does work well. When the filament is cold the resistance is so low that it doesn't affect the signal at all. But if you drive it hard enough to energize, well for that split second the resistance rises proportionally to the level of the signal and then immediately drops back off. The end result is you don't even know its there but it does a great job of protecting the speaker. Lonnie |
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KeithL
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Post by KeithL on Nov 2, 2016 9:42:34 GMT -5
You're sort of missing the point.... Assuming that the crossover filters are correct they're not going to be sending any DC out to the speakers.... And the amps won't pass pure DC anyway (so there's no need for a sharp low cut filter on a crossover channel that's already a high-pass filter). The point of adding a blocking capacitor on the driver itself is to block DC if the amplifier fails and its protection circuitry fails to operate or fails to operate in time. (While this is pretty unlikely, at least with one of our amps, it's possible.... and, if it did happen, there's nothing you could do in the MiniDSP that would protect you from it.) A light bulb protects the driver because it simply limits the overall power reaching the driver (and the only reason DC is dangerous to a tweeter is because there's too much of it). The light bulb will protect from an excess of ANY sort of signal, which includes both DC due to a malfunction, and simply sending too much audio power in that frequency range. Look at the PARAMETRIC EQ of the MiniDSP. Some have 5 channels PER INPUT and OUTPUT. Some have 10, like the 2x4HD. For 3-way speakers? You'll probably start looking at the 2x8 which will leave an extra output per side FOR A SUB, to be added later! Do you have the calibrated mic? you'll need that and a fresh install of REW. You should be able to provide a steep low cut at maybe 10hz to 15hz or perhaps lower. I'd ship off a quick note to either the MiniDSP support staff or join their forum and throw it to the crowd. If you want to spend MORE money, Solen makes good caps as does Clarity. If you are wealthy, you could always go for Mundorf or Duelund. For proper implimentation you'll need to know the INPUT and OUTPUT impedance of the devices you wish to insert the capacitor between. |
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Post by Wideawake on Nov 2, 2016 12:21:01 GMT -5
I read about the light bulb method on the net but I'm wondering whether one would need to monitor the bulb for a blown filament? If the filament breaks then you would lose the desired protection, no? While with a cap you would not need to monitor it. Is that a true statement?
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Post by jefft51 on Nov 2, 2016 13:39:34 GMT -5
If it's wired in series and the filament breaks, that would stop any signal. Your tweeter therefore would still be protected, but would be silent till you changed the bulb.
Jeff
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Post by Wideawake on Nov 2, 2016 14:12:59 GMT -5
If it's wired in series and the filament breaks, that would stop any signal. Your tweeter therefore would still be protected, but would be silent till you changed the bulb. Jeff Ah! Yes, ofcourse! Seems like I experienced a slight lapse in logic.  |
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Post by leonski on Nov 2, 2016 16:04:56 GMT -5
I was thinking the OP was conerned with turn off 'thump' which can be 1 cycle of DC.
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Lonnie
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Post by Lonnie on Nov 2, 2016 16:06:09 GMT -5
I read about the light bulb method on the net but I'm wondering whether one would need to monitor the bulb for a blown filament? If the filament breaks then you would lose the desired protection, no? While with a cap you would not need to monitor it. Is that a true statement? The bulb is wired in series, so if the filament ever breaks, then you have no sound at all. Personally I have never lost one yet and I use them extensively in show cars because you never knew who was going to be running the system. Lonnie |
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KeithL
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Post by KeithL on Nov 2, 2016 16:30:22 GMT -5
The bulb will block excessive amounts of power at any frequency - which will protect you from DC due to an amplifier failure and from turning the amp up too loud. However, if you specifically want to block the turn-on thump, then a capacitor chosen to act as a high-pass at somewhere around 1/4 of the crossover frequency will block that more completely. I was thinking the OP was conerned with turn off 'thump' which can be 1 cycle of DC. |
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Post by leonski on Nov 2, 2016 17:17:33 GMT -5
In days past, you would turn ON / OFF gear 'in order' to minimize or elliminate such thumps. In the days when a TT was IT for a high-end source, than a 'rumble filter' would be a good idea. I saw my woofer cones 'walking' back and forth at a VERY low frequency. When I had my NAD1700 Tuner/Preamp, it had a back panel switch which was labeled 'Infrasonic' and would stop such nonsense. I don't know the turnover frequency.
The problem with the bulb is that it limits ALL power. Somebody with medium to low sensitivity speakers and a large power amp? What do they do? use a 100 or 150 watt bulb? If you installed the bulb ONLY in the tweeter circuit? I wonder if that would help since the tweeter is the most vulnerable driver. Even my panels have a 4 amp fuse on the high end, and NO protection on the low.
I'll remember this.
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Post by Chuck Elliot on Nov 2, 2016 23:30:57 GMT -5
This thread has become a mix of different protection modes, including:
Amp DC protection
Peak-Limit protection
Bandwidth protection
Amp DC protection is present in almost all HQ amplifiers. This was an early problem with SS amps. Gone are the power-on thumps and the catastrophic failure modes that could put out rail DC voltages to the speakers.
Peak-Limit protection is the light bulb solution although there are other ways to achieve the same result. Klipsch used back-to-back zener diodes to protect the K77 tweeter in some designs. Go above the zener voltage, blow a fuse.
Bandwidth protection is the in-line capacitor solution. An in-line capacitor blocks frequencies that the driver cannot handle. This affords protection from wiring mistakes from an active crossover too.
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