Members moody Posted December 8, 2003 Members Share Posted December 8, 2003 I know nothing about them, but I wouldn't mind a bit of protection the next time an amp goes DC. How do I go about it, what are the pros and cons, how do I know what sort of light bulb etc. Link to comment Share on other sites More sharing options...
CMS Author Craig Vecchione Posted December 8, 2003 CMS Author Share Posted December 8, 2003 Light bulbs are used mainly as over-power protection for HF drivers. An 1156 bulb can be used to limit the driver to about 40 watts or less. They are not to be used as fuses; they work by dissipating increasing current as light and heat, not by opening the circuit. Link to comment Share on other sites More sharing options...
Members agedhorse Posted December 8, 2003 Members Share Posted December 8, 2003 They will do nothing to protect a speaker from an amp failure like you describe. They are used in the high frequency portion of the crossover to limit the power to the horn driver by acting as a non-linera resistor. The high frequency section is already inherantly protected from a DC fault by the action of the series capacitor(s) in the high pass section. Link to comment Share on other sites More sharing options...
Members barefoot Posted December 8, 2003 Members Share Posted December 8, 2003 The main disadvantages are: 1. Dynamic compression - the whole point of using light bulbs is to reduce the rate of increase of power delivered to the speaker as power increases. This is the definition of dynamic compression. This does help protect the speaker from thermal overload, but the large temperature coefficient of a light bulb filament causes dynamic compression at lower levels as well. Similarly, the thermal mass of the filament can yield very unpredictable time lags between the onset of short term power spikes (kick drums for example) and the induced compression. 2. Distortion - coiled filaments act very much like little loudspeakers. The AC signal in the coil creates a fluctuating magnetic field, which then causes the coil to vibrate. The mechanical vibration modulates the magnetic field, which in turn modulates the electrical signal. The light bulb essentially acts like a tiny spring reverb. 3. Passive Crossover misalignment - passive crossovers are typically designed to look back into a small stable output impedance from the amplifier. The light bulb presents a larger, unstable output impedance, wreaking havoc on the alignment of any passive crossovers following it. Thomas Link to comment Share on other sites More sharing options...
Members agedhorse Posted December 8, 2003 Members Share Posted December 8, 2003 Originally posted by barefoot The main disadvantages are:1. Dynamic compression - the whole point of using light bulbs is to reduce the rate of increase of power delivered to the speaker as power increases. This is the definition of dynamic compression. This does help protect the speaker from thermal overload, but the large temperature coefficient of a light bulb filament causes dynamic compression at lower levels as well. Similarly, the thermal mass of the filament can yield very unpredictable time lags between the onset of short term power spikes (kick drums for example) and the induced compression.2. Distortion - coiled filaments act very much like little loudspeakers. The AC signal in the coil creates a fluctuating magnetic field, which then causes the coil to vibrate. The mechanical vibration modulates the magnetic field, which in turn modulates the electrical signal. The light bulb essentially acts like a tiny spring reverb.3. Passive Crossover misalignment - passive crossovers are typically designed to look back into a small stable output impedance from the amplifier. The light bulb presents a larger, unstable output impedance, wreaking havoc on the alignment of any passive crossovers following it. Thomas In my opinion, your comments focus on theoretical issues that have not proven to be problematic in actual applications, especially compared with the benefits that this may provide to the "average" user. 1. Dynamic compression is actually the intent of operation of this device. Due to realatively the large thermal mass (affects the attack time constant) and the non-linear transition between low inpedance (cold) and high impedance (hot) properties of the filkiment (threshold) a PROPERLY designed application of this "crude" technology can actually deliver very good perfromance. By properly designed, this means that the driver is sized adequately, and that the properties of the bulb allow for uncompressed operation in normal use of the loudspeaker. Since this approach is generally applied to the high frequency portion of the crossover output, kick drum and bass do not have a significant effect on the overall operation of this scheme. The time constant of a snare is very short and is 100+ x the time constant of the bulb filim,ent so pumping due to transients of this nature are not an issue either. Upon a "gross overload", the bulb will transition from low resistance to high resistance in order to protect the driver... at this point audible effects from the bulb will be preferable to destroying the diaphram. 2. Distortion... again in theory this is possible, but restricting to the HF portion of the signal, self-excited vibration is unlikely. Now external excitation of the filament by the LF acoustic coupling from the cabinet could be possible, but the circuit impedances are so low that any such externally coupled modulation would be horribly inefficient at best. With yor spring reverb analogy, the return circuit element is too inefficient to transfer significant energy. 3. The stability of the crossover is not affected significantly when the bulb is properly sized to prevent action unless presented with a gross overload. At that point, being a series component, the crossover will shift somewhat, but the energy level will not increase with respect to the drive signal protecting the driver from destruction which is the goal of this type of protection device. This type of protection really gets a bad rap from the factthan some manufacturers use it to reduce the size of the HF driver and then end up operating the system well into the non-linear portion of the bulb's curve. The proper application of this type of protection (IMO) is as a fall-back type of protection that buys the user added time to correct an improper (especially accidental) situation, but ifthe user then chooses not to, it will not protect over a long period of time from their own ill-actions. Link to comment Share on other sites More sharing options...
Members FX doctor Posted December 8, 2003 Members Share Posted December 8, 2003 I may be wrong, but wouldn't a fuse in a speaker cab essentially fry your amp if you blew the fuse? Link to comment Share on other sites More sharing options...
Members barefoot Posted December 8, 2003 Members Share Posted December 8, 2003 Well, I can't really disagree with you much agedhorse. I do think the drawbacks I stated are real and valid, but their magnitude relative to other issues are certainly up for debate. And I have to confess that I'm heavily biased towards the design of high linearity studio monitors. I don't like the sound of probably 95% of the PA systems I hear. And their deficiencies are usually so numerous that the light bulb problems I listed above would tend to be just drops in the bucket. Not that I'm criticizing live sound engineers. I know the issues are very complex and difficult. Still, I always wish things could be better . Anyhow, I guess you're right. For all practical purposes in most applications my concerns are "theoretical". Thomas Link to comment Share on other sites More sharing options...
Members agedhorse Posted December 8, 2003 Members Share Posted December 8, 2003 Originally posted by barefoot And I have to confess that I'm heavily biased towards the design of high linearity studio monitors. I don't like the sound of probably 95% of the PA systems I hear. And their deficiencies are usually so numerous that the light bulb problems I listed above would tend to be just drops in the bucket. Not that I'm criticizing live sound engineers. I know the issues are very complex and difficult. Still, I always wish things could be better . Thomas I would certainly agree with your comments in the context of critical (and presumibly expensive) studio monitors. Fortunately, there's the bi-amped avtive path which eliminates all opportunity for such concerns! Link to comment Share on other sites More sharing options...
Members D Aussie Posted December 8, 2003 Members Share Posted December 8, 2003 He he.. I knew nothing of sticking light bulbs inside speakers till one show I thought I was going nuts because I kept seeing a flickering of light cvoming from some TR JBLs. What a relief to find out that it was actually a lightbulb! Link to comment Share on other sites More sharing options...
Members barefoot Posted December 8, 2003 Members Share Posted December 8, 2003 Originally posted by agedhorse I would certainly agree with your comments in the context of critical (and presumibly expensive) studio monitors. Fortunately, there's the bi-amped avtive path which eliminates all opportunity for such concerns! You must be reading my mind! Link to comment Share on other sites More sharing options...
Members tlbonehead Posted December 9, 2003 Members Share Posted December 9, 2003 Originally posted by nirvanas silence I may be wrong, but wouldn't a fuse in a speaker cab essentially fry your amp if you blew the fuse? No,it shouldn't. It is basically like unplugging the speaker. Not a big deal with an SS amp. And,if the amp goes DC like the original post was about,it is already a problem. Link to comment Share on other sites More sharing options...
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