A comment by Perry in another thread raised an interesting question, what parts are run too hard, by design, in various car (and home) amplifiers?
Resistors heating up
It would be perhaps interesting to expand on the theme a bit to design flaws in general.
For example, the Soundstream rubicon series seem to pack dc protection on the output, madness!!
Resistors heating up
It would be perhaps interesting to expand on the theme a bit to design flaws in general.
For example, the Soundstream rubicon series seem to pack dc protection on the output, madness!!
Regulator components and resistors in general
The original question was related to resistors running too hot and a question of how they get something so simple, so wrong.
The linear voltage regulators are probably the simplest circuits in the amp. There is no change in their operating conditions, no matter how hard the amp is run. The temperature of the amp may vary slightly but not enough to account for the problems caused by the heating of the regulator components.
In some amplifiers, the components run so hot that they damage the circuit board. I've seen regulators in expensive home receivers ($2000+ in the 90's) have regulators run so hot that the regulators charred the board badly enough for the regulators to start to fall through the board. They had no heatsinks. In other receivers, the regulators were on heatsinks but the heatsinks were not flat from the punching/manufacturing process and there was no heatsink compound so the regulator components again ran hot and failed. The equipment was otherwise well designed but they got this simple thing wrong.
The fix is often simple. For some amplifiers, you can simply use a clip-on heatsink (with heatsink compound) to help dissipate heat. For some you can use larger components (resistors, transistors...). In some amps, I move the regulator transistors to the main heatsink. For voltage-dropping resistors, you can leave the leads longer to keep the resistor body farther from the board and/or use higher wattage resistors which will have more surface area and therefore run cooler.
For some situations, the hot components can simply be moved to the other side of the board. Look at the way the heat will rise when the amp is in operation. Moving the resistors (and/or transistors) to the heatsink side of the board (with proper insulation and thermal conductors like gap pad or heatsink compound), can not only reduce the heat rising to the board but can pull the temperature down through conduction.
In some instances (this applies to driver components in some amps), you can add a small fan to help cool the area. Even a small amount of airflow will make a big difference. Remember that most components can safely operate at relatively high temperatures (with proper derating) and be perfectly reliable. What's hot to the touch is icy cool for the components.
Back to board damage... The damage to the board is often more than simply the board becoming brittle. It sometimes becomes conductive. When it does, the conductive part of the board has to be removed. It's not generally pretty but there isn't much you can do about it. Where there is enough good board left to support the components, removing the conductive board may be enough. In others, in addition to removing the conductive material, you have to relocate or hardwire components to reestablish connections.
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DC offset protection or lack thereof
For some amps, there is no offset protection. One reason that the manufacturer may not have included it is that the over-current protection along with the feedback circuit can generally be relied on to serve the same purpose. The feedback circuit is designed to correct for minuscule variations in the DC on the output of the amp. When an output transistor or some other component fails and causes DC to be driven to the output of the amplifier, the dive circuit drives as hard as it possibly can to correct for the offset. This will generally trip the over-current protection. Dedicated DC offset protection circuits are often a plan B. For someone using extremely expensive or rare speakers, it's better to have the DC offset circuit in case the output stage is destroyed (including emitter/source resistors), which will make render the over-current circuit useless to protect the speakers.
The original question was related to resistors running too hot and a question of how they get something so simple, so wrong.
The linear voltage regulators are probably the simplest circuits in the amp. There is no change in their operating conditions, no matter how hard the amp is run. The temperature of the amp may vary slightly but not enough to account for the problems caused by the heating of the regulator components.
In some amplifiers, the components run so hot that they damage the circuit board. I've seen regulators in expensive home receivers ($2000+ in the 90's) have regulators run so hot that the regulators charred the board badly enough for the regulators to start to fall through the board. They had no heatsinks. In other receivers, the regulators were on heatsinks but the heatsinks were not flat from the punching/manufacturing process and there was no heatsink compound so the regulator components again ran hot and failed. The equipment was otherwise well designed but they got this simple thing wrong.
The fix is often simple. For some amplifiers, you can simply use a clip-on heatsink (with heatsink compound) to help dissipate heat. For some you can use larger components (resistors, transistors...). In some amps, I move the regulator transistors to the main heatsink. For voltage-dropping resistors, you can leave the leads longer to keep the resistor body farther from the board and/or use higher wattage resistors which will have more surface area and therefore run cooler.
For some situations, the hot components can simply be moved to the other side of the board. Look at the way the heat will rise when the amp is in operation. Moving the resistors (and/or transistors) to the heatsink side of the board (with proper insulation and thermal conductors like gap pad or heatsink compound), can not only reduce the heat rising to the board but can pull the temperature down through conduction.
In some instances (this applies to driver components in some amps), you can add a small fan to help cool the area. Even a small amount of airflow will make a big difference. Remember that most components can safely operate at relatively high temperatures (with proper derating) and be perfectly reliable. What's hot to the touch is icy cool for the components.
Back to board damage... The damage to the board is often more than simply the board becoming brittle. It sometimes becomes conductive. When it does, the conductive part of the board has to be removed. It's not generally pretty but there isn't much you can do about it. Where there is enough good board left to support the components, removing the conductive board may be enough. In others, in addition to removing the conductive material, you have to relocate or hardwire components to reestablish connections.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DC offset protection or lack thereof
For some amps, there is no offset protection. One reason that the manufacturer may not have included it is that the over-current protection along with the feedback circuit can generally be relied on to serve the same purpose. The feedback circuit is designed to correct for minuscule variations in the DC on the output of the amp. When an output transistor or some other component fails and causes DC to be driven to the output of the amplifier, the dive circuit drives as hard as it possibly can to correct for the offset. This will generally trip the over-current protection. Dedicated DC offset protection circuits are often a plan B. For someone using extremely expensive or rare speakers, it's better to have the DC offset circuit in case the output stage is destroyed (including emitter/source resistors), which will make render the over-current circuit useless to protect the speakers.
Lack of heat sinking.
Sony multi channel home amplifiers frequently used STK amp chips as drivers for their discrete output stages, they run hot as hell and eventually short causing the amp to go into protection. Simply add a sink to them and they last forever. Same thing with Clarion car amps (the purple ones with powerguard) and Clarion made McIntosh branded car amps.
Sony multi channel home amplifiers frequently used STK amp chips as drivers for their discrete output stages, they run hot as hell and eventually short causing the amp to go into protection. Simply add a sink to them and they last forever. Same thing with Clarion car amps (the purple ones with powerguard) and Clarion made McIntosh branded car amps.
a lack of decoupling can allow through spikes off the mains and put transistors into breakdown.
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