Herman makes some good points, but i'd like to take his explanation a bit further.
Think of an electrical circuit as a hose for electrons. Then think of the music as a fire. Different size hoses can deliver different volumes of signal at different pressure levels. Volume and pressure are different things, so don't confuse the two. Fires also burn at different rates in different areas, making some harder to control than others. Music is much like a fire as you have different intensities in different parts of the audible spectrum, all taking place simultaneously.
Think of how fast the signal can make it through the hose ( circuit ) from one end to the other on demand. This is a measure of accelleration and is equivalent to the rise time. In other words, we are clocking how fast the circuit can go from just above a trickle of flow to almost wide open flow. Obviously, when we need water ( electrons ) to put out a fire ( reproduce the signal ), we need to get it there fast. The faster that the circuit can deliver maximum flow, the higher the rise time. The higher the rise time, the more responsive we are to putting out the fire. There's one problem here though.
What happens when we have a hose of a given speed, but different size fires? Hmmm.... Obviously, a garden hose could be used to put out a fire, but whether or not it could keep up with the demand of flow required for a very large fire that changes intensity or direction rapidly is another story. This is where slew rate comes into play.
Slew rate is equivalent to how much flow the hose ( circuit ) is capable of. The bigger the hose ( higher the slew rate ), the more volume we can flow. The more volume that we can flow, the larger the fire ( signal ) that we can put out. After all, it is possible to get water onto a fire in rapid fashion ( fast rise time ), but if the fire ( signal ) is bigger than the volume of water ( limited slew rate ) that we can deliver, that fire ( signal ) goes at least partially unquenched.
The end result would be that our fast response time ( fast rise time ) wouldn't be enough in itself to handle the amount of fire ( signal ) due to a lack of volume of flow ( slew rate ). In effect, we not only need to be able to deliver the signal fast, but we must also be able to deliver the quantity of signal needed as the situation varies.
With all of that in mind, rise time and slew rates are not one in the same and shouldn't be thought of as being directly tied together. While one can derive horsepower figures by looking at the torque rating of a motor at a given rpm, the horsepower and torque curves do not run parallel to each other at any given time. Such is the same with slew rate and rise time. Obviously, rise times and slew rates are two different measures of a circuit i.e. speed and capacity. Having good performance in one area without the other means a limitation in performance somewhere down the line though.
In most cases, a circuit that has a faster rise time will demonstrate a higher slew rate by design. That is, if the engineer / designer is smart enough to take both aspects of circuit performance into account. The faster the rise time and the higher the slew rate, the more responsive the circuit is to any given signal and the less challenged it will be in trying to reproduce that signal. Combining great speed and agility ( fast rise time ) with a capacity for brute force ( high slew rate ) makes for a very well rounded performer.
There are many other spec's that are taken for granted when it comes to circuit speed and finesse that never get mentioned. That is, just as a circuit has a rise time ( how fast it can go up the hill ), there is a fall time to ( how fast it can come down from the peak ). Most circuits offer pretty symmetrical levels of performance here, but not always.
If symmetry is lacking in this area, the peaks will tend to be reproduced TOO avidly, resulting in overshoot. When you get a high peak that wasn't meant to be that sharp, the sound gets "edgy". This has to do with a lack of damping i.e. too slow of a fall or "recovery" time. To continue on with our hose analogy, we can get the water to the fire in both the time and quantity needed, we just couldn't regulate the quantity of flow from the hose before the water itself created further damage. This is sometimes referred to as excessive leading edge energy when one can find a qualified reviewer. They might not understand the what's & why's, but they can hear it taking place : )
Another overlooked spec is Td or the Time Delay of the circuit. Like the hose mentioned above, different circuits have different length paths to them. A shorter hose or circuit can deliver what we need faster with less potential for delay whereas a longer hose or circuit will take longer to deliver the goods. Much like a hose, a longer circuit may end up having more losses along the way with greater variances in flow due to all of the various connections made.
For best results, we want to keep the path short and as simple as is feasible. This results in mininal Time Delay with the least potential for loss or smearing. Most circuits don't do this and the end result is not only a loss in signal, but also timing. The fact that a circuit can have different loss rates at different frequencies along that path can really play games with what we hear. That's because not only can the timing be altered as frequency varies, but the amount of loss can differ as frequency is varied too.
This has to do with dielectric absorption of the parts and why changing passive parts like resistors, capacitors, inductors, wiring, component jacks, etc... can change the sonic presentation that we hear. Not only in terms of tonal balance, but transient response, speed, coherency, spectral purity, etc... These effects also take place in active parts like transistors, diodes, tubes, etc, too. That's why "doping" or the application of chemicals to the external parts of the device can change the electrical and sonic characteristics, etc...
While one could conclude that a longer path is both slower and more lossy ( logical conclusion ), there are also other factors here too. Not only does negative feedback increase the complexity and parts count of the circuit, it also increases the length of the path and the response time of the circuit. The more negative feedback that you have, the more time delay that you'll have. The more time delay that you have, the more loss that you have of various time and frequency related artifacts.
This is why high negative feedback designs sound less "liquid" as they suck the life out of the music. That is, due to the above mentioned non-linear losses with increased parts count and circuit complexity, the timing, harmonic structure and transient response are all mangled to a much higher degree. This makes the music sound lifeless, even though most of the distortion type spec's look great on paper.
This is where the design prowess and listening skills of the manufacturer come into play. It is also the reason why products that are built only to measure well don't sound all that good i.e. the "distortion wars" that resulted in all of the horribly hard, bright and sterile sounding SS gear of the past. That is, the engineers didn't count on all of the negative sonic aspects of electrical error correction and circuit complexity that are involved, nor the non-linearities of the low quality parts that they were using. They assumed that the end would justify the means, which in reality, it really did. Unfortunately, the end sums up the means and the means weren't well thought out or properly executed to begin with. That's why they had to rely on so much negative feedback to get the job done.
This is also why products that can measure similarly don't always sound alike i.e. two different design routes can produce different complexities of circuits with different parts used. As such, the sonics of the circuit are even more important than the measurable accuracy that most engineers base their designs on. This is why many circuits that measure poorly actually sound pretty good. That is, the distortions and non-linearities that they introduce are actually not as detrimental as the means used to achieve some of the better measuring, but poorer sounding gear.
In effect, the "PRAT" or "musicality" of the circuit are directly correlated to all of the above. While there is no real way to quantify the term, "PRAT" is a term that takes into account both the musical accuracy and the electrical accuracy of the circuit. That is, musical notes don't sound like musical notes, whether it is due to non-linear circuit design, using low grade parts, relying on error correction circuitry to compensate for lossy parts and / or non-linear circuitry or any combo of the above. The end result is always audible.
Obviously, there are different levels of PRAT based on how well the circuit does all of the above and preserves the signal being fed into it. With that in mind, once you've heard a system that has even a smidgen of PRAT, you'll know the difference between a "sound system" and a "music reproduction system". One reproduces sounds that emulate music and the other reproduces music. The former is what i refer to as measurable accuracy and the latter is what i refer to as accurate musicality. Sean
>
PS... I did this at 5 in the morning after waking up in the middle of the night. Please cut me some slack if it's a little lacking in flow and / or specificity : )
Think of an electrical circuit as a hose for electrons. Then think of the music as a fire. Different size hoses can deliver different volumes of signal at different pressure levels. Volume and pressure are different things, so don't confuse the two. Fires also burn at different rates in different areas, making some harder to control than others. Music is much like a fire as you have different intensities in different parts of the audible spectrum, all taking place simultaneously.
Think of how fast the signal can make it through the hose ( circuit ) from one end to the other on demand. This is a measure of accelleration and is equivalent to the rise time. In other words, we are clocking how fast the circuit can go from just above a trickle of flow to almost wide open flow. Obviously, when we need water ( electrons ) to put out a fire ( reproduce the signal ), we need to get it there fast. The faster that the circuit can deliver maximum flow, the higher the rise time. The higher the rise time, the more responsive we are to putting out the fire. There's one problem here though.
What happens when we have a hose of a given speed, but different size fires? Hmmm.... Obviously, a garden hose could be used to put out a fire, but whether or not it could keep up with the demand of flow required for a very large fire that changes intensity or direction rapidly is another story. This is where slew rate comes into play.
Slew rate is equivalent to how much flow the hose ( circuit ) is capable of. The bigger the hose ( higher the slew rate ), the more volume we can flow. The more volume that we can flow, the larger the fire ( signal ) that we can put out. After all, it is possible to get water onto a fire in rapid fashion ( fast rise time ), but if the fire ( signal ) is bigger than the volume of water ( limited slew rate ) that we can deliver, that fire ( signal ) goes at least partially unquenched.
The end result would be that our fast response time ( fast rise time ) wouldn't be enough in itself to handle the amount of fire ( signal ) due to a lack of volume of flow ( slew rate ). In effect, we not only need to be able to deliver the signal fast, but we must also be able to deliver the quantity of signal needed as the situation varies.
With all of that in mind, rise time and slew rates are not one in the same and shouldn't be thought of as being directly tied together. While one can derive horsepower figures by looking at the torque rating of a motor at a given rpm, the horsepower and torque curves do not run parallel to each other at any given time. Such is the same with slew rate and rise time. Obviously, rise times and slew rates are two different measures of a circuit i.e. speed and capacity. Having good performance in one area without the other means a limitation in performance somewhere down the line though.
In most cases, a circuit that has a faster rise time will demonstrate a higher slew rate by design. That is, if the engineer / designer is smart enough to take both aspects of circuit performance into account. The faster the rise time and the higher the slew rate, the more responsive the circuit is to any given signal and the less challenged it will be in trying to reproduce that signal. Combining great speed and agility ( fast rise time ) with a capacity for brute force ( high slew rate ) makes for a very well rounded performer.
There are many other spec's that are taken for granted when it comes to circuit speed and finesse that never get mentioned. That is, just as a circuit has a rise time ( how fast it can go up the hill ), there is a fall time to ( how fast it can come down from the peak ). Most circuits offer pretty symmetrical levels of performance here, but not always.
If symmetry is lacking in this area, the peaks will tend to be reproduced TOO avidly, resulting in overshoot. When you get a high peak that wasn't meant to be that sharp, the sound gets "edgy". This has to do with a lack of damping i.e. too slow of a fall or "recovery" time. To continue on with our hose analogy, we can get the water to the fire in both the time and quantity needed, we just couldn't regulate the quantity of flow from the hose before the water itself created further damage. This is sometimes referred to as excessive leading edge energy when one can find a qualified reviewer. They might not understand the what's & why's, but they can hear it taking place : )
Another overlooked spec is Td or the Time Delay of the circuit. Like the hose mentioned above, different circuits have different length paths to them. A shorter hose or circuit can deliver what we need faster with less potential for delay whereas a longer hose or circuit will take longer to deliver the goods. Much like a hose, a longer circuit may end up having more losses along the way with greater variances in flow due to all of the various connections made.
For best results, we want to keep the path short and as simple as is feasible. This results in mininal Time Delay with the least potential for loss or smearing. Most circuits don't do this and the end result is not only a loss in signal, but also timing. The fact that a circuit can have different loss rates at different frequencies along that path can really play games with what we hear. That's because not only can the timing be altered as frequency varies, but the amount of loss can differ as frequency is varied too.
This has to do with dielectric absorption of the parts and why changing passive parts like resistors, capacitors, inductors, wiring, component jacks, etc... can change the sonic presentation that we hear. Not only in terms of tonal balance, but transient response, speed, coherency, spectral purity, etc... These effects also take place in active parts like transistors, diodes, tubes, etc, too. That's why "doping" or the application of chemicals to the external parts of the device can change the electrical and sonic characteristics, etc...
While one could conclude that a longer path is both slower and more lossy ( logical conclusion ), there are also other factors here too. Not only does negative feedback increase the complexity and parts count of the circuit, it also increases the length of the path and the response time of the circuit. The more negative feedback that you have, the more time delay that you'll have. The more time delay that you have, the more loss that you have of various time and frequency related artifacts.
This is why high negative feedback designs sound less "liquid" as they suck the life out of the music. That is, due to the above mentioned non-linear losses with increased parts count and circuit complexity, the timing, harmonic structure and transient response are all mangled to a much higher degree. This makes the music sound lifeless, even though most of the distortion type spec's look great on paper.
This is where the design prowess and listening skills of the manufacturer come into play. It is also the reason why products that are built only to measure well don't sound all that good i.e. the "distortion wars" that resulted in all of the horribly hard, bright and sterile sounding SS gear of the past. That is, the engineers didn't count on all of the negative sonic aspects of electrical error correction and circuit complexity that are involved, nor the non-linearities of the low quality parts that they were using. They assumed that the end would justify the means, which in reality, it really did. Unfortunately, the end sums up the means and the means weren't well thought out or properly executed to begin with. That's why they had to rely on so much negative feedback to get the job done.
This is also why products that can measure similarly don't always sound alike i.e. two different design routes can produce different complexities of circuits with different parts used. As such, the sonics of the circuit are even more important than the measurable accuracy that most engineers base their designs on. This is why many circuits that measure poorly actually sound pretty good. That is, the distortions and non-linearities that they introduce are actually not as detrimental as the means used to achieve some of the better measuring, but poorer sounding gear.
In effect, the "PRAT" or "musicality" of the circuit are directly correlated to all of the above. While there is no real way to quantify the term, "PRAT" is a term that takes into account both the musical accuracy and the electrical accuracy of the circuit. That is, musical notes don't sound like musical notes, whether it is due to non-linear circuit design, using low grade parts, relying on error correction circuitry to compensate for lossy parts and / or non-linear circuitry or any combo of the above. The end result is always audible.
Obviously, there are different levels of PRAT based on how well the circuit does all of the above and preserves the signal being fed into it. With that in mind, once you've heard a system that has even a smidgen of PRAT, you'll know the difference between a "sound system" and a "music reproduction system". One reproduces sounds that emulate music and the other reproduces music. The former is what i refer to as measurable accuracy and the latter is what i refer to as accurate musicality. Sean
>
PS... I did this at 5 in the morning after waking up in the middle of the night. Please cut me some slack if it's a little lacking in flow and / or specificity : )