slew rate and rise time

Rise time is defined as the time it takes a signal to go from 10 percent of it's peak value to 90 percent of its peak value. For example, if you have a 0 to 10 volt pulse, the rise time is the time it takes for the pulse to go from 1 volt to 9 volts. For a 0 top 5 volt pulse, rise time is the time it takes the pulse to go from 0.5 volts to 4.5 volts. Fall time is similiarly defined, but in the other direction. Rise and fall times are just that, time measurements specified by a single unit. Slew rate is a units per time (eg. volts per time) measurement, specified with two units; eg. volts / time.

Slew rate is typically used to describe an amplifier (look at a spec sheet for an op amp). Rise and fall times are typically used describe signals.

One more point as far as interrelationships. If it is not already obvious from the previous posts, if you are using an amplifier to process pulses you would first determine the rise time and fall time of the fastest changing pulse and the change in volts of that pulse, that you expect to encounter and then select your active devices based on that time. That is a starting point. In many instances the passive components that you select and the pcb layout and physical construction may also need to be considered, depending on those times.

So rise time and slew rate are not related to each other? Does an amplifier that has very high slew rate very good transient response? Does it sound more dynamic?

Chris

Dazzdax: For the most part the slew rate of just about any amplifier you are using at home is going to be a non-issue. Any signal, no matter what it looks like, can be broken down into a series of sine waves at different amplitudes and phases (Fourier analysis is the mathematical method for proving this). You can only hear the components out to 20 kHz. The max rate of change of a 20 kHz sine wave is 7.5 v/ usec

If you go through the calculations and then double the value for a safety factor, you arrive at an amplifier with a slew rate of 10 v/usec a capable of accurately reproduce a 20kHz sine wave while providing 400 W rms into 8 ohms. So whether you have an amplifier with a slew rate of 10 v/usec or 10000 v/usec, makes no difference.

If you want to get extreme, an amplifier with a 80 kHz bandwidth and providing 1000 W rms for an 8 ohm load will accurately reproduce all of the frequencies in that bandwidth if it has a 64 V/usec slew rate.

In short, slew rate is not something to brag about in an amplifier. Unless the design is simply terrible, there is not going to be a difference in the sound - the amplifier will be fast enough. The placebo effect is of course another story. More important are such specs as output impedance, distortion, and current capability. Find an amplifier that has low output Z, a low distortion spec and a wattage rating for driving 8, 4, and 2 ohm loads that comes close to doubling for each halving of the load Z.

Pay attention to the specs that make a difference. High slew rates and high damping factors are meaningless.

"Find an amplifier that has low output Z..."

What is "low output Z" and how do you find it? Jeff