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. |
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. |
Jj2468 & Rleff : In general, the specs for output Z will be listed in the owners manual of the amplifier or on the manufacturers web site. One should also note the input Z as this can come into play when interfacing a preamp or source to the amplifier.
As to damping factor, a damping factor of 200 or 2000 will be indistinguishable in practice. The damping factor is just Zspeaker / Zout amp. One could say that this is equivalent to specifying Zout of the amplifier, and, technically that is true. The problem that I have with paying a lot of attention to the damping factor is that it is a step removed from the fundamental parameter, the output Z of the amp. One must keep in mind a caveat - the damping factor has to be stated with regard to a given load impedance, usually this is 8 ohms. But, no speaker has a constant impedance from 20Hz to 20 kHz. In deciding whether the changes in speaker impedance over frequency will affect the sound, it is simpler and more straightforward to look at speaker impedance curve or the minimum and maximum speaker impedance specs and then compare those to the amplifier output Z. So long as the ratio is better than 100 or so, you should be fine. Whether it is 200 or 2000 is not going to matter. When it comes to figuring out how to select components for a system and what could cause problems in a system, thinking in terms of base parameters rather than derived parameters makes life easier because it lessens the tendency to unnecessarily complicate the the issue. |
Rleff - Keep in mind what "damping" is, as opposed to "damping factor". If you look at the response of a system, any system, to an input signal, the question is how well does the output follow that input - both in terms of time and magnitude. At the extremes systems that are not properly damped are will either be too slow or will have too much overshoot. Damping can be accomplished by mechanical, electrical, pneumatic, etc, methods. Damping factor on the other hand, at least in terms of audio amplifiers (as opposed to the definition found in control system theory) is simply a ratio of impedances and at least one of the impedences is never fixed.
Jj2468 - in response to the other half of your question - a low output impedance will be somewhere less than 0.04 ohms. Most solid state amps will be there without a problem. You are more likely to have high output impedances with tube amps. |
My guess is that the term "damping factor" was at some time in the past appropriated by audio manufacturers for marketing purposes. The term is defined in automatic control system theory where as the product of the damping ratio and the natural undamped frequency. The damping factor is the distance in the left half of the s or plane along the real axis. It is a term specified wholly in terms of mathematics and is completely generic. It's application is in determining the transient response of a system to a step imput and requires that the system first be expressed in terms of a characteristic equation. Factors that are found in a typical electromechanical system include the torque constant, load intertia, amplifier gain, armature resistance, back emf constant etc, acted upon by appropriate factors.
In short, damping factor as it is understood by engineers in all disciplines is not reduced to a simple ratio of two numbers. Since the term has an actual scientifically accepted meaning, my guess is that the term was appropriated to lend scientific status to what someone was trying to sell.
That all being said, knowing the input and output impedances of the components of a home audio system is helpful in choosing which items to connect together in a system, how to connect the items, and how to avoid or address problems. |
For the accepted engineering definition of "damping factor" see for eg. Automatic Control Systems, Kuo; or, Digital Control System Analysis and Design, Phillips and Nagle. Both published by Prentice Hall. The first edition of Phillips and Nagle has a very nice graphical description of how the transient response of a system changes depending on pole location - of course in the z-plan rather than the s-plane seeing as in that instance the analysis is based on on a discrete rather than a continuous analysis (z in that analysis has nothing to do with impedance but comes from the name given for tranforming number sequences to the frequency domain).
Once you review the definition accepted by the engineering community, you will see that damping factor has to do with a system, not merely one component of the system or one set of measurements of a system. The point is that the term, as it has been 'borrowed' is a marketing tool. This is common in the marketing to audiophiles - see the recent thread on the guy who was trying to sell his product to correct for the "doppler effect". The doppler effect has a real scientific definition and by that definition it makes no sense to talk about the doppler effect occurring within an electronic amplifier.
The appropriation of terms in this manner works in marketing like this - from the naive purchasers viewpoint "Oh, the (name the borrowed term) is something I have not considered in my system. It certainly sounds official, and it's "science" so it must be important. Now there is something that I can purchase that will deal with this. If I don't correct for the (name the borrowed term) then my system will not be optimized. I better buy it. Then I will have dealt with another aspect of system degradation." Of course when engineers and scientists debunk the practice, the response shifts to "Oh, I don't know why it works it just does - you can hear it - although sciene and engineering may be able to send man into space they cannot explain the complex world of home audio" or "but these are not simple sine waves - music is complex." |
Krkus: Very nice. I like the speaker damping - stuffing leading to damping factor for an amplifier etymology. I also like your reasoning based on the multiple tap amplifier technology of bygone times. |
