Slew rate and rise time

Bigtee: There have been questions here that i've wanted to respond to in this fashion and i've had private requests to try and explain this too. This specific thread caught me at the right time and in the right mood to attempt this type of "article". I was able to write all of this in about 45 minutes ( including additions and revisions ), so it wasn't as bad as i had thought it was going to be. Obviously, it is not all inclusive, but it should help explain a few things and give people that are interested in learning something to work with.

This might be a good thread to keep going with other technical questions pertaining to spec's, measurements and what we hear. This would give newcomers a point of reference as to what spec's are, how to interpret them, how they interact with each other, etc... Only problem is, getting people to post the pertinent questions & comments within this thread and then keeping those that shouldn't be posting comments & questions in this thread away from it.

I don't have a problem with people presenting alternative points of view, but i do have a problem with those that present information in dogmatic form that they've read elsewhere without having any first hand knowledge on the subject and / or those who refuse to either consider alternative points of view and / or won't learn on their own.

Having said that, most all points of view have some form of validity to them and the key factor is being able to fit all the different pieces that make up the puzzle together in a cohesive manner. Too many people want to grab hold of one aspect of operation and / or theory at the expense of how that part of the equation factors into the sum product. The end result is a partial understanding that is based on fact, which is what makes it believable, but may not be completely applicable when one considers all the factors involved in the grander scheme of things.

This is why i've said that spec's can be used as a very useful tool, but only if the spec's are valid and the type and quantity of spec's are suitable for a meaningful interpretation.

The only variables beyond the spec's would be the actual parts used and the lay-out of the circuit, which can surely present some variables to the sonic outcome of any given product. None the less, and as i've shown on several different occassions, "reasonable guesstimates" of what to expect sonically are possible even with those variables entering into the equation.

Part of this has to do with common sense. If one starts off with a junk platform, even the best parts and excellent execution of the circuit can't make it an awesome performer. On the other hand, even an excellent design that is saddled with poor parts and / or less than optimum execution isn't going to perform the best either. That's why we end up with so many mediocre products i.e. all the factors aren't considered evenly and the end result is a "happy medium" that the bean-counters can live with. Sean
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Thank you Sean for your descriptive analogy. I'll keep it also. You know, another issue for discussion is also what the relationship is between gain, volume and amount of power that an amp delivers. Let's say an amp is capable of delivering 200wpc into 8 Ohms. Is this with the volume knob wide open or doesn't it relate to the position of the volume knob? But this is as I say another issue to discuss. I'll start a thread about this.

Sean, your analogies are very colorful, one might say even fiery, but I am afraid they are not exactly right. Given the fact that part of what you say is correct and some of it isn't makes it hard to follow.

"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." No, how fast it makes through the hose would be propogation delay. The signal could take all day to make it from one end to the other and still have a very fast rise time once it got to the output. The two are unrelated.

"Slew rate is equivalent to how much flow the hose ( circuit ) is capable of." You also refer to it as "volume of water." No, this would be the equivalent of electrical current.

Slew rate, by definition, is simply the maximum rate of change of the output voltage for all possible inputs. In simple terms, how fast the voltage can change.

Mathematically it is the slope of the line on a graph plotting voltage versus time. Take the amount the voltage has changed, divide it by the time it took to change, and you have the slew rate, usually expresed in volts per microsecond.

Since music is made up of sine waves, and higher frequency sine waves have faster rates of change (the fastest being where it crosses zero) at some frequency this rate of change will exceed the slew rate of the amplifier. It will distort this frequency and all of those higher.

Therefore, we can say that the bandwidth of an amplifier is slew rate limited.

Rise time is how long it takes the voltage to get from 10 to 90 percent of it's peak as I stated above. You are correct that there is also a fall time and it can be different.

I admit I may have oversimplified the relationship between risetime and slew rate. Even though you can calculate a rate of change based on the risetime it is not the same as the slew rate. It is possible that a circuit can have a high enough slew rate to handle a signal but still be unable to do so because it is limited by the rise time, and vice versa.

From a practical point of view it makes more sense to talk about rise time with digital circuits since they are always switching between 2 voltage levels. For instance, a zero represented by zero volts and a one represented by 5 volts. The rise time would be how long it takes to get from .5V to 4.5V. The falltime how long it takes to get from 4.5V to .5V. In an analog circuit that has an infinite number of different levels it probably makes more sense to describe it in terms of slew rate.

I could go into a long winded mathematical explanation of the two and show how they are both related to the RC time constants of a circuit, and therefore both are limiting the bandwidth of a circuit, but that is probably more than is needed and it is very difficult to do mathematical equations on this forum.

Sean, I think Herman is right: http://www.amplifier.cd/Tutorial/Slew_Rate/SlewRate.htm.
But I also understand your analogy, Sean. Is slew rate analogous to a car's acceleration speed and rise time to the time a car needs to go from 0 to 90 mph? The slew rate and rise time are a bit related to each other, but if plotted a device with a certain rise time can have another slew rate if compared to another device with the same rise time.

The term acceleration is not used correctly in Sean's analogy. The correct term would be velocity, but the analogy is still wrong. Velocity is how fast something is moving, how much distance you can travel in a given period of time. Acceleration is how fast the velocity is changing, you are moving but either speeding up or slowing down. Neither slew rate nor rise time have anything to do with speeding up or slowing down, only moving at a constant rate.

Velocity is a rate of change.
Acceleration is a changing rate of change.

If you want to use a speed analogy then slew rate is the velocity of the voltage. It is not related to the "volume" of the signal, at least I don't think it is because in 25 years I've never heard anyone talk about the volume of a signal except in terms of current.