PS AUDIO / Paul McGowan
In a recent post, we discussed some types of distortion that are not typically part of a manufacturer’s specifications. One of those terms, slew rate, generated a bunch of questions and requests for me to explain what it is.
I will do my best in as simple of terms as possible.
Slew rate refers to the rate of change in voltage over time. It is typically measured in volts per microsecond (V/µs). For example, a slew rate of 1V/µs would mean that the maximum rate of change for an amplifier circuit in 1 microsecond is 1 volt. If the amplifier circuit was a preamp or, worse, a power amplifier that needs to get to higher voltages (like 40 volts for a power amp) it would take more time than is available and we would get distortion.
The higher the slew rate the better.
How high is enough?
1V/µs is 1MHz, which is plenty fast for audio but only if you need to get no higher than 1 volt in one microsecond. Higher volts take longer and thus you can easily do the math. The ways around this are to limit the input frequency so the amplifier isn’t required to handle higher frequencies or to raise the slew rate.
My rule of thumb is a 10X greater slew rate than the system needs (the old 10X rule).
Low slew rate is not a good thing. If the system can’t follow the incoming signal it goes into what we call slewing: resulting in slow and sluggish voltage changes, leading to distorted and smeared audio signals. This is especially noticeable in high-frequency and transient-rich musical signals.
Hope that’s not too technical.