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RE: Slew Rate

Posted by tomservo (M) on December 1, 2013 at 06:47:52

In Reply to: Slew Rate posted by middleground on November 21, 2013 at 11:36:14:

Slew limit is the maximum rate of change the amplifier (usually the output stage) can produce. At slew limiting, any signal you feed in, is converted to a triangle waveshape who's slopes are at the slew limit.
The issue is that during slew limiting, any information in the input signal is lost other than fundamental frequency of the triangle wave.
Early SS amplifiers before feedback had a very low high frequency corner but they also had a lot of gain .
That gain could be exchanged for bandwidth using negative feed back and so with a sine wave, those amplifiers could go to 20KHz or beyond (although most used a De-coupling choke wound over a resistor to disconnect the load higher up).

Tubes have no bandwidth limit problems, they have bandwidth limiting issues as nearly any audio tube is happy working at least into the AM radio band.
The limiting things are capacitor coupling and especially the transformers which have level and frequency dependent issues and add a unique "spectrum" of harmonics which has the same envelope shape regardless of input freq (kind of weird). In any case, Tube amps rarely use huge amounts of negative feed back and it isn't needed to bump the amps inherent open loop corner response of say 100Hz, out to 40KHz as with SS.

Anyway, Slew limit distortion (was also called TIM once upon a time) slew limit became another hifi sales buzz word where it is assumed more is better.

How much do you need?
Lets pretend one had recordings made with modern electronics, microphones and like all CD's your upper bandwidth ends at 21Khz more or less.

So next take the maximum RMS voltage your amplifier can produce and recall the peak voltage in that sine is that RMS voltage times 1.414.

Then recall the maximum rate of change for a sine wave is 2 *pi times the peak Voltage.

From that info, you can pick a high frequency like 21KHz, figure out it's time period for one cycle, figure out what the rate of change is for that undistorted sine wave at full Voltage swing and then convert the period to Volts per micro-seconds.

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