Technical Question about "Slew Rate"...

[Lowcountrypsulion]

Hey everybody. I recently read an Ebay listing for an old Alpine "Juba" amp that was describing the sound quality by pointing out a fast "slew rate".

What is "slew rate" and how would you describe the slew rate of the old-school PG amps, specifically the M or MS series?

Thanks

[ttocs]

slew rate - wow back to the tech school days lets see if I can recall.... To make it easy, its how fast it can change direction on the signal and is measured by the change in the voltage/change in time. These are normally used to describe how accurate a signal generator on a square wave or some other quick changing signal. It really doesn't apply much in an audio amplifier because if the slew rate was that bad then it would make it sound bad by not accuratly reproducing the input signal.

[Lowcountrypsulion]

Thanks, ttocs! I appreciate the explanation. I was suspicious of that selling angle because I looked in the M44 manual and saw nothing about slew rate.

[wooferdog]

remember wow and flutter in the old tape decks

[ttocs]

pretty sure it was nothing more then some snake oil that alpine put in their amps that year to help them sell.

[Mr. Wild]

PG boasts using fast 25MHz devices in their manuals, which boils down to the same thing as high slew rate.
Using fast devices reduces TIM distortion because a typical output stage uses large amounts of feedback. Even though an audio amplifier is used to output only low frequencies in the kilohertz range, it must be able to handle much higher frequencies internally (megahertz range) for the feedback to work without creating too much distortion.

Dr Matti Otala from Finland discovered and resolved Transient Intermodulation Distortion and designed the famous Harman Kardon Citation XX amp.

May this be the reason or not, I think the best bit sonically about PG amps is the beautifully clear highs they reproduce like hi-hat and cymbals and shit .

[Lowcountrypsulion]

Mr. Wild, thanks for the detailed explanation and additional information. I appreciate it!

Here is a link to the Ebay listing in case anyone is curious about the "slew rate" sales pitch:

http://www.ebay.com/itm/Alpine-3545-Leg ... 138wt_1068

[ttocs]

not sure about the sq but I know that those old alpines never seem to die. Its amazing how many of them are still being used and half of them come in looking like they survived some old missle tests but always seem to work....

[shawn k]

PG boasts using fast 25MHz devices in their manuals, which boils down to the same thing as high slew rate.
Using fast devices reduces TIM distortion because a typical output stage uses large amounts of feedback. Even though an audio amplifier is used to output only low frequencies in the kilohertz range, it must be able to handle much higher frequencies internally (megahertz range) for the feedback to work without creating too much distortion.

Dr Matti Otala from Finland discovered and resolved Transient Intermodulation Distortion and designed the famous Harman Kardon Citation XX amp.

May this be the reason or not, I think the best bit sonically about PG amps is the beautifully clear highs they reproduce like hi-hat and cymbals and shit .

That's a pretty good explanation right there!

In addition, here are a couple of things to keep in mind when talking about "slew rate": I wouldn't go as far as saying slew rate is not important. Adequate slew rate is essential for undistorted high frequency as well as high transient response. Granted as frequency drops slew rate becomes less important, but if the intent is to use the amp at all for high freq. reproduction then it's something that at least should be considered! Now it's something that perhaps one shouldn't necessarily lose sleep over, but a high slew rate is at least one clue of how well the amplifier is designed (mainly the feedback circuit). Actually, when talking about class A & AB (full bandwidth) amps, the slew rate is more important than damping factor! Zeff has stated multiple times that he prefers a high slew rate over a high damping factor whenever he designs an amp

Also keep in mind that the more power an amplifier can produce, the higher the slew rate needs to be since the voltage swings will be larger. Good slew rate can range anywhere from 5v/us up to 500v/us, but you should only compare slew rate with amplifiers of approximately the same power to get an idea of what's good.

[ttocs]

I was not trying to say that it was not important, it is just basically assumed that any amp worth its weight would have a good slew rate. The idea of an amp is to reproduce an amplified signal accurately and if it had a horrible slew rate it would not be able to do this very well.