Another hypothetical? What natural source is this supposed to model, some random combination of chaotic sources?renx said:Hi,
I have revisited my claims in the last couple of posts, and I did the math again, and these are my conclusions:
There exists such a complex signal that when "passed" through a 1st order butterworth lpf of 20kHz @(-3db), gets minimally linearly distorted (a couple of degrees of phase shift and less than 0.5db in amplitude) but requires 10 times larger slew rate than the slew rate required by the 20kHz pure tone signal of the same amplitude peak.
Much simpler to start with an arbitrarily fast rise time square wave (rifle shot, lightning strike), then LPF that with one pole at some still fast but reasonable rate. If you want to go old school jangle some keys in front of a fast microphone. If the circuitry can't keep up you'll hear the IMD under the key noise.
Sounds like some marketing blather. An amplifier with marginal or inadequate slew rate, will likely have other shortcomings (like gain bandwidth product, etc) that can impact sound quality. Like PRR already mentioned, amplifiers at their slew limit are essentially clipping wrt to their rate of change so severely distorted. While i don't want to generalize, the slew rate limiting behavior of different amplifiers in the region just before slew limiting can also be different, so it is always prudent to have a safety margin.As soon as I get my Mathematica graphs uploaded, I will post them here to back up my claims, and would kindly ask all of you to check for any errors in my theory or calculations.
The reason I am doing this is that I remember reading an article from Yamaha, I think, on how the slew rate impacts the sound of power amplifiers. If I recall correctly they were playing the same material through a couple of amplifiers different by their slew rate only, and same speakers of course, and have concluded that larger slew rate is needed than that of a max freq. Unfortunately I can't find it anywhere!
Normalizing for power supply voltage a 20khz sine wave peak or max rate of change is only around 60mV/uSec/Vp-p. I personally like to design for a power bandwidth of 40kHz so maybe double that for a hard target, but we want a safety margin on top of that.
The good news is that even inexpensive widely available opamps (NE5532/TL074) have been spanking those slew rates for several decades.
An earlier rule of thumb suggested 1V/uSec/V of power supply. That seems more than a little conservative to me.
Nobody is saying make it "just" fast enough for 20kHz, and for decades we haven't had to.
The slow part of this thread's dynamic duo at 15V/sec will deliver a power bandwidth a few octaves above human hearing (>160khz clean sine wave for +/-15V rails). This seems adequately fast to me. While I would also include input RF filtering since even lowly AM radio is higher slew rate than that.
The fact that I have been designing electronics under wrong assumptions for so long is just killing me!
That is why I have to get to the bottom of this.
Regards, Renx.
I'm afraid to ask what that premise is. I recall when we actually had to deal with slow opamps, but thankfully that was a long long time ago.
JR
PS: I seem to recall a thread around here a few years ago about making a bench circuit with variable slew rate that could be listened to with the slew rate necked back just to learn what it sounds like. But this artificial slew limiting would be be cleaner below slew limiting thresholds than we encounter in practical circuits.