Bandwidth question?

I personally find that amps with a usable bandwidth out to 150 KHz hit a sweet spot for speed, clarity, and transparency without opening up the can of worms that ultra-wide bandwidth introduces nor the too-sweet, closed-in sound of amps that just make it out to 20KHz.

Amps linear to 150 KHz (and I'm simply relating my personal experience from preference--I've owned around 20 amps over the last 40 years) have a combination of body and warmth, plus transparency and clarity that I like. Although few of us hear *frequencies* beyond 20Khz, I suspect all of us can hear the difference that the wider bandwidth confers on square wave *rise time* in regard to clarity and transparency that accompanies a bandwidth of 150 KHz and beyond.

Amps with ultra-wide bandwidth have an unmistakably stunning clarity, but are sometimes accompanied by over-ring and treble brightness at the expense of warmth and body, both musical values. And sometimes an ultra-wide bandwidth design can run into an oscillation problem with the speaker interface. It's my understanding that this is why MIT designed a special network for their speaker cables to prevent oscillation in their collaborative audio show system with electronics from ultra-bandwidth Spectral and speakers by Hales.

For Your Information!

http://www.cco.caltech.edu/~boyk/spectra/spectra.htm

High frequencies above 20Khz matter!

So what is your concern? Do you feel like you are missing something in the high end? Air, space around instruments? Not always the amplifiers fault. A super tweeter might resolve issues.

There is a good article on wide bandwidth here:

http://wilson-benesch.com/downloads/whitepapers/Wilson_Benesch_ACTC60_Loudspeaker_White_Paper.pdf

So far I've not seen this mentioned yet, IMO this is one of the most important reasons to have some bandwidth:

Phase shift accompanies bandwidth limitations. In general, you can expect phase shift components to manifest to 1/10th the upper cutoff frequency, so if the amp cuts off (begins its rolloff) at 50KHz we will hear artifacts announcing that at only 5KHz.

This is true on the bottom end as well, phase artifacts will be heard at 10X the cutoff frequency, so a 20KHz rolloff will have artifacts up to 2KHz.

A premature HF rolloff will indeed manifest to our ears as a darkness or slowness in the amp at high frequencies. Oddly, if the rolloff is severe, it can have a brightness and a darkness at the same time! Phase shift can have a pronounced effect on the amp's ability to portray an accurate soundstage as well.

An LF rolloff is heard as a lack of impact. So yes, 2Hz is the minimum cutoff frequency if you want the bass to play right.

The fact that a speaker does not have this sort of bandwidth really does not seem to be relevant- you can hear these artifacts easily despite a lack of bandwidth in the speaker.

Excessive bandwidth on the high end can lead to RF problems. If the amplifier is capable of amplifying broadcast frequencies, you can expect to fry a lot of tweeters. You can also expect the amp to run hot, in the case of transistors. It is for this latter reason that many SS amps are bandwidth limited. It is for the former reason that we limit the bandwidth in our amps (our output section goes well into the megahertz region). Another reason to limit bandwidth is that if the amp is amplifying an out-of-band signal, it can gooble up power that might otherwise be used to drive the speaker with music.

Finally, if you have too much bandwidth you do run the risk of stability. This is particularly true if the amp runs feedback (ours get the bandwidth without feedback, BTW). Due to propagation delay times in the amp, negative feedback can become positive feedback if the frequency gets too high! For this reason feedback loops have to be handled with extreme care by the designer to prevent oscillation (heck, you *always* have to be careful with that anyway), as super HF bandwidth is tricky- you don't want the amp to have a reputation for blowing up, or blowing up speakers!

Thanks Atmasphere for taming famous American "Bigger-Better"!