Bandwidth question?

I agree that most of us can hear only up to perhaps 15kHz but higher system bandwidth guarantees low phase shift. For instance my Rowland 102 amp has -3dB bandwidth of 65kHz but at 15kHz has phase shift of about -20 degree. Even at 10kHz it is still around -15deg. This phase shift affects sound (summing of harmonics). I would feel more comfortable with at least 100kHz bandwidth but agree with Al that bandwidth too high has problems of its own. Obvious problem is HF noise pickup but less obvious is increased chance of Transient Intermodulation (TIM) since reduction of the bandwidth at the input reduces TIM.

Magfan,
That's true, but sensitivity of the amp does not end at -3dB point. Also antenna reception still exists at 1/10 of the wavelength. If we take, in your example, level drop of 20dB/decade it will be only -43dB at 10x10x150kHz=15MHz. Your full wave antenna is now 60ft offering reception up to 1/10 of the wavelength = 6ft.

Noise might be also capacitively coupled. At high frequencies any connector might become an input. Speaker output for instance is also an input of negative feedback. It has very low output impedance but only for low frequencies. Properly designed amplifier will have filters either RC and/or common mode chokes etc., but it only reduces noise pickup and not eliminates it completely. Some amplifiers (common to most of opamps) exhibit rectification phenomena where small amounts of very high frequency signal that is modulated (radio stations) converts to even smaller amount of audible signal because of uneven rise and fall times.

First obvious remedy is to avoid long cables if possible (where cable is still some antenna but skin effect does not provide shielding). Using shielded balanced ICs helps as well as power supply filter/conditioner, but it is better to avoid 500kHz amplifier because it is just asking for trouble. 44kHz seems a little low to me - I would settle for 100kHz. On the other hand newest Rowland 625 extends to 350kHz and Jeff Rowland is definitely a guy who knows.

"Why 10 times the audbile bandwidth is neccessary" - Because of phase shift. Rule of thumb says that phase shift at 1/10 of -3dB cutoff frequency is about 5 deg and that might be significant for proper summing of harmonics.

Magfan, my class D Rowland 102 has 5Hz-65kHz -3dB bandwidth but I'm saving my Pesos for Rowland 625 with small possible temporary setbacks in the future like divorce, bankruptcy and foreclosure.

Orpheus10,

We're all talking harmonics here. I mentioned summing of harmonics twice while Al mentioned it also few times. Highest fundamental is around 8kHz (church organs) but most often it is only up to about 2kHz as it is shown in this interactive chart:

http://www.independentrecording.net/irn/resources/freqchart/main_display.htm

I noticed that Harp has absolutely incredible range - never expected that.