By 90<W, do you mean under 90 watts?
Elliot sound products has some good hi-fi amps, at
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I believe they sell amps, but they also have a lot of schematics for DIY projects. You can click on the projects link to see them.
All of their amps that I have seen posted reportedly have a flat frequency response, with very low THD.
About distortion at higher volume, when an amp is turned up, it will clip the signal when the AC signal has a higher peak to peak voltage than the supply voltage of the amp.
Check the level of the input signal, probably about 1/2 to 1 volt peak to peak for line level. Then look at the voltage gain of the amp you are considering. Whatever the voltage gain, multiply this number by the input signal level. The supply voltage has to be a little higher than this or the amp will clip at maximum volume.
Example: input signal at 1 volt p-p. Using a power amp with a gain of 20, 1 volt x 20 equals an output signal of 20 volts peak to peak at max volume. If the supply voltage of the amp is 18 volts, whenever the volume cranked over the 18 volt threshold, the ac sign wave will be cut off, or clipped, at the top and bottom, and it will sound like many cheap amps do at high volume.
Also, no amp can convert all of the supply voltage to an ac signal. Some of the voltage is lost when passing thru the transisitors. An amp with a gain of 20, used with an input of 1 volt, should have at least about a 24 volt supply voltage to ensure no clipping of the signal.
I don't think you will find any simple designs for hi-fi power amps. The closet thing would probably be some of the integrated circuit chips from National semiconductor. They have a couple of high performance amp chips that can work from different supply voltages, from about 20 to 60 volts.
They have a section called overture power amplifiers that list them here.
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