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In Reply to: RE: Misunderstood posted by deafbykhorns on June 10, 2017 at 16:13:20:
" We all know music is under 12k but I'm sure there's a reason why we choose transformers with extended bandwidth, sometimes 100khz and above."
If we look at a simple, single pole 6db per octave low pass filter we see that the amplitude is still down 1db one full octave below the -3db point.
A OPT that is rated for -3db at 15kHz will still be down 1db at 7.5kHz
Then there's the matter of phase shift. A filter causes not only a loss of amplitude but also a shifting of phase. The music's phase is shifted for a decade from the -3db point. So if the amplitude is -3db at 15kHz the phase will be shifted all the way down to 1.5Khz
If we want the response (phase and amplitude) of a system to be flat 20Hz-20kHz we need the filter's (high pass and low pass) -3db points to be 2Hz-200kHz.
If we want response of a system to be flat 50Hz-15kHz we need the filter's (high pass and low pass) -3db points to be 5Hz-150kHz.
These are "tall" orders, especially for an output transformer, but there are real world benefits in meeting them whenever possible.
BTW There are low pass and high pass filter everywhere in a circuit. Every cathode bypass cap forms a filter, every coupling cap forms a filter. The Miller capacitance of each stage forms a filter. Stray inductance, stray capacitance etc. It's not just OPT's.
Tre'
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- Engineering reasons - Tre' 07:27:58 06/11/17 (0)
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