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mixing them. This is most pronounced when at higher volumes. I thought that I might be overloading my amplifier, but it acts the same if listened to through headphones.
If the sample values going into [dac~] are outside of [-1, 1] then you are clipping the signal... (that's the soundcard clipping not the amp) solution; place before the dac~, [/~ n] for n = the number of signals you've added together.
andy