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Comment: Re:Cool but... (Score 1) 126

by mjmahon (#36020694) Attached to: AppleCrate II: Apple II-Based Parallel Computer
Bingo on the annunciators and the button inputs. But I did intercept the TTL output of the speaker flipflop. That is a much cleaner signal than what comes out of the AC-coupled speaker amplifier, and it is readily available on a solder pad intended for an integrated audio amplifier chip that was never populated on the Apple //e boards. This TTL level is actually full of digital noise, as you would expect, so it is used to drive an inverter powered by a "quiet" +5v level created from the +12v supply to the audio processor board. The output of this inverter is clean, and is mixed with other similar outputs prior to being filtered by a three-pole Sallen-Key lowpass filter to eliminate the 22.05kHz PWM pulse frequency. -michael

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