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AVR Drum Synth Improvements

There were some immediate problems to address in my first draft of the drum circuit and fuelled by some Easter feedin’ I sat down to trawl through them.  Here’s the reworked schematic:

Summary of changes:

  1. The resistors on the output of the ATTINY45 have been tweaked to give a broader output PWM of around 1.6v to 5v
  2. These saturate the first unity gain op amp to generate a PWM signal with a range of 1.6v to 4.3v.
  3. The resistor after the low pass filter is much bigger, 1M so it doesn’t load the filter and screw up the signal.
  4. The second op amp gain is fixed at -0.5 (inverting 470k/1M).  This gives a nice clean signal for the transistor.
  5. The level adjust is now just a linear pot over the second op amp output.

Because the output level adjust pulls down to ground, a reduced signal is much lower so the transistors draw less current.  Now the Instead of always draining a constant 80mA the circuit now drains 20mA at low volume up to 80mA for absolute max.

First draft of this circuit in progress:

The rightmost pot is the level control for the drum – the leftmost two are parameters which will plug into the ATTINY45.  The leftmost switch is the power, the middle one select output on the amplifier or direct from the TL072 (not quite sure how to wire this up yet), the final switch selects whether the output is stereo/mono.

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