Clock, Step and Hold logic

Preamble

Following on from Sequence Dancing – Schmitt_trigger_NAND_implementation, and the number of issues with that schematic (from CMOS 4017 Based 8 Step Sequencer)…

clk_input

Basically, the HOLD function of the schematic above does not work:

  1. An inverted (inverted by the HOLD circuit debouncing Schmitt trigger – see below) active low button press to the HOLD control (i,e, where the input to the circuit above is active high),  does not hold, but, rather, enables the clock. The active low HOLD button press is inverted by the Schmitt trigger NAND gate (see schematic below), providing an active high HOLD input to the schematic above. However, this actually enables the clock, rather than holding it.
  2. A positive edge is created when the HOLD is released, thereby advancing the clock asynchronously
  3. If the active low button press is passed through an additional inverter, so that the input to the circuit above is now active low, then the action of pressing hold gives a rising edge clock, thereby holding on the subsequent step, rather than the current step..

Continue reading Clock, Step and Hold logic

Synth Power Supplies

Preamble

Sort of following on from VCO madness, I was really looking for the Super Simple VCO (which seems to be actually a 555 timer as a VCO, see the GetLoFi LFO in Ha Ha Ha… 555 fun). However, I got distracted and ended up analysing PSUs and buses for synths.

For ease, in order to power my synth, I had been planning on using a couple of single rail 12V SMPS designed for LED lights, in order to make a dual rail supply, but I started to wonder about the noise implications – as all designs for synths appear to use the old fashioned transformer, rectifier, and linear regulator approach.

(As an aside, Julien Ilett also points this potential noise (as related to buck convertors and SMPS) issue out in Julian builds: a Split Voltage Power Supply (+/-12v and 5v) @5:20. See also, Julian’s Postbag: #85 – E.Ink, Spring Onion, Split PSU (@7:33-8:00), where he mentions that high frequency noise is not removed by the linear regulators in the Split Rail Power Supply Kit Build – SEPIC Cuk Topology).

Continue reading Synth Power Supplies