LF398 Sample & Hold ICThe National Semiconductor LF398 Sample & Hold Amplifier is an inexpensive ($2) monolithic S&H IC that provides the sample and hold functionality. With a logic high level on pin 8 the output tracks the input; with a low level applied, the output holds the input level as it was when the logic low was first applied. For typical synth S&H applications, you want to apply a very short low to high pulse so it quickly acquires a new value and holds it.The Pink Noise output was used for the default input, as the white noise gave a bit too much input swing. LFOThe Rate Generator is the same basic triangle-square oscillator used for the LFO. Additional circuitry was added to provide the GATE output and a narrow pulse to trigger the LF398. My original circuit took the signal off the squarewave output at the Q9/Q10 emitters, but that was 90 degrees out of phase of what it should have been.The new circuit uses an LM311 comparator to turn the triangle output at S8 into a square wave; this becomes the GATE output. A transistor inverter follows this, and then is capacitively coupled to trigger a 555C (CMOS 555) timer. The 10K and .022uF timing components give about a 250uSec positive pulse. The 555 pulse feeds two transistor inverters. These are really unnecessary in the new circuit; the 555 could have driven the LF398 logic input directly, but it was more convenient to tap into the existing circuit at this point. Glide CircuitA glide/portamento circuit was added |