Doepfer A-188-9 High Speed VCO Eurorack Module

Manufacturer's description

Module A-188-9 is a so-called high speed VCO that tracks prefectly to the 1V/oct scale in the frequency range of about 15kHz ... 500 kHz. The main application is the exact drive of BBD modules (A-188-1/2) to generate sounds realized by means of the so-called Karplus-Strong synthesis. But even in cooperation with other modules featuring high speed inputs (e.g. digital noise, delay, frequency divider) the A-188-9 makes sense.

In principle the module is made of a precision audio VCO followed by a PLL (phase locked loop) unit that multiplies the frequency of the audio VCO by 32. Thus the module can be used also as an audio VCO as it features the standard waveforms sawtooth, triangle and rectangle with adjustable pulse width.

Controls

• Frq: frequency control (coarse)
  • Control range (without external CV): about 13kHz...450kHz at the HSVCO output in HSVCO mode (range switch bottom position)
  • Control range (without external CV): about 400Hz...14kHz at the VCO outputs in HSVCO mode (range switch bottom position)
  • Control range (without external CV): about 12Hz...400Hz at the VCO outputs in VCO modes (range switch top position)
• CV: attenuator for the frequency control input CV
• Fine: frequency control (fine), small control without knob
• PW: pulsewidth control for the rectangle output of the audio VCO, in HSVCO mode this control has to be in the range marked "HSVO", small control without knob

Sockets

• 1V: frequency control input 1V/oct scale
• CV: frequency control input with attenuator CV
• Socket with rectangle symbol: rectangle output of the audio VCO, the pulse width of this output is adjusted by means of the control PW, output level ~ 7Vpp (~ -3.5V/+3.5V)
• Socket with sawtooth symbol: sawtooth output of the audio VCO, output level ~ 7Vpp (~ -3.5V ... +3.5V)
• Socket with triangle symbol: triangle output of the audio VCO, output level ~ 7Vpp (~ -3.5V ... +3.5V)
• Socket with inverted comb symbol: high speed VCO output, waveform rectangle, output level ~ 5Vpp (~ -2.5V/+2.5V)

Switches

• Range
  • Upper position: audio VCO mode (frequency range of the audio VCO about 15 Hz to 6 kHz, the high speed output is not usable in this mode ! )
  • Lower position: High Speed VCO mode (frequency range of the high speed output about 15 kHz ... 500 kHz, frequency range of the audio VCO about 500 Hz ... 15 kHz), in this mode the audio VCO and the high speed VCO can be used simultaneously
• Slew
  • Upper position: medium slew time
  • Center position: short slew time
  • Lower position: long slew time

Technical Note reqarding the Slew parameter

Besides the audio VCO the module contains a so-called PLL circuit (Phase Locked Loop) and a 1:32 frequency divider. A slew limiter is part of the PLL. It defines how fast the frequency of the PLL follows the frequency of the VCO. The PLL cannot follow the VCO without any delay. Rather one has to find a compromise between the resudial ripple of the PLL frequency and the frequency follow rate. That is the job of the slew limiter (works in principle like a glide or portamento unit).

For this the module is equipped with a 3-position switch that is used to switch the slew time in 3 ranges. When mostly higher frequencies are used ((~ 100kHz ... 500 kHz) a short slew time can be chosen. When also lower frequencies are required (< 100kHz) a longer slew time may be used to reduce the resudial frequency ripple. More details about the working principle of a PLL can be found in the information about module A-196. Module A-188-9 is in principle an A-196 combined with an audio VCO and a 1:32 frequency divider.

By means of a jumper also other PLL multiplication factors can be selected (8, 16 or 64). But the circuit is optimized for factor 32.