Doepfer A-110-1 Standard VCO Vintage Black Panel
Module A-110-1 is a voltage-controlled oscillator. This VCO's frequency range is about eight octaves (ca. 15Hz ... 8kHz). It can produce four waveforms simultaneously: rectangle, sawtooth, triangle, and sine wave (triangle and sine shapes are not perfect, see remark below). The output levels are typically 8Vpp for saw and rectangle, and 10Vpp for triangle and sine. The frequency or pitch of the VCO is determined by the position of the octave (Range) switch and tuning (Tune) knob, and by the voltage present at the CV inputs. Frequency modulation (FM) of the VCO is therefore a possibility. Footage (the octave of the fundamental) is set by the Range control in five steps, and Fine tuning controlled by the Tune knob. The tune knob range is about +/-1 semitone for older modules. Since about middle of 2015 the value has been increased to about+/- 1/2 octave. In the A-110 service manual is described how to modify the range.
You can control the pulse width of the square wave either by hand, or by voltage control - Pulse Width Modulation or PWM.
For more detailed information please look at the user's manual A110_man.pdf found on Doepfer's website. In addition the A-110 service manual is available as an example for the A-100 service manual that is available at extra charges. This document describes also how to modify the sensitivity of the tune control (changing the value of the resistor R5), how to re-adjust the 1V/octave scale and the frequency offset (i.e. the absolute pitch). Such modifications should be carried out by experienced users only!
The core of the A-110-1 is a sawtooth oscillator (in contrast to the A-111-1, which is based on a triangle oscillator). The other waveforms are derived from the sawtooth by internal waveform converters. As the sawtooth reset (i.e. the back-to-zero slope) is not infinite fast but takes a little bit of time the derived waveforms triangle and sine are not perfect ! At the top of the waveform they have a small glitch or notch that is caused by the sawtooth reset and cannot be eliminated by the waveform converters. The sine is derived from the triangle by a simple diode-based converter and the sine shape is not perfect (only a rounded triangle). To obtain a nearly perfect sine signal the triangle-to-sine converter module A-184-2 is recommended. It includes also a voltage controlled crossfader to fade between two waveforms.
If a perfect triangle is required the A-111-1 or A-111-2 is recommended. For a perfect sine wave the thru zero quadrature VCO A-110-4 or the quadrature LFO/VCO A-143-9 is recommended.
The sawtooth output of the A-110-1 has a falling (or negative) slope as shown on the front panel.
A simpler VCO (without sine, without rotary switch, but with linear FM input) is the module A-110-2.