I opted for musical capabilities similar to the classical MiniMoog analog synthesizer, with fully analog sound generation and signal path and also an analog control signal and modulation path.
The analog sound part is built from different physical modules:
More details can be found in the section about the analog audio path.
There are further two auxiliary modules
The modules are interconnected in a fixed manner like in the MiniMoog.
I did not implement the MiniMoog "external input" and 440Hz test tone features though.
Unlike the MiniMoog, there is no separate "audio mixer" module as this function is implemented in a different way.
The analog part does not have any physical potentiometers or switches or buttons. Rather, the analog part is fully voltage controlled. The Analog Patch Control Interface is not a physical module per se, rather it is a concept of standardised control inputs of all analog hardware modules. The control voltage lines are all inputs and are standardised as two types:
These control voltages are generated by the "Digital Interface".
The Digital Interface links the Analog Control Interface to the outside world.
The Digital Interface is very basic. It has the following features:
I followed four principles:
The main technology used for the analog part is bipolar transistors, either as single devices or in OpAmps (well I also use some with JFET inputs). CMOS devices are mostly used in electronic switches for analog signals or as digital circuitry.
A major design consideration is signal voltage range.
Given the transistor technology, I decided for an analog signal range of −5 V < Vsignal < +5 V symmetrical around 0 V ground for both the audio signal and the analog control voltages. The audio signal may not use the full 10 Vpp range. In fact, I often work with a signal current in the range of −0.5 mA < Isignal < +0.5 mA. Flowing into a load resistor of 10 KOhm, this will yield the mentioned voltage range. Working with current sources has the benefit to be able to easily sum audio signals. I use this for the audio signal path and also for modulation signals.
The −5 V < Vc < +5 V range applies also to the control voltages that replaced the manual potentiometers. These control voltages are generated by digital to analog converters (DACs) in the Digital-In module.
There is another type of control voltage "Vs" that is digital and either 0 V or +5 V. Those control voltages are generated by CMOS type digital circuitry in the Digital-In module.
Classical VCOs usually contain wave form generation, amplification and level shifting. Classical VCA designs also employ level shifting and usually attenuation to employ voltage controlled transistor pairs for "amplification" control.
I decide to pull the VCA functionality into the VCO modules to save some of this amplification & attenuation.
Classical synthesizers have the VCA after the VCF in the signal path. Pulling the VCA into the VCO thus reverses the order of signal processing. In an ideal linear world, the order would not matter. But these circuits are not ideal. Thus UBZR1 will behave differently to a classical synthesizer when pushed to the extremes: effects to be discovered!
For the signal voltage range, I also considered a signal range of 0 V < Vsignal < +5 V, symmetrical around a notional 2.5 V. That would allow using standard DAC circuits running from +5 V only.
The power supply could then simplify to +10 V, +5 V and −5 V with further reduced power consumption.