UBZR1 Analog Synthesizer Architecture

Chelys Lyra

Analog Sound Generation and Modulation

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.

UBZR Architecture

The analog sound part is built from different physical modules:

Three Voltage Controlled Oscillators (VCOs) with built-in VCAs
One noise generator with built-in VCA
One VCF
One Output module

More details can be found in the section about the analog audio path.

The control voltage section which effects volume contour control and modulation control is built from

Two ADSR type envelope generators
Modulation Matrix

There are further two auxiliary modules

Keyboard interface. Please see the section about the keyboard pitch control and how the keyboard pitch control voltage (CV) is routed to the different modules
Digital Interface. The Digital Interface has a communication interface, storage for the current patch set, digital to analog converters and digital outputs.

The modules are interconnected in a fixed manner like in the MiniMoog.

I did not implement the MiniMoog "external input" and 440 Hz test tone features though.

Unlike the MiniMoog, there is no separate "audio mixer" module as this function is implemented in a different way.

Analog Patch Control Interface

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:

Analog voltage inputs - shorthand V_c - in the range of −5 V < V_c < +5 V
Logic level inputs (usually CMOS) - shorthand V_s - with levels 0 V or +5 V.

These control voltages are generated by the "Digital Interface".

The Digital Interface

The Digital Interface links the Analog Control Interface to the outside world.

The Digital Interface is very basic, with the following features:

It contains just enough functionality to receive MIDI control messages to generate 32 analog voltage outputs, using digital to analog converters, and 40 bits of logic control signals (CMOS level)
No intelligence. All sophistication needs to be built into the controlling digital device.
It has digital storage for just the current set of settings (patch)
It is fast = very low time latency.

Design Principles and Choices for UBZR1

I followed four principles:

Build on designs of other people
Use parts I already got
Use only parts that I could purchase easily
Achieve low power consumption

Technology

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.

Signal Voltage Ranges

A major design consideration is signal voltage range.

Given the transistor technology, I decided for an analog signal range of −5 V < V_signal < +5 V symmetrical around 0 V ground for both the audio signal and the analog control voltages¹. This results hence in a signal amplitude of up to of 10 V_pp.

However, the signals are often a specific current in the range of −0.5 mA < I_signal < +0.5 mA. Flowing into a load resistor of 10 kΩ, this will yield the mentioned voltage range. Working with current sources has advantage to be able to easily sum signals. I use this for the audio signal path and also for modulation signals.

The −5 V < V_c < +5 V range applies also to the control voltages V_c that replace 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 "V_s" that is digital and either 0 V (Low) or +5 V (High). The control voltages are generated by CMOS type digital circuitry in the Digital-In module.

Analog Signal Path

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!

Footnotes:

For the signal voltage range, I also considered a signal range of 0 V < V_signal < +5 V, symmetrical around a notional 2.5 V. That would allow using standard DAC circuits running from +5 V supply voltage.

The power supply could then simplify to +10 V, +5 V and −5 V with further reduced power consumption.