Multimode filters, Part 1: Reconfigurable filters

Posted by & filed under Elka, Filters, OB-8, OB-Xa, Oberheim, Synth DIY, Synthex.

What is a “Multimode filter” anyway? It’s a filter which can provide more than one response. So it might offer a choice of 2-pole or 4-pole lowpass responses, or it might be switchable between lowpass and highpass, or it might have lowpass, bandpass, and highpass outputs. There are basically three techniques for building multimode filters, which I’d like to look at in the three parts of this series. How do we build a multimode filter? The three techniques can be summarised… Read more »

A study of Sub-Oscillators

Posted by & filed under Oscillators, Roland, SH-101, Synth DIY, Vintage synths.

This article is a look at sub-oscillators, a common tactic for fattening up the bottom end, particularly  in synths with only one oscillator, or only one oscillator per voice. Classic examples include the Roland Juno 106 and SH101. More recently, sub-oscs turn up on a lot of modern analog synths since they’re cheap to do and can add more punch and depth. The Arturia MiniBrute, Moog SubPhatty and most of the DSI synths include them. My synth’s sub-osc only has… Read more »

Adding Vintage Hiss, Crackle, and Pop!

Posted by & filed under Noise, Projects, Synth DIY.

In general, we’re trying to keep noise out of our circuits. Somewhat inspired by this thread on DIYStompboxes, I thought it might be fun to build a circuit that could generate that “vintage background noise” sound so that I could make things sound like they’re being played back off a phonograph cylinder or an ancient 78rpm record or something. It’s been an interesting little challenge and made me look at some different circuit elements and try a few different things. First,… Read more »

Adventures in Top Octave Generation

Posted by & filed under Oscillators, Synth DIY, Vintage synths.

Can we make a modern reproduction of the classic top octave generator / top octave synthesiser chips of the 1970s, as used in classic string synths and combo organs? What would be involved? Can we improve on the originals in any way? This page is a summary of what I learned by doing it. A little background In the late 1970’s, many synth manufacturers were thinking about producing polyphonic instruments. One musical area where that was already done was in… Read more »

Timbral Evolution: Harmonic analysis of classic synth sounds

Posted by & filed under Phase Distortion, Synth DIY, Vintage synths.

This post is a place to gather my thoughts about one of the most important parts of synthesizer design: the timbral evolution of sounds. It also presents the results of my analyses of some classic synth sounds. There have been many methods developed over the years to achieve timbral evolution. The voltage-controlled filter is the first and most obvious. Pulse width modulation and oscillator synchronization are two more from the analogue era. Other later approaches include wavetables, vector synthesis, FM, phase… Read more »

CEM3340 VCO (Voltage Controlled Oscillator) designs

Posted by & filed under Memorymoog, MKS-80, Moog, OB-Xa, Oberheim, Oscillators, Prophet 5, Prophet T8, Roland, Sequential, SH-101, Synth DIY, Vintage synths.

To celebrate the 2016 return of the CEM3340 chip, I thought I’d do another page in my series looking at how various synths implemented classic synth chips (the others being CEM3320 Filter designs and SSM2044 LP Filter designs). This is especially useful right now, since many people have bought a few CEM3340s for their own use and are looking around for circuits to use them in. If that’s your purpose, note that these are all synth voicecard circuits, so they’re used in… Read more »

SH-101 replacement processor – a feasibility study

Posted by & filed under Roland, SH-101, Synth DIY, Vintage synths.

The Roland SH-101 is a late-era analogue monosynth which uses a microprocessor to perform keyboard scanning and various related functions, much like the Sequential Pro-One. Since the original uP is a 40-pin DIP, it would be possible to replace it with a more modern 40-pin DIP uP, like the PIC 16F777, or with a modern surface-mount processor on a daughter board. This page is a look at the feasibility of this idea. Original Processor pins and functions The datasheet for the… Read more »

MIDI clock to analog gate pulses

Posted by & filed under Projects, Synth DIY.

How do you synchronise your modular synth with MIDI equipment? MIDI provides the MIDI clock message, but you need something to convert this to a format an analog modular synth can understand. Most MIDI-to-CV convertors will do the job, but that can be expensive. Here’s a cheap way to do it. The Druid MIDICLK chip converts MIDI clock signals into 0-5V analog pulses. It provides two separate analog clock outputs, which can be set at various division ratios of the… Read more »

RC Filtering for SSM2164 CVs fed from DACs

Posted by & filed under Filters, Synth DIY, VCAs + Amplifiers.

I typically use the 12-bit MCP4822 DAC to produce a CV for the SSM2164 (or CoolAudio V2164, it’s the same thing) . This is a good, cheap combination. However, the 4822 produces an output of 0-4.095V, whereas the 2164 needs 0-3.3V. It is possible to use a simple voltage divider to reduce the voltage to the correct level, and it is also possible to add a capacitor to give an RC 6dB/Oct rolloff whilst we’re doing it. However, it’s not… Read more »

Investigations into what a BBD Chorus unit *really* does

Posted by & filed under BBDs+Delays, Chorus, Flanger, Stompbox stuff, Synth DIY.

I’ve been curious about Chorus for a while, since I’ve been working on and off with chorus design myself. There were a few things I didn’t understand, like what the relationshp is between the modulation LFO’s waveshape and the frequency modulation of signals going through the chorus. You’d think that if you use a sinewave to modulate the BBD clock, you’d get a sinewave modulation of frequency, right? Wrong! So what do you get? At this point, I realised that… Read more »