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 »

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 »

CEM3320 Filter designs

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

Like my SSM2044 page, this page is a look at how various synths implemented the CEM3320 filter, with the datasheet design as a reference standard. Whereas the SSM2044 is a dedicated lowpass filter, the CEM3320 is just a set of filter building blocks. In this respect, it is more like the earlier SSM2040. The CEM equivalent of the SSM2044 is probably the CEM3328, which is basically the same as the 3320, but wired up as a dedicated lowpass filter, thereby… Read more »

SSM2044 LP Filter designs

Posted by & filed under Filters, Korg, MonoPoly, Opera, Polysix, PPG, Siel, Synth DIY, Trident, Wave.

This page is a look at how various synths implemented the SSM2044 filter, with the datasheet design as a reference standard. I’ve redrawn all the schematics to make comparisons very easy. First, the datasheet design.   The datasheet suggests using the two differential inputs (+audio and -audio) as individual oscillator inputs for a two-oscillator synth (or synth voice). It also suggests that these two inputs should be provided with slightly different resistor values (68K Vs 100K) so that the oscillators… Read more »

Technical aspects of the Hammond Organ

Posted by & filed under Hammond.

This page is intended to be a look at some of the key technical aspects of the B3 Hammond organ that contribute to the “Hammond sound”. My personal slant is towards copying this sound using other technology. Perhaps one day I’ll build a “clonewheel” organ, but then again, Clavia might have nailed it! Still, building an organ would be a nice project. Contents Drawbars Foldback Key Click Harmonic Leakage Chorus/Vibrato scanner The problems with copying the B3 Generating 91 individual… Read more »

Phase Distortion Synthesis

Posted by & filed under Casio, Phase Distortion.

I heard about this and could find little about it, so I started to investigate. Phase Distortion is one of those forgotten synthesis techniques that someone once built a few instruments with, and then abandoned. In the case of PD, Casio were unlikely synth pioneers to start with, and after a brief flirtation they gave up on the professional market and went back to making home keyboards. The real question is whether, if they’d stuck at it, phase distortion could… Read more »