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Tim Stinchcombe

Doepfer A-100 Synthesizer

The Doepfer A-100 system conforms to the smaller of the two most popular sizes, modules being 3U high and using 3.5mm jacks, as opposed to the larger 'Moog-style' modules using the large 1/4-inch jacks. This is not to everyone's taste, but there is no denying that they offer a very wide range of modules, currently at over 100, and they are competitively priced. My system has been steadily growing (go here for my current list) since I bought the original batch in 2001 - click on the small photo below for a larger version (204kb):

A dedicated Doepfer A-100 group has been active at Yahoo! Groups for several years (access requires group membership). Dieter Doepfer himself is a regular contributor, providing feedback to users' queries (if he has the time!) and news of up-and-coming modules.

I have typed-up my story of how I came into the realm of synthesizers - it may well not be particularly interesting reading for others, but it was quite a cathartic and liberating exercise for the author!

DIY Modules

Ladder SVF A 12dB lowpass VCF, formed from two transistor ladder filter sections arranged in a state-variable-like structure.

LMF100 Switched-Capacitor Filter A 24dB lowpass VCF, built around the National Semiconductor LMF100 switched capacitor filter chip.

DC Voltmeter A large, centre-zero, illuminated, moving-coil DC voltmeter.

Phase Meter & Phase Shift Oscillator A ±180° phase meter, with a simple phase-shift oscillator with four phases.

Stepped CV Generator Generates a uniformly stepped CV, to aid the plotting of filter response curves.

Stereo LED VU Meter A VU meter built around a Velleman dual LED VU meter kit.

Modifications

I have modified a few of my modules for a variety of purposes. Most of the instructions for doing the mods are available at the Yahoo! group, but are rather spread about there, so I will collect them together here as time allows:

A-110 VCO: improving the sine waveshape

A-117 Digital Noise: to allow clocking from a 'sloppy' input

A-127 Triple VC Resonant Filter: switchable LP/BP selection for each filter

A-135 Voltage Controlled Mixer: obtaining the individual outputs from each VCA

A-140 ADSR: adding an automatic re-trigger

A-150 VC Switch (old type): to allow switching of signals greater than +8V

A-151 Sequential Switch (old type): reset on 2, 3 or 4 steps

A-155 Analog sequencer: addition of a permanent 16-step facility

A-163 VC Frequency Divider: switchable DC-coupling for the output to allow division of slow clock-type signals

G3/G6 case: identifying/replacing a leaky IEC power inlet

Plan B Model 9 Mixer: reducing the amount of bleed-through when a signal is supposed to be 'muted'

Links for other modifications

Doepfer have a dedicated A-100 DIY page, containing not only various modifications to specific Doepfer modules, but also ideas on how to modify modules more generally, and basic explanations of some simple circuits.

Nick Keller has a page of interesting mods at the analoguehaven site, the distributor for Doepfer gear in the US.

Miscellany

Diode Ladder Filters

A page dedicated to diode ladder filters, including my views on the perennial TB-303 'dispute' between 'its an 18dB 3-pole filter' versus the 24dB 4-pole reality... (Note also that my recently updated paper, which is detailed below, now has a good deal about the diode ladders in it.)

A Study of the Korg MS10 & MS20 Filters

I have moved the link to my paper onto this new page, which also contains many other links to material relating to the Korg35, the MS-10 & MS-20 filters, and various copies/clones of these filters, and the like.

You can't move far in the world of synthesizers without coming across the fabled 'Moog transistor ladder filter', or one of the diode ladder filters derived from it. In Feb 2004 the first version of this paper came about because I couldn't find the derivation of the transfer function of the Moog filter anywhere on the web, so I had to work it out for myself. As of Oct 2008 I have re-titled the paper 'Analysis of the Moog Transistor Ladder and Derivative Filters' to reflect the fact that it has had a major re-write: not only have I now included derivations of the transfer functions of some of the diode ladder filter configurations, but there is lots of stuff on poles, frequency responses, and comparisons between the functions and the like. [The document is about 50 pages, 2.1Mbyte big, is paginated for double-sided printing, and for best effect should be printed in colour.]

Here is a page of links to research papers on digital implementations of the Moog and other ladder filters.

Filter Pole Animations

I spend a lot of my time trying to understand how filters work, and analyzing various filter circuits. Recently, in order to try and better understand how the pole positions affect the frequency response of the filter, I produced some 3-D plots of the transfer functions as a surface over the s-plane, using Mathematica. Then I discovered I could put many together and actually animate the surface as the pole positions moved. Not only was this educational, but it was also a good deal of fun: I have put a load of them together on a separate page.

How the Serge VCS Works

Having become fascinated by this module, I found it a real challenge to fathom out exactly how it worked, and eventually I decided to write up a page detailing my findings. I am in complete awe of anyone with the level of intuition that it seems to me you need to design something like that from scratch - I can only dream of getting to that sort of level of understanding!

EDP Gnat Synthesizer

I have a few of these now, and when I acquired them they were in various states of repair. Finding absolutely no possibility of acquiring the schematics and/or a service manual off the web, I eventually decided to sit down and reverse-engineer the entire thing. The resulting (hand-drawn) schematics are linked on this page.

Old Synthesizer Brochures etc.

As a lad I was forever writing off to various companies to obtain their glossy brochures on whatever product. I had quite a collection of synthesizer-based ones at one time, and I kept some of the better ones for nostalgia's sake - I have recently scanned some of these and made a page of them here.

Electronotes Indices

Electronotes is a newsletter-like publication produced by Bernie Hutchins, covering technical details of music synthesis and sound processing: back-issues up to the very first issue in 1972 are available, and the complete set is a rich repository of circuits, techniques and ideas. When I bought mine a few years ago, I came across an index of them on the web: subsequently I expanded the files and brought them up-to-date, and they may be of use to others (simple text files which makes searching for a particular topic easy):

Electronotes Index

Indices of Mid-Month Letters, including 'Perspectives' and 'Bibliographies'

Application Notes index

Supplements index

Investigations Page

Here I will post any interesting findings of investigations, experiments etc. that I may do from time-to-time.

[Page last updated: 11 Jan 2010]