AM8071 Snow Fall VCF

IntroductionBack in the late Winter of 2009/2010 there were heavy snow falls in south east England for the first time in many years. During this "white out" I designed a diode ladder voltage controlled filter similar to the ones that appeared in a number of analog synths in the 1970's, and named this new module "Snow Fall". In 2009 I had already built a number of prototype 4-pole diode filters from the EMS VCS3, Roland SH-3 and the Practical Electronics Minisonic 2. I improved the designs with modern components and added a fifth pole and slew delay on the CV input. This AM8071 protptype was very sucessful and delivered a great sounding and flexible VCF.

During Summer of 2011 the prototype was worked up into a production design using the new AMSynths panel design which has a mix of rotary and slide pots, with momentary push buttons. The production units were launched on 20 September 2011 and this EuorRack module is available to purchase from AMSynths web site.

AM8071 Module
The filter is a Diode Ladder Low Pass with a cutoff rate equivalent to 18dB per octave in 4-pole mode, and 24dB in 5-pole mode, thanks to the characteristics of diodes. The filter has a large Frequency rotary control with smaller rotary controls for adjusting Resonance and Resonance CV (QCV) which enables voltage control of Q. The filter has a maximum Q of 20 and will self oscillate at higher settings. There are 30mm slide potentiometers for adjusting two signal input levels and two frequency CV amounts. There are five input jacks and one output.

The module uses momentary push buttons to control slew and slope rather than clumsy toggle switches. The enables analog switches placed close to the filter to activate these functions (keeping the analog signal path short) and for green LED's to show the status - on or off.

AMSynths PanelThe panel is professionally manufactured in 1.6mm thick alumnium in Germany with black lettering. The module is EuroRack sized and fully compatible with many other EuroRack modular synthesizer systems such as Doepfer.

More Information

Visit the AM8071 web page on my web site for more information.

http://www.amsynths.co.uk/Products/AM8071/am8071.html

Korg Poly 6 VCF Replica

Introduction
This module is designed around the infamous SSM2044 VCF chip that appeared in a variety of analog synths (and drum machines) in the early 1980's, including the Korg PolySix, PPG Wave 2.2 and the E-mu SP1200. A direct descendent of the legendary SSM2040 chip, and designed by Dave Rossum and Ron Dow in 1980, the design was patented in the USA (4,404,529) and described as a low pass filter circuit, employing feedback current mirrors as dynamic resistive elements, and characterized by high accuracy, low noise, and low distortion.

Whatever the specification, its a very nice and fat - low pass filter! SSM2044 chips are easily found as many thousand were made for Korg in 1984. But these large orders never materialized as Korg made their own OTA 4-pole filter chip the NJM2069 and then went digital with the M1.

AM8044 Module
The AM8044 module is based around the original SSM2044 Datasheet and an article that appeared in Polyphony Magazine in 1982. The chip delivers a 4-pole voltage controlled low pass filter, with the added feature of the resonance being voltage controlled, but this is a reverse log response. Designed to be used with micro processors this is not a problem in poly synths such as the Korg Poly Six. However when used in a analog module a micro processor is not really cost effective, although I did consider this option initially. I then tried out the Polyphony design (using an OTA), which was rather useless. The Q was voltage controllable but the response curve was dreadful. Other designs on the Internet attempt to replicate the reverse log control with Op Amps...but I have found that the original Datasheet design of a reverse 5K log pot and some resistors to define upper and lower voltage ranges works extremely well - and its cheap! So that's what I use...

AMSynths Panel
I built this module around 2006 using a hand made PCB panel sprayed with blue car aryclic paint and using SafMat for lettering. However in September 2010 this module has been fitted to the new AMSynths professionally manufactured alumnium panel, which you can see in the photograph. The panel is EuroRack sized and fully compatible with many other EuroRack modular synthesizer systems such as Doepfer.

More Information

Visit the AM8044 web page on my web site for more information.

http://www.amsynths.co.uk/Filter_Bank/AM8044/am8044.html

A video will be uploaded soon. I plan to manufacture a short run of 10 of these modules in 2011 as the AMSynths AM8044, so please drop me an email if you would like one reserved.

Jupiter 8 Low Pass Filter - AM8109

Introduction
This module is a clone of the Low Pass Filter from the legendary Roland Jupiter 8. This is a 2 and 4 pole OTA design that Roland first used in the Jupiter 4 in 1980, with a dedicatde filter chipe (IR3109) replacing a discrete version of the filter which took up more PCB space. The IR3109 chip featured in the next series of Roland polyphonic synthesizer (due to the small space needed for 6 or 8 voices), including the Jupiter 6 and 8, as well as the Juno 6/60. It was also used in the Roland SH101 monosynth, Boss phasers and Roland guitar synths.

I have kept the design of the AM8109 to just the LPF rather than replicating the HPF from the Jupiter 8 as well, that's simply because I don't have space on the PCB. The IR3109 chip has a set of 4x OTA filter stages, and an exponential CV generator to control the frequency cut off, as well as a VCA to control resonance.

Description
The first stage was to buy an IR3109 chip (well 2x actually so I could build two identical modules), and read the Jupiter 8 schematics - cool! I designed the circuit using Eagle CAD and laid out the PCB, and managed to fit the design onto a 100 x 80mm PCB.

The core of the filter is based on a single voice of the original Jupiter 8, as is the resonance control circuit using a BA6110 as a VCA. The frequency control circuit is from the SH101 with temperature compensation via a 10K NTC. I have used high quality capacitors for the filter stages and their are high quality Op Amp buffers to translate the signal levels to and from the higher levels used in a modular synthesizers. I have used high quality audio grade Panasonic capacitors in the signal path, and the quality of all these upgrades pays dividends in terms of sound quality - excellent!

There is a simple flip-flop circuit to switch between the 2 and 4 pole modes, a 4013 CMOS chip drives a M5201 switching Op Amp. This is different to the Jupiter 8 design which uses FET's to switch between the poles. A front panel LED shows whether the filter is in 2 or 4 pole mode.

Outcome
The design went together very well, and only a few gain settings in the Op Amps needed a bit of adjustment, and then a perfect sounding low pass filter - with a lot of presence and weight emerged. This is a very smooth sounding filter which breaks into full sine wave oscillation at high resonance settings. The difference between 2 and 4 pole modes is not dramatic - with the 2 pole mode providing a more gradual cutoff slope, but nevertheless worthwhile.

More Information
If you want to build your own Jupiter 8 filter module then visit AMSynths and drop me an email. http://www.amsynths.co.uk/Filter_Bank2/AM8109/am8109.html
PCB's are available as well as EuroRack modules. If you'd like to see and hear the filter, check out the video.

Jupiter 6 Filter - AM8060

Introduction How would you like to trasform a boring Boss Phaser pedal into the awesome multi mode filter from the Roland Jupiter 6? Well read on....just buy yourself a second hand Boss PH-2 Super Phaser off eBay for 30 - 40 UK pounds and carefully unsolder the two rare IR3109 chips. Then buy an AM8060 PCB from AMsynths and build the Jupiter filter. It sounds a lot better than the light weight phasing effect from the pedal, with 4 filter modes each with resonance into full on self oscillation. This filter was such a beast in polyphonic synths that they replaced in with the much tamer IR3R05 chip in the MKS80 REV5 and JX8P. However you can build one or buy a completed module in EuroRack format later in 2010 from AMsynths.

Description The AM8060 module is a clone of the Roland Jupiter 6 voltage controlled filter which comprises two 2-pole OTA filters in series, and was launched in 1983. Each 2-pole filter can be switched into either Low Pass or High Pass modes providing three VCF modes all with variable resonance:

• 24dB Low Pass Filter
• 24dB High Pass Filter
• 12dB Band Pass Filter

The original filter in the Jupiter 6 (and MKS80 REV04) use two IR3109 chips, one as the filter cores and the other to control RESONANCE level as a dual VCA. The filter modes are controlled by two push button switches, which provide an extra 4th mode (Band Pass) than in the JP-6. The filter is easy to set up with a simple cutoff frequncy trimmer. The prototype is shown on the right, AMsynths will release 10 completed modules later in 2010 in EuroRack format with high quality aluminium panels.

If you want to build your own Jupiter 6 filter module then visit AMSynths and drop me an email. PCB's are available from late June 2010. Project Notes are online.

So what does it sound like? Visit this web page for an online demo..

http://www.amsynths.co.uk/Filter_Bank2/AM8060/am8060.html

Snow Fall - EMS VCS3 Filter Clone

During the exceptionally snowy weeks of early 2010 I built a redesigned version of this classic filter which was used in the EMS VCS3 and AKS synthesizers in the 1970’s. With snow falling and the wonderful bubbly sound of the filter, I named this module - Snow Fall. It was a 4-pole diode ladder low pass filter using all discrete and rather old components. The Roland SH-3 filter (AM8003) and PE Minisonic 2 (AM8050) filters are close copies of this legendary design.

EMS originally (in 1970) described the filter as Low Pass and with a cutoff rate 12dB for the first octave and then 18dB per octave thereafter, with band pass filter response at medium Resonance levels and a maximum Q of 20. From early 1974 EMS updated the filter with an extra capacitor at the base of the diode ladder to provide a steeper 24dB per octave response, this appears to have changed the character of the filter. The filter frequency response is stated as 5Hz to 10kHz.

In terms of electronic design the standard filter is 4-pole but the characteristics of diode ladders makes this sound more like 18dB, and the 24dB version is really 5-pole. No matter, the filter sounds fantastic....

Visit the AMsynths web site for the full story here:

The Snow Fall has a CV input for controlling Q as well as push button switches to provide Lag on the Frequency CV input and 4 or 5 poles. The prototype has been built as a FracRac module with a PCB front panel and SafMat lettering. Later this year I will have a 2 colour aluminium panel made in my workshop.

Roland 100M Replica - Major Progress

With a few days off over Xmas 2009 I was able to make some progress in completing my Roland 100M analog synthesizer clone. The main taask has been to de-bug the Roland SH-5 filters which I have been developing for the last 12 months. The SH-5 has a diode 4-pole voltage controlled low pass filter as well as a HPF, a BPF mode with the VCF and a dedicated BPF, making for some fantastic sounds - check out many Ozric Tentacles tracks.

The latest set of PCB's (4 in total) needed a few corrections, but I now have all 4 filter modes working - albeit I have configured the HPF as a direct output and not mixed in an inverted HPF output taken via the VCF - as per Rolands thinking.

The filters souds amazing and I will post some MP3's as soon as I can. In the mean time here is the updated web page, with a picture of the completed AM8005 and the AM8002 Mixer and Ring Modulator. These are Frac Rac modules with temporary white card and PCB panels. I will either spray paint the PCB panels or have aluminium versions made.

The Roland 100M replica is nearing completion with just the MIDI CV converter to be completed (waiting on the right PIC chip from MIDIBox) and I still need to design and build the LFO and Mixer modules. There is also an analog delay and phaser (M172) and the 8x2 analog sequncer (M182) to be built later in 2010.