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AM Filter Bank - The Subtractive Core
Overview The voltage controlled filter is the subtractive core of the analog synthesizer, shaping the sounds of the oscillators to give a
unique sonic signature, giving each synthesizer it's unique character.
AMSynths Filters I have a soft spot for filters....they are sonically so different and full of character that I can't stop building new ones and
replicating old ones....Well I started with an ARP 4023 back in 2002 and went on to many more. Analog filter designs tend toward the sculpting power of the 4-pole LPF, but there are many designs for smoother sounding 2-pole LPF, the
single, dual or even 4 pole HPF and the multi-mode SVF. Twelve of my filter designs are housed in a special EuroRack Filter Bank Cabinet.
Design Details The Filter Bank is built as a standard EuroRack as follows:
- The modules are all standard 14HP wide EuroRack.
- The panels are professionally manufactured with printed text and graphics.
- The two 3U 19" racks are manufactured by Rittal
- The +/-15V power supply is Oakley Sound with Oakley Dizzy.
- The MDF cabinet is covered in black Tolex.
- 12 filters are contained in the Filter Bank.
Here is a bit of history on the subject of filters along with links to the modules:
Transistor Ladder Filters The traditional 4-pole voltage Low Pass Filter invented by Bob Moog is the mainstay of the analog synthesizer, and
the original Moog 904A was subsequently improved and copied by most synthesizer manufacturers in the 1970's. The Moog design from the early 1960's is totally based on discrete transistors, as it predates the arrival of Op Amps and
matched transistor arrays. Moog developed the filter into the mid 1980's and the Moog Source programmable analog monosynth, but retained the basic design and used different approaches to the ladder buffer (OTA, transistors, FET Op Amp).
The design lives on in the current Moog Voyager.
AM1006 ARP were one of the first to copy the Moog ladder filter and
it appeared in the ARP 2500 as the 1006 module in 1970. It uses discrete transistors in the ladder and output buffer (which also has a final Op Amp). I plan to build a clone in a MOTM 5Y high module, it will contain the VCA as did the
original. AM8003A Even Roland had a brief flirt with the transistor ladder filter in 1974. They used a clone of the Moog design in the SH-3A Analog Synth as well as the SH-2000. It uses discrete transistors. I have added an Audio Mixer and Ring Modulator - inspired by the Roland SH-5.
AM8012 The ARP 2600 used the 4012 filter for the first few years of its life, until ARP moved onto the OTA design and the ARP 4075. The AM8012 accurately replicates the ARP 4012 using discrete transistors, and it achieves the classic Moog sound.
AM8035 This ladder filter made it into the Mark 2 Odyssey, and boy does it sound good! Law suits seem to have stopped this filter being used in the Odyssey for very long, but its brother the 4034 was the core of the ARP Explorer for many years. The design uses transistor array(CA3046) and a dual FET and Op Amp as the ladder output buffer.
AM8904 The original and the best! A clone of the Moog 904A using THAT matched transistor arrays for the ladder and output buffer and closely matched Mylar capacitors.
Diode Ladder Filter These low pass filters are very similar in design to the classic Transistor Ladder but use matched diodes in the ladder
rather than transistors. This produces a rather different sound made famous by the EMS VCS3 and Roland TB303 - squelch! Most designs are 4-pole, but there is also a 5-pole version.
AM8003 Roland, rather surprisingly, used a diode ladder filter in the first version of the SH-3 synthesizer in 1974 with a 4-pole design. I have replicated the design in the AM8003 module and added an Audio Mixer and Ring Modulator - inspired by the Roland SH-5. The Roland SH1000 Preset analog synth also has this filter.
AM8005 The Roland SH-5 has a great set of filters; a traditional Band Pass Filter (not voltage controlled) along with a 2-pole High Pass Filter and finally the main Low Pass Filter - a 4-pole diode ladder that uses transistors wired up as diodes. The design also made it into the Roland 100 synthesizer.
AM8050 The VCF in the Practical Electronic Minisonic synthesizer of 1974 has a 4-pole diode filter. I originally built this synthesizer from a kit in 1974 as my first synthesizer. It had dreadful VCO's that never did go into a musical octave, but the filter turned out to be like a VCS3 rather than the Moog sound I was after. So I gave up trying to play lead lines and emulated VCS3 filter sweeps. 35 years later I found the schematics and realised it was a 4-pole diode filter, and for old times sake I have built an exact clone, even using the 9V power supply. I am afraid the rest of the designs are really not worth cloning as they use rare parts and believe me are not that good.
AM8071 This the EMS VCS3 and AKS diode ladder filter.
State Variable Filter The 2-pole State Variable Filter(SVF) has been in existence for many decades but it was not until the early 1970's that
synthesizer designs started to make it voltage controlled and usable in an analog synthesizer with its need for exponential control. The SVF has the disadvantage of only being 2-pole but has the advantage of providing 3 modes at the
same time; High Pass, Band Pass and Low Pass. Designers usually provided all 3 outputs, although sometimes they mixed them together or even just used one mode.
ARP were the first in with the Multi-Mode filter in the ARP 2500, followed by a version by Dave Rossum at E-mu Systems in 1972 - the Universal Active
Filter. These designs used transistor VCA circuits to enable the voltage control of frequency and. Q By 1974 it was possible to use the newly available OTA chips (CA3080 designed by RCA in 1969) to simplify the design, and Tom Oberheim
was the first to get this idea into a commercial synth - the SEM module.
AM8023 Now this is an edgy one. The 2-pole LPF from the early ARP Odyssey, a State Variable Filter wired up with just the LPF output. It simply screams! The resonance cranks up the volume, the reverse of a 4-pole dumbing down response.
AM8703 This is the version from the Roland 700 analog synthesizer.
I have used BA61110 OTA chips and has all 3 filter modes which can be selected as the output. There is no voltage control of Q. AM8140 and AM2140 A very nice Dave Rossum 2-pole state variable filter which just is so smooth. A true classic
design that is based on the SSM2020 VCA/OTA chip and it appears in the E-mu Systems Audity as well as the E-mu Systems Modular, very rare! The filter provides all 3 modes and a nice audio mixer to mix these into one output. Originally
the Q was not voltage controlled, but I have enabled this.
OTA Filters With a patent on the Moog transistor ladder filter, synthesizer manufacturers looked for other 4-pole Low Pass filter topologies
and came up with the OTA as the basis for each filter pole. Although the OTA chip was available from 1969 it was not until the mid 1970's that designs started to appear. Roland lead the way in 1976 with a 4-pole Low Pass Filter in the
the Roland 703 module based on the CA3080 chip. They subsequently refined the design over the next 10 years by placing it all into one single chip (IR3109) and then added a VCA. The design lasted up until the last Roland analog synths
of the mid 1980's, like the JX10. Dave Curtis and Dave Rossum put the design into single chips, such as the CEM3320 and SSM2040, and Korg built the NJM2069 OTA 4-pole chip which was very successful in the mid 1980's.
AM8040 The legendary SSM2040 chip and Dave Rossum's contribution to analog filtering. The ultimate low pass filter, with infinite character and presence... Famously used in the SCI Prophet 5 Revision 1.
AM8044 In the mid 1980's the analog VCF chip was further
refined before being totally replaced by the digital filter chip. SSM brought out the SSM2044, and it was used in the Korg Poly Six, PPG Wave and E-mu System SP1200. The filter has been carefully designed for the same signal output
even at high resonance. AM8069 Korg developed an OTA based 4-pole low pass filter for use in their mid 80's digital oscillator based synths and samplers, such as the DW6000 and DSS-1. This chip was the NJM2069
which includes 3 VCA's and it has a reputation for being rich sounding. I raided an old DSS-1 and retrieved 8 perfect NJM2069's AM8075 Smooth as velvet, the ARP 4075 4-pole LPF from the later Odyssey and of course the Quadra. This is such a polite filter, which works well with strings and organic sounds.
AM8105 This is the VCF and VCA from the Roland JX8P and later
MKS80. It uses a rare and original Roland IR3R05 chip. This is a 4-pole low pass OTA filter with external capacitors just like the IR3109 and a VCA. AM8106 This is the Low Pass and High Pass Filters from the Roland Juno 106, it uses a D80017A clone chip. This is a
single chip with a 4-pole low pass OTA filter and a VCA, its use of SMD chips and capacitors gives it a dry sound. AM8109 More Rolandness... This is the 2 and 4-pole Low Pass Filter from the Jupiter 8 using a real IR3109 chip. This chip is a 4-pole OTA design and it compares well with the Dave
Rossum designed SSM2040 as the ultimate filter with its airy presence. AM8320 The cutting filter from the infamous SCI Pro One. Very carefully cloned with some extra care to make this a truly edgy 4-pole LPF. Works well with ultra short ADSR timings.
AM8372 The massive Oberheim Matrix 12 filter that has 16 different filter types based on the CEM3372 chip, it transcends any other filter on the planet with it's ability to subtract and modify.
So what have I have missed out? Some of the early Korg filters and various 2-pole designs such as the Oberheim SEM as well. Maybe one day...
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