AM1047 Universal Audio Filter

ARP 1047 Module

Overview The AM1047 is a replica of the ARP 1047 Multi Mode Filter and Resonator from the ARP 2500. This 2-pole filter has Low, Band and High pass modes, along with voltage controlled resonance. The module provides distinctive filter sounds and will operate at a Q of 500 without drifting into oscillation, at any frequency from 16Hz to 16kHz.

The 1047 has a variable Frequency Cutoff and 2x voltage control inputs and pots for frequency cutoff modulation. There are 2x audio inputs with variable controls and 2x audio outputs. Resonance has a front panel control along with a CV input and level pot. There are individual outputs for each filter mod and a slide switch to limit the filter resonance level and a lamp to indicate the output audio level. The audio levels should be turned down when the lamp indicates overload.

The filter has a unique percussive feature which enables a keyboard CV, gate and trigger to be patched in and to “ring” the filter, this is most effective in band pass mode. The impulse response of the filter at a high Q and a center frequency of 1kHz, is a 1kHz sine wave that takes over 1 second to decay to 60dB below the initial amplitude

Percussion Mode

AM8047 Prototype

AM8047 Prototype I originally recreated the 1047 as a 21 HP Euro Rack module during late 2011 using THAT matched transistor arrays to create the discrete OTA’s , and TL071 Op Amps replace the 1339/LM301 Op Amps. I was pretty brutal at replacing all the old components with new ones. The filter worked well, but the high pass mode was not quite right, so the module never made it into production as I did want to release a perfect version.

The module has a mixture of slide and rotary potentiometers and there are slide switches to limit the filter, and to select filter mode at the two audio outputs. There is a LED to monitor the audio output. I attempted to improve the resonance CV circuit at R46/R47/R48 but didn’t make much progress, and blew up a few THAT arrays in the process. I was also incorrectly using 4K7 resistors at R91/R92 and R93/R94, inspired by how the 4019 works. The project was shelved.

AM1047 Module

Feedforward One of the challenges in the original design was the need for a fast slew rate in the core Op Amps of the filter. Such Op Amps were expensive or hard to locate in 1969. The design uses a slow Op Amp the LM301 with feed forward compensation which boosts the slew rate from 0.5V/us to 10V/us at higher frequencies. A 220pF ceramic capacitor is used and it might partially explain the unique sound of the original.

The Dave Rossum version uses the LM318 with a slew rate of 50V/us which easily surpases the ARP design, and the Behringer version uses TL07X Op Amps with 13V/us. Feedforward compensation is no longer used in modern Op Amp designs as better parts are available.

AM1047 Prototype In 2017, when I restarted the AM2500 project, I laid out the 1047 onto a large 150 x 150 mm PCB as a second prototype (V2), using the schematic from the 8047 but with Linear Systems matched transistors and the addition of the keyboard percussion circuit. It was laid out for carbon composite resistors and LM301 Op Amps, using a plan PCB without solder mask. Initially this PCB did not work – dead!

Controls The AM1047 has the following controls:

  • Frequency Cutoff Coarse and Fine
  • Resonance
  • Notch Frequency
  • Audio Levels x2
  • Frequency Cutoff CV x2
  • Resonance CV x2
  • Resonance switch (Normal/Limited)
  • Keyboard Percussion (On/Off)
  • Final Q amount
  • Overload Lamp

Connections The AM1047 has the following connections:

  • 2x Audio Inputs
  • 2x Frequency Cutoff CV inputs
  • x2 Resonance CV inputs
  • LP, BP, HP and Notch filter outputs
  • Gate and Trigger inputs
  • External input on front panel
  • Rear mounted keyboard CV input

AM1047 PCB Close Up

Build Outcomes The V2 PCB build was paused in 2018 whilst I moved into a new business workshop, and it was not working. I restarted the 2500 build in the spring of 2019 and I was about to order a new V3 PCB based around the 4005 sub modules and the original PCB layout, but I fired up the V2 PCB and added in the resonance pot and replaced a failed Op Amp. It worked immediately creating amazing filtered sounds, and each filter mode worked perfectly. Looking back I think the low gain of the THAT340 PNP pair versus the NPN pair is the downfall of the 8047 and its weak HP mode.

The V2 prototype is a bare PCB from Euro Circuits with no solder mask or screen printing, so its tricky to build up! It does include the lamp circuit (converted to LED) and the keyboard percussion circuits. It does not have the external input circuit as I originally excluded this from the AM1047. I completed the V2 build and calibrated the filter, the drive circuit on the Q pots is important to get the right range of Q response. The V2 was built up into a completed module and panel, as a full prototype. I upgraded the Op Amps to a mix of TLE2071, OPA134, LT1012.

AM1047 PCB REV04

2019 Update A production version of this PCB (REV4 without sub modules) was ordered in early April 2019 with all the circuits on board. Two 1047’s will be commissioned into my 2500, the second will use the potted 4005 modules, MC1439 Op amps and carbon composition resistors (REV05 PCB). The front panels were carefully drawn out (BUT I made a typing mistake!) and ordered. them. They arrived on 5 April and one was used to fit the REV04 PCB in October 2019.

The production modules use a mix of 1/4″ shaft TT potentiometers for the big controls, and 270X pots for the small controls (with smaller nuts and washers).

Here are the setup instructions: AM1047 Setup Instructions

2022 Update The V2 prototype was the basis for the Behringer 1047 which I collaborated on in late 2019, and the production modules sound very good. The Behringer Manchester team added a trimmer for Q CV adjustment, which makes setup easier. Comparing the V2 version with an original revealed that the sound was not exactly the same. The V3 version whilst an accurate layout of the original schematic failed to work, with a constant -14V output.

In 2021 I developed the AM1047K Keyboard Percision module to augment the B1047 and provide the “ping” capabilities. More details are here.

2024 Update I developed an expanded version of the 1047 circuit in 2024 and released it as theAM1047 Eurorack module. The design has been implemented in SMD and has a voltage controlled Notch circuit and a soft clipping input, as well as the usual features (including Keyboard Percussion). More details are here.


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