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

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 with no 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

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