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.

2019 Version In 2017 as I restarted the AM2500 project and laid out the 1047 onto a large 150 x 150 mm PCB as a second prototype, and with a more careful approach. I went back to an accurate replication of the 1047 components, except where I knew upgrades were safe, such as the CV and Audio Op Amps where I have used TL071 Op Amps and 1% metal film resistors. I used Linear Systems matched transistors and a dual matched (to 1mV) FET’s (2N3954A). silver mica 1nF capacitors and LM301 Op Amps for the filter core.

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

Build Outcomes The PCB build was paused in 2018 whilst I moved into a new business workshop, and then restarted in the spring of 2019. I ordered a new PCB based around the 4005 sub modules and the original PCB layout.

Copyright AMSynths 2017