AM8104 JP-4 VCF


Roland JP-4 Filter Panel

Overview This module is replica of the filters from the legendary Roland Jupiter 4 released in 1978. The 4-pole low pass filter is a typical 1970’s Roland 4-pole OTA design, with an additional single pole High Pass Filter. The original low pass filter uses matched BA662 OTA chips, and ceramic capacitors, which we have upgraded to polypropylene, along with the 2SK30 FET’s.

The high pass filter is a simple 1 pole design based around a BA662 and JFET. The ceramic capacitors give the filter a slightly harsher tone, contributing to the legendary sound.

AM8104 Module

Euro Rack Module:  16HP wide
User Manual:             Download Here

Detailed Description 
This module is an accurate re-production of the JP-4 low pass voltage controlled filter, using matched BA6110 OTA chips in the filter core and (unusually) ceramic filter capacitors. The design includes buffering Op Amps to interface to modular systems, and uses an OTA in the resonance feedback loop. It delivers a warm but sometimes harsher sound.

There are front panel controls for adjusting the Cutoff Frequency and Resonance, as well as the level of 2x control voltages and 2x audio signal inputs. There is a small slide switch to select the keyboard follow into cutoff (Off, 50%, 100%). Resonance is smooth and controllable into self-oscillation. There is a LED slide potentiometer that controls the High Pass filter cutoff frequency and the filter uses traditional skirted aluminum control knobs

There are five 3.5 mm jack sockets on the base of the front panel:

  • 2x Audio inputs
  • 2x Frequency control voltages
  • Audio output

Module Outcome & Availability  The prototype tested out well, with a great sound and effective HP filter (as its voltage controlled), with only one minor defect that was fixed for production. A batch of modules were manufactured in 2021/2022.

JP-4 Notes on BA662

Transconductance Initially I used BA6110 chips without matching the transconductance(gm) and this worked fine for the first few modules. However once I started using the next batch of chips it became clear that matching for transconductance (equivalent to gain) is important. Matching to within 150 gm enables the filter to self-oscillate down to 20Hz, there were also issues in the HPF possibly due to the +3dB hotter output.

The availability of the AS662 Dual OTA in spring 2022 enabled the filter to be redesigned with this chip, and a new transconductance testing rig meant I could test chips for gm and match them up. The production of AM8104 modules restarted as a result, and both the low pass and high pass filters sound amazing.

 

 

 

Copyright AMSynths 2022