AM8102 – SH02 VCF


Description The AM8102 module is faithful replica of the filter from the legendary Roland SH-2 that was released in 1979 and proved very popular with over 7300 sold. The AM8102 is a fantastic sounding 4-pole low pass filter with that unmistakable Roland filter sound, in a narrow skiff friendly Eurorack module.

The original SH-2 uses four Rohm BA662 chips cascaded into a 24dB low pass filter. Roland first implemented this design in the large 700 Modular ( 703 filter) using CA3080 chips. Variability in the offset of these chips required expensive manual trimming, so Roland designed their own improved OTA chip with Rohm and used this in the 100M, SH-1, SH-2, SH-09 and Jupiter 4 before it was replaced by the IR3109 filter chip.

Each of the filter implementations across these synthesizers is slightly different, with the SH-2 using both a JFET buffer and the internal BA662 buffer on each filter stage. Roland switched to using Mylar filter capacitors rather than the smaller ceramic capacitors that they usually used.

Eurorack Module:    10HP wide
User Manual:              AM8102 User Manual

AM8102 Module

AMSynths Design The AM8102 module was the first module in the world to use Rpar AS622 chips in the filter core, to replace the originals BA662. The AM8102 includes buffering Op Amps to interface to Eurorack modular levels. The filter does not have Q compensation but the signal level does not decrease dramatically with higher Q settings. The resonance is smooth and controllable into self-oscillation, and the filter will oscillate from 10Hz to 20kHz.

It is very important that the AS662 chips are matched for transconductance (gm) which is effectively the gain of the OTA. Failure to closely match the chips results is self-oscillation collapsing at 400Hz and below. The transconductance for all 4x OTA’s has to be within 150 gm, which is within one colour of the Roland BA662 colour dot scheme. Various Roland service manuals make it clear that the gm must be in this range for filters.

I built a simple test rig which has a SMD to THD adapter for the SMD AS662 chip and measures the output current across a 22k load with a 500uA bias current. The output current usually measures around 200uA. The gm of the AS662 varies across samples and across the 2 OTA’s within the chip. To achieve the 150 gm match over 50 AS662’s must be measured, as the range of 100 chips is 300 gm. I use a 5.5 digit multi-meter to measure the currents and the gm does reduce during the first few minutes of the chip warming up, then it stabilizes.

There are slide potentiometers (with green LED’s) for adjusting the Frequency Cutoff, and the frequency modulation from Keyboard, LFO and Envelope CV signals, There are rotary potentiometers for adjusting the mix between two audio inputs and the degree of resonance. There is a small slide switch to select whether the envelope CV input is Normal or Inverted.

The AM8102 is hand built with high quality components, including Wima polypropylene capacitors, Alpha potentiometers, and Eagle control knobs. The development of this iconic module identified a 40 year old error in the Roland SH-2 service notes – the filter capacitors are marked as 4,700pF (0.0047) but are actually 470pF.

There are six 3.5 mm jack sockets on the front panel:

  • 2x audio inputs
  • 3x frequency CV inputs
  • 1x audio output

Outcome & Availability The AM8102 VCF module completed development on 21st April 2022 and is in production. Check our webstore for more details and pricing.

Copyright AMSynths 2022