AM317 Voltage Controlled Filter

The PPG 317 Voltage Controlled Filter dates back to 1976 and the beginnings of PPG when they were a modular synthesizer company based in Hamburg. The The first set of modules was named the 100 series which are visually similar to the Moog modular but with different circuits and panel lettering in German. Some early modules were marked W.PALM rather than with the famous PPG logo. The 100 series modules were reworked into a short lived 200 series before Wolfgang settled on creating the 300 series of modules with a new visual identity.

The first PPG filter module was the low pass filter 104 based on the Mini Moog filter, which became the 303 Voltage Controlled Modifier with the addition of a VCA section. The 104 was also developed into the 217 Voltage Controlled Filter, which contains two 104 circuit boards. Edgar Froese bought one and used it in his Moog Modular rack. The 217 was then renumbered into the 317 with the same functionality and sold at DM1,560.

Description The PPG 317 is two 24dB low pass filter PCB’s augmented with additional circuitry, a cheaper and narrower version of the Moog 904A, B and C. One low pass filter has been converted to high pass by mixing in the original signal out of phase at the output using an Op Amp. These two filters can be used independently, with the front panel mode switch set to SEPARAT (German for separate).

The front panel switch can also put the filters into parallel or series configuration, to create band pass and band reject filters. The switch also converts the high pass cutoff potentiometer to bandwidth and the low frequency cutoff to band center frequency. The third CV input (C3) for each filter remain dedicated to cutoff frequency in band pass/reject mode. A small daughter board sits between the two filter PCB’s to provide the extra functions.

In SEPARTAT mode both signals ins and outs are used, in BP mode the signal ins and outs of the LPF are used, and in BR mode the filter makes use of the signal in of the HPF and the signal out of the LPF.

The PPG 317 is a powerful filter providing the classic 24dB ladder filter into four different filter types and it was used by Tangerine Dream for many years. Initially be Chris and then Edgar Froese. It was subsequently sold into private ownership. Modifications were made to these filters by Tangerine Dream, typically there is a toggle switch or a “magic eye” switch in the high pass filter section which switches this filter into low pass mode, to create a 48 dB low pass filter!

AMSynths AM317 There are no schematics for the 300 series modules, so the AM317 is based on tracing the PPG 303 PCB layout, pictures of a PPG 317, and then adding the electronics needed to create the different modes. The pictures confirmed the 317 was two 303 PCB’s (actually labelled 103) with an additional PCB with 3x op amps to create the bandwidth and band center frequency functions.

The original module used a 3-way 5-pole rotary switch which is difficult to source. I have used nine high quality analog switches to do the same job, controlled by a 1-pole 3-way switch. A THD prototype was developed in July 2021 and partially worked, the CMOS switches were not right for the module and the project was paused.

In early 2025 the project was restarted, with the high pass filter quickly corrected and working perfectly. A manual patch of BP filtering sounds amazing, and the project was quickly prioritized into production. The design was upgraded with improved analog switches, all SMD components and the panel width reduced to 12 HP. The circuit design make use of Q compensation by using V2044A low pass filters, so they do not have the bass “cut” of the original at high resonance settings.

The 2026 prototype was working (of sorts!), but needed quite a few gain changes and some rewiring how the High Pass Filter works and some decluttering of the toggle switch which was badly routed. Finally in June 2026 the filter works and its now going forward to production.

The filter is available in two formats;