AM8172 Phaser & Delay

Introduction Back in 2009 I replicated many of the Roland 100M modules and built 20 modules into two FracRac cabinets. In 2010 I attempted to clone the 172 module with its 6-stage phaser and BBD delay. I separated them into two modules (172A and 172B) and built the BBD delay as a prototype using a rare MN3004. It didn’t work, so it never made it into a completed module. Subsequently I find out this is likely to do with the 4013 chip. The phaser never made it to prototype stage as it is only 6-stages.

In 2018 I started recreating my 100M designs into Eurorack but the launch of Behringer 100M clones in 2020 meant these plans were scrapped. In 2022 I bought some of the Behringer modules secondhand to use as a new test rig and to check how they are designed. I also restarted my 100M plans using powder coated panels which are more in keeping with the originals.

Behringer 172 Module Let’s first take a look at the Behringer 172 module to see how it has been designed, and if there are any shortcomings that can be fixed or deserve an AMSynths module. The Behringer module uses a single PCB with all the hardware mounted onto it and the space between used for all the SMD components.

The 6-stage phaser uses two TL074 JFET Op Amps and three dual matched JFET transistors (probably the 2SK2145-GR). This JFET is a neat way to reduce the component count and get matched JFETs that are needed for accurate phaser operation. The modern part has the same IDSS range as the 2SK30A that was used in the original, and presumably modern manufacturing tolerances mean they do not need to be selected.

The delay circuit in the original is really a chorus, as the MN3004 BBD chip provides only 20ms of delay. Behringer have used the same circuit (with 4013 based clock) but swapped in a Coolaudio V3207 chip which is usually used in pedals (June 60 Chorus). This should give a longer delay time (50ms) but depends on how fast the BBD clock is. The LFO for both the Phaser and BBD Delay is authentic and uses a dual Op Amp. Big 470uF power supply caps are used to stop this type of LFO modulating the power rails.

There are four trimmers near the Phaser and BBD, which is as expected from the original designs. There is also a jumper which is probably used in setup.

The Gate Delay circuit is a copy of the original with 1MA pots, so presumably the SMD timing capacitors at C76 and C76 have been doubled to 4.7uF. There is customer feedback that the gate threshold does not work correctly, as the upper threshold is too high. The potentiometer is the correct 10KA value, and the resistor from the pot to GND measures 220R (close to being correct), however the resistor to +12V measures over 5k, when it should be around 200R. This explains the incorrect behaviour.

AM8177 Module  The Behringer module is 95% accurate and only requires a small modification on the Gate Delay, so there is no reason for me to attempt an AMSynths 172 module. The Gate Delay circuit with 2MA slide pots does feature in the AM8141 module. There are plenty of third party BBD delays in Eurorack, so I won’t attempt this part of the 172.

The AM8177 is a Dual Phaser module that replicates the 8-stage phaser (Op Amp and JFETs) from the System 700 and the 8-stage phaser from the Boss PH-2 Phaser (using AS3109 chips). This pair will deliver a set of richer phaser sounds, with the PH-2 being more subtle. I have used the 2SK2145 as a cheap dual JFET in the System 700 phaser (nice idea Behringer!).

Outcome & Availability The AM8141 module was launched in early 2023 and the AM8177 will be available later the same year. Check the webstore for details.


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