AM8005 Diode Multi Mode VCF


Overview This module is unique and very powerful diode ladder VCF, which has three different modes selected by a front panel slide switch:

  • Low Pass
  • Band Pass
  • High Pass

All three modes have voltage controlled frequency modulation. Frequency Cutoff and adjustable Resonance into self oscillation. There are 2x audio inputs and 2x CV inputs for frequency modulation. The module has a mixture of slide and rotary potentiometers, and it is a replica of the voltage controlled filter in the Roland SH-5 analog synthesizer.

EuroRack Module:   18HP wide
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Description This 18 HP wide module has at its core a 4-pole low pass diode filter, along with a 1-pole High Pass Filter which can be used in conjunction with the diode LPF to create Band Pass and High Pass filter responses. In LP mode the signal is simply sent straight to the Low Pass VCF although the signal level is reduced down at higher Resonance levels, by use of a dual Resonance potentiometer. The filter will self oscillate and it sounds rather like a traditional EMS VCF.

Prototype PCB

In HP mode the input signal is sent to the high pass filter, and a non inverted HP output goes onto the LPF. An inverted high pass signal output is mixed with the low pass-signal output. This adds back in the high harmonics that the LPF removes thus approximating a resonant high pass filter. The inherent phase cancellation does most of the “high pass” filter work and creates a steeper cutoff and has the added bonus of resonance in a HPF.

In BP mode the input signal goes via the HP filter and then the LP filter but with no HP signal mixed in at the last stage. This provides the traditional BP response. All three filter modes have voltage controlled frequency cutoff and variable Resonance with self oscillation. Quite a filter!

Prototype 2011

Technical Design The AMSynths version follows the Roland schematics, but has additional input and output buffer Op Amps, and the horrible TA7136 Op Amps are replaced with modern TL072’s, watch out as the pin outs are different! The filter uses the same green Mylar film capacitors in the LPF ladder and a high quality polypropylene capacitor in the HPF (rather than Mylar). Matched transistors are needed at the base of the ladder, and I have used LM394’s.

The choice of buffer transistor is critical, with the original using 2SC100GR’s which have a slightly higher gain than the standard 2SC945. However 2SC1000GR’s are expensive (1 GBP each) and when I used them in the 8005 the filters had some offset problems, possibly because they are rather old stock. I have used BC547C transistors instead, which have a higher gain and worked well. Of course the BC547C is now obsolete, so a modern replacement is the BC550C.

Filter Levels The three modes have slightly different gain structures which means the BP and HP outputs are lower than the LP output.  On the SH-5 the BP is 80% and HP 60%, whilst on the 8005 its 70% and 80% of the LP level. I have added a bit more gain into the HP mode in the 8005 and overall the filter performs better than the original, for example the onset of resonance is smoother.

AM8005 VCF (2019)

Outcome & Availability The initial idea and schematics for the 8005 were created way back in December 2010, and various prototypes built over the subsequent years, with the PCB at right angles to the panel. Whilst these prototypes proved the LP and HP modes, the BP mode was consistently very low in volume.

So the module did not go into production and got placed into the backlog. In 2017 I redesigned the PCB to be parallel to the panel and skiff friendly at 35 mm depth, but the BP mode was still too low in volume. In the spring of 2019 I finally had sufficient time to get the filter working perfectly.

The main issue was the low BP volume (at 600 mV), which is due to an error in the Roland schematics were the input of the Op Amp in BP mode is left open rather than being grounded. It took me literally years to find this out. This was simple to correct, along with an increase in the HP signal level to align to the LP output signal. Now all modes work well and have the same output levels.

The AM8005 module was released to customers in July 2019, with a small production run.

2022 Update The AM8005 module was completely re-engineered in 2022, just 16HP wide, with the addition of the dedicated BPF, and the use of LED slide potentiometers. The resonance circuit has been redesigned to use a VCA based on the AS662D chip. This provides Q compensation (like the original), without the need for a dual reverse log potentiometer. The HPF circuit has also been changed to use the AS662D chip, replacing the move expensive CA3080.

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Copyright AMSynths 2022