VC Fixed Filter with Distortion and Feedback

Zlob Modular F3DB can be ordered assembled or Do It Yourself. I suggest going to the Zlob site to read the full description. It is basically a six-band voltage-controlled filter bank with a lot of extra nice features. Each band has its own mute switch, gain slider, and output, plus individual signal out and envelope follower out. The input section has adjustable gain and even voltage-controllable feedback.

I bought the PC board set and panel from Modular Addict.

BOM and Parts Sourcing

After sourcing the parts for the F3DB, I would recommend getting a full kit. The Bill Of Materials (BOM) is pretty good, but it omits a few specific part numbers. Most parts are available from Mouser, and the rest from Thonk. I had to order the tiny slide switches directly from Zlob.

Mouser had all of the resistor arrays and capacitors, though I had to look up a few equivalents from the ones specifically listed in the BOM. I needed to buy a few resistors, but I had most of them in my own stock. I couldn’t find the black pot washers and nuts, so I decided to go with Davies clone knobs, which work fine, even though closely spaced. I happened to have enough right angle “Btype 1PINSIP” single pin parts to use for the anchors. Fortunately, Mouser had all the special low profile fork headers, as given in the BOM. One part number missing from the BOM was for the trim pots. These are the ones I used.

531-PT6KH-100K

I ran into trouble with the right angle male headers, for which the BOM does not list a part number. So I used my eyes the best I could.

The right-angle header problem

These headers are important, because they solder into the four daughter boards and plug into the fork header sockets on the main board. The build documents (which are very detailed, with lots of photos), depict these headers.

Taking my best guess, I ordered these from Mouser.

649-1012937892001BLF

They are the right size, but the plastic that binds them together is on the wrong end.

When I found this problem, I spent quite a lot of time searching Mouser for the correct parts, but never found any. The so-called mating pins were all too long. So I decided just to cut them off. This worked.

Remainder of the assembly

Just go board by board, following the detailed build documents (one PDF for each board). I didn’t follow the order exactly, because I like to use Kester 331 organic flux solder and wash the boards. Each small board had to be partially built and washed about three times, so as not to wait too long after soldering before washing.

I could not find the adhesive strips, described in the docs, and this actually made final assembly easier. The anchor pins are not soldered until after the board is tested and calibrated. Calibration was easy.

Jumper options

Two sets of jumper options are available. Rather than install headers for these options, I decided in advance and simply soldered them with a loop of bare wire.

Each of the six bands has a jumper for the input to the envelope follower. It can come from the band filter output (pre-VCA), or it can come from the VCA output (post-VCA). I chose pre-VCA, because I want the followers looking at the filter outputs, which show the amount of energy in each band. The alternative, the VCA output, means that the followers are affected by the VCA level. The LED on each slider is driven from the envelope follower, which means with the pre-VCA option, the LED is indicating the amount of energy in the band, independently of the slider position or CV.

The second set of jumpers are for choosing pre- or post-distortion section for Input 2 and for the feedback, the post-distortion meaning that the signal (should be) mixed with the distortion section into the band filters. I chose post-distortion, because I thought that then Input 2 and the Feedback would mix with the distortion section, like having three mixed inputs. But…

The manual is wrong about the pre- and post-distortion jumper options

It says the jumpers must both be set to pre or to post. So I set both to post. But then I got a surprise when I tried using Input 2. I was monitoring the oscillator signal I was sending to Input 2, and saw that something caused it to zero out! Yes, the output of the oscillator was snuffed. Investigating, I found that with both jumpers set to post, Input 2 is shorted directly to the output of the feedback input buffer op amp. With that at zero, it essentially grounded the input signal.

After further investigation, I found that you should have both set to pre-distortion, which is the default. It seems that the post-distortion settings do not work as described.

Finally assembly

Testing it out

After calibration, I have it on the bench, looking at the input and output signals on my scope and listening to the output. I am varying the frequency and wave shape going in from an oscillator. Input 2 now works, but I prefer using Input 1, because of the gain pot. Everything works as expected until I start using gain feedback or feedback from the filters. Whoa, the input signal starts getting yanked around. That is weird. Apparently the feedback signals are being passively mixed and can alter the input signal. This is good to know, so that if I want to preserve the original input signal, I’ll need put an external mixer or buffer in front of the input. I have requested the schematic from Zlob, which should clarify this.

Envelope follower outputs

I was happy to find that the envelopes seem to be quite good at following the energy levels coming out of the filters. The maximum envelope appears to top out at 10 volts. This is independent of the VCA, the way I configured the module. I did notice a bit of ripple feed-through riding on the envelopes. This may not matter for some uses, and when it does matter I can post-process the envelope with a slope limiter, etc..

Well how does it sound?

This module is really, really good at distortion. Just playing around with the sliders and feedback controls, varying the frequency of the input, affords a huge array of timbres. And considering that the mix of the bands is all voltage-controllable, this will be an amazing animated distorter. I’ll make a separate post, demonstrating sounds.