Sunday, June 9, 2013

J3RK's 291 clone build

A Wiggler asked me to build a pair of these; I hadn't seen this board before due to having already etched several of Scott Stite's and Luka's modification to Mark Verbos' design.

I'd always had trouble with this filter and have recently found a partial culprit...a bin of parts at the local supplier, indicating .022uF which were in fact .0022. D'oh.

Using Scott's output buffer design to allow for lowpass and highpass modes in the boards "muck" experimenter's section (with the power most thoughtfully brought out next to it), I found that using a dual opamp resulted in an unexpected result: the highpass section, which is basically just a 1K resistor between one input and the output, put out 12V+ for no apparent reason, even with the input unconnected, even with the 1K resistor disconnected, using a 1458, 072, etc. No solder bridges, no mis-connections. The same on both boards/builds. I've ended up adding a single opamp IC solely for the highpass section and it works just fine. Go figure.

Both boards are now built and wired to a Buchla-style front panel. I've changed R39 and R43 to 47K (originally 68K). Both are to set the range of CV to Fc and to Width. As early Buchla systems used a 15V unipolar control range, and the later modules (281, etc.) output and accepted 10V (the 200e as well AFAIK), more range was required to allow a 10V system to fully sweep the intended control ranges. This was not a scientific substitution. It works. Depending upon the input signal, the filters will self-oscillate with bandwidth at minimum and resonance at maximum. The Width CV input allows self-oscillation at will.

I've opened the FM range through altering the limiting resistor to 220R instead of 47K. Haven't yet tested the results but am aware that the vactrols themselves limit (slew) fast signals so audio-rate change simply reduces to not much at all as the modulator frequency is increased.

The 258B schematic says that resistors with multipliers of 1, 2.2, and 4.7 should be 10% carbon composition. I've applied this standard to the build, using modern Kamaya 5% carbon composition 1/4 watt types (vintage Allen Bradley suffer from rising values beyond their tolerance range).

For example and reference, a Kamaya 470R resistor is mouser part number

As I may have mentioned earlier in this blog, I'd built one of Scott etc.'s clones and put in several carbon composition resistors and found a bit of a sonic difference. I'm not sure exactly what or why so I suggested that this build use as many as were possibly indicated. (This is nice because vintage CC often require the leads to be cleaned of dirt and age in order to solder well.)

I've complained at Muffwiggler because upon hearing the first build, it lacked the oomph, depth of frequency center range into the bass, and overall character of the 291 section I remember from the 212 Dodecamodule I once owned (and as also exhibited to an extent in the Scott etc. clones). Upon wiring up the second build, I find that (as some also found at MW), one build may sound very different from another. The second build is meaty; the first is thin-sounding. Going over it now to detect the culprit; has to be user error.

I noted that the better-sounding build did not have the two resistors modded as indicated in the MW thread, to 330R and 100R, so I've restored the other.

The MW thread indicates the use of .047 capacitors in place of the .022 (as well as the resistor change) to lower the overall Fc. I've settled upon .068 types and with resonance at maximum and bandwidth to minimum, it sweeps nicely down into the bass range, allowing the highpass to be used to fatten bass sounds and drums, without losing the upper range Fc.

I believe the original Buchla module used silver mica capacitors for the .022s. Will attempt to find .068 which aren't the size of a Buick...

I actually installed vintage Allen Bradley carbon comp resistors as indicated on the first build, and had to swap them out. The J3RK boards are great, fun to build, and durable, but as with many PCBs, cleaning out the pads of excess solder once resistors etc. have been removed can be a pain in the ass. I've since taken to simply using the drill press (#65 drillbit) to open the solder in any blocked pad(s). I'll be using this technique in the future instead of solder wick, the solder sucker, and even attempting to blow the heated solder out of the way.

I've added 1/2 watt 2R2 Allen Bradley resistors at the positive power input on each board, per the original. Photos of the original module indicate an even larger 2R2 resistor in grounding the front panel to one of the audio jacks...may attempt this once the build is finished.

The recipient wished for vintage sound so I've used a 741 at the input (per the original) and a 1458 (dual 741) for the CV section. The 074 remains; have not checked for a vintage plug-and-play substitute.

The rest of the resistors are 5% carbon film, as there are no 1% resistors listed in the schematic.

Overall, this is an excellent filter made ever sexier through bringing out the highpass and lowpass responses. Most recommended! (Photos to follow.)