OTA HPF Proto

This is a reference to my previous article about 24dboct-high-pass-filter. I built this two-layer proto of that circuit and it works great. Front panel layout is different because here I have a dedicated layer for pots, switch and connectors where I have more freedom on placement of components.

Crumar Performer

I have been repairing my Crumar Performer string/brass synthesizer. One zener-diode was dying and made the master-oscillator to change the pitch randomly. Now this is fixed and it's time to make some modifications. I replaced the MK50240 top-octave generator with FlatKeys replacement box FK50240. This box has 12 synthesizers that create the desired notes from the master clock with a worst case error of 0.00005 Hz. There are also some extra features switches (scale,transpose,octave) that you can activate if you want.

Other modification comes from SoundDoctorin in this "Crumar Performer easy modification for release envelope" video. In the original Performer when you switch the brass-section on it kills the sustain on both brass and strings. With two switches you can choose from three options:

1. original setup where brass-section kills sustain on both brass and strings
2. Sustain works for brass-section but kills the strings sound
3. Sustain works equally for brass- and strings-section

I think the third option is most useful. Brass section adds nice fatness to the string sound.

P621-board where is the brass-switch. The white/black/red cables goes to the both separate switches.

Explanation of what to cut and where to connect the switches.

LM723 heater circuit

I have been breadboarding several different heater-circuits for temperature stabilisation of LM3046 transistor array. They all use the same principle of comparator circuit that compares the measured temperature against the desired temperature set by a trimpot. Comparator then works as an on/off switch for heater-transistor that keeps the 3046 chip in steady temperature. I must say that I haven't found an bulletproof circuit that could accomplish this basically simple task. Until now that I found this LM723 based heater circuit.

LM723 is a voltage regulator designed primarily for different power supply applications. But it can also be used as a temperature controller. I have these great McGraw-Hill Publications books "Circuits for Electronic Engineers" where from I found this "Using transistor arrays for temperature compensation" article by Mahendra J. Shah. With 15V power supply it worked right from the box very good indeed. With only one heater-transistor it could rise the temperature from 30°C to 50°C in 5 minutes and reach the final 50.5°C temperature in 10 minutes.

LM723 has also some kind of comparator inside it called Error Amplifier. It seems to work more smoothly than just an on/off switch. There is also a reference voltage generator inside this IC that generates a constant voltage of 7.15V from where the desired temperature CV is adjusted with trimpot R1. I modified this circuit a little to be used from 12V power supply. The pin numbers of LM723 in my schematic refers to PDIP version of this IC. The original article claims that with this heater circuit the LM3046 array exhibits a temperature coefficient of -3.1 ppm/°C from 33°C to 50°C. Peak current that this circuit draws from 12V is about 80mA where from it comes down to 52mA when the circuit stabilizes. In Moog Prodigy you need two of these with total current need for 104mA.  I haven't compared this current consumption to Prodigys original heater circuit. But that's sure that it is a better circuit than the original. Next I must try this inside my Prodigy clone.

24db/Oct High Pass Filter

Most mixers have that fixed 75/100Hz high pass filter for cleaning audio from extra low rumble. What I need is an sweepable HPF covering at least 10Hz - 1kHz region. The slope should be quite deep so that I can make the sound lighter without totally killing it. CEM3320 (http://electricdruid.net/cem3320-filter-designs/) would be ideal but I don't like to use this now obsolete part. So here is my LM13700 OTA-based 24db/Oct HPF. It's made from four serial 6db/Oct OTA-filters. With a switch you can choose between 12db and 24db slope. I have simulated it with LTSpice and it seems to work. Here is my preliminary front-panel and PCB for Eurorack. Here you can find some interesting stuff about OTA's in two parts (Understanding and Using OTA Op-Amps.pdf, Understanding And Using OTA OP-Amps PartII.pdf).

Robertsonics WAV Trigger

Under works is PCB and frontpanel for Robertsonics WAV Trigger that I bought from Spark Fun Electronics (https://www.sparkfun.com/products/13660). The PCB and panel creates a frame where WAV Trigger can be fitted. Simple transistor circuits work as comparators that can be fired either from frontpanel switches or from external CV/GATE input voltage (anything between +-10V is OK). My idea was to use this sample player to play some pre-recorded sounds in live-situations. It is polyphonic (max 16 voices) so several samples can be played at the same time. This could also work as a drum machine where drum samples can be triggered from external triggers.

Moog Prodigy PCB version 5.8

This should be the final version of my Moog Prodigy Eurorack clone. Here is what I have done:

- some corrections and enhancements to the PCB
- fastening holes to allow some other frame format than Eurorack
- all the pots (Alpha 9mm PC mount right-angle) and switches (standard PC mount toggle) are now more common format that are easy to source
- new modifications
--> MW MOD SHAPE where LFO is replaced with modulation wheel adjusted CV. So it is possible to sweep the pitch or filter cutoff with modulation wheel. This works also in my previous version and the original Prodigy when you set the MOD SHAPE switch to center position.
--> OSC MOD PW1 where LFO modulates OSC1 pulse width (modulation wheel controls the amount of modulation)
--> External Audio input with level control

FrontPanelDesigner and HPGL

If you are designing frontpanels with FrontPanelDesigner (https://www.schaeffer-ag.de/en/downloads/front_panel_designer/) as I am, here you have instructions how to save about 50% in the costs with grouping and HPGL. In my Prodigy-panel most of the engravings are done with 0.2 tool. The lines and bigger texts use 0.4 tool. Normally every engraving creates a new object, so your panel has many objects and the factory charges by objects. It's possible to group similar engraving objects to one HPGL-object, so eventually you have only two engraving objects (one for 0.2 and one for 0.4 tools) and the drilling holes.

1. When I am ready with the design, I save the design in FrontPanelDesigner to another file named Prodigy_tool02.fpd where I leave only the 0.2 engravings and delete all the other stuff. I also save the design to Prodigy_tool04.fpd file where I leave only 0.4 engravings. Third file that I save holds only all the drill holes. Make sure the origin in all these files is in the lower left corner!

0.2 engravings

0.4 engravings

drill holes

Here is the end result!

2. Export the tool02 and tool04 files to DXF files. Do not export the reference points!
3. Import these files to some CAD-tool that can read DXF-files and create HPGL-files. I use the free CadStd Lite-program (http://www.cadstd.com/). In CadStd I leave only the gravings layer active and export only visible layers to a single pen HPGL-file.
4. In FrontPanelDesigner I open the drill holes file and select insert HPGL engraving. This activates a crosshair tool with which I click the origin (X:0.00 Y:0.00). Select the tool02 HPGL-file, set the tool to 0.2 width, select the color and click "Origin of HPGL file" and OK. Then I repeat this with the tool04 HPGL-file.
5. If you want to save more you can leave the in-fill color out. With this black panel the colorless engravings look very good. You can fill the engravings afterwards manually with Lacquer-Stick paintstick color from Micro-Tools (it is little bit messy but it works) http://www.micro-tools.de/en/Paint-oxid/Lacquer-Stik-Paintstick-White.html

Panel is ready!