DDRM Expander

I got the DDRM expander (ready build smd version) 2weeks ago, first jams where real cool....
But after some jamming I found several weak points and was then realising that the unit is far from perfect...

• Some fader behaviour where far from useful and started to nerve... especially => ADSR Release /  RM-Speed  / DL-Tone. I fixed it with just some added resistors
• As above, not having a single LED to see the unit switched on is just too minimalistic....
  I decided to add: A LED showing the ADSR Envelope, A power LED and a very helpful overload LED to signal any overload of any of the three FX chips (FV-1  Pin5)
  (see the schematics for the LED mods below)

Unfortunately during my lab investigations I found more...

• Level matching of each FX stage is far from optimal, there is at least a 12db waste in S/N ratio (I fixed the level matching internally) You get either more noise or distortion... hence where the overload LED came from
• Big level drop when switching to Tremolo mode (not fixed yet, needs additional analog switch)
• Erroneous SW in the Delay FX. If you put the fader fully up there is a fix delay of about 160msec with absolute strange behaviour of feedback/tone (adjusted the fader range to start with proper delay modes)
• Unfortunately, the delay time can not be adjusted without disturbing zipper noise (could probably be fixed with proper delay software)
• Master Volume sets headphone and FX output in parallel but headphone is extremely loud, making it impossible to use both outputs at the same time (I split headphone and line out with line out having fix level)
• Mono Output has a 6db higher output level than L/R and can easily run into distortion (I tuned the output driver for 6db less output level)
• The input circuit monos the two inputs and limits the high frequency range too strong for my opinion. Trough-out the whole unit there is a substantial loss of high frequency on the direct signal!

But now to mention the good things

• The midi to gate conversion time/latency is with 1.2msec and hardly any jitter; excellent! Most developers using Midi neglect the critical timing (latency/jitter) and don't tell you the specs
• Overall noise figure is quite good (with my level mods even better)
• The RM is what we wanted and its great
• all the fader settings are CV based leaving room for great further mods... Next I will add at least CV pedal inputs for RM-Speed and RM-Balance (maybe others) to add performance possibilities

Possible future Improvements (my personal favourites)

• All of the above mods
• The unit could easily be layed out in full stereo from stereo in to stereo out without adding much more electronics instead of monoing the 2 inputs (mono switchable)
• A bypass switch that gives the pure Input that I still want to use (without juggling around jacks...)
• add a midi function (with on/off) that adds frequency offsets to the RM speed based on highest/lowest note played, thus changing RM Speed with your playing and not just the ADSR.. imagine!

The MCU's (PIC16F1805) 1.1 firmware has a bug, where the frequency of the ring mod follows the "Sustain" slider but never releases. This can be verified by monitoring the RA0/ICSPDAT line on the PIC - this starts at 0V at power-on, but as soon as any MIDI note is received it goes high to 3.3V and never drops. It should go low after the note is released, following MIDI sustain rules.

Version 1.5 of the PIC resolves this - the file as provided by Black Corp is available here. The original PIC can be programmed with a PICkit3, or via an Arduino using this GitHub project.

Hit a couple of queries (so far):

1. No LEDs are in the BOM - any guidance on colours?
2. All potentiometers are listed in the BOM as 10K-100K - any guidance?
3. 12V-!2V DC converters are currently OS at most suppliers - quite long lead times - such is life!

• Original DDR Expander does not have any LEDs. Choose whatever Colour you like (Classic 3mm red will just be fine)
• With R5,6,28 LED Brightness may be adjusted to whatever you like.
• Pots are just used as variable voltage sources. Take 10k or max 20k! 100K will not have a linear behaviour any more due to the heavy 10K load!

Thanks TKW! All fine - so far! Does anyone out there have a setup document or any information regarding the setting of the pcb presets?] I am struggling to find any information or suggestions for testing. 

Previous problem identified, I think! Obvious really that until the TL071 has 12V applied it will not complete the voltage inversion.

Awaiting on DC-DC  converter to obtain the 12+ and -