Captain’s Analog


Posted on August 1st, by Mark Verbos in 20, Analog, Mark Verbos. No Comments

Captain's Analog with Mark VerbosThe Baby 10:  Part II
Hello again. I have had my biggest response ever with the last issue’s article.  I made an effort to make the schematic as simple as possible in hopes that the number of builder errors would be minimized. Most responses I have received were comments like, “this thing doesn’t work!” and “I think you screwed up that schematic!”  Well, as sorry as I am to say it, this thing does indeed work.  However there are a couple simple extras I think are nice to add on to the basic circuit in order to hot rod it into a real usable instrument.

The DISCLAIMER
Well, I did say the schematic worked as is, but I lied, a little.  If you look at the pot all the way to the left, you may notice that the wiper is grounded.  This is a mistake.  The wiper should only go to the diode.  Sorry about that!  Also, make sure that you ground the reset input if you are not using a step to reset the sequence.  In fact, tie it right to ground and if you do send a pulse to it, that will still work.

EASY TRIGGERING
In the original article, I suggest that you trigger the Baby 10 from a 606.  I have had good results using this method, however not too many other sources will work!  As a fix to this problem, I have provided you with an input comparator in FIGURE 1.  A comparator is a simple circuit that, oddly enough, compares to voltages together.  If the + input of the Op-amp involved goes higher than – input, the output goes high (+15 volts in our case).  This allows us to set a threshold for the trigger input.  The raw CMOS input has a threshold of about half the power supply (7.5 volts).  We have set the threshold to just below 1 volt.  Just about anything will trigger this thing now.

I suggest you put one of these comparators on the step input as well as the reset input of the Baby 10. Since the LM358 Op-amp is a single supply friendly type, you still can use you single ended power supply.  Also, this wonderful chip is a dual Op-amp, so you can build both of these comparators from one chip!  If you so please, you can even tie pins 2 and 6 together and only use one pair of resistors between the 2 of them!  This is the biggest adjustment I suggest you make as it will make all the difference in how useful this thing will be.  Also, note that you won’t need to ground the reset input if you use a comparator on it.

SINGLE STEP
Sometimes when setting up an analog sequence, it may be convenient to be able to step through the voltages from a button.  Well, knowing this, I have provided you with a “manual step” circuit in FIGURE 2.  The circuit uses an entire 4001 CMOS chip.  This is the most common CMOS package available.  It is actually a quad NOR gate.  We are actually only using 2 of the gates in this circuit, but I think that is fair, since this chip is so cheap.  The button should be a normally open type.  The capacitor values are not too critical, they really only adjust how long the button will need to be held down.  I suggest you don’t stray too far from the values shown.  This is just a little convenience feature, but you will appreciate it.

TRIGGER OUT
Last time, I suggested that you use a multed split of the trigger input for the trigger output, but FIGURE 3 shows a circuit for buffering this output to prevent any weirdness from taking place between all the gear involved.  This circuit sends out a huge 15 volt trigger so be forewarned and if this scares you add a resistor divider to suit.  I, however, have found this to be perfect as it will trigger ANYTHING. It is based on a simple transistor design that I saw on the Polyfusion sequencer.  The Polyfusion also has a S-trigger output, but I decided not to include it because a converter from V-trigger to S-trigger is so simple to build.  This circuit is not entirely necessary either, but it is nice because it freshens up the trigger output and protects the CMOS from you accidentally frying it with voltage going in the trigger out.

ACTIVE SUMMING
Finally, I suggest you replace the original passive diode ladder on the CV output with an active summing section.  The circuit in FIGURE 4 is what is in my 1st home-brewed sequencer and has served me well.  It is based on the classic 1458 Op-amp.  Although many people would frown at the use of a 1458 in this day and age, the output is only slow moving CVs so noise and slew rate are really not an issue.  You must replace the diodes with 100K resistors and the rest should be self explanatory. Unfortunately, this will require a negative power input, eliminating the single ended power supply we have been using.  All good things must come to an end, and it may be time to invest in a dual output (+/- 15volts) linear power supply.  This output section sets the range and output impedance to desirable.

The original summing circuit works OK, but using some synths (like my VCS3) the CV outputs can be drained so hard that the controls get all ridiculous.  An active summing section like this one, will make this more of a professional circuit.

IN THE END…
On my sequencer, all of these circuits are present.  Although none of them are absolutely required, I suggest that you add them as they make the world a whole lot more reliable.  The Baby 10 may actually become a usable machine instead of an experimental toy.  I suggest you at least try all of these.  I can be reached verbos@on-q.com with any questions.  Happy soldering.