The circuit fragment to add glide to simple control voltages is EZ:
HOW IT WORKS: the incoming CV is buffered by the unity gain op amp stage on the left. The 100 ohm resistor protects the circuit when the 1M pot being shoved all the way CCW--doing that means the output of gain stage one goes directly to input of stage two, without current limiting, so without the 100 ohm resistor you risk damaging your buffers.
The 1M pot and 2.2uF cap form a basic low pass filter--the DC is "slowed down"--which is buffered by the last op amp stage. Turn the pot and get more glide. Simple.
With all that I created a super simple PCB using Eagle (stripboard or perf this if you want, but me? Um no I will avoid breathing the fumes):
|Put an SPST switch the SW1-SW2 to bypass glide.....jump R3 with a wire for more accuracy but less protection for your op amp against short circuits to ground....|
OK that was easy, next, I whipped up a second glide circuit that uses a 2N7000 Mosfet (interesting part--1001 uses?) to allow "FET ACTION" to turn the glide on and off.
Here Q1 is acting like a switch--when the mosfet is turned on (Q1's gate to V+) there is a great deal of resistance between Q1's source and drain and the value of the pot becomes the path of least resistance, turning on the glide.
With Q1 off the source and drain act a lot like a wire and consequently there is very little glide.
T1 inverts the logic, so gate high = glide on. It also buffers the incoming gate signal. So when you have anything above about 1V DC at the gate input the glide pot "works"; otherwise it is ignored.
Running around in the Buff(er): Op amp over current protection for a glide circuit is a bit tricky: if you keep R3 in circuit and hook the output of this glide into your V/oct VCO, you have current limited the V/oct source, which will probably knock your VCO a bit out of tune. For me this was most noticeable at higher pitches and so R3 had to go....it took me a bit of time and some forum discussion to realize this. So:R3 should be replaced with a wire jumper if you need good accuracy at output.
I will write about output buffer current limiting in a subsequent post, as it is a a slight bit complex and warrants its own discussion.
And now on to the obligatory bench photos....
|The SMD board is to buffer the LED input. More information on this tiny PCB here.|
|Notice the crude Dremel cuts for the front of the RAP2CV board. This to make the PCB/panel fit mo better. Turns out these aren't necessary. I'll cover that in a future post.|
|Why not use SMT? Maybe later, but now, too soon--"indolence pays". Fixes? Mods? Kludges? Tweaks? Easier with through hole.|
|Lumpy and Bumpy with Lazertran (left) but good enough for now.|
What next? There is plenty of discussion on the E-M DIY forum about how to improve this circuit. Since the glide here is in series with V/oct devices that need to stay in tune, you may not be able to slap any old damn thing into your circuit and have it sound good right? Let's see what we can do to improve this:
One thing: move the fet. This might help sidestep any unwanted resistance it creates when on, have to mess with it:
IXTERN from the E-M forum suggests this form of protection for the IC1A:
On Ebay I bought some metal film 2.2's, but generally large value metal film caps can run big in size, too big for a lot of what we do, if the voltage exceeds say 100V. The ones I found are pretty small in size, and I am curious if I will be able to hear a difference between the expensive cap and the "cheap as you can get" Aliexpress components. Stay tuned for that.
Another thing is to buffer each output stage individually using something like a TL074.
And/or: How about using an op amp with less temperature dependency like a TL052? So on a really cold day your VCO won't go flat when you glide? Maybe--but I think we can get carried away here. I try to remember that we are designing audio (CV, really) circuits here, not RF, not TV, not gravity wave detectors, not sensors for the LHC, or whatever, yes, you can get a bit sloppy with your PCB layout and parts selection and live to tell the tale.
I'll probably build more of these to experiment with accuracy, but at some point it'll be time to move on. For now these 2 glide modules are in my rack and they work.
Enough for now! Until next time: glide between your days, life is most often non linear, non quantized.