Glide Circuits and Portamento Basics

This time let's DIY a staple of analog synthesis: portamento, glide, lag, it's the "O Lucky Man" ELP drunken, swooping sound at about 3:40. You know what I'm talking about.

The circuit fragment to add glide to simple control voltages is simple:

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 turned 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 op amps.

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.

With that I created a simple PCB using Eagle (stripboard or perf this if you want, but me? 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 so let's now fabricate a variation: a second glide circuit that uses a 2N7000 Mosfet (interesting part) to allow a control voltage 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 the glide on.

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 be removed....it took me a bit of time and some forum discussion to realize this.  

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 better. 

My philosophy about testing new circuits is changing--I used to drag the partially completed circuits onto the bench to test but now I like to create a temporary front panel, with knobs on the pots, maybe even labels? to keep my sanity. Worth the extra time and effort, maybe.

Why not use SMT?  Maybe later, but now, too soon--"indolence pays".  Fixes? Mods?? Tweaks? Easier with through hole.

The front panels turned out 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 put  anything into your glide circuit and have it sound good.  

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 the glide is turned on:

IXTERN from the E-M forum suggests this form of protection for the IC1A:

We could replace the 2.2uF cap with something other than an electrolytic, since electros are a poor choice for high accuracy designs (see the video here for more information about this).

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 their voltage rating exceeds 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. Update: So far I can't.

Another thing is to buffer each output stage individually using something like a TL074.  How about using an op amp with less temperature dependency like a TL052? With a TL052 on a cold day your VCO won't go flat when you glide. But I think we can get carried away here. I try to remember that we are designing audio (CV, really) circuits, not RF, not TV, not gravity wave detectors, not sensors for the LHC, or whatever, so yes, you can get a bit sloppy with your PCB layout and parts selection and things will still be decent enough musical applications.

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. 

Until next time: glide between your days, life is most often non-linear, and non-quantized.

Morph-lag: Easy shape shifter!

Now for something completely different: a repop a circuit fave of mine from 2004 or so: Harry Bissell's "Morph-lag".

Finished, front panel courtesy Mr. Label.

Test rig with temp front panel.....

Morph-what? I was initially attracted to morph-lag back in the day by its simplicity and ability to produce interesting Attack-Release voltages at output. I built my first M-L back in 2005 or so using stripboard and mounted it into a 2U frac panel, a low cost "large format" modular synthesis standard.

I've been messing with the morph-lag ever since. 

Like a lot of other circuits I fab'd back in the 2000's my original M-L is getting tired and corroded.  15 years later and like it or not global economy has made strip-boarding a thing of the past.
So let's fab a new PCB for this.

I laid out the schematic in Eagle for a small/quick/easy build. Then I shipped the gerber off to China for a PCB....$2 USD before shipping for five of the damn things.... 

How did I do?  Not too bad.....Rev 1 has the LEDs reversed and screwed up ("nice work") but otherwise the M-L was "A-OK".

I fixed the LED issue in Eagle and created a REV2 PCB file which I will send out for fab real soon.

HOW MORPH-LAG WORKS:

Looking at the schematic (other than using LEDs instead of diodes, what you see here is unaltered from Harry Bissell's original design):

Control voltage comes in through R7 and hits IC1a, set up as a comparator.  From there D1/D2 (you can use diodes or LEDs) and the 1M audio taper pots and C1 form a simple attack/release  circuit fragment.

IC1B op amp is a buffer for the A/R; the signal is inverted by IC1C, mixed back with the original signal, and fed back to IC1A's inverting input.

With R8 all the way up signals are summed and you get the source mixed with the inverted AR output fed back into IC1A. With R8 the way (towards ground) the comparator output is bled through R9 and R10, allowing more of the initial signal to get through and less of the A/R output and hence less "lag". So R8 forms a "shape" control.

The result of all this is a recursive signal mix; allowing variations on the original, most often with a big peak following whatever formed the signal's peak at input. Sometimes you get an exponential bang, other times, inversions mixed with the original get you back to an amplified linear response. Set shape and A and R at will; you can't go wrong....this is a simple, clever circuit.

USING THE MORPH-LAG:

I have had best luck summing morph-lag's output with the original signal (so, mix the input signal via a CV mixer with Morph-lag's output).  To get the desired result, screw with levels.  Sum it, invert it, feed it back into itself, shape LFO's, change audio frequency square waves, use it to create odd slews, put the whole thing through a logic gate, are a few uses. I am yet to find a really bad use case for a morph-lag. Glad to have a few of these in my rack to play around with.

But--what happened to Harry? I have lost touch with him and don't see him any longer on the SDIY forums. If anyone knows where he is, and think he's up for getting email, please comment below, I'd like to contact him again, he was always very helpful and enthusiastic vis a vis what we do here. And yeah, Harry's kung fu is the best! Curious what other gems he's been working on.

Also, Larry Hendry of stooge bracket/MOTM fame was a big proponent of Morph-lag but tragically Larry died in a motorcycle accident many years ago. Nevertheless as of the writing of this post larry's"wiseguy" site is still up and is a source for the original morph-lag design. I remember exchanging fun emails with Larry many years ago. He is missed.

Kludge wires fix the misplaced LEDs.....

WHAT'S NEXT?

I have drawn up a revision of morph-lag for Euro, with the Euro +/- 12V setup, there is no reason that won't work exactly the same as frac +/- 15V.  Also a REV2 of what you see here that fixes the LED mistakes. For a future project, I may put a summer after the M-L outputs, to mix the signal back with the original CV used as a trigger or gate, or maybe put some sort of attenuverter in series. I have been thinking of calling this "CVGBs", a takeoff on the legendary club from my youth that has very little to do with synthesizers.  

I'll keep posting and once I have some stability for the Frac and Euro designs I'll put the schems etc. on my website.

OK, see y'all next time, until then, don't breathe the fumes.

Benched!

I have been retiring some circa 2004 DIY modules, but, what to do with them? I could rob them of parts and toss what's left, but instead decided to make a mini modular out of them and use it to test things on the bench like new filters or whatever.

Finished--working! L to R: breakout to amp; power and CV; stripboarded Irwin VCA; crappy mixer; TH ADSR, ASMVCO with 3900 waveform mod

First up, I took an ancient ADSR (read more here, it's a really old Thomas Henry/PAIA design). Stop the presses!  Run, don't walk, and immediately download all the Thomas Henry SDIY reprints here. Anyone doing audio DIY needs to download and at least glance through this PDF!) I think this ADSR was finished in about 2005. Back in the day I hand made the PCB....using press-n-peel, acid and a drill press...I can't believe I used to do that, but the ADSR still works.

To the ancient ADSR I added a much needed buffered LED to get a visual on CV at output; OK how to do that?  Had a very small PCB fab'd up--the IC is SMD:

Punched a hole in the ADSR front panel and mounted it:

Yea I tied wrapped all the wires down after I took this photo.

OK that worked!  Get the Eagle files, PDFs of board, etc., here.

Next built up a buffered CV board, information on my website here:

..and mounted that next to a PAIA power supply relic I had in my junk box in a 1u FRAC panel.  Tight fit....

The 9770 power supply board is from like 2002? I had it in a junk box labeled "Linear power supply".  Don't remember what if anything it was ever used for.  But it works!

Q: will it all fit?

Had some blank alubase panels from lying around--punched more holes to accommodate the power supply and CV board in a 1u Frac.

Ready to test--yes it fits and it worked first time! Bought the wall wart for something like $1.50 USD from the Goldmine

Still need (maybe) to Lazertran the powersupply/CV module? Not sure, so many other things to work on.

OK let's assemble our castaway modules. Almost ready to go, let's rack 'em!

The **not DIY** BGW power amp was purchased on Ebay for $20. A steal! BGW makes great power amps that can be had for next to nothing if you look around.

Wow that was easy.  The whole project start to finish maybe took an evening.

The VCO is a really old ASMVCO but i finished it in 2017, only to replace it with a more stable 3340 VCO, a design I am still working on (next few posts?).

The mixer got pulled out of my main modular racks because I disliked its ergonomics--I put the jacks and knobs too close together, and the lazertran art came out really crappy. But it's fine for a workbench, where it won't get a ton of use.

Good news, everything here, ADSR, mixer, power, VCO, etc. doesn't smoke (it didn't when I pulled it from my modular, and it still doesn't). Good to know 15+ years later there is still a use for this crusty old frac rak crap. I will use this going forward to see how things work with other modules I build. The island of misfit toys? Who cares!

See you next time!