Tuesday, 2 September 2014

A-143-9 Lin-FM & Buchla Saw Modification

How to add a linear FM input to Doepfer's quadrature oscillator and get the weird waveshapes of Buchla's 258 VCO.



The A-143-9 quadrature oscillator is a favourite for FM. Its sine wave is pure and the DC-offset is minimal. This makes it the perfect choice as the modulator in dynamic FM patches. It's a shame then that it itself doesn't have a linear FM input. Fortunately, the fix is simple and the result sounds great, especially in quadrature 'stereo':

A-143-9_linFM.mp3
A-143-9_stereo-linFM.mp3



Most analogue VCOs are linear at heart. To get the response we need for musical pitches, CVs need to be translated. If we bypass the part of the circuit that does this - the exponential converter - we should be able to modulate the VCO in a linear fashion.

This depends on the actual circuit design but, happily, this 'dodge' is possible with the A-143-9. I asked Dieter Doepfer and he identified pin 6 of the top quad opamp (TL084) as the target (see A-126 connection PDF for board layout). A resistor determines the modulation depth. Dieter suggested 100K but I ended up using 68K to allow for some over-modulation.



There is one caveat with this simple modification, but it's a limitation that can also be used to musical effect. Pin 6 is also connected to -12V via R8 (270K). This provides the current needed for the oscillator to work. If the modulation voltage 'robs' the VCO of this current - i.e. at 0V - the oscillator will stop. This is good and and bad: at low audio-rates, over-modulation could sound choppy. On the other hand, we gain the ability to gate the VCO/ LFO on or off, similar to a track-and-hold.



The reason FM-ing the A-143-9 sounds so good is that there's no waveform converter messing things up. Even with the best schemes, the sines on triangle- or saw-core VCOs show their heritage by being either brassy or buzzy. So, a sine-core makes sense if clean FM is a priority. Question is, what do you do if you also want a saw or square?

The latter is easily generated with a comparator. For the saw, we can take inspiration from the Buchla 258's 'funny waveshapes'. We can use a triangle-to-saw converter circuit or a patch:

Patch the sine (0) output of the A-143-9 to a mixer. Patch a multiple of the sine to another mixer (e.g. mh01). Set the channel to 50%. Mix in a 2.5V offset and patch the result to the signal input of a polarizer or DC-coupled ring modulator like the A-133. Patch the cosine (90) to a comparator (A-167, Sport Modulator etc.). Set its threshold to 0V. Patch the comparator's output to the CV input of the A-133 and send the result to the first mixer. Now balance the amount of original sine to get the shape you want. Altering the comparator threshold will yield other shapes. Here's how it can sound when modulated:

A-143-9_buchla258saw.mp3

It's not thru-zero, but the quality of the A-143-9's sine-on-sine linear FM is clean and stable. Given the right C:M ratio it can cover some of the sounds generated by wave-folding. With a little more circuitry to provide a permanent saw/ square output, a pair of these quadrature VCOs could form the basis of a budget timbral VCO.

Saturday, 22 March 2014

Berlin Nights

Had a cracking time with new and old synthesizer friends at Schneiders' Messe Aftershow in Kreuzberg on Thursday night. Look forward to a video including Mark Verbos and Koma Chrisi's funky set.



Highlight: Schneider & Splitradix improv on funk bass and megaphone. Low point: the whisky selection at Zum Goldenen Hahn post aftershow.



If you're in Neukölln next weekend, I'll be playing live at Clockwork#1, a modular synthesizer night hosted by Koma Elektronik.

Hope to see you there!

Clockwork#1
29 March, 20:30
at Drumherum
Lichtenraderstr. 49
12049 Berlin

Wednesday, 5 March 2014

CGS77 Serge 1973 Filter Demo

Sounds of the CGS77, Ken Stone's adaptation of the original Serge filter, are scarce. I hope these recordings help anyone considering this DIY build.



I'm really not a filter man, but the Serge VQVCF is something special. This circuit is its 1973 ancestor. The state variable found in today's systems has been refined with gain compensation and a trigger input. But Ken Stone tells me that the CGS77 is essentially the same core and my ears aren't about to argue.



If you want to hear more, please buy the EP for the unedited recordings and pictures of my build.



Building CGS modules for Eurorack presents the DIY-er with the problem of how to accommodate the PCB. Others have opted for 'best-of' panels, combining several PCBs behind one large interface. My solution was initially based on a false assumption - that I could cut below the CGS77's 'dotted line' to remove the unneeded buffers and mount it vertically.



I should have done this before populating the board, so I settled on another tactic. Using stand-offs and making sure the module in the row below is less than 4 cm deep, my '73 filter is a dashing 10HP. Here are the guts and here its installed.



On the mechanics, I used Charlie The Bad Producer's method of attaching the power via a perfboard add-on. I considered normalling the band-pass to the input for self-oscillation, but found in testing that the '73 doesn't make a particularly stable oscillator. The HP & BP are nice, but not as good as the Plan B Model 12, so I might leave them out on a future build. Other casualties might be the DC mode and LED. This filter responds well to modulation, so an additional CV input would be more useful: this can be added in at the points marked S & R on the schematic.

Thank you Serge & Ken for this wonderful filter, negativspace for BOM help and my brother for the boards!

Sunday, 17 November 2013

Nord Coast - Micro Modular Patches

Performance patches created on the diminutive yet powerful Clavia Nord Micro Modular.



I'd seen these odd shaped and coloured boxes lying round in studios gathering dust before. But it wasn't until I saw Rastko Lazic's inspiring video that I took the Micro Modular seriously. Here is a collection of patches, driven solely by the Micro Modular's three knobs:



Yes, it sounds digital and you need to jump through hoops to get the editor to work on a modern Mac operating system, but it's worth it. I've used the G2 demo for years to try out ideas, so I was used to the workflow. The NM engine doesn't have some of the conveniences or modules of the G2. But, as ever, there are workarounds. If you get stuck, consult the Nord Modular Book, edited by James Clark or Rob Hordijk's workshops.

In his video, Rastko uses a new Faderfox controller to play his patches. The two make a perfect pair, but I didn't want to be tied to a USB host i.e. computer. The older FF controllers are less flexible when it comes to custom assignments, so I built my own. I used a Doepfer Pocket Electronics kit and a perspex sandwich. The joystick is a small, game controller type.



With that built (and an old Faderfox controller now bought ...), I've realized that the beauty of the Micro Modular is its simplicity. With some canny patching you can get a lot of mileage out of three knobs! Download the album to access the patches and try them yourself.

Here is a link to Clavia's sound-bank. If you're using a Mac, the V3 Editor will run with varying degrees of stability and frustration under emulation. I have both Win XP and 10.6 versions running under VMWare Fusion on a 10.8 system.

Monday, 14 October 2013

Mungo d0 Delay Demo

Sounds and thoughts on the Mungo Enterprises d0 dual channel delay.



The d0 can be a simple echo for audio or CVs, but it also allows the times and depth of control needed for tuned Karplus-Strong synthesis and DX7-like audio-rate phase modulation.

Here's how it sounds:



Download the EP for more recordings (basic runthru, random tap tempo, flanger, audio-rate phase mod, cv delay (envelope), comb cancellation, karplus, filtered repeats).

There's been discussion on the d0 here and the Mungo range in general here. The d0 is a quality delay but some users are frustrated with the Zoom function and a lack of documentation. The controls do take some getting used to. I'm grateful I got to try the module first, buying it 2nd hand locally. jnlkrt, outlines most of the d0's quirks here. Bear these in mind to get the best out of it.

Here are some of my thoughts:

The Zoom function is essential and should not be sold as an optional extra. Because of the way it works, the panel knobs are not always WYSIWYG. It took me a week to get a feel for Zoom: switch down to get into the right zone, mid for 'normal' changes, up for fine tune.

Once you have zoomed a parameter, the knob range becomes 0 to 10, even if it is nominally -5 to 0 to +5 like the bi-polar controls. So, if you have selected a zone around plus 3, when you release the Zoom switch the knob covers that range from fully CCW to fully CW.

The slew setting influences the delay time response. This might sound obvious, but I caught myself wondering why the delay time wasn't changing only to hear it catch up seconds later! PM sounds require a fine balance between mod depth, delay time and slew.

To gain voltage control over the slew time, null it (fully CW) and use an external VC lag like the Sport Modulator. Slewing even seemingly smooth control voltages like sine waves can have a radical effect on the end result.

I made some of the recordings in my demo before I'd located a source of noise in the d0's output. Musically, they were interesting so I kept them. When unpatched, the CV inputs and Z outs carry a DC offset. Depending on the settings (gain, feedback) this can cause crunch and clipping. For a cleaner sound, null unused inputs or use a dummy plug. A filter can be useful in the feedback path.

The build quality of the board is good, attention to detail on the panel less so. Despite having slots for the mounting screws, they are 2 mm shy of the Doepfer standard. Keyed headers on the PCB power connector are welcome, but the orientation might confuse: -12V is 'up'.



The Mungo d0 is unique in its sound and range. Other modular delays I've tried were limited by comparison. I didn't find the controls or interface insurmountable. When something odd happens in use it's usually a welcome and musical surprise. In future, I'd love to try the rest of the Mungo range at my local dealer.

Saturday, 28 September 2013

Patch Tips #26 - Let's Make an Envelope

Deconstructing the AD envelope. An extravagant but enlightening exercise in understanding how triggered slopes work.



I've been looking into the circuits used in attack/ decay envelope generators and thought it might be interesting to patch one from its basic building blocks. I took my clues from a design described by Barry Klein and René Schmitz and Ray Wilson's Skew LFO to come up with the Skew-velope. Here's how it sounds (MP3).

And here's how it works:



A trigger sets a flip-flop high. Its output is slewed. When the slew signal hits a peak threshold, a comparator turns the flip-flop off and the signal starts to fall. When that voltage hits zero volts - i.e. the envelope ends - another comparator turns the flip-flop on and the process begins anew. Feedback helps shape the envelope. Here is a PDF of the patch and modules I used. Ironically, you may find yourself turning to Maths for its secondary functions in your experiments!

So, why use 8 modules to replicate something that can be done with one? It helped me understand the characteristics of these sorts of contour generators. For example, why re-triggers are ignored during the rise phase, making use as a delay/ divider possible. It also helped demystify Maths & its Serge forbears. Analogue envelopes rely on some form of logic and switching to work. These multifunction modules make some of those processes available to the user.

For more, read Tim Stinchcombe's paper on the Serge circuit.

Thursday, 22 August 2013

Fonitronik Triple Vactrol Resonators

A short demo of the forthcoming Fonitronik mh21 Triple Vactrol Resonators - three vactrol controlled band-pass filters, as found in the Korg PS3100.


















The layout and features of the mh21 are not finalized, but this recording should give you an idea of the resonator's sonic character.
In a word, juicy.



First you hear a summed signal. From 1:00 onwards, you're listening to two detuned VCOs which I patched into two filter inputs. I panned the outputs left and right. Both VCOs receive the same pitch CV. Depending on the frequency of the oscillators and filter cut-off, the signals seem to disappear and reappear. Let it swirl!

Thanks to Matthias for the loan of the prototype.