Continuing my suddenly obsessive System 100 stripboarding with a boring bit, the voltage controlled amplifier.
I wasn’t expecting this to be amazing sounding, being not much more than a CA3080, but I thought it was worth a go as I’m trying to replicate the original as far as I can.
Here’s the original VCA, with the blue marking on the 3080 presumably to show that it has been selected for low CV feed-through. I’ve seen circuits for matching OTAs, but I’m not clear how best to select for low CV bleed. Maybe it’s just a case of trying a bunch of 3080s to see which is the least clicky.
I wanted to keep the mix input from the 102, so I built the thing around Q337. Also not having a TA7136 SIP opamp to hand, I just subbed in the usual (non-inverting) TL072-based CGS DC mixer on the end, which was a mistake.
As I understand it now, the System 100 VCF inverts the signal from the VCO, and the VCA inverts it again, so it’s all back the right way up. As it stands, my layout inverts it twice, so input to the VCF eventually ends up inverted.
Incidentally I can’t imagine the TA7136 will make much difference, but now I’ve just had some arrive in the post, I’ll build a version with it.
To give an idea of my general confusion, I spent an evening wondering why the output from the VCF was crazily offset by a number of volts, before realising that this would be sorted out by the capacitor C324, which is on the VCA board. Just because it seems all nice and modularised doesn’t mean that it is.
Here’s a demo with the envelope on the System 100 101 keyboard modulating the VCA cutoff.
It gets clicky like my SH-5 with the envelope on a short release and the filter cutoff low, it might be that I need to find a better 3080.
One mod that might be interesting is to boost the input to the VCA from the VCF to see how it overdrives. Maybe messing with R378 on the input to the CA3080 might do the trick.
Here’s a stripboard layout for the inverting version using a TL071, any corrections or improvements joyfully received in the comments below, taaaa.
I’ve read loads on the various forums about fat, vintage-y sounding voltage controlled oscillators. What is the secret? ask the people with too much time on their hands. “Let’s measure it” say some. “There’s some undefinable mojo” say others.
Does the System 100 VCO sound vintage and fat? What does that mean anyway?
Enough of all that, here’s a schematic.
My System 100 sounds generally lovely. The oscillator sounds slightly different to my Analogue Systems RS-95 oscillators, apparently itself a “vintage-y” sounding oscillator. I’d prefer to quantify such woolly terms with measurements, but when I started building this, I lacked test equipment beyond my laptop/audio interface inputs and a really cheap DSO Nano oscilloscope. Which actually turned out to be a problem.
(It must be vintage – it’s brown and the opamps are in cans).
‘Cos it’s bigger than the things I’ve stripboarded so far – and I couldn’t get it to work on a breadboard – I was super-careful about laying it all out and checking for shorts before plugging it in.
The expo converter at the heart of the System 100 oscillator uses this…
…the Fairchild uA726 heated matched transistor pair. It was apparently always quite costly, and has been officially out of production for a long time, which means that it’s even more expensive and harder to get hold of.
There are other ways to skin the expo converter cat, but I’d hoped to stick with the uA726. Mainly because it’s easy, it’s what the System 100 had, and I didn’t think I was clever enough to work out how to adapt the circuit for a tempco or a heated CA3046 (like on the Curetronic version of the 100m VCO). Also I fretted about the possible power draw of a heating up a 3046 to working temperature.
All this hand-wringing lead me to searching eBay, where a seller in Hong Kong was offering a pair for fifteen quid with a no-questions-asked refund policy.
With my usual timing, I read all the comments in the analogue-heaven archives about dodgy uA726s after I’d clicked buy-it-now, so it seemed quite likely I’d ordered a pair of fakes.
Here’s one of them – they’re outwardly identical.
Before I plugged the uA726 in, I bunged in a matched pair of BC547 with no temperature compensation just for testing the circuit.
Here’s the arrangement I used for testing, with a 10 pin DIP socket in the uA726 position.
I used some electrical tape just to stop the legs of the left hand transistor from touching.
I flipped the switch, the power supply lights glowed happily, and the VCO made some sounds like it was trying to oscillate, but not quite managing it. When waggling the pitch knob it would cut out, and then distort and glitch out in a mad FM sort-of way. This is as good as I got it.
After a lot of head scratching, on a whim I swapped the comparator CA3130 at IC205 for a CA3140, which made it oscillate without glitching. With the BC547 matched pair in place, the pitch drifted in a comical way.
On installing the 726, it warmed up nicely and the oscillator tracked reasonably. I’ve still got some fiddling to do to see if I can get it tracking as well as the oscillator in the 101 keyboard.
Although the CA3140 worked, it didn’t seem quite right. On taking tuning measurements, I found that the pulse width increased with the pitch, from 50.3% at C1 to 51.3% at C6. I measured the pulse width on my System 100, and that stayed steady up the octaves.
I checked my photographs of the 101 keyboard, and there really are two CA3130s there – here they are in the bottom left corner of the shot – so there must be something else going on.
Boringly I spent a couple of months (no really…) to get the CA3130 working with no luck. I looked at the SH-5 for inspiration and tried a bunch of different things, including swapping the CA3130s, checking the value of R227, using original 1S2473 diodes, and checked all the voltages, but nothing worked.
Then I started having a look at the VCOs from the Rolands from around the same time and found an indication of the length of the reset pulse in the SH-1 service manual, again with using a small picofarad capacitor on the integator feedback:
Slinging in a 10pF capacitor across pin 3 and 6 of IC205 made it work. I was so glad, it was pathetic.
About this time I finally got a decent oscilloscope (which would’ve really helped with tracking down the problem), so here’s a video of the reset pulse with extra cap, dancing about like a four year old at a wedding after too many sweets:
And here it is with a 10pF capacitor on the integrator feedback, just a picture because it stays still. Although the trace glows brighter as the pitch goes higher.
3µs is a bit long, so trying again with a 5pF capacitor we get:
which seems pretty close to the SH-1 spec. Going smaller with a 2.2pF capacitor it still resets happily with a pulse length of 1.3µs, and it’ll probably go lower than that.
I’m still interested to know why it didn’t work the first time round, or even more how my 101 keyboard works at all.
I note that the SH-5 and the System 100 have a similar arrangement of diode + resistor round the integrator, but most of the Roland VCO cores that come after use a low picofarad capacitor. Reading the 1S2473 datasheet, it seems like there would be some inherent capacitance there, maybe it’s not quite enough in this case to keep it cleanly resetting.
I made a more basic mistake when comparing the DIY VCO frequency with my System 100, wondering why it was wobbling around like that. Looking at it on the oscilloscope I could see the square wave flexing in and out in a suspiciously 50Hz-ish kind of way. I realised that stringing a long wire across the room to my (switched off) MC-4 for pitch CV was a bad idea – turning it on or removing the wire mostly fixed it, with the rest of the wobbliness coming from the unshielded pitch offset wire coming from a pot on a breadboard.
While researching pitch wobbliness, I stumbled across a post in the AH archives: Ritchie Burnett did some testing on analogue synths to check for oscillator pitch drifting, and found that the VCO in his SH-09 was modulated by interference from the nearby mains power cables. There are some power lines routed directly underneath my table which probably don’t help, so I’ve placed a grounded metal place underneath the circuit board in the hope that’ll cut down on the interference.
The voltage levels are close enough to the original, and the waveforms all look pretty similar, apart from the triangle which suffers from a much larger reset glitch, presumably from the capacitor I’ve added for lengthening the reset pulse.
Here are some octaves of C, all taken through the filter fully open, then through my version of the VCA, which unfortunately inverts at the moment. Not ideal but you get the idea. Warning – the tuning isn’t perfect… Saw:
And a triangle – the glitch makes it fuzzier than it should be:
And some random PWM fiddling:
The scaling measures as ok until it gets to the sixth octave where it goes sharp. Adjusting the scale has sort-of minimised it, but I’m wondering if increasing R224 (resistor in the integrator feedback loop) to the SH-5 standard of 3.3K or adding a trimmer here might help – I note there’s one on the SH-2 at this point which controls “linearity”. Starting off from C0 isn’t terribly realistic so it’s not as bad as it might seem.
The high notes don’t quite sound as pure as the original System 100, which I’m putting down to a 50Hz modulation being picked up from the mess of cables on my desk.
I had a quick go at just comparing a couple of loops from the original and from my clone. Here’s the original, sequenced from the MC-4 with resonance set about half-way:
and here’s the clone
And the original with a bit more resonance
and the clone
The pot positions are different on both, partly because some of the pot values are different on the clone (lower resistance pots = more control over the snappy area, especially on decay) but also some of the rotary pots on the clone don’t have any knobs on yet.
Despite having left both on for half-an-hour before I started this, the clone had drifted upwards slightly inbetween takes, which wasn’t too great.
This thing has been sitting on my desk for over a year now while I fret about sliders vs. rotary pots for a front panel for my 102 clone, and I’ve changed my mind about the uA726. Initially I thought it was a real one but it seems unlikely that they are exact clones given how expensive it would be to start making such a component again, so I wonder how it came to be. At the very least it’s some similar arrangement of transistors shoved into a can with a possibly dodgy heater circuit.
Now I’ve got a bit more confident with changing circuits, I’m leaning more towards remaking it with a heated CA3046 (looking at the Doepfer A110 for inspiration, see below for the relevant part of the schematic), or an LS318 matched NPN pair with a tempco.
But anyway, here’s the stripboard layout, and rather more helpfully here’s the DIYLC file in the rare case that you are masochistic enough to want to build this and you’ve somehow happened upon a bunch of cheap uA726.
If you find anything wrong with it or if you know why my oscillator needs the extra capacitor to stretch the reset pulse out I’d love to know.
I bought a Roland System 100 101 keyboard years back, so I’ve been after a 102 expander since, with no luck. The last one I looked at on eBay went for over £1500 – it’s nice, but that seems a bit ridiculous. Although look at the panel, it’s bloody lovely, in the style of some austere-looking 70s science equipment.
So a while back I spent the evenings of a couple of weeks obsessively working on stripboarding the Roland System 100 filter.
This is a bit of the schematic, I’ve been staring at it a lot.
Most of the time has been spent wondering why it wasn’t working – but I happened across a post in the s-diy archives from Tony Allgood saying that all the System 100s he’d seen had the collector and base of the 2SC945Q transistors in the ladder connected together, much like the 303 – and this made it spring into life. (update: see comments at the bottom, I was being an idiot).
I had a look inside the keyboard just to check some values.
You can see the matched 945Q transistors at the bottom, marked with white paint.
Here’s a side view of the ladder, just to see how the transistors are connected.
If you squint a bit you can see the blob on the right hand legs where the middle leg is soldered. It’s even marked on the circuitboard, although the line goes the wrong way.
While I had the 101 open, I had a go at calibrating the square wave. The 50% trim pot is really touchy, trying to get that equally spaced waveform for the hollow square bonk was a trial, but it’s amazing how the ears can pick up on such tiny differences. It’s easier to get it just the wrong side of 50% and use the PWM slider to tune it in, although I think I’ve pretty much got it. Better still to replace the single turn 104 with a modern multi-turn preset, but I’d prefer to leave it alone while it works.
I used 2SC945GRs in for the 945Qs in the ladder. I didn’t match them but I did measure the betas and select the lower ones. The 945Q has a range of 135-270, and the 945GRs were mostly coming out between 260 and 290, so I selected a bunch between 210 and 250. I don’t know if it matters, but I was trying to cut down on any differences. The 945 were all pretty similar, it wasn’t hard to find matches (using Ian Fritz’s technique – PDF) for the matched pairs at the top of the ladder.
I put some old pulled 2SC2240 in place of the strictly unobtainable 2SC1000, although it’d be worth trying BC547 instead.
The dual ITS1276 transistor in the expo converter is also impossible to find, couldn’t even locate a datasheet for it, so I bunged in some matched 945GRs here and hoped for the best. Rob Keeble of Amsynth was planning on using a THAT transistor array (presumably THAT300) in his take on the SH-5 filter.
The input single op amp on the original is a TA7504M, so I stuffed a 741 in here. A TL071 works here too, it doesn’t seem to be critical. I used a CA1458 for the CV op amp.
Some of the things I got wrong were putting an electrolytic in the wrong way round, the TL071 powered from pin 7, not pin 8, as on the TL072, a track not cut on the output, and all of the 2SC transistors the wrong way round. I’d socketed most of them, so it wasn’t too bad to swap them, but… really.
Also I’d got the 2sk30a-y in the wrong way, which meant a drop in gain. Or at least, I thought I had – until I realised that it’s part of a sub-circuit to boost the gain going into the filter as the resonance increases, so the output level remains reasonably consistent.
Apart from the base-collector thing on the ladder transistors, the 101 schematic was wrong in a couple of places. The feedback capacitor C318 should be 47uF instead of 10uF – although I can’t think that would matter so much – and R371 in the VCF checkpoint output is marked as 10M, it should be 2.2M, or else you’ll get a distorted output. The 102 schematic looks right.
I’ve spent ages trying to get the resonance pot response right. It seems partly dependent on input level, needing 10vpp to sound authentic. The specified dual 100kA pot went nothing-nothing-nothing-slight-FEEDBACK, rather than the gentle increase in twang of the original.
Looking at the schematic for the LPF in the SH-5 which is really similar, the rotary for resonance is a 100kC reverse audio log pot. This makes more sense to me than a log pot, from memories of playing with guitar feedback and how twitchy it was.
Substituting a dual 100kC pot made the resonance much more controllable. I found it useful to unplug the feedback side of the pot (connections 90 and 91 on the right hand side) just to check that the gain increases when the resonance pot is turned up.
So does it sound like the original? I think so. The knob positions are a bit different because they’re tuned slightly differently, but just as a demo – here’s a square wave pattern recorded from the VCF check point of my System 100, no VCA
And here’s the same pattern, System 100 VCO check point to an input on my stripboard clone, recorded from the VCF check point, no VCA
Here’s a slow filter sweep at high res, the resonant peak picks out the harmonics in the square wave bass note.
Some more square wave acid-y silliness (now I’ve got that square wave tuned in I can’t help playing with it) – again no VCA, but with a TR-606 keeping time, the slow-ish envelope from the 101 keyboard struggling a bit to keep up.
And a bit more acid nonsense, some sawtooth wave cobblers, playing with ADSR and CV modulation of the filter, through a breadboarded version of the CA3080-based VCA.
Finally, the layout. I used DIY Layout Creator to lay the stripboard out, which is a bit buggy here and there, but works ok. Here’s the layout in PDF and DIY Layout Creator format. (updated 8th Sept 2014 – thanks Paul K).
I wouldn’t recommend it as a first build. In trying to save board space, I’ve made the layout rather tight. If you’re going to build it, I’d suggest zooming in really close and checking for track cuts underneath components. The numbering broadly follows the schematic, which is worth studying. Let me know if you find any problems with the layout.
I’ve been using 0.1″ headers for connecting wires from the stripboard to pots/sockets/power, which has worked out pretty well for testing and making alterations. I bought a bunch of rainbow coloured wires with the connectors already attached off eBay.
This seems obvious now, but next time, rather than jumping straight to laying out a stripboard, I would breadboard the circuit first. It’s what everyone says you should do, and I didn’t. Mostly because I couldn’t be arsed with fiddly breadboarding on my too-small slightly-melted board. It would have saved me time in the end.
There’s no panel design yet, mostly because I have all kinds of (possibly expensive) ideas about recreating the entire 102 expander on stripboards, and what kind of grim looking 70s style Roland front panel I could get built.