I Dream Of Wires

A great film and well worth watching even if you're determined not to get sucked into that whole different mess of wires.

http://www.idreamofwires.org/


Also a Garry Numan song and a made up (by the NME) band.

Anyway, as I may have pointed out once or twice there are a lot wires in the SS30. A lot.

I've had a couple of sessions since the G boards were brought back to life just rewiring the K boards.

First off, two of the G boards where disconnected from their corresponding K boards.

De-soldering 

Lots of old, seventies solder. 

Finding a good method for stripping wires as thin as these can be a case of trial and error. Luckily I quickly hit upon using these tweezers to just yank the plastic sheath off.

Strip and twist, strip and twist.

Note the wires are tied with fine string dressing.

Ready to solder

And on the other side, another 49 wires from the K boards to the key switches.

The muck was there when we bought it. Also you can see more loose wires to be reconnected. That yellow one should be attached to AT1.

I had cut all the wires from the key switch PCBs on the assumption I wouldn't need them again. It was also to make handling the whole tangle of boards and wires less difficult. Now I decided to reattach them temporarily to make it easier to test things out.

I removed the switches as they were in a bad way.

And the finished job. A keyboard (if your fingers are earthed).

More power?

The MIDI interface needs a power supply between 8 and 35 volts. The supply current needs to be 15 mA for the interface itself plus whatever is drawn by the output loads.

The loads are connected to a common positive supply switched to ground by the darlington pair drivers.

If the driver voltage is 15V I'd need the 24V rated Coto reed relay which has a coil resistance of 2KOhm. That would be a current draw of (15 / 2000) 7.5 mA each. Assuming worst case of all 49 relays being on at once that would be approx 368 mA.

That's worst case but the question is this: Can the existing SS30 PSU be used to power the interface?

The SS30 PSU has a +15V rail (this is all carefully worked out, you understand) which is rated up to 500 mA. 

What's not clear, is how much of that 500mA is used by the SS30. I know that the +15V rail is not used for the K boards (-7V) or the G boards (all voltages are derived form the -26V rail) but it is used on the LF, OR and F boards. If the interface took the best part of 400mA (absolute maximum) is it likely the SS30 is using under 100mA?

I'm going to have to get the whole thing working and then measure the current on that rail before I can decide whether I need another power supply.

G Board Restoration

Here are the SS30's main PCBs.

It's about 50% wires!

G boards on the left, K, boards at the back and the rest in the middle. During the hiatus I'd cut out a piece of MDF to fit everything to.

I'd checked the power was okay so I decided to start with G boards and see if the oscillator and tone generator ICs were all working. It's a good place to start as all the sound originates there so anything else would be hard to test until they were verified. Also a faulty tone generator chip might be difficult to replace. Yamaha said they had spares when I got the service manual from them way back when, but would they still have these chips? I;d need to find out early on.

I took everything out of the case to gain access.

Don't worry, I've got a wiring guide.

I prodded the scope probes around the G2 board, which is the top most of the stack, and straight-away found that there was a problem. Only one of the tone generator chips was outputting anything. The other IC had no input clock. I wasn't surprised, a quick look around the G boards and I could see several wires that had worked loose at some point.

-26V and -9V supply rails broken on G4

 These wires were all easily fixed, although I did have to check a few more than once on the schematic and wiring diagrams. One group of wires in particular seemed to be linked in a circular fashion. I'm not even sure what they do - something to do with suppressing noise I guessed - so I decided to move on and come back to that mystery later.

I checked the traces again and was disappointed to find that there was still no output on the tone generator ICs. Now I was worried. Damaged ICs would slow things down and a potentially be expensive. However, I've spent enough time fixing electronics to know that you need to be careful not to just swap out an expensive IC because it's appears faulty.

By the way, the SH-101 I'm also fixing does seem to have a broken oscillator chip, but I double checked everything around before condemning it. When I get the replacement I'll still be nervous until it's in and working though.

I went all round the circuit and traced the problem back to the oscillator on the G3 board. There are two identical oscillators built from discrete components, one on G3 and one on G4. This is great as I could A/B compare them to see where the problem started. The first problem I had was using my scope! I was comparing two wave forms and getting one at the right the frequency (around 500KHz) and the other was... well it seemed to be beyond the range of the scope! It was already late, so I went to bed.

Spot the mistake? (the chocolate wrapper is not a clue)

The next evening I started again and got the same problem. This time I went through and systematically worked out that the scope was not showing me the right trace if both signals were connected. I'd got the trigger set-up incorrectly. In the picture above you can see that only channel A is being used as a trigger source. The oscillators are not synced so triggering one from the other gives the problem you can see in the photo.
Once I triggered each input off it's own signal I got two comparable traces. The night before I'd quickly ascertained that the signal from the G3 oscillator was DC shifted negatively by several volts. Having excluded any other issues I was sure this was the problem. It's supposed to be between 0 and -15V, so what was causing this anomaly?

G3 Oscillator being negative.

I eventually disconnected a cable - which I'd just reconnected - that feeds the clock from G3 to G4. Now the signal was biased correctly. So, it was something on the G4 board.

There's something

 When I was fixing that cable I'd noticed this capacitor on the underside of the G4 PCB. It was obviously a modification and didn't appear on the schematic. It also wasn't present in the same position on the G3 board (where the G4 clock enters that board). So, I took it off. Now everything worked perfectly. This cap was clearly an afterthought. It was pulling the signal too low and served no apparent purpose. I suspect that once everything is running I may find out what it was for, but that bridge can be crossed if I get to it.

Test Points 1 and 2 (A and B triggering!)

So, I could now see clean, strong signals from both of the first two test points on the schematic, indicating all was well with the oscillators.

The next job would be to reconnect the wires from K1 and K2 to G1 and G2.

Power-up

It's been an exciting couple of evenings (yes, that's how I roll) as I've had the actual SS30 hardware out and powered up for the first time in years.

I had done some work a few years back that didn't make it on to the blog. I had put the power supply into a temporary enclosure. It's never going to fit in the rack case so one day it'll get it's own, proper enclosure.

The original back-panel (upside down), power switch and lamp

 Which is a re-purposed carry box

Inside you can see everything looking good

 

 

Note the Japanese characters on the PCB.

 I checked all the rails and was relieved to see them all looking healthy

But what about the synth itself?

Switching negative voltages - Concluded?

Keep it simple

 Back in this post http://ss30m.blogspot.co.uk/2009/04/how-are-you-to-switch-negative-voltages.html I was talking about the issues with switching negative voltages with the simplest circuitry possible.

The guy at J-Omega recommended opto-isolators and I like the sound of that but in the interests of trying to limit the number of pins and foot-print (can't we just use three pins and keep things small?) and the price (transistors are cheap!) what, if any, are the alternatives? Also, I don't know about you, but despite studying these blighters at university I can never quite remember the exact rules governing their use. In particular there are precious few examples with negative voltages and certain factors, that don't come up when everything is above the ground rail, were obscure to me.

Transistor biasing basics.

It seems almost counter-intuitive but you can't switch a negative-ground current with a positive-ground current. The problem is that to turn a transistor switch 'off' the gate (or base) voltage must be lower than the lower end of what you are switching. This is as well as having the 'on' voltage higher.
Normally switching a positive current to a ground is a doddle because your 'off' voltage is ground. No current flows (in enhancement type FETs, anyway) because there is in-effect no bias. When you apply a positive voltage the transistor is biased and the switch is 'on'.
With a negative current to switch the 'off' voltage has to be lower that the lowest voltage. In this case it will have to be lower than -7V (or thereabouts). Because the MIDI interface drivers use Darlington pairs which themselves are driven on from TTL logic levels there is no way to generate an off voltage that is less than 0V.

Previously I mentioned analogue switches which can (in some examples) do this kind of magic. Normally analogue switches are subject to the same limitations as laid out above and the logic switching voltage must be between Vss and Vdd. I referred to the Modern CMOS Circuits Manual book which mentions that some devices - the 4051 and 4053 - have a logic level converter. In effect a 0-Vdd input logic level can be translated into a Vss-Vdd level internally. Those devices are multiplexers though and take a binary 3-line logic input to decode which of the 8 switches to enable. This is no good for this design as we have a line per switch and moreover need to be able to switch all the keys on or off independently.

In summary neither analogue switches or simple transistor circuitry can be used to switch a negative current. I'm glad I checked though as it would have bothered me. I actually spoke with a few colleagues who are full-time electronic engineers and they all initially thought it must be possible, only to conclude that it wouldn't be. It's such an unusual situation that it's easy to get caught out.

Reed relays?

Metal Vs Silicon

 So, opto-couplers it is! Or maybe not. One suggestion a  colleague gave me was reed relays. The main advantage of a reed relay would be that in electrical terms the mechanical switch contact in the keyboard would be replaced by another metal contact switch and not silicon. Introducing silicon might affect the circuit in some way. Perhaps. Or perhaps not much, but the simplicity of this solution is attractive and it might remove some possible risks.

Slim Jims

 One other nice thing about relays is that some come in narrow, SIP (single, in-line pin) packages. So you can see that they could be lined up neatly and compactly. This Coto part http://www.cotorelay.com/datasheets/Coto%20Technology%209007%20Spartan%20SIP%20Reed%20Relay.pdf is 5mm wide*.With a maximum of 13 switches per line (from the four K' boards) and assuming that I would stack the boards vertically, that would be a minimum* of 6.5cm per board, which is around the length I would want it to be.

* Actually they are 5.08mm wide which is 2 x 2.54mm, the standard pitch for through-hole PCB components and boards. At a minimum you could sit them side-by-side, on adjacent lines on a strip-board with no gaps.

Tracking out

 The only down-side of this package is that that the coil pins are on the inside (pins 2 and 3) so you can't as easily track to the outside edge of the board.

To be clear, I'm working on the assumption that the board will have wires from the MIDI interface going in one side and wires to the K boards on the other. Because both of the coil pins are on the inside of the device if you line then all side-by-side there is no easy way to get one of those pins out to the edge.

It would have been great to just use veroboard for this and avoided any kind of tracking out. It's frustrating but they all seem to be designed like this so it looks like I will still need a custom PCB.

Quick enough

 There is one potential downside - timing. This Hamlin part http://www.hamlin.com/specsheets/he3600%20revised.pdf , which is currently the cheapest at Farnell, has a maximum turn on of 1ms. The next is half that at 0.5ms. To be frank I don't think the SS30-M is going to be played at high tempos and the in-built attack and release are already quite slow. A millisecond is not going to be noticeable.

Decision?

So, reed-relays it is.Is it? Am I decided?
I think so but I need to work out the PCB and think about the options a bit more before deciding once and for all.

Tone Generation

Whilst looking for some clues on the key switching problem I came across this.

https://archive.org/details/synthmanual-yamaha-ic-guide-book

https://archive.org/download/synthmanual-yamaha-ic-guide-book/yamahaicguidebook.pdf


The Internet Archive has a mission to provide "universal access to all knowledge". Amen to that!

The document seems to have come from here : http://www.loscha.com/scans
which has a number of other interesting synth related documents, including another version of the IC guide book.

The manual contains details of the two chips used on each tone generator board. Ten years ago (blimey) I wrote about the polyphony. I mentioned in passing that the SS30 has two oscillators per note. This post expands on that and, although it still isn't the whole story of how the sounds are generated in the SS30, it is where it all starts.

The SS30 tones start with a pair of oscillator circuits built from discrete parts. These are free-running VCOs with control for the pitch and detune. They share a common master tune and vibrato input and one also has the detune control input . The output frequency of these VCOs is much higher than what you will end up hearing though.They both produce 500 KHz signals.

These two signals are then feed to a pair of YM25400 Digital Tone Generator chips as the master clock input.

Each cascades of these Digital Tone Generators output a divided down clock which is passed on to next generator board for further division. 500KHz - 250KHz - 125KHz - 62.5KHz

The YM25400 derives 13 tones (an octave + 1, C0 - C1) from the master input clock. Each YM25400 then feeds a further pair of LM3211 frequency divider chips.

In summary: the G (tone generator) boards convert a pair of master clock inputs into to a pair identical octaves from two YM25400s. These octaves are then divided down further by a two pairs of LM3211s. This creates another identical pair of octaves one octave lower than the first pair. You end up with two groups of 26 semi-tones (2 x 13).Only G1 uses the extra semi-tone to give octave  + 1, the other boards discard the extra C tone and just output an octave.

The two octaves are named using the eight-foot-pitch naming convention. The initial octave direct from the YM25400's at the 8' and the one below is the 16'.

By now you may be thinking we're going to end up with not with 49 tones for the 49 keys but more like two hundred tones! And we do, but not all at once. Firstly the outputs of the YM25400s and LM3211s are mixed in pairs. Each tone that come out of the G boards is a combination of two square waves - and quite a strange combination too, which I will cover in a later post. If I can actually figure out what is going on!

So, there are actually half that number, but still double the number of keys. The reason for that is the G boards generate Violin, Viola and Cello tones. The various outputs of the G boards are split, switched in and out and combined in various ways to provide the various options selectable from the front panel. At it's simplest you can play the bottom octave, the G1 board, as violin/viola or Cello. When playing as Violin/Viola you only use one half of the G1 boards output. And when you change the split and play Cello you use the other half of the G1 board. It becomes more complicated on the other boards. G2 is split twice, so you get Cello half way through an octave at F as well as at C, and G3 and G4 don't out put any Cello. It gets very confusing in the schematic but is quite simple in the end.

Five years - and then an oscilloscope

Five years is long time between posts. I'm not going to try and explain that gap. Life has it's own priorities and this project wasn't one of them, I guess. I mean, I have done other things with my time beyond work, family, home-life and friends. In fact I have done a bit of work on the project in that period, but I didn't post about it. I can't really explain why but I do know that I get interested in something, spend a lot of time on it and then lose momentum suddenly. Or more accurately, I get distracted by something more important or interested in something else.

However, this project was always intended to be long-term. It took me years to go from a pile of bits to getting a case and thinking seriously about how to progress it. So, a bit of a gap is no great concern to me, although every year I don't have the finished article is another year I could have been using it.

On the other hand I'll be forty next year and if I aim to finish this project by then it would give me a date to work towards. As I type I have around 11 months to go, so it's attainable. Finishing would also free me up to do something else. I can't think what that would be though. 

Anyway, enough of this introspection.

One thing that has held me back from continuing with the project is having a decent scope to work with. In my early twenties I spent around three years of my work-life with a scope or soldering iron in my hand. When I'm working on electronics it's a scope that I reach for to see what's happening. It's the right tool for the job. I say a decent scope because I do have a scope, a GBDSO - Elektor Gameboy Digital Sampling Oscilloscope. This project was fun to do and produces tolerably good results but it's also a far cry from the professional kit I'm used to and, whilst a bad workman might blame his tools, a bad workman usually has bad tools. The issue is that I need this to be a pleasurable experience and using the GBDSO can be frustrating. I also want to be able to see audio traces cleanly and even some fairly pricey and professional digital scopes do a poor job of that.
So I bought a second hand Philips PM 3050 60Mhz analogue scope instead.

As you can see it's dual-trace and there's a nifty LCD display to show you the current settings. The traces on screen in that photo are the square/pulse and sawtooth output of the CEM3340 voltage controlled oscillator chip on my Roland MC-202.

The 202 was modified by me with some CV inputs years ago and has always been a bit flaky. As part of a general sort out in my studio I resolved to do something about that. I also realised that an SH-101 that I have on loan from a friend isn't working any more. No, I didn't break it. Well, I don't think I did. It's hadn't been used for years so I'm not sure what happened to it. So, I'd like to repair that too. It was these repairs that set me thinking about a scope again and how annoying the GBDSO was to use.

I've fixed the 202 now. The main issues were actually to do with removing some of the battery circuitry and disconnecting the internal sequencer. The sequencer was zapped when I did the mods originally so I decided to live without it but made a mess of the way the battery state is monitored and disconnected when the mains power is applied. I also fixed the filter audio input which never worked because I hadn't realised that the 1/4" headphone jack socket re-purposed to be the input was shorting the input to ground!

The 101 is next on the repair list and then - back to the SS30-M.

How are you to switch negative voltages?

I've been wrestling with the problem of key switching again. Because the key driver circuits switch a negative voltage to ground it creates a bit of a problem.

When I first looked at the j-Omega MPT8 I thought it could switch negative voltages but after thinking again and e-mailing then it seems not.

What's all the fuss about though? I can use a solid-state relay, optocoupler or CMOS switch package right? You don't even have to think to hard to get it working. The issue here is that I have 49 keys and very limited space. I'd really just like a transistor and maybe one or two resistors per switch. CMOS switches only come in quad packages at most so I'd need 12 and all the trracking back and across each other to get everything wired. If I must have a PCB at least I'd like it to be simple.

Generally switching negative voltages to ground is not something you get a lot of talk about when looking up these things. Everything is geared to positive voltages and how to bias your transistor that way. It's not impossible just less usual and if you wan to use a simple +nv/0V logic level your options are limited.

The reason for this is not that you can't do it (just switch from n-channel to p-channel FET) but that transistors that switch negative voltages themselves need a negative voltage to switch. which takes you back to square one.

Well, If a CMOS switch can switch a negative oltage with just a +nV power rail how does it do it? I've been wondering.

Google books have a Modern CMOS Circuits Manual online and chapter four has the answers

External Power Supply Unit

I'm thinking life would be much easier if the power came from an external unit, rather than having the PSU inside the chassis. Space is really at a premium and that's before squeezing in the Midi converter and switch circuits.

There are six lines (rails) coming from the PSU so if I had an external uni I'd need a six-way connector.

These ones from Rapid look good... http://www.rapidonline.com/Cables-Connectors/Connectors-Multipole/Circular-Connectors/Circular-locking-multipole-connectors/66501

I powered up the PSU last night it looks good but I am worried about the extra cutrrrent draw when I add the new modules. It's all on a 0.5A fuse which seems quite low. Might have to do a bit of measuring...

PCBs

Is it practical to get a custom made PCB built up?

These UK company's do a custom service:

pcbpanel.co.uk

pcbtrain.co.uk


They both accept Gerber and Excellon format files.

pcb train recommend using Easy PCB for layouts but that runs to hundreds of pounds.

Instead, I found KiCad which does the job and is GPL.

Switch circuits

I will need 6x SPDT, 2x DPDT and 2x SPST switches.

The 4066 contains four SPSTs which can be connected as required to make SPDTs and DPSTs.

2 x SPSTs = 1 SPDT
4 x SPSTs = 1 DPDT

So, I will need (6 x 2) + (2 x4) + 2 = 22 SPSTs in total. So, I'll need 6 4066s in total giving me 24 SPSTs to use.

It would be simplest to make a single circuit board up with all 6 switches on but you also need to accomodate the logic invertors. The 40106 invertors have 8 not-gates per package and you need two per switch (irrespective of what type you're making) which makes 20 gates or three hex invertors.

In total thats 9 x 14 pin ICS so in a single strip there'd be at least 63 rows on the board or in other words it'll be about 9 x 20mm = 180 mm. Which is okay, as Maplin do an 81 strip board 213 mm long .

Switches cont...

After looking around at switches I can see that there's alot of choice but it get's quite limited when you start narrowing down to DPDT types. This is another reason for looking at the switch IC idea.

I want to choose switches that will look nice and the biggest choice is in SPST.

Like this one from Rapid Electronics which has built it LED. Not blue though :-).

Switches

I'm going to need ten switches plus the power switch.


Orchestra:

Speed - SPDT
Cello - DPDT
Violin - DPDT

Cello:

Attack - SPST
Cello 1 - SPDT
Cello 2 - SPDT

Violin:

Attack - SPST
Viola - SPDT
Violin 1 - SPDT
Violin 2 - SPDT

So, that's 6x SPDT, 2x DPDT and 2x SPST.


When I breifly worked in pro-audio I noticed that they almost never had audio signal going through the actual mechnical switch. The switch was usually controlling a DG201 or DG211 audio swicth IC. This was high-end gear designed for the live use so I suppose there was a concern that meachical switches would degrade over time geting damaged or dirty. You don't really want audio going through anything that might colour the sound or produce noisey clunks on switching.

The original switches are all mechanical though, so why worry? Partly because I won't be using the same kind of switch and partly because there may be a way to improve on the original.

This guy calling himself The Tone God has a design called Wicked Switches that uses 4066 analogue switch ICs. He also explains how to add debounce and LED indicators.

Spec reset.

Over the years, I've been thinking I should really make this project easier to finish.

The two main issues have been the awkward meachnics of fitting the original switches and knobs and the challenge of conjouring up a 49 output, velocity sensitive MIDI decoder.

The meachnics iossue could be resolved at stroke by moving to a set of new off the shelf compoenents.

Similarly there are several polyphonic MIDI decoders available from one company or another.

Therefore, I will continue this project on a simplest is best principle in order to finish it sooner rather than never.



j-Omega Electronics make a reasonabley priced MIDI decoder that will meer my needs.

j-Omega Electronics MPT8

The MPT8 has 64 outputs in an open-collector 'pull-down' arrangement. I'll be pulling up, from -7V, but to ground which is okay.

Matrix synth comments.

I added some comments to Matrix Synth:

"some sources describe this synth as a duphonic/ multiphonic ,others describe it as a 4 voice polyphonic. I have a feeling it is a 4 voice polyphonic but the 4 voices are split between cello (two tones) and violin (2 tones) . It then uses divide down chips enabling all keys to be played at once)"

Nearly. It has two master oscillators which are both dived down to give you the full set of notes, twice.

You need two waveforms to get a stringy effect so the same note outputs of the dividers are mixed.

When you press a key you're switching in a mix of two divided down oscillators.

The detune affects both oscillators but not equally hence you can detune one against the other and get to the good old thick sound but more importantly set the PWM type character you need for strings.

The outputs of these mixed waveforms are sent to either the Violin, Viola, or Cello mixers. In some cases the outputs can be sent to either one or the other mixer as set by the split selector switch.

The cello, viola or violin character is set by filters after each mixer. Violin 1 & 2 just are different (1st order) filters fed from the mixer, so when you switch them in you're just mixing and matching different filtered outputs of the same thing. the same goes for Cello 1 & 2.

So far it's just a paraphonic organ but with two tones per key. However the SS30 rules over other string synths because it has an AR envelope generator per key. When you set a long attack each time you press a key it starts a new attack just for that key. same with Release.

There's no VCF per key but that's what we have Polymoogs and, err, GX1's for.

Also on MatrixSynth

Matrix synth entry

There's an eBay auction y'see

So these pictures and videos may be up for only short time.

"some sources describe this synth as a duphonic/ multiphonic ,others describe it as a 4 voice polyphonic. I have a feeling it is a 4 voice polyphonic but the 4 voices are split between cello (two tones) and violin (2 tones) . It then uses divide down chips enabling all keys to be played at once"

Well, I maintain this is a fully polyphonic keyboard keyboard. See here for more. Essentially each sound (Violins and Cellos) have their own voice board which are then switched to a key range.

"The outer pot adjusts the overall tuning and the inner pot detunes the voices against the pitch which can produce a wonderful thickening effect"

This is interesting as I hadn't thought about this and since I haven't played my in some years (sigh) I'd forgotten what it sounded like. I'll have to check this but if memory serves correctly each note is mix of two oscillators (that's how you get a stringy sound) so the detune is just off-setting the two.

Video demo.

A spiffing demo! From Synthopia via my good friend Barker


http://www.synthtopia.com/content/2009/03/08/yamaha-ss30-rare-string-synth/

Greetings MatrixSynth gawpers!

Sorry, I've been too busy to keep this up.

Don't give up hope though, I haven't.

Yeah, I'm lame. Okay, okay.

Here's some red hot videos from YouTube featuring the wonder of the SS30.

http://www.youtube.com/watch?v=eRIJW5bYWN4

http://www.youtube.com/watch?v=eRIJW5bYWN4

The first one is Simple Minds on the Old Grey Whistle Test which I remembered seeing the SS30 on a Best Of... program about 10-15 yeras ago and due to magic of YouTube (yes, it's magic) I was able to find. Umless there's another one and this is not the one I saw. The second one is Ultravox on TOTP.

Front panel mock-up

As I mentioned I've been busy working on the front panel. The first fruits of this labour are now starting to show. Below is a screen grab from Schaeffer AG's Front Panel Designer program. You might need to zoom in to get a better look.

This has been laid out with all the original controls. I still need to add the power switch and indicator lamp as well as any other controls I might need such as MIDI channel settings. The biggest concern is getting everything to fit both in relation to each other on the panel and in relation to the internals of the case at the back of the panel. Rather than just send the design off to Schaeffer and hope I'd got it right I printed out a simplified version of the design and pasted onto a piece of card of the right dimensions. You can see the results below.

At this stage, just the switch banks are fitted. I have also laid out a design for the reverse of the panel. Schaeffer will fit either threaded bolts or bushes to the panel so I've positioned cavities on the reverse of the panel for these so that the switch and pot assemblies can be mounted without anything showing on the front. This design was also printed out and pasted to the back of the card. I then punched holes through so I could check that everything matched up. You can see the holes more clearly in the close-up below.

So were there any problems? Firstly I was a bit concerned about the holes for the switches. They seem big enough but cardboard is a bit more forgiving that 4mm aluminium so I may open these up by another millimetre on both dimensions just to be on the safe side. Secondly and more seriously I'd made an error of measurement for the height of the switch banks at the rear...

As you can see the top bank of switches protrude above the level of the case. The lid will hit the top of the PCB. I think the error was partly bad measuring but also a miscalculation of the usable area of the front panel. The panel protrudes above and below the level of the case and it's obviously wider so there's only a window on the panel which is usable. Because the switches bank up at such a steep angle I should have moved them down by around 10mm. There's barely 10mm between the bottom of the top switches and the top of the bottom switches. In order to allow a little wriggle room, I really need to move the bottom row down. But is there room? In fact, there are a few mm available if I cut a strip off the front of the base board. This means I can move the bottom row down by 5mm and the top row by 10mm leaving a good few mil' spare at the top. It's going to be tight, but it should work. I'll need to do a new mock-up to check it over but it that should be it.

Overall it's gone quite well. The basic dimensions are good and apart from needing to move everything down, it fits into the case.

Next time I promise a solution to the MIDI channel control issue and a finished front-panel design.

Re-comencement & front-panel design

Well, it been a long time since I've made an update and whilst there hasn't been much progress there has been some.

Back in March I was stating to try and choose the PIC and I had to make a decision about the method for selecting the MIDI channel. Clearly Omni mode is useless in a set-up with a few different MIDI instruments. Also, I didn't want to have a fixed channel as this too can cause complications. So I was trying to decide how best to make the channel selectable - would I have a rotary switch or a pushbutton and LED display arrangement. There was problems with both and I decided I needed to see what space was left on the front panel. I also wanted to get the mock-up front panel in place so that I could fix the PCBs in place.

I had decided to get an aluminium panel cut and engraved by Schaeffer (http://www.schaeffer-ag.de ) and so I set about laying out the panel using their excellent free program. I soon ran into problems though.

In order to produce a pleasing and profesional looking final panel I don't want to put screws through the front panel. Nor do I want to fix pots to the front panel. The correct way to do this is to have a sub-panel behind the front-panel with all the controls fixed to that and then the frontpanel is kept clear for all the text and markers. Schaeffer can fix bushes (clinch nuts) to the rear of the panel so I can fit the sub-panel(s) to the back of front panel. Great! Let's designing.

If you look at the pictures below you can see that the pot's are already fitted to sub-panels. This is how the SS30 was built and is another reason to go with the sub-panel idea. When I sat down to try and lay-out the front panel I had to ensure that the holes for the pots were kept in line with these sub-panels. Also the position of the bushes has to be lined up with the fixing holes. This was starting to get tricky as the positions had to be relative to each other. On top of this the space on the front panel is tight if I want to use the orignal chunky tablet switches so the placement of the sub panels and switches needed some fine tuning. I needed templates for the sub-panels that I could move around and line up. This a bit of stretch for Schaeffer's Front Panel Designer (FPD) program so I decided design templates for the subpanels in AutoCAD and then I could slide everything around till it fitted. Luckily my partner is a dab hand at AutoCAD and she had me up and running in no time.

It took some time but eventually I had all the SS30 sub-panels and switches placed. I could then take the co-ordinates of all the holes from AutoCAD and transfer them to FPD. Finally I put all the text and marker engravings on the design too, using teh original SS30 panel as a guide.

So this is where I've got to. I'd like to post up an image of the panel but I need to do a screen grab as you can't print to file or anything like that in FPD. Eventually I'll put up the screen grab and a link to the actual FPD file.

That all took me a couple of months but I've done nothing for the last two months owing to other things taking priority.

Next : I need to get a print out of the design from FPD then paste that onto a piece of MDF. I can then drill and cut all the holes to a) check the measurmenets and b) have a mock-up that I can use until I take the plunge and get the real panel made.

But what about the MIDI channel control? Is what room is there? Well, I'm 90% sure that I'm going to for a push-button and LED arrangement but I need to check whether the PIC has Non-volatile that can remeber the setting. If not, then it'll have to be some sort of mechanical switch.

The interface nitty-gritty

Okay, I'm a bit bored of all this mechanics talk. I'm still working on the case, but I thought I'd talk about the interface for a change.

To recap, the aim of the inteface is to provide a MIDI in port that will take note on/off and velocity data messages on a specified channel and not only individually switch the keys of the SS30 on and off but also individually set their amplitude from the velocity data.

I have found from experimenting that if instead of simply shorting the key inputs to ground (as with the original keyboard) place a 10K potentiometer between the input and ground you set the loudness of the note played by that key. I'd suspected this was the case previously as the volume of the keys across the keyboard wasn't uniform and I'd put this down to the dirty contacts creating an impedance where there should have been a zero ohm contact.

So, if I can set a resistance across the key inputs from a combination of the note on/off and velocity data then I'll have a velocity sensitive MIDI interface for the SS30!

There are severals way to do this but I think they boil down to these two:

1) Use a DAC to set the gate voltage to a JFET acting an amplifier with gain between 0 and 1
2) Use a dedicated digital potenetiometer.

So which is better? The dedicted digital potenetiometers can be found in packages containing 6 pot's each. They would ceratinly make wiring easy and would keep the number of components low. I will err on the side of simplicity and persue these for now. If they become unworkable I may have to switch to the DAC-FETs solution.

So how am I to get the right MIDI data to these digital pots? I need a microcontoller to decode the MIDI data and then drive the pots and the simplest and most popular solutiuon with other people doing this kind of task is the range of 8-bit micros from Microchip Technology known as PICs (Programmable IC's) . I'm not going to say much more about them now but I will want one with an SPI (Serial Peripheral Interface) bus to drive the digital pot's.

Time for some maths! Hooray!

MIDI is transmitted at 31250 baud (31250 bits/s). Which means each bit's period is 1/31250 = 32us. Messages are transmitted in 10 bit packets with 8 data bits plus start and stop bits. So the message period is 320us.

Typical clock rate of a higher-end PIC is 40MHz (e.g. 18F458). This gives a clock period of 25 ns but a 100ns instruction cycle period with some assembler instructions (e.g. goto) taking 2 instruction cycles.

The SPI bus on a 40MHz PIC has a max clock rate (for 40MHz device) of 10Mbps. Which is 100ns per bit.

So what? Well, part of the key to this design is going to be the time avalibale to set a key after receiving a MIDI note message. The time avaliable would be 32us without a UART module in the PIC. Without a UART the program will have to be available every 32us to read the next bit but with a UART it only needs to read every byte which will arrive at the message rate of 320us.

320us is time to carry out 320 x 10e-6 / 100 x 10e-9 = 3200 single assmebler instructions or transmit as many bits on the SPI bus which should be plenty of time to decode the bytes and when neccessary update the digital pots for the 49 keys.

Analogue Devices make a 6 pot device, the AD5206 . The 5206 takes 11 data bits on the SPI bus, 3 to select the pot and 8 to set the postion of the wiper. So each pot is randomally accessable giving a fast access time. 9 of these could be used in parallel with a 4-16 line decoder selecting which device the data is for. This would give a write time in the region of a few micro seconds. With 9 AD5206’s daisy-chained (which might be easier to wire up) over 99 instructions would be needed per write but we're still well within the 3200 available.

But the PIC can only output a whole byte at a time so 11 bits will be tricky to manage.

The Maxim have a 6 pot packege DS1806 which clocks in 48 bits to set up the 6 pots. This equates to 4.8 us per device or 43.2 us for 9 devices in series which is still comfortably within the 320us. Good.

The only issue with the Ds1806 packge is that the resistance is linear and there are only 64 steps. If the key inputs respond in a logarithmic fashion, which I would expect, then I would have to re-scale the input velocity logarithmically. This can be done but as there are only 64 steps this may make the response a bit 'grainy'. The AD5206 is better in this respect as it has 256 steps but there remains the problem of writing 11 bits on the SPI bus.

An approximate log scale can be made from the linear taper by adding a resistor between the the wiper and ground as shown here. The flaw in this approach is that the 'pad' resistor is present even when the potentiometer is at zero ohms. More work needs to be done to see if this can be made to work.

That's enough for now...

Vinatge Synth Explorer & More Case Notes

Well, my comments have been added to the Vintage Synth Explorer site and perhaps even more excitingly this blog has been linked from that page. So, thanks to the guys over at VSE.

I've now ordered all the screws, nuts, washers and pillars I think I'll need to get the PCBs mounted. I've cut a piece of MDF for the base so all I'll need to do now is get the drill out. Previously I mentioned the four bushes fitted to the sides of the case. I postulated that they were for mounting something inside but if I'd read the data-sheet properly I would have seen that they are actually for fitting support rails to the sides of the case. Not to worry though as you can screw right through to the bushes and I can still use then for supports if I need to.

The next problem is wiring. You can see from the pictures that the wiring is something to behold and it's all been carefully tied into bunches with lengths of cotton. The looms were obviously laid out in the best arrangement for the keyboard case but now they're just making life difficult as I have a eleven PCBs all tied together and then all the front panel controls. All the moving about of boards and the years I've been fiddling with them have put a lot of stress on the wires and quite a few have come free. So in an effort to wrest control I've started to cut all the cotton ties and when I come to start fitting the PCBs to the base-board I'll undertake some resoldering and maybe desoldering and reattaching of various parts. Also, I think I'll disconnect all the front panel controls and get those wires loose until I'm ready with the temporary front panel.


I haven't thought much about the power supply lately (still need to consider the needs of the interface at some point) but once it's fitted into the case I'm going to have to put some sort of guard over the mains side of things. It'll probably just be some sort of cardboard box but I need something so that I can get my hands into the case when it's on and stay alive. I also need to get an RCD (Residual Current Device) to be on the safe side.

SS30 on the web

I haven't looked for any SS30 stuff on the web for while and there's a couples of nuggets I've unearthed.

Firstly a discussion over on http://elists.resynthesize.com/analogue-heaven/2003/05/ threw this up...

From: Michael E. Caloroso ( [EMAIL REMOVED] )Date: May 7, 2003Subject: Re: [AH] yamaha SS30Jim Black wrote:
> Someone told me they were never available in the states - only sold
> in Canada and Europe. Considering their hybrid synth's came out at
> about the same time over here - that makes sense.
My guess is it was more that the SS30's ensemble FX was too close to the
patent ARP and Solina had on the ensemble FX used in their string
machines. They skirted the US patent system by restricting distribution.
This happens a lot in many industries.
MC

This lack of distribution in the U.S. may explains why the SS30 has remained relatively obscure.


Secondly there is now an entry for the SS30 at http://www.vintagesynth.org . I've already had word about putting it down as 4-voice polyphonic so we'll see if they change it. I've since noticed that they've also said there's no filter. Well the brilliance control could be considered a filter so I might mention that too.

PCB mounting

Here's a picture of the case!


I'm now worried I won't be able to get all the original switches to fit on the front panel.
I'm planning to make a mocked up front panel in MDF to see if it will all fit using the original switches.

So, now I have a case it's on to the mundane but vexed question of how I mount the PCBs in the case. I originally though about mount the PCBs vertically (along the long edges) as they were in the orignal case but the wiring terminals scattered around the edges make this difficult. So instead I'll stack the boards in horizontal layers. This still leaves one problem.

The case has a completely blank base (apart from an earthing point) so there's nowhere to fix anything to it without drilling through. If I drill though the base any nuts or bolts used to fasten into it will stand proud of the base and ultimately scrape the lid of whatever the case is resting on top of. As eth base is only 0.9 mm thick counter sunck screws won't pass muster here either.

I could drill holes and then fit bushes (AKA clinch nuts). These are nuts that are stamped into sheet metal and basically give you something to screew your bolts into. RS do bags of 50 for around £3.50 part number 827-625. Or they sell a 'nutsert' kit for abot £7.50 which has 100 pillars with threaded stubs which are also stamped into sheet metal like the bushes. You get a tool with kit too. This is a bit of scary option though and I'd rather not drill 30 or more holes into the base of my unit espescially when I'm a) not that experieced at metal work and b) new bases are expensive.

Fortunately the case already has bushes fitted on the inside of the side panels. There are two on each side about 1-2 cm above the base. You can just see the front two in the picture. I'm guessing that these are fitted for the purpose of mounting your own base plate and at present this is what intend to do. I can use a piece of MDF, around 5mm thick and then attach it to the side bushes with angle brackets. The MDF is easy and safe to drill and cut and I can fit counter sunk M3 bolts from the underside to fit M3 PCB pillars to on the top.

This is the next task then. I have to cut a piece of MDF to size and calculate how many nuts, bolts, washers, lock-washers, pcb-pillars I need and of what sizes. I also need to determine what size nuts the side bushes take and find suitable angle brakets to fix the MDF to the side bushes with.

The Case

At last another post and it's good news - I have a 19" chassis case.

After failing to find one of the right size from any of the usual suppliers (RS, Farnell etc) I got in touch with APW www.apw.com. Farnell stock their cases but not in the 450 deep version which I need to fit all those long (over 30cm) Generator boards with room to spare. APW put me on to their distibutors, Surtech http://www.surtechdist.co.uk were happy to ship me a single case for the pricley sum of £66.92. No exactly cheap but I don't intend to skimp too much on this project and will probably spend as much again on the front panel and internal fixings.

You can see the technical details of the case here but I'll post soem pictures when I get around to it.

I ordered the case sans front-panel as I intend to get the panel made by Schaffer - http://www.schaeffer-ag.de. More on them later.

Polyphony

As I mentioned in the requirements there's no real short-cut to triggering the SS30 and every key will have to be addressed individually. I thought it would be a good idea to get my thoughts on polyphony down in writing and put to bed some misconceptions about polyphony in general and the SS30 in particular.

A good part of my understanding of polyphonic synths and their foibles came from the excellent Synth Secrets series of articles by Gordon Reid, published in Sound On Sound magazine. Some of what I'm about say is taken directly from the parts 20 and 21 published in the December and January 2000 issues.


The first question most synth people ask is "how many notes can it play at once". This seems an odd question to an organ or piano player. They can always play all the notes at once (should they choose to do so) but it isn't ever that simple for synths.

In order to cut down on the amount of circuitry required in creating polyphonic synths manufacturers came up with a scanning mechanism using digital logic that meant they only needed a small set of circuits that could be assigned to play notes on the fly. So, one of the early such synths, the Oberheim 4-Voice had four voice circuits and was 4 voice polyphonic. When you pressed the first key the first voice module played. If you then pressed a second key before releasing the first key the second voice module would play and so on till you had four notes pressed down. Many later synths were 8 note polyphonic.

This sort of thing is necessary for synthesizers designed to give you as much programmability as possible as you need a large amount of control, but the SS30 is dedicated to one sort of sound and puts it's effort into that one sound. Instead of trying to be clever the SS30 takes the sledgehammer approach of having an individual oscillator for every note (infact two oscillators for every note! But that's not important right now). So, the SS30 get top marks. It's fully, err, multi-phonic. I've read in some places people saying that the SS30 is 8 note polyphonic but this simply isn't true.

So far the the SS30 is only as good as an electronic organ. However it does have one more polyphonic trick up it's sleeve.

In subtractive synthesis (what most analogue synths practice in) there are two ways to change the sound you hear from the oscillator: 1) Alter it's volume (or amplitude) with amplifiers and 2) Alter it's tone, with filters. In order to bring a sound to life you need to alter these attributes over time. The most obvious way is with an envelope. An envelope determines how the something develops over time from the moment you press the key. Typically they have settings that determine how quickly the envelope contour increases and then decreases.

For a keyboard to be truly polyphonic it must have a individually triggered envelope for each key. Monophonic keyboards only have one tone and they restart the envelope each time you trigger it with a new key, as you'd expect (this can be changed on some monosynths though). On a polysynth you can still just have one envelope which starts when the first key is pressed and finishes after the last key is released but it is rather limiting. It's okay for pads but, with slow attacks for example, overlapping notes wouldn't sound right. If a so-called polyphonic keyboard has just one envelope it's only para-phonic and many synths were built like this including one of the SS30's rival strings machines the Solina Strings Ensemble. To be truly polyphonic it needs to have individual envelope generators for each key. They need not be individually controllable, as setting the envelope for every key would be highly tedious. Instead a global setting is used for all keys.

The SS30 is not a synthesizer in the true sense. It has synth like controls but it is fixed to one type of sound and variations of that basic sound. In particular the oscillators are fixed to one wave form. A real polysynth would have envelopes for each notes volume and each notes filter, possibly even it's pitch too. The SS30 has envelope control over the volume but not the filter. This is simply because bowed string sounds don't markedly change their tone as they develop. There's just no need for a filter envelope and it would mean more circuitry. The volume, or amplitude, envelope attack is locked to two settings - Fast and Slow and the sustain is variable .

The envelopes are generated by the the K boards which you can see in the picture below. Look! There's a circuit for every key.

You can see a bunch wires attached at one end and then another bunch leading off to the other. These are the connections from the physical key switches, (some of which you can see at the bottom of the picture) each going through a circuit and then leading off to the G (Generator) boards. The circuit is an envelope generator with settings for attack and sustain. Over on the top right of the board are some more wires and these run back to the attack and sustain controls. There's also a trigger line which switches the vibrato circuit in and out. I'll maybe write in more detail on these boards later

So, is the SS30 a truly polyphonic synth? Yes and no. To be a truly polyphonic synth the SS30 would have to have a individual VCA and VCF envelope for each of it's 49 keys. But, then again, it's not a synth it's strings machine and it does score over a few other strings machines and even proper synths with it's polyphonic envelope generators.

Pictures.

I now have a digital camera so here's a few snaps of the box of junk that used be functioning SS30. Annoyingly some of these came out a bit blurred but you get the idea.

The box of junk.

Examining the entrails. Note the key switches at the bottom.

Switches.

Pots sans knobs.

These are the key driver boards. I've used PCB pillars to stack them up whilst I did some investigations. Originally they were laid flat under the keyboard which flipped up to allow you access.

Here are the 4 voice generator boards laid out with the 3 mixing/FX boards. This is how they were laid out in the original case except they were mounted vertically rather than flat like they are here. Check the 30 cm rule for scale.

And here's a shot of the back of the generator and mixer boards.

I've put the power supply in a case so I could work safely with the mains on.

I fitted the back panel too.

I've tracked down the user and service mannuals from Yamaha. The service manual is original whilst the user is just a photocopy.

The service manual pages fold out!

Tucked in the back of the service manual is this double-sided sheet with a block diagram.

The Requirements

The basic idea is to: a) put the electronics into a rack mountable 19" case with the controls accessible from the front and the connectors at the back b) Build a MIDI interface that can at least make all the keys/notes respond to MIDI on/off messages on a selectable MIDI channel.

This presents a number of unique problems to overcome.

Firstly, the physical size of the circuit boards and their wiring requires a fairly large case. Most readily available 19" cases aren't big enough.

Secondly the front panel should look nice. This is a something that will take up space in my house and a well engineered product should look the part. It's problematic because I intend to use the original tablet switches and getting clean square cuts will not be easy.

Thirdly it's important to note that this is an fully polyphonic keyboard. There is a separate tone generated for each key and if you liked you could play every key at once. CV/Trigger interfaces are only used on monophonic keyboards that can only play one tone at a time. This MIDI interface will need to switch every tone individually rather than having one switch and a control voltage (CV) to set the note. I say this is a fairly unique problem as CV / gate interfaces are readily available and in both ready made form and from designs. More on this later.

I raise these three issues because they are the most particular to this project. If this was a small monophonic synth that only used rotary switches there would be nothing to write about You can already read about such projects here - http://www.synth.clara.net/resources.html and elsewhere.

The project is in two halves really. The mechanical and the electronic. I really need to address the mechanical problems first as they will make it easier to carry out the electronics development. Having all the boards in a cardboard box does not make it easy to work on them

Why?

Why bother.

Why not just get a sample CD of analogue strings sounds? Or get a decent soft-synth which can recreate the sound.

To a certain extent there is no point in resurrecting the SS-30. If I just want the sound I can use a sample or a soft-synth. Even my CS1x does a pretty good analogue strings.

Also it's going to cost a bit of money to carry this out and quite a lot of time. Shoudln't I just be saving that money and using the time to write music instead of just tinkering about?

Yes! Of course I should, but the tinkering about will teach me alot and the next project will be more ambitious and I'll learn even more and so on. Maybe one day I'll be able to make a living doing this kind of thing. Probabaly a pipe dream but it's worth thinking about.

Also the point about sound is interesting. Does a sample or soft-synth really ever replace the real deal analogue? It's a huge debate but right now I'd say no. I recently splashed out on VST of the Yamaha CS80 (see here - http://www.arturia.com/en/cs80v.lasso ) and whilst it's great I personnaly still don't find it as analogue sounding as my analogue synths. It's a personal thing and eventually it may get to the point where I change my mind but for now I'm saying I can hear the difference.

So that's why. I want the sound and I want the experince of doing it.