Tuesday, August 30, 2016

Altogether now: Chorus! - A stereo chorus?

Josh, call the Bucket Brigade

I was re-reading the superlative Synth Secrets articles by Gordon Reid again to see what he had to say about Bucket Brigade Delay Devices. I was interested because I have a Roland System 500  (lucky me) and it's 572 module is based on the System 100m's 172 module, which has a delay section. 
"the audio delay has independent knob control of delay time and resonance (or feedback) for short chorus-like modulation delays." 
Josh from Malekko Heavy Industries has posted on the Muff Wiggler forum that:
"The original 172 used an MN3004 and really was a chorus. For the 572 i used a (much more available yet still 15v) MN3007 and overclocked the crap out of it. From 0 to around 5 will be that chorus detune range but after that it goes into "delay" territory (yes i know choruses are delays too...you know what i mean)". 

I do know what he means. That statement "The original 172 used an MN3004 and really was a chorus." caught my eye because the SS30 uses three MN3004 BBDs for it's chorus. Although it's called Orchestra in the SS30.  I wanted to look at how a single BBD becomes a chorus and how I might use the 572 in that way. I then started thinking about the SS30 chorus and that's where this post is going to end up. But first, the single delay 572...

What's your secret Gordon? 

The synth secrets part 62 covers choruses and explains how a single delay line can be used to create a basic chorus effect.
"We can patch Figure 3 very easily using just four modules from a modular synth: a multiple to split the incoming signal into two paths; an LFO to modulate the rate of the Echo unit (which combines the delay line and clock generator in a single module); and a Mixer to recombine the two audio signals."
Figure 3: Adding the unaffected signal to the pitch-modulated signal.

The important point is that the delay time has to be in a the relatively short "10ms to 50ms" region and be modulated by the LFO by only tiny amount and at a slow rate - from around a busy, vibrato style 600ms to a much slower, imperceptibly shifting 7s.

Gordon goes on to say that with this arrangement:
"you'll obtain a serviceable chorus, reminiscent of the cheapest '70s string synths."
So, not great! He then explains that whilst your modular synth modules might not be cheap - and the Roland 572 is not - the effect of a single delay is not "deep" or "lush" because our ears can easily perceive the effect of just two frequencies together.  To get a more convincing chorus you need multiple delays just as a true chorus consists of multiple singers or instruments.

 You've got to have a system, Roland

The Roland 172 module uses a single MN3004, as you can see in this schematic. The misleading thing, perhaps, about this module is that whilst it's called an audio delay the range it delays by is quite limited, 0 to 7ms. This is not what you normally expect from an audio delay if you think in terms of obtaining an echo effect. Remember what Josh said though: "The original 172 used an MN3004 and really was a chorus." A short delay which you modulate slowly and mix with the original signal is a basic chorus; this is what you get with the 172 and is made even more explicit in the design of the 572.
The 172 LFO has a range of 0.4Hz to 10Hz and it's even hard wired to the delay time modulation. As long as there's nothing plugged into the external CV input that is.

In the 572 Josh has made the, so called, chorus design of the delay module more explicit still and added control for the mix of dry and wet signals.

Let's get some feedback on that.

Of course, the short delay available in BBDs can be used for flanging or just, err, short delays, but with just one delay the chorus is limited, right? Yes, but don't forget that resonance control! This control adds "resonance (or feedback)". The signal out of the delay is fed back into the delay so you can overlay the signal again and again. To what effect though? Are we obtaining a more chorus-like effect by doing that? Yes and no. The modulation is slow, so whilst the signal being fed back is from a earlier part of the LFO's cycle it's not that much earlier. It's more like a phase shift of the original signal. Unless the feedback was also delayed and that delay modulated it's not adding true complexity. Our hearing can discern such simple relationships and whilst it's an interesting effect it's not adding the right kind of depth.

Because you're Gordon, I mean gorgeous.

In the largely monophonic world of modular synths the aims are different to those of a polyphonic instrument where chorus is a vital ingredient. The economics of making each note sound as rich as possible is a challenge in a polysynth. Complexity that sounds convincingly natural and pleasing means more cost, so polysynths and string ensembles need some help. And if you're using the simple divide-down architecture of most string synths that help is even more vital.

As Gordon goes on to explain, more delays are only part of the solution. Having more LFO's and some phase shifting of the LFO waves are how the more advanced designs implemented the lush chorus of 70's legend. Naturally, this adds cost again so to get a reasonable compromise phase shifters and mixer circuits are employed to create the illusion of more LFOs. As Gordon explains: 
 "This involves the use of just two LFOs (which cuts costs) and four phase-shifters (which are cheap), and generates three instances of a single complex delay modulation. As before, these are out of phase with one another, typically by 120 degrees, and the result, while not quite as lush as you can obtain using six independent LFOs, is nonetheless gorgeous."
Figure 13: The classic three-phase chorus unit.

Chorus or Orchestra?

We now come to the SS30 which is almost exactly as described above. The only difference is in the relationship between the LFO and phase shifters and how they are mixed.

Below is a diagram of the SS30 Orchestra section.

SS30 Orchestra section

The LFO signal to the first delay line (A) is a simple mix of the two, phase shifted LFOs.
The LFO signal to the next delay line (B) is also as simple mix of the two LFOs but with a different phase relationship.
The LFO signal to the last remaining delay line (C) is a mix of all the phase shifted LFO signals.

It's not obvious from the schematic what phase each LFO is shifted by (an exercise for the reader perhaps) but they are all different.

One of the LFOs is fixed at 0.6Hz and the other is variable from 0.6Hz to 6.2 Hz. The SS30 control simplifies this and provides a slow and fast option.

And there you have it. The SS30 Orchestra chorus is a tweaked version of a classic design. Presumably this particular variant was arrived at after trying a few different combinations.Yamaha doubtless had similar designs in their analogue electronic organs of the same era.

LFO customisation

At this rate...

The switched LFO speed provides the first opportunity to customise the SS30 Orchestra section and add extra control in the SS30-M. The variable LFO is based around the Yamaha custom iG00150 VCO chip.

As you can see from this design from the Yamaha CS-15, reproduced in the Yamaha IC Guide Book, the VCO is capable of a range of frequencies with sine and sawtooth outputs.  The CS15's LFO has been designed to provide 0.2Hz to 100Hz. 

 In the SS30 the circuit is similar but with switched control of rate instead of the variable control and a lower and narrower frequency range. 

I'll have to do the maths, but basically putting a linear pot in place of the the speed switch (SW1) should make the range variable. For fun, more extreme effects it might be worth having VR3 as a switchable control too, to get more coarse control of the VCO. It's not clear to me yet what the full range of that VCO actually is. I need to play around with those trimmers and see what I can do with it.

Wave hello (to saw-tooth?)

With a bit of hacking it will also be possible to make the waveform switchable from sine to saw output. I'm not sure how useful that would be though. The Moog ClusterFlux chorus has such a wave option and there's a modification for the CH-1 Boss Super-chorus pedal that goes from the normal triangle to saw-tooth at the higher rates. No idea what it sounds like though!

Modulating the modulator via modular modulations?

Another possibility is a CV input to modulate the LFO externally or straight LFO inputs to replace the built in ones. If I was removing the chorus section altogether to just use that it might make sense but things can get out of hand and the front panel will start to grow and grow.

Stereo - Typical chorus

The section of part 62 of Synth Secrets  - More Creative Synthesis with Delays - that got me thinking most was that on stereo chorus. 
It's quite simple to take a 3-phase mono chorus and create stereo chorus and you can see how below.

Figure 14: The output stage of a triple-path stereo chorus unit.
As Gordon says:

"In this scheme, you can leave out the 'straight-through' signal, because the dual inputs to each mixer will be chorusing differently, and — far from contributing to the result — the original might actually damage the impression of width and depth."
I think this will thought this would be easy to do on the SS30. The 'mixer' in this case is simply a 12K resistor on the output of each delay and a 22K on the straight-through signal. The mixed signal then goes through the Depth control. If I tap-off the delay outputs and mix them with similarly rated resistors I can create a stereo chorus output. In practical terms it might be best to remove the 12K resisters, take wires out to a new board and have a switch circuit to handle a stereo or mono signal and control the mix of signals via the Depth control. To keep things simpler the current mono output would stay the same and a new stereo output would be added. This would need a new pair of output amplifiers though.

BBD devices 

TC350 - 185 stage - This ITT device was used in the Solinas, Crumars, Elkas, Logans, Vermonas, Hohners and the modern recreation Elkorus

MN3009 - 256-stage - low noise
MN3002 - 512-stage - standard type 
MN3004 - 512-stage - low noise

MN3005 - 4096-stage - low noise
MN3007 - 1024-stage - low noise
MN3008 - 2048-stage - low noise

Mono Choruses

SS30 - 2 x LFO - 4x phase shifters - 3 mixers - 3x clock generators (50KHz) - 3x delays (3x MN3004)

Boss CE-2 - 1x LFO - 2x phase shift (1xMN3101) - 1 x delay (1xMN3007)

CS70M - (3x MN3009)

Solina - 2 x LFO - 2x phase shifter + 1x inverter (0 degrees, 120 degrees and 240 degrees.) - 3 x clock generators - 3x delays (3xTCA350)

Korg Polysix - 3x LFOs - 3x delays (3x MN3004 with loop-back)

 Stereo Choruses

Boss CE1 - 1x LFO - 2x clock generators - 1x phase shifter  - 2x delay (1xMN3002)

Roland VP-330 -  2xLFO with 2x mixers 4x delay (4xSAD512D)
"the Roland VP330 (1978) has a thinner string ensemble sound generated by just two delay lines with dual LFOs." Synth Secrets 62
Elkorus - 2xLFO - 3xPhase shifters - 3x delays (TCA350)

Engraving objects - Front-peanel Designer, Inkscape and HPGL files

An apology

This post was drafted some months ago. Since it was all too difficult and now that Schaeffer are offering printing to their front-panels now I didn't really come to any conclusion. Engraving tools, HPGL files and using Inkscape is a frustrating process and I just gave up. I'm posting this anyway, in case someone is looking for advice on using Inkscape with HPGL files and FPD, and hoping it might be of some use.

Oh no! Logo no-go?

I was thinking about having something more interesting than just the labels on the front panel. But what? How about a logo? If I use Schaeffer panels in theory anything can be engraved and in-filled with colour.

I found someone had created a Yamaha logo for CNC

They posted it as a dxf file so using Uniconvertor I made an HPGL file.

Front-Panel Designer can import HPGL engraving objects, so that works easily.

The cost is
HPGL engraving54.4462.98Yamaha 2 HPLG.plt1.83 €
Other--Engraving infill5.69 €
Other--1 tool changes1.03 €

Engaging in engraving

But this kind of engraving is easy because it's a single-line drawing. What if I wanted something more elaborate?

This is the Yamaha logo and logotype

If I convert this file to just show the outline though..

In this file the outline has not been cleanly closed off and it only makes sense when you view the 'fill' without the 'stroke'. To engrave this design you need to start with a clean outline and then have lines inside which are cut out.

You can do this in Inkscape but FPD recommends using Corel Draw as it has better support. But Inksape is free so... Let's try with Inkscape.


First there are some top tips in this document MANUAL FOR HPGL FILES WITH COREL DRAW
There is an explanation on engraving and cutting tools.

Something I missed earlier when doing this is that you need to set the stroke style's 'Join' to rounded. Doing this gives you a realistic picture of what the engraving will look like.

Also there is a fundamental problem with cutting a line in-side another line. Although the outside of an angle will have a rounded edge the inside will be straight and sharp. When you draw a line inside the corner that comes to meet that inside join it will be rounded and you get a gap. There's no way to prevent the gap so you have to fill in the triangle with another but of engraving or cutting to remove it. Alternatively go for designs with no acute angles inside.  

The S shape used below is a simply hard to do and there are much easier designs with fewer problems.

Starting with a single character 'S' in the Faktor font favoured in some of the Yamaha advertising of the same period as the SS30 let's try and engrave and cut a custom font.
This is how the text appears in Inkscape to start with.

Then I set the fill to nothing, the stroke colour to black and the stroke width to 0.2mm (which is the smallest engraving size on FPD).

That's not quite correct though. The thickness of the stroke works from the middle outwards.

A thicker stroke would start to unnoticeably distort the shape. This is at 3mm.

To be accurate the stroke needs to be centred 0.1mm inwards. Here's a close up showing the outline in black (at 0.05 px) and the proposed engraving in read (at 0.2 mm).

You can see clearly that the engraving is going outside the bounds. So let's fix that.
The Path->Inset command moves the selected path inwards by a set amount. You have to set the amount in the Preferences (Edit->Preferences->Behaviour->Steps->'Inset/Outset by'. For 0.2mm I've set it to 0.01.

Now the engraving line fit's inside the outline but it's lost the sharp edge. If we engrave this the tool will follow that curve instead of sharply turning the corner.

If you try and do this inset another way you get a different problem though. In blue I created another outline in blue, set it to 0.2 mm wide and then reduced the size of the outline by 0.1mm.

This still have sharp edges but as you can see it's not the same as doing an inset and it's not inside the lines. Reducing the size is nothing like doing an inset command. 
To get exactly the right size it is better to start with a shape which is 0.1 mm too small and then the engraving will keep it's sharp edges. But there are other problems with the inset function in Inkscape as we will see later. 

What happens when we actually engrave an inset outline? I took a copy of the drawing (deleting the blue engraving first) and offset the guide outline to the right from the engraving. Then I saved it as HPGL format.
This is what it looks like in Inkscape. The inset outline on the left the original shape on the right.

And then imported to FPD

You can see the problem. The line created with inset has lost it's smooth shape.

Now we know that the outline is better of being scaled correctly with allowance for the engraving tool width and not using inset we can turn our attention to the in-fill.

The cutter tool options are 0.6, 0.8, 1.0 , 1.5, 2.0, 2.4 and 3.0 mm.  Bigger tool means less cutting which is cheaper, but the bigger the tool the bigger the contour radii.

Here's is a close-up look at the outline cut with the 0.6mm tool.

The shape isn't too bad but the outer edges are rounded off. How will that fit inside the outline cut by the engraver?

Firstly let's create an inset path which is the first cut inside the engraving. The cut will be 0.6 mm and therefore 0.4 mm inside the outline (0.1 mm + 0.3 mm).


Not too bad, but you can see a gap. When this is cut and engraved that gap will be left over metal. This is because the inset has miscalculated and not followed the outline properly.

Let's convert to HPGL and see what happens.


We now hit problem with Inkscape and HPGL files. Each line in the HPGL file can be assigned a pen (from the idea that this was a file format for plotters). In FPD each so-called pen is assigned a tool (engraver or cutter). Unfortunately when you save the file in Inkscape you can only choose one pen and not multiple pens with a different one for each line.
Instead to create the engrave and the cut line you have to create two files. One file is just the engrave line and one is just the cut line. When you save the file you assign each one a different pen so that when you import then to FPD you can get two files loaded and placed at the same time with different pens. Actually not at the same time exactly. You have to place each one in the same position individually, which may be an issue if the reference points don't match up.  However, you can manually combine the two files. Open each file in a text editor, like Notepad++, and then append one after the other in a new file. Then you can open both as one file and you will have two pens, one for each. The pens are identified as SPx where x is the number of the pen, so SP1, SP2, etc.

In any case, bringing the file into FPD the result is poor.

I have done more experiments where the shape is much larger and the result is a bit better. But that is no use. You cannot reduce the size of the outline and it's still not perfect.

To be clear using the automatic inset function is not possible. But there is another way.

There is a path function called Linked Offset. Using this function you can manually drag the shape an inset. This has a much neater result and fewer errors. The only thing is that you have cannot measure the inset with 100% accuracy. You can move the amount of offset but also the position from which the offset is being dragged. This give a fair amount of variation and can create problems but is still better than the automatic option. Selecting this tool also creates a copy of the selected path so it's quite quick to use. There are some strange effects though. There seems to be a bug which causes all kind of artefacts to appear and make a mess of things. They can't be removed and appear scattered around the original shape for no apparent reason.

  And here I ran of interest in doing this....

Thursday, August 25, 2016

Yamaha's Quality Design and Manufacture

I was browsing around the web looking at Yamaha organs (uh-oh!) today and came across this image which tickled me.

It's from an fact-sheet/flyer of their new organ around 1978, so contemporaneous with the SS-30.



Tuesday, August 23, 2016

A switch to variable controls?

A bit on the slide?

As I've been too busy to get into the garage again these past few days (and will be until next week) I was instead meditating on the idea I had during the front-panel rewiring exercise.  The idea was that for some of the switches I could instead have a variable control. I liked the idea of using sliders for some of these as you can more easily switch them on/off.
Let's see how that might work.

Orchestral Manoeuvrers 

Here is the Orchestra section.

Orchestra Section - Controls

 The Depth control set's the mix of signal that's going to the output unaltered (dry) and the amount that is going through the of the effect (wet).  The only difference between the minimum setting and having the instruments bypassed is that when with the minimum setting a small anout of signal will still get through from the Orchestra block.

The other controls are all switches, but what would happen if they were variable controls too?

Speed Control

SPEED is a switch which alters the voltage to a Yamaha IG00150 LFO chip. It's speeds '1' and '2' could be any value between those two voltages but with the switch they are fixed. It could even be a voltage beyond those limits.  Replacing the switch with a suitable potentiometer would be a useful addition.

Switch controls

The instrument switches are more problematic. Each tablet switch is dual-ganged and has two 2-way switches.
For each instrument (CELLO or VIOLIN) when the switch is 'off' the Orchestra effect is bypassed. In bypass mode the signal goes through T1 switch, through a resistor and then on to the T2 switch and onwards to the output.
When the switch is 'on' the signal goes from T1 switch to the Mixing Amp, then it's split to go to both the Orchestra section and the depth control. The depth then balances between the original dry signal and the wet. Then it's back to T2 and onwards to the output.
In other words, when it's bypassed the Orchestra inputs and output are disconnected. Otherwise you would get both Cello and Violin input when you only select one and the noise from the choruses would leak through even if nothing was being input.
Therefore the level of each instrument into the orchestra could be controlled but there will need to be some care taken to ensure that 'off' still means off. The easiest way would be to retain a switch and add a variable control. A rotary pot with a switch would allow both to co-exist but I want a slider then a button would be needed. 

In the mix

Here is the mixing section:

The best form of attack is a switch?

The ATTACK switches set the period of the 'attack' phase of the envelopes - i.e. how long it takes each key to rise up to full volume. When not engaged the AT signal to the key switch circuits is switched to ground. In the keying circuit that means that when the keys switch to ground the transistor controlling the signal to the VCA switches immediately to ground too and the attack is period is fast. However when the switch is engaged and switched to SLOW the AT signal is disconnected from anything. Now a capacitor in the key switch circuit comes into play. This capacitor sits between -7V and the emitter of the transistor.  The ground from the key switch will now cause the emitter to rise to 0V but because the collector is no longer at 0V there is no current through the transistor. In order to bring the signal to the VCA up to 0V now the capacitor has to be charged up which slows the increase in voltage from -7 to ground. Phew!
In short the switch governs if the charging is significant to the switching, but the time period of the charging is set by the RC constant of the capacitor. As the AT line is not connected to anything in the SLOW mode there's no point trying to replace the switch with a potentiometer.
I'm not sure my circuit analysis is totally on the money there but it might work with suitably large pot' if the so some experiments are in order. I might have to see if there's a non-trivial but still feasible way to do this with some sort of circuit to control the current. Maybe... 

Mix selector

The selector switches turn the signals from the 'Mixing Amp & Filter' circuits on or off. 'Off' means the signal is switched to ground. 'On' means it is routed on to the next part of the chain.
Therefore it's a simple matter to replace each switch with a suitably rated potentiometer which would allow each instrument to be balanced rather than just switched.    


Most, if not all, of the switches could be replaced by controls, but should I? Putting aside the fact that I've swung back to not reusing the original controls again, are these useful changes? I think changing the Orchestra speed is useful. No respectable chorus effect would really want to limit the user to two speeds so this is definite improvement. Similarly all effects allow you balance how much signal is fed in so the orchestra switches would be better off as variable controls (or both). For the attack control is would be nicer to be able to control the time. again no self respecting synth has only a switch and the sustain time is variable after all, so that is preferable too. The mix of instruments is easy to do but is it worthwhile? I do think it is. It seems that no stringers offer this feature though. They were seen as organs and followed the switch per instrument approach (where they offered anything at all). But why not? Synths almost always allow you to mix the tones, even when they come from the same oscillator. In summary then, I should seriously consider this option.

Thursday, August 18, 2016

Rewiring update 2

Cello, again!

Back one day later with more updates!

I fixed the issue with the Cello. Well, almost. The issue with broken wire to the volume control was easy to fix. However the signal had another break further down the chain. At the Orchestra section the signal is either passed through the chorus it bypasses the effect. In other words the it's a 2-way switch*. The problem was that the open backed switch for the Cello has had it's contact bent at some point and it was only operating as a one-way switch. The Cello Orchestra (wet) worked but not Cello without Orchestra (dry).  I bent the contact back and it's work okay now. No great, but okay.

Vibrato delay fix

Next, I wasn't sure that the Vibrato was working right so I did some measurements around the LF board and found a couple of broken wires. An earth wire and a control line for the Vibrato Delay. After that fix the Vibrato seems okay too.

The 100Hz Mystery

Yesterday I said it sounded like there was a noise floor of all the voices bleeding though. Last night after fixing the Cello and the Vibrato wires I again turned up the volume to listen to that noise. I can't be sure, but I think it has changed. Now I hear a single tone coming through. Like the stuck note I have on A# on K4 board, but this time much quieter. To be clear maybe my ears were deceiving me previously but now, for sure there is something there at one tone.

100Hz noise

There could be a bleed of everything else under that, but as you can see there's something quite loud. It's around 25mV at 100Hz and crucially it varies frequency with the pitch and detune controls. That means that it must be coming from the voices and not the power supply or some other circuit. I traced it back and it is present all the way back to G1 board.  I started there as it seemed likely that this would be coming from a whatever note is around 100Hz, and that is the lowest frequency board. However, I could also see it on the G4 board outputs. This isn't surprising actually. The outputs are mixed together (crudely without buffers), but I would have expected to see something stronger at the source, probably on G1. I didn't, so I will need to go deeper.

Make like a banana...

Something else I noted is that as I switch the split-gate upwards it lessens. So at split 0 it's loudest and at split 3 it's quietest. It's also present from all sources - Cellos and Violins.The split gate is quite fiddly to describe without a few pictures and tables but some voices are always open to be switched. The lower part the G1 board 16U and 16L are always present. In split 0 G1 16U/L are present as part of the Viola, mixed with the G1 8U/L. At split 1 (and above) G1 16U/L is present (alone) as part of the Cello sound. 

On the down low

If we assume some note in the lower part of the G1 board is the source of the noise then it should go away if only the Violins are switched in and the split is 0. This sort of matches the effect I got when playing around. It didn't go away completely though. I could get a trace of it in G4 so that does make sense. It could still be bleeding back even though the Viola and Violins are mixed separately.

How low can you go? 

Therefore the theory that it's coming from G1 makes sense, except for one thing. If I played through the notes on G1 I should have found the same note somewhere on there. I think I did, actually, but it wasn't quite right. I played note G and got something much louder but sounding the same. But I couldn't get a trace that matched. I think that the lowest G is G3 not G2 and my ears tell me it's more like 200Hz than 100Hz. I must redo this but I think it might just be because that G is a fundamental of 100Hz I'm hearing. G2 is 98Hz and G3 is 196 Hz. I think I'm hearing the first fundamental at ~200Hz because of the natural high-pass filtering in my listening environment from strip lights etc. (and yes, I tried turning them off to make sure it wasn't EMC). What I measure is 100Hz, but what I hear is a mix of harmonics from that pulse trace - I think! 

All very interesting (in a way) but if the fundamental is 100Hz then where is it coming from? The lowest note is C1 at 130.81. If it was the low G and I was wrong about the lowest note frequency it would probably be because the key circuit was broken. But I also shorted that key to -7 and it made no difference.

Pulse, a ting?

Finally the shape of this tone is not 'right'. Firstly it's a pulse, not a square -which is the source of the tones - or the more saw-tooth like shape - from the wave shaping (the subject of another post). If some wave shaper on the G board is faulty and bleeding through the raw oscillator then maybe a pulse is the result. The most obvious fault would be a shorting capacitor, but how that would get to a pulse I don't know.

Enough. Let's see what I find, next time...

* Idea - Replace the switches with a mix control? In fact all the switches could be replaced with such controls. Then instead of switching things in or out they could be balanced instead. Sliders would be faster for a quick switch in/out.

Wednesday, August 17, 2016

Front-panel rewiring update


Temporary front-panel rewiring

Progress has many enemies: perfection; comfort; greed; fear; indecision; indifference and (it says here) free-time(?). 

I can't point to any of those and blame them for the lack of progress on the SS-30M. Free-time is the thing I lack the most.  Okay, maybe comfort. I don't like working in my garage when it's below 10 degrees. No, the real enemy of progress has been Eurorack.


 I made a rack case

Eurorack case sanding

Eurorack case rear

Eurorack case ready for loading

And I made some DIY modules

DIY Mutable Instruments Braids

And I got a Roland System 1M and then a System 500.

My Eurorack set-up taking shape

I even started another blog about the System 500 - http://system500.blogspot.co.uk/

Now all that is (mostly) done (for now) I am keen to get the SS-30M show back on the road!

Front-panel rewiring

You probably won't remember that last year I was wiring up the temporary front-panel when I stopped. Most of the controls had been removed some time back and the challenge was to reattach them all. In the bigger picture this would allow me to test the SS-30 and see if there were any major problems before starting on the MIDI interface.

Rewiring was quite slow because although each wire is colour-coded I still wanted to check that it came from where I was expecting. With the black ground wires this was quite hard as most are still bunched together and it's hard to tell where they go to. Note that there isn't just one ground for all the controls. They are all grounded to different points on the F, LF and OR boards so there are several black wires. I think I got them all in the right place in the end.

After a couple of evenings work everything seemed to be back in place and I powered up to see what was happening.

A sharp pain

The first thing wrong was that there was a stuck note. A# on the top K4/G4 boards was on and staying on. I traced it back and couldn't see any wiring issues with it. I fixed another couple of broken connections on K4 and then disconnected A# from the G board. The output should have been -7V but it was 0V. I poked around on the circuit for a while and after a bit it sprang into life. Hmm! Well, it is a very dirty board. K4 sits at the top of my stack of K boards and has collected a lot of dust and fluff over the years. I will just have to wait and see if it fails again, but for now all is well.

Yo! Yo! where's my Cello, Ma? 

The next obvious problem was that although I could switch in the Violin section (Viola, Violin1 and Violin 2 were present and seemingly correct) the Cello switches weren't making any difference to the sound. I switched through the KYB.SPLIT options to no avail and it was clear that the Cello audio was missing in action.
I traced it back to the Cello Brilliance control. At that point the Cello signal comes from the Cello Amp circuit and splits. The signal goes into the Brilliance control as part of the low-pass filter circuit. The control sets the resistance to the signal going in to the capacitor to ground. If there's nothing going in, no filtering - brilliant! Another wire goes direct to the Cello volume control and from there onwards to the Orchestra section. When I looked, the two wires had split apart and only one was connected. The signal in to the volume control was waving in the breeze.
It was too late to fix that so it's next on my to-do list.

Bleeding hell?

All the Violin section was working though and I checked the Pitch, Vibrato, Brilliance and Orchestra which all seemed well. I was particular happy to hear the familiar woozy chorus taking over as I increased the Orchestra depth. I was tempted to immediately try and patch in another synth and have a play with it as a stand-alone effect. I resisted though and pressed on with my checks.

The only other problem I found was that the noise floor was seemingly quite high. I say noise but in fact it's the sound of all 49 keys bleeding through at once. The only thoughts I've had so far are that either this is normal or that there's something wrong with the -7V or 0V around the key switches. If -7 isn't low enough you will get a bleed through. I need to measure and think about that. Maybe I have 49 capacitors that need replacing :-(