A milestone

When I started this project I had a simple aim. I have expanded on that scope now, but the basic idea was to put the whole SS-30 into a rack-mount case, with MIDI. I have had MIDI working since last year but the front-panel was temporary. It was a proof-of-concept and not something I wanted to really play. Not ready for the studio.

As of last night, I have achieved what I set out to do. An SS-30 I can play in my studio from a MIDI enabled rack mount.

I had to replace almost all the faders. I suspect that the wobbly, ill-fitting, fader knobs just did something horrible to them.
Anyway! The panel was as good, if not better, than the original design. Although I hadn't added the additional features yet - pitch CV in, external Orchestra input, Orchestra feedback, individual voice outputs, and yes this isn't the final front-panel and no the wiring isn't perfect and so on and on and on (but now is not the time to dewll on what is left to do.) - but I had got the optimised control of voices with faders and dry/wet control of the Orchestra inputs.
And so, I decided to call a pause here, fit the front-panel and lid, bolt the panel into my rack and enjoy the SS-30M!

I should have some new fader caps tomorrow and I will work up some audio or videos soon to show off what I've got.

Good, eh!?

Noise Flaws

Bring The Noise!

This post will be me looking at why the SS-30M is so infernally noisy.

Since I brought it inside and started fiddling with output levels a bit I've become far more distracted by the noise-floor of the SS-30M. With a quiet 'studio' space with high-quality monitor speakers and headphones, the noise is far more obvious. Maybe not unignorably bad, but certainly not very good.
 
So far I have identified four types of noise

1. Unidentified 'ticking'
2. The Orchestra section's bucket brigade delays
3. The 'swarm of bees' effect 
4. General hiss

I'll start analysing each one and return to this post as and when I identify the root causes or fixes for each.

MIDI Interface 

The MIDI interface is the major new component in the system. On that basis alone it's a prime suspect for all sorts of issues. However, the first thing I did was disconnect the power to the interface and none of the problems were even slightly better.

Just a tick

This was a new (to me) and unexpected sound when I turned the gain up on the mixer and really listened. What is it? I don't know, but it sounds like a kind of click or tick sound every quarter of second or so. 

Possible sources - PSU switching, an oscillator (such as the orchestra LFO), other

PSU switching

If the PSU was a switch-mode type you might expect some switching noise (a ripple voltage), but that would be occurring at the switching frequency, which is much much higher. 
The reason I'm thinking of this is that when faulty they can exhibit similar ticking sound. It can't be that because the SS-30 supply is a linear supply.

But in a linear supply, there's no switching at all. It simply rectifying the 50Hz AC and filtering that. 

So, if it's coming from the PSU, I need a better explanation. 

LFO

 This occurred to me after switching off last night, but could this regular tick be coming from something the circuit which is already regular, like an LFO.

Orchestra LFO

The  Orchestra LFO is a pretty good candidate because a) it's operating in the right frequency range and b) there's a loose wire hanging off it at the moment. The wire is for the LED circuit, which I have yet to implement fully. You will recall that I had a breadboard set-up to demonstrate this. I remember when I got this design wrong, to begin with, drew too much current and caused ticking sound at the frequency of the LFO. 
So, I really need to check that wire and play with the Orchestra rate knob. If the ticking changes with the rate, then I have my culprit. If. 

Vibrato

The Vibrato LFO is a fixed frequency and is much higher than the Orchestra one. At around 6.4Hz, it does not fit what I'm hearing




That's all I have on the ticking, for now...

Orchestra Noise

 http://www.stefanv.com/electronics/oh-the-noise.html


That link is a nice article about reducing noise in a similar situation. The SS-30 has a high-cut filter on the output but is it enough? And if I change it, what then? A noise gate would be one way to go, but I can do that externally. 
The main issue that even when there is no signal going in there is noise coming out. Using the separate voice outputs should solve that to some extent, but it's not ideal. Preferably I'd be able to switch off the orchestra output altogether. I say preferably but it would lead to a set of control settings which produces no sound, which is something I'd prefer to avoid. The only way around that would be to have another balance control to replace the wet with a dry signal, but that's getting silly. A switch could be used instead which would bypass the orchestra and the orchestra depth controls. All more wiring, but possible.

The other thing about this noise is that it's masking the other noise. If I decide to leave it on, then there's not much point working on the others. 

 Swarm of Bees

 The so-called swarm of bees effect is the sound of all the notes buzzing away. As we know, the string synth design depends on keeping all the notes running continuously and raising their normally negative DC level towards 0V, so that they start to pass through diodes. If the negative DC voltage is too high you get all of the notes bleeding through the diodes even when no key is pressed. 

A possible source of the problem: supply voltage too high/positive, switching circuits not switching 'off' properly; sustain circuit not decaying fully, switching voltage from MIDI interface not 'off' enough leading to some bleed from the optoisolators, oscillator signal to large (peak-peak). 

So, lots to got at, there! This will be hard to isolate. As they are apparently all going at once, fixing one switching circuit will be hard to hear. However, with careful use of the scope, it should be possible to trace if they really are all bleeding through and if not all which ones. It will take some work though. 

Hiss-trionics

 General noise (hiss) is the bain of all analogue designs. Starting with the power supply and infecting every part of the design, noise is a fact or life. But some things are there to help and must be proven to still be working - i.e. de-coupling capacitance - and somethings can be upgraded - capacitance again, but also newer ICs. Not to mention the noise from electromagnetic interference. 
Good grounding, shielding, decoupling and so on will all help, but I'm not too hopeful. There's a risk of doing a lot of work to get a small benefit. I can't replace some components easily (or even at all) and working on each board is a major task, unless I desolder the wires and test each in isolation. Big jobs! Like, 'I need to quit the day-job' big. And finally, everything is now squashed into a small space. I need to be realistic and decide how bad it really is. 

I suspect that compared to the Orchestra noise the background noise-floor is quite small. 




 


 

2019 - Reawakening and wiring hell

And we're back.

Since last autumn, I've been busy with the usual life stuff, but also: building a pair of Eurorack clones of the Pearl Syncussion, engaging in a little friendly remix rivalry between friends and visiting China(!).  

Now all that is out of the way (well, the remixes will continue, I hope) I decided to put everything else to one side, for at least a week, and see if I could get the #3 temporary front-panel wired up and enjoy the use of an SS-30 with MIDI for a while. Maybe I could use it in my next remix challenge!

As it turned out, I burned through a lot of night-time hours this week battling with the wiring and even managed to blow two fuses and possibly a CMOS chip in the process. Grrr. It's a been difficult return to this project but after another epic session last night I seemed to have got it more or less under control. By which I mean that I know whats' going wrong and now have to fix it all! The good news is that apart from one real fault, the core of SS-30 is still working well and sounding great.

SS-30M panel wiring part 128,765

 I nightmare of wires

General issues with short circuits due to bad soldering and wiring management. 

• This continues to be a big challenge. What is the solution? 
• Ideally, all the wires would be terminated at a PCB and then all the controls would be on either the same PCB or another one connected via a ribbon cable, or several. 
• Instead, every wire is going straight to the pin of the control where it's needed.
• On top of that, most wires are shielded, and the shield is earthed and there's nowhere to 'tie off'' these shields on the panel
• In some cases, the shield is not earthed so the shield has to be trimmed off, but there's still a risk of loose braids touching something and causing issues

Viola voice fader faulty? 

•  I've disconnected most of these faders, for now, as they all seemed to be causing me problems of one kind of another.
• Something definitely is up with this fader though.
• I suspect it has taken a bad knock at some point 
• Or, it was maybe that the too large fader caps are causing stress on it.
• I will return to this one when everything else is sorted out. 

Lower Cello voices - disconnected wire.

• The whole bottom octave of the cellos was missing.
• This was fairly easy to find and was just the usual problem of a wire snapping off at the solder joint.

Wrong potentiometer used for the orchestra send. 

• I had stupidly selected a log part, instead of linear.
• This resulted in either all Orchestra or no sound for the Violin or no orchestra or no sound for the Cello. Or, something weird anyway.
• I have had to use a pair of 50KB pots I had in stock in place of the 100KA's I'd bought.
• I'm now concerned that too much sound is coming through to the Orchestra when it's supposed to be off and this design needs to be re-thought. 
• Even using a 100KB instead of a 50KB part (I only had one suitable 100KB to hand) there was ~400mVpp getting through to the Orchestra circuit. 
• Maybe I should use a dual concentric linear pot with ground on opposite ends so that I can get better isolation.
• I think I have some of these ready to go in!

Lower G2 Viola voice not switching off when split

• This is the worst one...
• Even when I was getting no sound from the Cellos, for a variety of reasons (see above), I was always getting the bottom half of the second octave coming through the Violin signal path.
• When I checked the switch gate, it was clear that the bus of notes (G2-8L, or the lower part of the 8' from the G2 board) was apparently not being switched off when it should be.
• The switch gate is mostly composed of two quad CMOS analogue switches - TC4016P parts. 
• On this particular switch, the G2-8L bus should be switched to the Viola's voice when the split is for Violin and switched off when the split is for Cello. 
• However, it is never switched off. 
• When Cello is selected, those notes are what you normally get normal and you obtain an even thicker sound. 
• That might be considered an optional feature - Cello + Viola, instead of either/or.
• However, I have no desire to redesign the switch gate!
• I have ordered a replacement 4016 but tonight I will double check that the wire from G2 that brings that signal in, is not simply shorting to the wire taking it back out.
• that would certainly account for the input matching the output!

In conclusion, I have still to get all the controls sorted out. I haven't even started on some of the additional features, such as Orchestra feedback, Orchestra external input, pitch CV and the Orchestra LFO LED. 

Update 1

Orchestra pot (like pit, but pot? Oh, why do I bother?)

Did I say dual-gang linear? Ha, of course, I was still not 'getting' what I needed here. What I actually needed was a dual-gang log taper pot. Thus, as the dry signal fades out, the wet signal fades in. Luckily I had exactly two such pots of the right size. Phew. That's working perfectly for both now. 

Coming in hot

I added a 100K to ground on the Violin Orchestra input and a 10K for the Cello Brilliance to bring the level down a bit as it seemed to be distorting a bit when a lot of keys were played. I'm not entirely sure why the violin one was needed, but the Cello volume control (now removed, of course) had some resistance to ground and it was coming through very 'hot' without it. I might tweak these again later. 

Fade to nay

I made a start on reconnecting the voice faders, but I still have problems with some. The Cello 2 fader was as far as I got, but it wasn't working right. 

Update 2 

Viola Split Fixed

Last night I fixed the Viola split problem by fitting a new 4016 analogue switch. As soon as I'd got the LF board free I noticed that I have already replaced its companion. A quick search of the blog reminded me of this post: Viola Woes

Attack issue

After checking the Viola was switching right I spotted another issue. The same notes that were coming through as Viola still sounded different. Not to worry though, this wasn't the Viola but something else. In fact, once I'd tried different splits and attack settings I realised that the whole bottom octave had no attack setting. Attack is short when switched to ground but this one wasn't. After a bit of messing around with the split switch, I suddenly spotted that there was loose brown wire next to the K1 switch board. Of course! although there was a brown wire attached to the correct pin on the K1 board, it was only the one coming across from the other side of that same board! The wire to the split switch was loose. Soon fixed, and everything was back to normal. 

In the past, I've been a bit unsure about the attack. How much is it really working and what is the point of it, really?
 I wonder now though. The more I play the SS-30M the more obvious it seems and the more I appreciate what Billy Currie was saying about the character of it better now. This all comes from being able to really just play it.