Orchestration

Let's look at the Orchestra. Firstly I was interested in a new DIY product from Oakley Sounds which leads me on a diversion around the Roland Juno chorus. Then I talk about the controls I will need for the SS-30M.

Oakley SRE330 - Enhanced Stereo Ensemble and Chorus Module

I just saw this new DIY chorus project available from Oakley Sound and it got me thinking about the Orchestra (AKA Ensemble or Chorus) part of the SS-30 again.
They also used an alpha Juno for the demos, which is also kind of interesting as I have an MKS-50.

The Oakley design has 4 delays which can be stacked or and split for stereo in various combinations, It has two basic settings.

"The three phase setting replicates the action of classic string machines originally from Europe such as the Solina and Logan String Melody."

And

"The Multimode setting is designed to replicate the actions of various classic synth chorus units and string machines from Japan."

A-ha! Like the SS-30? Not quite, no.

The multimode has three settings

• Quad Ensemble
• Dual Ensemble
• Stereo Chorus

The SS-30 is mono and the quad ensemble uses all four delays, whereas the SS-30 has just 3. In fact the SS-30 is closer to the three-phase mode, albeit with fewer phases.

You may recall from this post that the SS-30 has two LFOs with two phase shifters

So, that's two LFOs, although one is variable and only two phase shifts but the mixing is complex. It's not even clear from the Oakley description how they mix the phases in 3 phase mode, so I can't actually tell how they compare.

Overall it looks like a very nice project and one that I would be prepared to consider doing, but probably after the SS-30M is done.

Juno what chorus the Alpha has?

As mentioned above I have an Roland MKS-50, which is a rack-mount Alpha Juno. The chorus on the Alpha Junos/MKS-50 is apparently a different beast to that on the older Junos. Gordon Reid summed it up as "more flexible but less rich". And rich is the desirable thing, really. You can change the rate of the LFO (or LFOs?) but is that better than being able to switch between two LFOs? I would say the chorus on the Alpha is in keeping with the more restrained and cleaner sounds which were in vogue at the time. But, why is it like that?

The Alpha's chorus is a stereo-pair of single BBD delay lines. The MN3101 is the clock and the BBD is the 256-stage MN3009. And the Juno-6? The same! They use the same exact chips. There are two identical circuits, so you get stereo outputs. The JX-10/MKS70 had more-or-less the same design as Alphas too. So what was different?
The Juno 6 service manual helpfully makes it clear that there are three triangle-wave LFOs. Two at chorus speed of 0.4 and 0.6 Hz and a vibrato which is added at a lower level and runs at 8Hz. The schematic is a bit loopy by I think the same modulation LFO mix is is fed to both channels but out of phase.
On the MKS50 things are not as clear. There seems to be a single signal split into two out-of-phase signals driving the clock. The signal's origins are more obscure though. It appears to be digitally sourced from the CPU via a gate array and a multiplexer.
The gate array has a D-to-A converter built in and its here that the data from the CPU is converted into the LFO signal. On the block diagram, the multiplex is labelled as a sample and hold (S/H) and there is some kind of active filter after that - which makes sense, as there is no continuous CV to make up the LFO. It is smoothed out by the filter, I guess. The notes refer to it as chorus LFO and Chorus Rate CV.
In notes there is also basic diagram of what to expect.

Those arrows seem to indicate some sort of pulse-width is adjusted but I guess this is actually the frequency/wavelength.

It's possible to add a chorus input to the MKS-50 but there's a little debate about the wisdom of doing it and I haven't seen a really popular modification for it. Apparently there was a professional modification around, so enough people must have thought it was worth the effort of tapping into it to make that a thing. There's no info on what they did though.

Conducting The Chorus

In my mock-up, concept art panel design I had replaced the Orchestra section's Violin and Cello switches with faders. Looking back I'm not sure I knew what that was actually meant to do. As there is also a Depth control what was I thinking? Would the depth control also be required? What would the effect of the control be exactly and wouldn't those faders be pointless if the depth was set to zero? I honestly wasn't sure. And then the switches started to misbehave...

I don't  think I had ever looked at the schematic for the orchestra controls in any great detail and when the Violin switch started to behave strangely I realised it was doing more than I had imagined.

Mix Depth

To start with, in the Off position the switches bypass the Orchestra completely. In this mode the depth setting has no effect on the sound at all.

This might seem pointless at first sight. It's generally less confusing if all controls work at all times and there's no context in which they aren't having some effect. In this case the context is whether the switches are on or off and the key here is that there are two switches and two sources that pass through the orchestra effect. There are, of course, four options here and only one leaves the Depth control redundant. Still, it does return us to the question of what my two faders were supposed to be for.

Next, if one switch is on and the other is off there's a way to have one voice dry and one wet. This is why there are two switches.

If either switch is in the On position the Depth control sets the mix of wet (effected) and dry (unaffected) signals for both Cello and Violin. This means that unless one is completely off, there is no way to set the mix independently. You cannot have one at 50% and one at 100% wet, for example. Now I think I can see the point in the faders. If they controlled the amount of mix for each voice then there would be more control. However the depth control would be redundant. And yet there is another option.

LFO Depth

Referring back to the Oakley Sounds SRE330 there is a depth control there too. However this Depth control is not the mix, it's the amount of LFO being applied to the BBD. So, could I rethink the depth control as LFO depth?

There are two LFOs. And then they are mixed together and the result is three signals which go from LF board to the OR board. I could easily intercept these wires and send them through a 3-gang potentiometer. Or I could have a depth control for each of the two LFOs - but then I would have to pull up some resistor and I;m not sure it's what I need anyway.
I reckon the LFO depth would be a good change. Along with the LFO rate settings I thought through in the previous post on the chorus I'm starting to get a better idea of a more fully featured Orchestra/Ensemble/Chorus effect. 

Switches to Faders?

If that's it for the mix depth control, there could be two fader controls to set the mix of wet/dry of each voice. Can I achieve this just by rewiring the existing connections though?

Unfortunately Yamaha took a design decision which makes that harder than you would think. The switches are actually double-pole (two switches which are actioned together at the same time). The first pole, on both switches, switches the incoming Violin and Cello signals to either a mixer which then passes them on to the Orchestra, or to another mixer which then passes them onto the other pole of switches. At this pole the mixed signal either goes directly to the output stage or to the Depth control.
To be clearer if only Violin goes to the Orchestra, Cello goes to the output stage directly and Violin goes through the Orchestra and to the depth control. The depth control is then mix of the dry Violin and wet Violin from the Orchestra.
I need to somehow have the completely dry, bypass signal at one end of a potentiometers and the Orchestra at the other. The incoming signals will then be sent to one or the other, or some mix of both. Then the output of the bypass would go direct to the output and the Orchestra would also go direct and and the switches and original depth control would be completely redundant.

Looking again at the diagram I simply removed the switches and Depth control then drew in what I wanted to do. 

After the Buffer section and before the (pre-Orchestra) Mixing Amp and Mixing sections I have replace the switches with two potentiometers. I will have to experiment with the values here.
After the Mixing section I have labelled the path to the next section as 'Bypass'.
The output from the Mixing amp is split two ways. One goes to the Orchestra board and the other to the mix Depth control. Now the one to Depth control line redundant. Instead the signal back from the Orchestra goes direct to the next section where it is mixed with the Bypass signal.

I will need try this, but it's mostly just disconnecting the switches and Depth controls then adding the sliders. It's not a big job. My main concerns are how well the inputs are balanced between bypass and whether having the orchestra always switched in makes and difference to noise levels.

External Input? 

One idea which has been in my mind for long time - and is again fired by the Oakley SRE330 - is to have an external input on for the Orchestra effect. I makes perfect sense and in terms of the value of the finished article will mean I have a top notch chorus effect available even when I'm not using the SS-30M strings. All it needs is an input jack and maybe a level control. It's something else to try, and whilst I have the wire's loose it make sense to at least try.

Summing Up

In summary:

1. Need to test out the ideas in the previous post about adjusting the chorus rate and making it controllable - LFO Rate
2. Insert a control on the amount of LFO reaching the BBDs - LFO rate
3. Remove the switches and Depth control and replace with a mix-depth control for each voice. 
4. Test an external input