Short summary: The Volume controller can be connected to a potentiomer in voltage-divider configuration via TRS jack or a control voltage can be applied via a TR jack. The Sustain pedal is a switch connected via TR jack, but this is not easy to control from other equipment due to the same negative voltage problem as the keys. An opto-isolator will be needed to make this work with a standard S-Trig input. The Expression output of the MTP-8 MIDI interface is of limited use but helpful so a CV output has been added to the rear-panel and could be scaled up and converted to drive the Volume pedal.
Foot Use
The SS-30 has quarter-inch jack sockets on the rear panel for two foot-pedal controllers. So far I have never investigated or used either. This is rather remiss of me and before the SS-30M comes together I need to look further into these optional extras.And whilst I'm here I will also take the oppotunity to test the Expression output from the MTP-8 MIDI converter.
Volume Control
The first foot controller is labelled ‘FOOT. CONT (VOLUME)’ and the User Manual suggests connecting a volume pedal, such as the Yamaha FC-3
Metal from the pedal
The connection from the pedal is via a TRS jack with the following signals
Tip - Volume signal
Ring - +15 volts supply
Sleeve - Ground
And this is the schematic:
After connecting a jack plug, this droppped to 0V. That's because pin 7 is disconnected from pin 6 when the plug is in. Hence, with no plug the 15V supply feeds into the volume control circuit via the 2.7K resistor and when the jack is inserted the resistor is disconnected and the foot controller's internal resistance sets the level of the signal.
Thus, the volume foot controller is just an audio log potentiometer wired as a voltage divider. The voltage being divided between the +15V and ground.
Volme Control Voltage Control
The Volume Control socket is designed for a foot controller / potentiometer, however there is no reason not to simply insert a voltage from somewhere else. The good news is that you can safely use a TR jack which shorts the +15V 'ring' to ground, because ground is not actually shorted to ground. There is a resistor there.
The voltage would ideally to be up-to and above 12V. It also needs logarithmic scaling. That can be tricky though, as we'll see.
This input opens up the possibility for external global envelope control or tremelo effects.
Expression
The off-the-shelf MIDI decoder I'm using has a output called Expression. This is mapped to the velocity of the last note event recieved and is limited to just 4 bits, or 16 steps, of resolution.http://www.j-omega.co.uk/Downloads/mtp8install.pdf
MIDI velocity has a resolution of 128 steps (0-127) so the Expression output reduces this to just 16, like so.
| MIDI note velocity | MTP-8 step | MTP-8 voltage (V) | |||
| 0 | to | 7 | = | 1 | 0.000 |
| 8 | to | 15 | = | 2 | 0.208 |
| 16 | to | 23 | = | 3 | 0.417 |
| 24 | to | 31 | = | 4 | 0.625 |
| 32 | to | 39 | = | 5 | 0.833 |
| 40 | to | 47 | = | 6 | 1.042 |
| 48 | to | 55 | = | 7 | 1.250 |
| 56 | to | 63 | = | 8 | 1.458 |
| 64 | to | 71 | = | 9 | 1.667 |
| 72 | to | 79 | = | 10 | 1.875 |
| 80 | to | 87 | = | 11 | 2.083 |
| 88 | to | 95 | = | 12 | 2.292 |
| 96 | to | 103 | = | 13 | 2.500 |
| 104 | to | 111 | = | 14 | 2.708 |
| 112 | to | 119 | = | 15 | 2.917 |
| 120 | to | 127 | = | 16 | 3.125 |
Those steps would be of no use for swells or gradual fades, without filtering, but there are some tricks that can be used here.
Velocity Sensitivity
It would be possible to build a sample-and-hold circuit for each note and build in velocity sensitivity per note. I'm not keen on building and wiring another set of boards to do this though. Tempted, but not keen!The output could be useful for setting a global level though.
Another suggestion from the MTP-8 guide is to smooth out the steps and the changes for each new note on event and then use the voltage to control a power amplifier which in turn drives the 'output common voltage' which sets the level to each note output.
Referring back to the opto-coupler interface design you may recall that the output for each note from the MTP-8 is this +15V, common voltage and this switches on the opto-coupler which in turn shorts the input of the keying circuit to ground.
Things start to get complicated with the optocoupler though. What is the effect on this device if the voltage is less than +15V and consequently reduced current to the internal LED? Is it linear? honestly it's hard to tell! I was aiming for around 1mA for each note but I can't find the actual current in my notes. If it was 2mA maxmum then it seems to be outside of the non-linear region, but I could only tell if I tried this.
Another consideration here is that the keying circuit doesn't expect anything but -7V or 0V. In theory, anything between -7V and up to 0V would result in a lower in amplitude for that particular note. However, because of the way the attack time capacitor charging works it isn't quite so simple. For a start, the transistor would change from being in saturation mode to active mode. This would have consequences not only for the output voltage of this circuit, but most imediately the collector-emiiter current and therefore the charging time/attack time of the circuit.
On the whole, I'm almost sure this approach won't work without affecting the attack time in ways that would make the attack time proportional to the velocity. in which case it's mostly unusable and not worth the effort.
Velocity Switch
Just saying that I did though...As a minimum, there would have to be a switch to enable this feature, as leaving always on just wouldn't be, err, on. Why is that important? It doesn't have any effect if the MIDI input device only has fixed velocity - such as the PSS-580 - but when there is a velocity sensitive keyboard there still needs to be an option to only use the SS-30M as designed.
Our Velocity
Whilst applying the voltage from the Expression output at the keying circuit is flawed, it seems obvious that wiring this control voltage to the overall volume control should at least be an option.The MTP-8 Expression output is not scaled for 0-15V, alas. Instead, it only goes up to 3.125 volts. Therefore, to achieve the full range of volume this will have to be scaled up by a factor of (15/3.125) 4.8. This is simple with an op amp and two resistors. However, this is still a linear scale. Can it also be converted to a logarithmic scale easily?
No. Not really. It's realtively easy - compared to some problems - but would call for a lot of components. It is a bit of a rabitt hole to go down.
Scaling up - brief notes
A constant-gain amplifier or scale changer is a basic op-amp circuit. In this case it must be non-inverting - and as we're not as worried about the frequncy response this is fine.In this configuration, the non-inverting input is used with the inverting input in the feedback path, which is formed as a voltage divider.
In simple terms the the feedback resistor should be a multiple of the input resistor. That multiple defines the scaling where Gain = Rf/Ri
There is another problem though. This control voltage must be logarithmic. This video makes it clear what this all about.
Changing the scale from a linear to logarithmic range is a little more challenging. I've made some headway with a design but that idea's been shelved for now.
Expression Yourself
In summary then, the Expression output could be of some use but I'm not in the mood to divert time and effort to it when I'm so close to finishing the whole thing. I could go back to it, but only if I really feel the lack and can make a case for using it, no matter how limited.
I have included a rear-panel output for this CV signal, just in case I want to use it, but I'm just as likely to externally create a control voltage from MIDI and send that into the Volume Control input.
Sustain Switch
The other pedal input is labelled ‘FOOT SW. ( SUSTAIN)’.For this the User Manual suggests the use of the optional accessory FC-3, a Sustain Pedal.
This one is simply a TR socket, so it's not a voltage divider like the volume control.
The tip is measured at the familiar -7V and the ring is grounded.
The -7V, when shorted to ground cuts the sustain off. I haven't analysed the sustain circuit much before so, here goes...
Sustain is set independently for the Cello and Violin voices. The sustain controls on the front-panel are linear 100K variable resistors which form a voltage divider between 0V and -7V. In addition to the 100K potentiometer there is between it and -7V at one end a 1.5K resistor and 18K in series with ground at the other end. These additional resistors are important.
The footpedal switch also has an 18K resistor in series, and is fed through diode into the base of the transistors.
A concern for this external control is positive going trigger inputs being added accidently, but that's easily blocked by this protection diode.
So, when the foot switch grounds, any current from either Sustain control is carried away from the transistor and the K boards.
In practice this is a very sharp cut in the sustain and creates a rather unnatural and abrupt stop. Obvioulsy it can be a case of adding sustain back in. A resistor as well as a switch could provide for more than one level of sustain.
Switching things up
For extenal control of the Sustain circuit that -7V level creates a problem, again. Not an issue if a foot-switch is used, with a physical switch, but if a synth gate control will not play nice with -7V. Synth type gates ae either S-Trig or V-Trigger. Neither will work with this input though.A V-Trig (Voltage Trigger) output wouldn't work, because there is no way a postive voltage will do anything to this circuit. As noted above, it would do no harm either, which is good to know because setting up a gate output can sometimes be a matter of trial and error.
Normally such an input, requiring a switch to ground, can be controlled by a gate output in S-Trig (Switch Trigger) output. Those gates are switching postive voltages to ground though, and this is a negative voltage. Where have we encountered this problem before?
Trigger Happy
Avid readers of this blog will know that the whole story of the MIDI inteface for the SS-30 was complicated by this switching of -7V to 0V. The positive gate signals for each note from the MTP-8 had to be used to switch opto-isolators with the -7V keying circuits on the other side.Thus, the obvious solution for the sustain switch is to use the same idea. Connect 15V through a current limiting resistor to the anode of the LED input side of the opto-isolator, and connect the cathode to the Sustain input. An S-Trig connection into the input will then switch the LED on when the S-Trig is switched to ground. As with the key triggers, the sustain circuit -7V is then connected to the photo-transistor of the opto with ground on the other pin.
I can add an extra opto-isolator to the key trigger boards and the rest is just wiring. More wiring...
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