Attack Formation

When I wrote this post about the concept art  I said "I've assumed I can replace all switches with potentiometers".  That assumption was wrong in the case of the Attack control though. In the last post I mentioned that "it's more of a re-design job than a modification".I shall now try to explain briefly, why that is.

The SS-30 Attack control is a switch and according to the user manual :

With this tablet switch on, the rise of the sounds of the Violin system will be softened.

 Softened is the adjective because although it is slower with the switch on the change is subtle. The normal attack is not particularly hard, but the Slow setting is only slightly longer. As we'll see below when I calculate the the rise time, it's not the typical attack range setting that you get on a synthesizer.

Another bow to the string?

Billy Currie put his finger on the reason for this choice in the interview he did with GForce Software. "You were able to create the feeling of a bow" he states. I had a bit of a look around at the synthesis of bowed strings but all I could really come up with was that you need a bit of attack to stop it sounding like an organ. Fair enough, but Curie's point is a bit more subtle than that. The Slow setting provides a softer attack option which his Elka Rhapsody 610 could not. In any case, an even slower attack is not typical of a bowed string. You cannot, for example, slowly increase the volume of each string. Therefore, an even slower attack wasn't needed, because that would stop it sounding like a bowed string (or bowed strings), and a shorter attack would have the same problem.

That's the design choice Yamaha made dealt with, but what about redesigning the attack circuit? Can it be done?

Not easily, no. I had a stab at analysing this circuit before, in the post A Switch To Variable Controls. I wasn't quite correct* in the description there, but I was right that no control could easily be added. Let's see why.

*Wrong

Charging into attack

Keying Drive circuit

 In the diagram above I've highlighted in red the part which controls the attack setting. Tr24 is a transistor which controls the current that charges the 6.8uF capacitor when the key is pressed. The same capacitor discharges through Tr23 when the key is released - this is the sustain control.



The key to the attack time is the charging time of the capacitor. Capacitor charging time is constant for a fixed voltage and is the product of resistance and capacitance. As the capacitance will not change the charging time is controlled by altering the resistance.

The collector of Tr24 is common across all the keys. This node is either open-circuit or grounded depending on the Attack switch.

Attack Switch - BL is the BLack ground wire, which is fed from VR3.

 The base of Tr24 is connected to the key switch. It's normally -7V (via the supply rail and the diode D12) and then switches to 0V when the key is pressed.

The emitter of Tr24 is normally at -7V (again via the supply rail and) so, as this matches the base voltage, normally there's no current flowing through the transistor. When the key is pressed and the base is set to 0V the base-emitter junction acts as a diode and current flows across it and thus the capacitor is charged . What about the collector though?

Let's start with the collector open-circuit. In this case we can treat the base-emitter as a diode and simplify the circuit somewhat. In this case the resistance is circuit with the capacitor is 12K Ohm and 2K2 Ohm,  in series, which give 14.K Ohm in total.

Now what happens if the collector is grounded? In this case we have provided a path from ground to the capacitor directly through the transistor Tr24. When the key is pressed the base switches to 0V as before and a small current is produced across the base-emitter, but now the transistor is switch 'on' and current runs through from the collector. In terms of the resistance to ground the 12K resistor has been bypassed and the total resistance is now just 2K2 Ohm. As the resistance is lower this will produce a shorter charging time, so the normal attack setting is grounded - as you can see in the diagram above.


We can now calculate the attack time for both the normal and Slow settings.

Normal  - R = 2K2 C = 6.8 uF RC = 1.496e-2 = 14.96ms
Slow - R = 14K4 C = 6.8 uF RC = 9.792e-2 = 97.92ms

Note that I haven't accounted for the Vce voltage drop of 0.2V. With that the normal attack time is slightly longer at around 15.3ms.That assumes that the supply rail is exactly -7V and to be more accurate will require some actual measurements. The important thing is that the attack time changes from approximately 15ms to roughly 100ms for the Slow setting.

The long and short of it

Now we know how it works, what about making the attack time more controllable? Can it be adjusted to be shorter or longer? The answer is that the circuit can be modified only to make it adjustable between 15 and 100ms. The RC constant of the circuit is limited by the 12K and 2K2 resistors and no matter what you do with the collector voltage those two impedances will still be there. The attack cannot cannot be shorter or longer than the setting already obtainable, without completely changing components on all the 49 switch circuits.

With 2K2 after the emitter the minimum attack time will always be 2200x6.8e-6=14.96ms. The resistance can't be set any lower so the CR time constant of the capacitor charging will never be lower than that  If I remove the 2K2 resistor from every key circuit, and replace it it with a short-circuit, I could replace the attack switch with a 2K2 variable resistor and vary the resistance from 0 to 2K2 Ohm, giving an attack time of 1-15ms - or thereabouts.
Similarly the 12K resistor into the base of Tr24 sets the maximum attack time. Increasing the resistance between the collector and ground will initially provide an increase in the time constant from15ms upwards, but the higher it gets, the more the total resistance is being determined by the two impedanaces in parallel. In practise the total resistance will never get back to the same value as when the collector is open-circuit and the maximum attack time will always be just a bit lower than the 100ms.
The only way to make the attack time fully controllable, via the collector would be to replace the 12K resistor with much higher value - like 1M - and replace the 2K2 resistor with 0 Ohm link. Then the collector resistance would be in complete control of the CR charging constant.

I don't think I'm going to do that. It would probably take several hours, and that could still be be worth my while, however the risk of damaging a PCB track when there's 4x49=196 joints to de-solder is, well, it's not good odds.

In conclusion, where there's a will there's a way but there's no will to do this one. The attack control will remain as a switch.

Organ Donor

Which brings me to a confession. I bought an entire organ to get hold of two switches.

I may have gone too far this time. It's quite something to discard the wooden case of the SS-30 only to later by a dual-manual organ because I need a couple of switches.

Not just any switches, of course...

These Yamaha switches are just the right sort  - being black and tablet, not too large and the right vintage - to go into my front panel. There will be more about this, and the organ, later.