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.

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