Add Muting, Decay/Release Isolation and/or End of Cycle Triggering to Your 4740
Here is a modification to the ADSR that will prove in making long attack settings easier and more comfortable to control. When S1 is closed, the attack cycle will terminate whenever the key is released, instead of rising all the way to a 5 volt peak before shutting off. For me, this allows a lot more expression in playing by giving a considerably wider control over dynamics. If a small toggle switch is chosen for S1, it could be installed in the manual trigger hole, or in the space between trigger button and output attenuator. The only drawback is that a step trigger must be used with the ADSR when the muting is on. If this is a problem, a pulse trigger could be derived by putting a capacitor in front of R2, but after the tap which leads to the muting modification. Also, a diode should be inserted to shunt negative trigger pulses to ground. The switch across the capacitor allows selecting a step or pulse trigger to be generated within the 4740.
Mike Genetti, Clawson, MI
If you have trouble visualizing how muting changes envelopes, a graph is shown below which depicts trigger times, envelope outputs for a stock 4740, and for a unit which has been modified for muting.
Also, an easier route to having muting on the 4740 is to lift one end of R6. This opens the feedback loop which normally keeps the 4740 triggered until the attack peak is reached. With R6 removed, the 4740 will stay in attack mode only as long as a trigger is applied to J1. A switch can be wired in series with R6 to turn the muting on and off.
The remaining modifications were sent in by Michael Bridgers, Elkhorn, WI. The first is used to make the decay portion of the envelope independent of the release setting. This circuit will only be effective when pulse triggers are used. As you may recall from the design analysis of the 4740, when a pulse trigger is applied, the unit will latch in the attack mode until the 5 volt peak is reached at the output. At this time, BOTH the decay and release discharge paths for the timing capacitor are engaged until the sustain level is reached, at which time the decay path is disabled and only the release discharge path remains active.
This causes some interaction to occur between the decay and release controls, and the decay time will always be less than what is produced by the same setting when using a step trigger for sustain mode operation. Shown on the right is a representative envelope using a step trigger and sustain mode on the 4740.
If the same 4740 settings are used, but a pulse trigger is applied, the resulting envelope would look something like:
Note that the decay section is considerable shorter than before. This is due to the timing capacitor rapidly dumping into the low impedance release circuit. Addition of Michael's isolation circuit will restore the timing of the decay cycle to it's intended longer setting as:
The schematic for the isolation circuit is shown below. Note that parts labelled with R3, R6, etc., refer to parts that are already in the 4740 circuit. This circuit can be assembled on a small piece of perf-board, and suspended above the stock 4740 board. Some of the circuit board paths and component mountings may need to be broken and rewired, so before tackling this one, carefully consider how much you need this circuit modification, and your ability to rework circuit boards.
The final chapter of our 4740 extravaganza is a nice, easy, fun circuit. This circuit generates a pulse trigger when the envelope generator has completed it's cycle and the output is nearly back to ground. The pulse output can be fed back into the trigger input of the generator, causing repeating or oscillating envelopes, or it can be used to trigger another envelope generator, sequencer, etc. The trimpot should be adjusted to allow the circuit to fire when the envelope output is as close to ground as possible.
By the time you get all these modifications put on your envelope generator, you'll have one super-duper 4740.
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