Add Shape control to EA Tremolo?

[Pedro Fontacos]

Hi all, is there a way to add a shape control to an EA tremolo for more chop i.e. square wave?  Thanks.

[jkokura]

Not precisely, but you can affect the minimum and maximum depth. There are two resistors coming off of lugs one and three of the depth pot. The one going to ground can be raised, and some have said that it can produce more of a choppy sound. It's not quite perfect, but experimenting with values is something you can do.

Jacob

[aballen]

Not exactly what you are looking for, but the cardinal trem has a pot for adjusting between sine and square wave. 

I have an EA trem, its great, but I think the cardinal is replacing it.

[midwayfair]

The bias modulation oscillator in the EA Tremolo puts out a sine wave. That's the type of wave you're dealing with, so anything you do to the wave will have to be within those confines.



You need more gain out of the oscillator -- enough to clip the waveform -- to square off the signal. But you don't want to make it too squished at the top. It's really the same as when you make a distortion circuit. Gain up the signal so that it clips and you lose dynamic range.

Another problem with just pumping up the gain is that the emitter is already grounded. You can mess around with the 10K in series with the diode. Decreasing it (e.g. 2.2K) will increase the depth to a point. But oscillators are a bit of a balancing act. You can't just gain it up into oblivion or the transistor won't oscillate. The depth is also interactive with the Rate in this oscillator -- so increasing the depth to its maximum point could make the transistor lock up in the fastest settings. The only really good solution to that is a dual gang speed pot (the other gang replaces the 15K -- take a look at the Univibe).

Finally, there's an actual sound issue to making the oscillator a really hard wave form: You'll get a popping sound when the bias on Q2 changes. Search for the original Electronics Australia article about the EA tremolo and they discuss this; it's the reason for the 120K resistor (or its equivalent in the original schematic).

The best way to increase the voltage swing is to stick an amplifying stage after the .47uF cap before the 120K. That's assuming that the voltage swing isn't already ~3v peak-to-peak (which is the most you're realistically going to get). And obviously that adds parts, which could be awkward if you're not making your own layout.

There's one more thing that can be done: You can stick antiparallel diodes to ground between the .47uF cap and the 120K resistor. I'm not sure what the voltage swing is, but two silicon on each side is probably a decent place to start. I have no idea what it'll sound like here, but it will certainly square off the top of the swing, and it's easy to switch in and out.

[culturejam]

There's one more thing that can be done: You can stick antiparallel diodes to ground between the .47uF cap and the 120K resistor. I'm not sure what the voltage swing is, but two silicon on each side is probably a decent place to start. I have no idea what it'll sound like here, but it will certainly square off the top of the swing, and it's easy to switch in and out.

Damn, that's a good idea. 

[raulduke]

There's one more thing that can be done: You can stick antiparallel diodes to ground between the .47uF cap and the 120K resistor. I'm not sure what the voltage swing is, but two silicon on each side is probably a decent place to start. I have no idea what it'll sound like here, but it will certainly square off the top of the swing, and it's easy to switch in and out.

Damn, that's a good idea. 

Agreed. I have an ea trem board ready to build so will have to try this.