Need help modding a pedal circuit! "Ramp Up" mod

[DavidJNichols]

I know it's not a tube amp, but you guys are smart!

I built an EA Tremolo a while back (http://www.generalguitargadgets.com/pdf ... proved.pdf) and want to add a "ramp up" mod to change the signal to a more sawtooth wave. It'll be controlled by a DPDT switch. The "ramp up" mod idea was taken from the Tremulus Lune pedal. Here are the "specs" for the mod.

http://fuzzcentral.ssguitar.com/tremulus.php
www.commonsound.com/kits/doku.php?id=co ... d:tremulus
http://commonsound.org/tremulus/tremmods.pdf

How would I add that to the EA Trem schematic to change the EA sinewave to a sawtooth wave? Where in the EA schematic would I put that? Would it even work? Is there a better way to get a switch-controlled waveform changer in the EA Tremolo?

Any help will be greatly appreciated!

Thanks, David.

[Cliff Schecht]

This is a pretty easy trick to implement. You can pretty much steal the entire oscillator circuit from the tremulus (including the ramping circuit) and slap it onto the EA. In the GGG schematic, everything before the DC blocking cap (C4) is for the oscillator circuit and so this is what you want to eliminate and replace with the tremulus circuit. You might have to adjust R7 and R19 so that the signal level feeding the JFET is in the same range as that of the original oscillator circuit.

You can't really change the EA sine generator into a sawtooth generator directly. You can *try* to add some DC offset to skew the waveform but this will most likely just kill the oscillator completely. You *can* use the sine generator to form a sawtooth generator but this is sort of going out of your way to accomplish a simple task.

If it was my project I would most likely include both oscillator circuits and have a switch to choose between the two. That way you can get back to the stock EA trem circuit (a fantastic tubey sounding trem) or insert pretty much any oscillator (or control voltage) source into the circuit.

[DavidJNichols]

This is a pretty easy trick to implement. You can pretty much steal the entire oscillator circuit from the tremulus (including the ramping circuit) and slap it onto the EA. In the GGG schematic, everything before the DC blocking cap (C4) is for the oscillator circuit and so this is what you want to eliminate and replace with the tremulus circuit. You might have to adjust R7 and R19 so that the signal level feeding the JFET is in the same range as that of the original oscillator circuit.

You can't really change the EA sine generator into a sawtooth generator directly. You can *try* to add some DC offset to skew the waveform but this will most likely just kill the oscillator completely. You *can* use the sine generator to form a sawtooth generator but this is sort of going out of your way to accomplish a simple task.

If it was my project I would most likely include both oscillator circuits and have a switch to choose between the two. That way you can get back to the stock EA trem circuit (a fantastic tubey sounding trem) or insert pretty much any oscillator (or control voltage) source into the circuit.

Aha! I would love to do that. Include two oscillator circuits and switch between the two. I love the EA one, but would also love to switch to something really choppy. The ramp up option seemed doable, but if it's easy to just ad a second circuit, maybe a square wave, I'll do that.

Now, my next question is where can I find a simple square wave circuit to copy that doesn't use an IC? (I'd rather build from stuff I have than order an IC).

[Cliff Schecht]

This is about as simple as you can get: http://hyperphysics.phy-astr.gsu.edu/hb ... quare7.gif

It's one op amp extra to build this square wave circuit and it works like a champ.

And then there is this circuit that generates a triangle and a square wave: http://www.circuitstoday.com/wp-content ... op-amp.jpg

Another classic circuit that works great (you see it in a lot of synthesizer circuits).

Because the tremulus uses a super slow LDR to generate the modulation for the effect, you won't have any problems making this choppy. But the EA trem, because of its use of a super fast JFET as a variable resistor, will not like square waves. If you don't slew limit the square wave (literally slowing down the rise and fall times of the square wave) then you will have an audible popping occur that is really hard to eliminate without (at least) second order filtering in the audio path (which reduces bass response at the least).

Can you tell I've been down this road before?

[DavidJNichols]

This is about as simple as you can get: http://hyperphysics.phy-astr.gsu.edu/hb ... quare7.gif

It's one op amp extra to build this square wave circuit and it works like a champ.

And then there is this circuit that generates a triangle and a square wave: http://www.circuitstoday.com/wp-content ... op-amp.jpg

Another classic circuit that works great (you see it in a lot of synthesizer circuits).

Because the tremulus uses a super slow LDR to generate the modulation for the effect, you won't have any problems making this choppy. But the EA trem, because of its use of a super fast JFET as a variable resistor, will not like square waves. If you don't slew limit the square wave (literally slowing down the rise and fall times of the square wave) then you will have an audible popping occur that is really hard to eliminate without (at least) second order filtering in the audio path (which reduces bass response at the least).

Can you tell I've been down this road before?

If I built that first one, would it be possible to run it through the depth and speed controls already in place with the EA Trem? Also, what opamp should I use? I have on hand TL072CN and NJM4558.

[DavidJNichols]

Because in theory couldn't I just add a square wave oscillator and switch between the two, using an SPDT that's placed just before the pots? I don't understand the EA circuit well enough to know where to put the switch though. Or maybe I understand it so incompletely that this isn't possible?

[Cliff Schecht]

Yes in essence you can just slap whatever oscillator onto the depth pot that you want. I like to be more studied about doing such things but if you adjust the resistors surrounding the depth pot correctly, it shouldn't be hard to use whatever you can find.

You will experience popping in the audio signal if you use true square waves. You have to slow down the rising and falling edges of the square wave in some way to prevent this. You could even try some first order lowpass filtering (passive or active) and play with the capacitor value to see if this is enough to give you a decently choppy trem without dealing with popping issues.

Also the switching method you are using will pop when you flip it because the two oscillators (even if synced) can be at different enough voltages to cause a step to occur (and thus a loud THUD!).