Wobble Box

The Accidental Chorus/Vibrato

Overview

Alright, you say, why is this the accidental chorus/vibrato? Well, I didn't set out to make this pedal at any point. In fact, I was playing with the ES56033 chip to try to understand its mechanism for time adjustment a little better to see if it could be used in the development of a flanger. In order to do that, I was trying to find a way to modulate the delay time using an LFO. I found out experimentally that I can get some modulation with a set delay time, but I couldn't find a way to actually change the delay time in a way similar to how a PT2399 can be done.

My thinking is that the ES56033, because it has the F_adj pins, probably has some sort of internal compensation, making techniques like an LFO on one pin, or a current sink somewhere ineffectual. I still don't understand exactly what is happening, but I do know that this chip won't work for a flanger. That being said, that doesn't mean the exercise was fruitless.

I found that an LFO into a current sink hooked up to the negative side of the pin 2 capacitor will create a lovely, wobbly effect. Thus, the Wobble Box. How does it work, you ask? Glad you asked. Here we go!

How It Works

A chorus is really just a delay with a very short delay time that is modulated with and LFO. If you look at the schematic closely, you will see that there is not a whole lot of difference here between this circuit and the Stalker. In fact, one could do a combination of the two and have both options in one. You could call it the Wobbly Stalker, or the Drunkard, or something like that. So let's walk through the circuit.

This is the same input buffer that many of my circuits use. It works well, so let's keep on using it.

Wobble Box Input Buffer

Next up is the delay stage. The multifeedback filters for input and output are the same, as are the modulator/demodulator smoothing caps. In fact, the only thing different here from the Stalker is that there is a fixed delay time (~20 ms), a slightly larger pin 2 cap, and the other side of the pin 2 cap doesn't go to ground. This is very important to the function of this circuit and we'll talk more about that later.

Wobble Box Delay Stage

Now that we have delay, we need to create wobble. That is first accomplished with and LFO. This is the same topology as the T60 and Stalker. It works ok, though a true sine shaped LFO would have a little smoother sound, but that requires either a dedicated IC or a programmed microcontroller, which is outside of the scope of this project. At least for now.

Note that the speed control is a reverse log pot. This is to give more resolution at the faster speeds. With a linear pot, there hits a point where the speed shoots off and it can make that range very hard to dial in. So don't sub this for a linear if you don't have to.

One other thing to note is that the output level of the LFO is fixed. I found that adjusting this doesn't have much effect on the sound because of how we are introducing the modulation.

Wobble Box LFO

With the LFO signal now generated, we are going to a little trick to mess with the current that the pin 2 capacitor can sink. Because the ES56033 (appears to) use an internally compensated VCO for delay time setting, we can mess with the delay time just a little until the internal compensation catches up. This allows us to create modulation while not actually changing delay time. To change the current that the VCO is sinking, we use a current sink. This is the same idea as the EchoWreck, except instead of using a smoothed PWM signal to change delay time, we are always changing the amount of current that the current sink's BJT lets through, so that the VCO's internal compensation is constantly on it heels. This creates the wobble sound.

The current sink uses the BJT as a variable "resistor" (how open/closed the collector/emitter connection is) to change the current allowed through. We also have a series resistor from emitter to set the maximum current to sink. With none, we don't have any modulation. Raising this resistor more makes the minimum depth setting more drastic. Setting it lower makes the minimum depth setting less noticeable. I found 68 Ohms to have a minimum depth right on the verge of being unnoticeable.

The depth pot is setting a limit as to the minimum amount of current that the chip will sink. I found that it is a very small resistance range (note the parallel 120 Ohm resistor!), and I also found that a logarithmic pot works best. With linear, the lower settings on the depth control go from mild to very noticeable over a very narrow range, so with the log pot, you get a much more even sounding distribution of the depth.

Wobble Box Current Sink

The remainder of the signal path is the summing of dry and delayed signal and output buffering. This is, again, the same output buffer that I have used several times now.

Wobble Box Output Buffer

Finally, we have the power section. This is pretty basic, but please note the use of the LM7805 instead of 78L05. This is done because it is pulling about 60 mA off the 9V line, which, with a 4V drop is nearly 1/4W of power dissipation, which gets that little TO-92 case of the 78L05 really hot to the touch. I prefer the larger, TO-220 packaged LM7805 for these ES56033 circuits just because they don't really even get warm. They are bigger, and you could probably just use a 78L05, but I'm just playing it safe. Who knows what could happen when it gets hot, maybe it can't supply enough current or something. I don't know.

Wobble Box Power Section

So there you have it! This accidental circuit is really just the combination of several building blocks I've used before. Don't believe me? Let's review:

Input/Output buffers from EchoWreck/T60/Stalker
Delay stage from Stalker
LFO from T60/Stalker
Current sink (with some modifications) from EchoWreck

See how easy it can be? Any circuit is really just a combination of smaller building blocks that make up something that does a specific thing to the signal. If you fancy building one yourself, grab the build documentation, layout, full schematic, etc. here. Happy Building!