The Stalker
The Easy DIY Double Tracker
Overview
Double trackers are essentially just a very short delay with some sort of pitch shifting or modulation. It's a very basic concept. However, in the DIY world they are a little more difficult because the very short delay times required (~10-50 ms) are difficult to achieve without using some form of a DSP. For example, the PT2399 has a minimum delay time of about 30 ms. While that can provide a reasonable double tracked effect, it can't achieve the shorter half of the typical time range.
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So what makes the Stalker different? Well, it uses a different delay chip. The PT2399 was originally designed for devices like karaoke machines and was adopted by guitar players. I figured I would pull the same stunt, but with a different delay chip. While looking at a broken Boss Ce-5(t), I discovered the 40 pin ES56028 which that pedal uses and that it has a 16 pin little brother, the ES56033. The data sheet is very sparse and has little information about it, so I took a chance and picked up 5 of them. This is the heart of the effect.
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The minimum delay time of the ES56033 is 14 ms. While the chip works very similarly to the PT2399 in principle, the package is not pin compatible and the time adjustment is different, but other than that, a lot of it is the same. The maximum delay time for the ES56033 is around 700 ms for reasonably clean repeats, but you will need a delay pot of around 100k to achieve that. With my oscilloscope, I was able to get 1.75 seconds of delay with a 1M pot, but the repeats were unusably dirty. Still, maybe you dig that kind of thing.
How It Works
Truthfully, the Stalker is not all that much different from a standard PT2399 delay, but with a different chip and an LFO that can engaged/disengaged. That being said, we'll still walk through it.
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The input buffer is the same input buffer that I use in the EchoWreck project. I find it works well and I see no need to reinvent that wheel.
Stalker Input Buffer
The heart of the Stalker is the delay section based on the ES56033 chip. As mentioned, this chip is similar to the PT2399, but some differences do exist. The most significant of these is how the delay time is controlled. Instead of a single pin with a resistor to ground found on the PT2399, the ES56033 uses a resistor connected between pins 2 and 3 and a capacitor from pin 2 to ground. The resistance and capacitance of these elements work together. With a smaller cap to ground, a larger minimum resistor is required. The data sheet does not give any info about component selection, so I found these values to work via breadboarding. These values give the minimum delay time of 14 ms. Note that if you go too low on the resistance between the two pins or too small on the capacitor from pin 2 to ground, the chip won't produce any delay.
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Other differences from the PT2399 are relatively minor. The assignment of power and ground pins and opamps differ, but the general circuit topology is the same as the PT2399: caps to smooth out modulator and demodulator noise, multi feedback filters for input and output opamps, etc.
Stalker Delay Stage
Stalker Time Control
The delay time is slightly modified by placing an LFO on pin 2. This is not nearly as effective as it is on a PT2399 circuit, due to how the delay time is set. Instead of getting a really noticeable modulation effect, it gives a slightly different texture so that it doesn't feel like just a straight delay, but it's hard to describe exactly how it sounds different. The LFO architecture is the same as the LFO's in the T60 reverberator and are a very classic LFO. I set up the speed to be a trim pot on my board, with the depth control being the external control.
Stalker LFO
Because this is a relatively simple project compared to some of the others I've posted, I created a perf board layout that will work in a 1590B, albeit a tight one. Note that if you do this, you have to cut the diagonal corners of the board at the top. That extra little bit of space is really critical.
Stalker 1590B Perf Board Layout
Stalker 1590B Perf Board Gut Shot
Fancy whipping out one of these guys? You can find full build documentation, schematics, etc. right here on Github.