CheckMate Vibrotrem
Synchronized Vibrato and Tremolo
Overview
I've had a hankering to try a tempo-synchronized tremolo and vibrato for some time. Together, the two effects can create some fun textures, and having the LFO's synchronized allows for some mild rotary-esque sounds, though this is not a rotary emulator. I wanted to make one that was easily DIY-able, but the vibrato can be a bit of a challenge. I decided to use the ES56033 and modulate its delay time due to its short minimum delay time of about 14 ms (half the PT2399). It's a little weird, but it sounds pretty cool.
How It Works
At the heart of it, this circuit is a combination of my Wobble Box circuit and Forrest Whiteside's Shoot the Moon tremolo. It uses the ES56033 delay stage from the Wobble Box and the LFO and general operational concept from the Shoot the Moon. However, to keep parts count down, the tremolo is implemented using one of the two onboard opamps in the ES56033.
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The input buffer is pretty standard stuff for a circuit like this. It's a simple inverting opamp stage so that we can split the signal between the dry input to the output summing amplifier and the delay stage.
CheckMate Input Buffer
The delay stage is really a combination delay and tremolo stage. The signal comes in to a multifeedback lowpass filter as usual, but the delay time is modulated by way of a current sink (discussed later), as is necessary for the ES56033 due to the internal compensation circuitry. The output of the ES56033 is amplitude modulated (tremolo) by way of an LDR on the input pin of the opamp, much like how the Shoot the Moon does its tremolo. However, the ES56033 opamp is also used as the summing amplifier with the dry signal. You will notice that instead of a traditional mix control that varies the wet signal between 0 and roughly unity with the dry signal, the dry signal is controlled between 0 and unity. This allows for a full vibrato-and-tremolo only output for really intense effects with dry at 0 and more chorus-y, rotary effects when there is some dry signal mixed in. Additionally, this was done to save the space of a dual opamp chip with having a separate output buffer.
CheckMate Delay Stage and Output
The LFO is taken from the Shoot the Moon. I really liked that it has shape control in addition to the standard speed and depth controls. Additionally, the way the output for the tremolo is configured made it compatible with the necessary current sink that is used for modulating the delay time of the ES56033. The ES56033 delay time cannot simply be modulated by putting a voltage directly on one of the Fadj pins because I believe it has internal compensation circuitry, so a current sink is needed to control how much current is pulled through the internal VCO circuit.
CheckMate Delay Stage and Output
Finally, we have the power section. This is pretty standard for a pedal like this, with +9V, VREF, and +5V. If you've seen any number of my designs, this shouldn't be a surprise to you.
CheckMate Power Section
If you want to build one of these guys for yourself, head over here and get the build documentation, Gerber files, and everything else you'll need to make one.