I am thinking of building a Tap-Tempo LFO module using the Pedal Sync MV-55 Tru-Foot LFO.  I got a part to prototype using a proto-board and a 3.3V supply.  This chip is kind of a "superset" of the Molten Voltage MV-52 Tap Tempo chip.  The MV-55 is setup for program storage and recall using a Master Controller and has more sophisticated functions.  I plan to use it as a Tap Tempo LFO with CV inputs and not to use the programmability.  Check out the Molten Voltage and Pedal Sync sites for other interesting parts.

The analog and digital decoupling for supplies and grounds is very important.  With the decoupling the waveforms are pretty clean.  Without, they are much noisier so take care.

Waveforms and Controls

The four output modes are selected by two control pins.  The cyan waveform is the quarter note clock and the yellow output is the LFO output.  The quarter note clock stays "fixed" at the tempo while the LFO output is modified by the Ratio control.  The Tru-Foot output is a positive half-cycle  with a flat portion "pause" between cycles to simulate the control voltage of a pedal wah movement.

Lowering the frequency a bit shapes the Tru-Foot output to a half-sine.

A nice LFO sine wave output.

The LFO triangle output.

The Offset control shifts the phase between the quarter note output and the LFO output.

The Duty control morphs the triangle to a saw output although with not a very sharp rise.  This is because the Duty control adjusts the duty cycle in 13  discrete "musical steps" of 1/20, 1/8; 1/6; 1/4; 1/3; 3/8; 1/2; 5/8; 2/3; 3/4; 5/6; 7/8; 19/20 and the output is distorted at high ratios.

The LFO square output.

The Duty control morphs the square to a pulse output.  This is the maximum duty cycle of 19/20.

Adjusting the Offset control 180 degrees inverts the LFO output mid-cycle.

Minimum and Maximum Frequency

With the Ratio control set to 1:1 the lowest frequency is 0.4 Hz.

With the Ratio control set to the maximum divide (/8) the lowest frequency is 0.05 Hz.  Note the quarter output stays fixed at 0.4 Hz.

With the Ratio control set to 1:1 the highest frequency is 4 Hz.

With the Ratio control set to maximum multiply (4X) the lowest frequency is 16 Hz but the sine wave distorts as noted in the datasheet.  Note the quarter note output stays fixed at 4 Hz.

Reducing the frequency down to 12 Hz restores a nice sine output.

These three images show the relationship of QTR OUT to LFO OUT with the Ratio control at the two extremes.

Inputs

The four inputs (Speed, Ratio, Offset, and Duty) are 0 to 3.3V CV inputs.  The pin specifications for over and under voltage are 0.3V from the supply levels.  5V CV inputs can be attenuated by 1/3 and clamped to the analog supply and ground with schottky diodes to protect the input.  A more elaborate scheme can use an inverting mixer to sum the CV input with a control followed by an inverting stage with clamp diodes.  My ComputerVoltageSource uses a similar circuit to sum and protect the inputs.

Duty and Ratio inputs are "backwards" so the potentiometers are wired "backwards" to compensate.  CV inputs need to be inverted so 0/+5 volts becomes +5/0 volts.  However, Duty behaves reasonably with increasing voltage increasing the positive duty cycle.  The Offset control seems backwards so my input design inverts the Ratio and Offset controls and corresponding CV inputs.

You can drive the MV-55 with a clock into the Tap input so you can set the tempo with the Rate control, Rate CV, Tap switch, or Tap input.  Tap always overrides the Rate control or CV.  Moving the Rate control or CV more than about 5% will then override the Tap switch or input.  A single tap will reset the waveform to the beginning of the cycle, even if the frequency was set by the Rate control or CV.

Sine Out while sweeping the Rate CV.

Square Out while sweeping the Rate CV.

Square Out while sweeping the Ratio CV.

Sweeping the Ratio CV faster results in non-periodic transitions as I suspect the CV input sampling only occurs at distinct points in the waveform generation.

Square Out while sweeping the offset.  The cyan trace is the Qtr Note Out for reference.

Square Qut while sweeping the Duty CV.

Outputs

The LFO output level measured 1.38V pk-pk with a DC offset of 1.74 volts.  To bring the level up to standard +/-5V waveforms it needs a gain of 7.24X with a -12V DC shift.  However, the Tru-Foot output is more of a CV control so it would work best as a 0 to +5 volt output.  I thought about using the waveform selection control to also adjust the gain and DC offset of the Tru-Foot output but decided to use a separate output level switch.  I also added an output level switch for the Qtr Note output.  I also added LED current drivers for the LFO and Qtr Note outputs.  Since Qtr Note is a square wave a current driver is not necessary but I had the spare op-amp.

Preliminary DJB-020 Tap Tempo LFO schematic  unverified

Panel Concept

I'm a planning a dedicated 2U module without programmability.  I've added the CV attenuators for each of the four controls, output level switches, a Tap input, and LED current drivers.  Bi-colored LEDs above the output jacks show the polarity of the waveform.  The LED blinks green/red when the output is +/-5V and blinks green when the output is 0/+5V.

DJB-020 Tap Tempo LFO FrontPanelExpress file  unverified

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