Do you remember BOSS Slow Gear pedal? If your a guitarist you most likely do or at least you’ve heard of it. It was a great pedal sold from 1979 to 1982 and it was made in Japan. The pedal would cut the attack of your notes giving a swelling sound. It god famous for making the guitar sound kinda like a violin.
I always liked that effect and i even made a clone a few years back. It is based on a 2SK30 JFET and it was a pain getting these transistors. It was a lot of fun though and i though i should make a Project Ryu swell effect pedal and so LAGGER was born!
Recently i worked on a few projects with LM13600/LM13700, one of them is a nice noise gate / compressor unit which i will present at a later date, and i really like the VCAs that can be built with these chips.
To cut the attack of a note and then swell the volume basically we need a triggered fade in effect. This means that we need to control our VCA with a rising voltage using what i call a ramp generator.
In a previous article (Monitoring Amplifier モニターアンプ P3: Speaker Coupling Delay) i described the circuit of such a ramp generator and it even has a command input. I will use this circuit with the LM13600 VCA all controlled digitally with a PIC18f1320.
Below you can see the block diagram of the Lagger:
The input is fed into an ADC channel to be rectified and averaged in order to detect when a note is played. Once it is detected, the ramp generator is triggered and provides the control voltage for the first VCA.
Since LM13600/LM13700 is a dual amplifier the second one is configured as a VCA with manually set control voltage. In the picture below you can see how the circuit works. The top signal is the input signal, the middle signal is the output of the ramp generator and the bottom signal is the trigger.
There is a problem with using the ramp generator circuit this way. The capacitor is discharged too quickly when the trigger is interrupted and this causes an audible thump noise when trigger goes off. Looking below at the schematic we can see the discharge current goes through CE junction of Q1.
We can lower this current by inserting a resistor between ground and Q1’s emitter but in our specific application that will cause an offset and the output will not be totally silent in absence of input signal.
Another way to solve the problem is by paralleling a capacitor with R3 (Q2’s emitter resistor) This will cause a fade out effect and eliminate the thump noise.
Below you can find the schematic for the Lagger:
U5 shows as TL071 but you need an opamp with higher output current sink capability. Something like HA17358 with 50mA capability is good:
Trigger for the ramp generated is created when the microcontroller detects a signal from guitar. In my last article i have explained a way to rectify and average an analog signal using ADC and software. If the input level is higher than a set threshold level then ramp generator is triggered.
In the first units the middle pot was used to set a sustain period but that was changed to sensitivity control as it proved to be much more helpful.
J1 is a push-button which will generate an interrupt for the microcontroller and provide a true bypass via the SPDT relay.
You will notice some unusual supply voltages. For example the microcontroller’s Vdd is set to GND and Vss to -5V. This is done in order to provide correct trigger levels and avoid using other active components to shift the level.
Below you can see the PCB for the unit:
Here are some pictures with Project Ryu Lagger:
Here is a short video with the unit in action:
I will be supplying the hex file for the PIC18f1320 microcontroller in my next newsletters so if you want to built the unit and your not a subscriber yet please use the top right form to subscribe.
Also in my newsletter you will find offer for kits and complete units for those who don’t do well with electronics.
Thank you for visiting.