I got a used Hartke A25 Bass Combo recently and it was in good condition but the pots needed cleaning so i decided to show you what is inside this little combo.
Once i took out the amp i noticed the power stage is actually using discrete transistors which is really really nice. The codes are not so well visible but i think it uses 2sd2058 and 2SB1366.
The opamp used is the JRC 2068 and a JRC 13700 is used for limiter (can be replaced with LM13700)
I also liked that the transformer is well shielded. After I cleaned the potentiometers I tested the unit and sounded great. I took some measurements to see the effect of the equalizer, range and center frequency.
I measured the output of the speaker from 50cm away and the speaker about 50cm from ground.
First i compared CD input with instrument input. Note that CD input is not affected by any of the controls, not even the master volume.
Red trace is the CD input and the other trace is the instrument input
Next i tested the equalizer controls.
The above is the bass control. We can see it is centered around 100Hz and indeed it has 30dB control range. Blue is +15dB setting, Red is -15dB setting.
Midrange seems to be center around 700-800Hz and again the control range is about 30dB. Blue is +15dB setting, Red is -15dB setting.
Treble center frequency looks to be 2kHz. The range is close to 30dB again. Blue is +15dB setting, Red is -15dB setting.
I also measure the direct output of the amp to see the frequency range of the preamp. It is quite linear as you can see below.
I can say it is well built and it has a nice tone. It doesn’t go that low but if placed on the floor it can be quite satisfying. In the future i will look more at the speaker and the enclosure as i think it can be improved a bit.
It has been a quiet period but a very busy period, very active and very fruitful. I will be launching the projectryu.com site soon with tons of free resources for the audio hobbyist.
Getting back to the title of this post, i would like to present a few improvements and a concrete design as a pedal unit. If you don’t remember about the Lagger project let me put the links below in case you want to check it out.
As a quick summary, Project Ryu Lagger is a guitar effect pedal that slows down the attack of the guitar (or any instrument for that matter) just like an automated volume pedal.
It does this with a VCA (Voltage Controlled Amplifier), based on the LM13600/LM13700. The fade in envelope is created with a ramp voltage generator that controls the VCA and this generator is triggered by a digital circuit created with a Microchip PI18F1320 MCU.
The digital signal conversion and rectification is explained in a previous article, please read it at the link below:
The new version i will present today was adapted as a real pedal with a 3 pole footswitch.
Another change was in the ramp generator replacing the 100k potentiometer with a 10k one since the other two pots in the circuit are 10k. This last change also implies changing some capacitor values. Please refer to the new schematics below:
I am pretty satisfied with this pedal so i have created the PCB for this circuit. The PCB gerber files and the MCU firmware is available for free as usual to subscribers in the next newsletters. If you are not a subscriber yet please register in the form to the right.
Some things to consider if you plan to design your own PCB is good shield of the LM13600/13700 input pins. It is good to surround it with ground plane like below:
With the new modifications on the ramp generator circuit the min – max values for attack control is 0.1s and about 1.4s. Below is the ramp waveform measured, this is actually the control voltage of the VCA:
1. full counter clockwise attack pot setting:
Horizontal resolution: 50ms/div
2. full clockwise attack pot setting:
Horizontal resolution: 200ms/div
Below is a screenshot that shows the triggering of the ramp generator, the red trace is the input signal:
As you can see modifying the attack control will adjust the length of the fade in effect. The trigger control adjust the input level above which the ramp generator is triggered. This is to prevent noisy pickups or other pedals used in front of the Lagger to trigger the effect.
The level control adjusts output gain from between +6dB to a max of +24dB so it is capable of a high amount of gain. In the measurement below voltage gain is 14 with an output voltage of 3.6V peak to peak (yellow trace). As you can see there is a bit of saturation happening.
Bellow are two videos showing signal traces using sine waves at 1kHz, one video triggering the effect manually and the other with the effect triggered by the input signal.
Project Ryu Lagger is one of my favorite pedals, i really like this effect and the latest version allows for a great deal of control for the guitarist in sustaining the effect or muting by simply applying muting effects with the palm for example.
For DIY-ers this project is of moderate complexity but you can subscribe or comment here and i will try to offer any assistance i can. Remember to read the Disclaimer though.
Here is the parts list for this project:
C1, C8, C15
C13, C14, C16-C18
R2, R6-R12, R15, R22, R23, R26, R28
R5, R29, R31, R32
R13, R14, R24
3 pole foot switch
I hope you enjoyed reading this article as much as i enjoyed building this pedal and hope to hear from you. To the subscribers, the newsletter will be sent in a few days with Gerber files for PCB and MCU firmware and a couple more details about this construction.
Here are 2 sample sounds of the effect played by one of my guitarist friends:
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.
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.