Please Read!

Experimental Values

You have purchased a second-run development board, the KS11 design.  That means that some of the component values have not been exactly determined yet.  In fact, you can tweak your sound by changing some capacitor values. 

Specifically, the output resistors of the expo converters are not fully specified and need to be determined for both 12V and 15V supplies.  Also the VCF capacitor can be changed to vary the range of tones that you get from the circuit.  Additionally, the two noise filter caps require tuning. 

In all cases, the published values are going to work just fine, but you’ll want to either play with the values yourself or stay tuned to the community for the discoveries of others. 

Errors

OK, the good news is that the KS11 boards have no cut traces or jumpers or any such unpleasantness.  The only bad news is that one or more of the power supply connectors is reversed.  At the time of this writing (7-1-10) I do not know which of the power supply connectors are reversed, and I need to rely upon the community to Ohm out their boards and report back to me what’s reversed.  It’s really confusing to get this right, so please tell me your experience and I’ll update the design accordingly, thanks! 

Also there is a minor screenprint error in that two of the capacitors next to U3 and U4 have overlapping reference designators.  Just solder in a 0.1 uF capacitor in both positions. 

Fun with BBD Chips

You’ve got some choices here.  The BBD chip used in this design is available in 1024 and 2048 stages commercially from Smallbear and other sources.  There are also 256 and 128 stage chips available as obsolete parts.  You may select any of these four chips for your KS board, producing different sound alternatives. 

I have found in practice that the 1024 chip gives the best range of sounds, however the 256 chip is great for lead guitar and percussion.  The 2048 chip produces such deep bass on the lower settings that even my bass-enhanced Sennheiser headphones could not reproduce it for my ears.  I have not tried the 128 chip. 

So if you crave deep bass, try a 2048 chip, otherwise get yourself a 1024 chip for best range of sounds.  The 256 and 128 chips are more for collectors seeking to expand their tonal options. 

Buying Parts

The parts selection is flexible on this board, so please scour your parts collection before ordering the remainder of the parts. 

I use 1% resistors only as my choice, but most resistors can be replaced with 5% tolerance, possibly all of them though I am not sure.  Perhaps the resistors in the expo converters should be 1%, and as for the rest of the circuit - nothing is so precise that it will matter much really. 

The thumbwheel potentiometer is a Bourns part available from DigiKey but you can substitute any pot that will fit in the footprint.  It is for volume adjustment only, so you can set it and forget it. 

The tempco resistors can be ordinary resistors if desired, or you may use tempcos.  Better temperature tracking will occur with the use of tempco resistors, however there is some debate about the necessity of that for the Karplus Strong circuit’s performance.  The choice is yours. 

Build Procedure

The KS11 board is an easy build for any moderately skilled builder.  Follow whatever your preferred build technique might be, usually resistors and capacitors first then sockets then discretes then potentiometers. 

Short the aux send and aux receive connections or bring them to the front panel if desired.  These connections enable you to put effects in the feedback loop to create your own novel sounds, but they are not at all required. 

The KS11 board is designed to use Alpha potentiometers on 0.2” pin spacing along the front edge of the board.  This enables you to mount the board to the panel without mounting hardware (or with it) and removes the necessity for potentiometer wiring.  Jacks must be wired to the corresponding connections.  Please note that provision is also made for using potentiometers with 0.1” spacing as a double footprint option. 

If you chose to use tempco resistors in the expo converters, I have made it easy for you to epoxy them to the appropriate transistors.  This is because the transistors are back-to-back with the tempco resistor between them.  Add some epoxy and a small piece of metal for thermal mass and you are good to go. 

Naturally, the 22 Ohm resistors in the power supply are for safety purposes at time of initial power-up.  Once the circuit is working, you will want to short these resistors by soldering a piece of wire in their place or in parallel. 

There is a screen print error in that two of the capacitors have overlapping reference designators.  These capacitors are just power supply bypass caps of value 0.1 uF and are located to the left of U3 and U4, the BBD 8-pin DIP chips.  Simply put a 0.1 uF cap in both positions and sorry for the error. 

Aside from the above details, the build is quite simple and should go smoothly. 

Testing Your Karplus Strong Board

To test the built board, connect power and apply a pulse of approximately 3 ms to the stim input.  The pulse can vary from 0.1 ms to 10 ms or so and can be positive or negative of value less than 5V peak to peak.  Then listen to the output signal and you should hear a sound generated in response to each pulse.  If the audio is distorted or silent, adjust the volume potentiometer on the center of the board. 

Then try adjusting the front panel potentiometers to get different sounds from your pulses.  Note that the feedback dial is a bit sensitive and likes to be on the left side of center position in order to get the most resonance without the warm distortion effect. 

Next try applying the control voltage (CV) of a sequencer to the VCO and / or the VCF inputs.  You will get the widest range of sounds by trying the VCO input first as the VCF input is less obvious in tonal variation.  It is also interesting to connect both inputs together to the same CV source. 

At this time there is no calibration procedure for the expo converters. 

Final Remarks

Although some issues like component values and calibration procedures remain to be addressed, I hope that you will find the eChucK Karplus Strong circuit board to be a great value in terms of quality sound for the cost in your modular system.  I have spent over a year developing this product (off and on as funds and motivation permit) and the design process has been a real electro-music community effort.  Best of luck with your build and remember - if you are not satisfied, I will do my best to make it right.  Enjoy!